Dissertations / Theses: 'Variable amplitude loading (VAL)'

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Hooper, J. "Fretting fatigue under variable amplitude loading." Thesis, Cranfield University, 2003. http://dspace.lib.cranfield.ac.uk/handle/1826/11046.

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Fretting fatigue is a major concern in the design of engineering components that will be subjected to clamping pressures whilst operating in a vibrational environment. The type of loading environment generated in these applications is generally of a variable amplitude nature. Therefore it is important for engineers to understand how such loading affects the severity of fretting fatigue. In the absence of this understanding, engineering designers are left to apply simplistic plain fatigue life prediction rules, such as Miners linear damage law, to try to estimate the damage generated from a particular loading history. In order to try to establish a baseline for the understanding of fretting fatigue under variable amplitude loading, a structured series of CAL and V AL tests were designed and carried out. The aim of the CAL tests was to characterise the fretting fatigue test rig (designed by the author) so that an experimental foundation could be developed, upon which the fretting fatigue study was based. A structured investigation into the effects on fretting fatigue of applying various specifically designed loading histories was carried out and some very distinct relationships between the applied loading spectra and fretting fatigue life were found. For the application of overloads it was observed that overload size, overload application interval and the number of overloads in each application all had a sizeable effect on the severity of fretting. Two-level, three-level and five-level block loading tests were also investigated and numerous loading history dependent results were obtained. The applicability of Miners law to fretting fatigue was assessed. Combining Miners law with well-defined CAL stress-life results gave reasonable predictions for all of the V AL fretting fatigue tests carried out, with damage summations at failure ranging from 0.64 (non-conservative) to 3.19 (conservative). However, a more detailed investigation into the relationship between fretting fatigue lives and damage summations due to the individual loading levels highlighted that the assumption of linear damage may not be applicable to fretting fatigue, especially during crack initiation and early crack growth. A hypothesis was proposed to explain the fretting fatigue life and Miner damage results observed for the different loading spectra, and reasonable success was obtained for tests where the Miners law proved conservative (in 27 of 36 tests), although the hypothesis was not able to explain the non-conservative results obtained in the other nine tests.

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Hooper, Jeremy. "Fretting fatigue under variable amplitude loading." Thesis, Cranfield University, 2003. http://dspace.lib.cranfield.ac.uk/handle/1826/11046.

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Fretting fatigue is a major concern in the design of engineering components that will be subjected to clamping pressures whilst operating in a vibrational environment. The type of loading environment generated in these applications is generally of a variable amplitude nature. Therefore it is important for engineers to understand how such loading affects the severity of fretting fatigue. In the absence of this understanding, engineering designers are left to apply simplistic plain fatigue life prediction rules, such as Miners linear damage law, to try to estimate the damage generated from a particular loading history. In order to try to establish a baseline for the understanding of fretting fatigue under variable amplitude loading, a structured series of CAL and V AL tests were designed and carried out. The aim of the CAL tests was to characterise the fretting fatigue test rig (designed by the author) so that an experimental foundation could be developed, upon which the fretting fatigue study was based. A structured investigation into the effects on fretting fatigue of applying various specifically designed loading histories was carried out and some very distinct relationships between the applied loading spectra and fretting fatigue life were found. For the application of overloads it was observed that overload size, overload application interval and the number of overloads in each application all had a sizeable effect on the severity of fretting. Two-level, three-level and five-level block loading tests were also investigated and numerous loading history dependent results were obtained. The applicability of Miners law to fretting fatigue was assessed. Combining Miners law with well-defined CAL stress-life results gave reasonable predictions for all of the V AL fretting fatigue tests carried out, with damage summations at failure ranging from 0.64 (non-conservative) to 3.19 (conservative). However, a more detailed investigation into the relationship between fretting fatigue lives and damage summations due to the individual loading levels highlighted that the assumption of linear damage may not be applicable to fretting fatigue, especially during crack initiation and early crack growth. A hypothesis was proposed to explain the fretting fatigue life and Miner damage results observed for the different loading spectra, and reasonable success was obtained for tests where the Miners law proved conservative (in 27 of 36 tests), although the hypothesis was not able to explain the non-conservative results obtained in the other nine tests.

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BAKHTIARI, SAEEDEH. "Fatigue behaviour of welded components under variable amplitude loading." Thesis, KTH, Maskinkonstruktion (Inst.), 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-141773.

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The current thesis work is based on fatigue testing of welded structures which is completed in collaboration with Volvo CE. Improvement of the technology and customers demands for higher quality products with lower price makes the VCE to invest on the product development projects. One of the projects in VCE focuses on the weight and production cost optimization of the bogie beam on the hauler. The present project is a completion of the previous project with the objective of the fatigue testing of the bogie beam of the hauler under oscillating loads for verification of the expected fatigue life assessed with FEM. In the process of this work, an existing test rig is modified. The strains over the beam are measured through strain gauges mounted on the beam. The simulated model, the stresses and strains are investigated. The fatigue life is evaluated with different fatigue assessment methods. Finally, the outcomes of all the methods for fatigue investigation of the beam are compared and verified.
Detta examensarbete, som utförts för Volvo CE, behandlar utmattningsprov av svetsade konstruktioner. Teknikutveckling och förändrade kundkrav på högre kvalitet till en lägre kostnad är drivkrafter för Volvo CE att investera i produkt- och metodutvecklingsprojekt, som exempelvis det aktuella projektet att optimera vikt och tillverkningskostnad för boggi-balkar till dumprar. Detta projekt är en fortsättning av ett tidigare arbete som hade som mål att verifiera utmattningslivslängden för en boggibalk till en dumper predikterad med FEM, genom att utmattningsprova balken för oscillerande last. Som en del av projektet modifieras en existerande provningsrigg. Töjningar på balken mats med ett antal töjningsgivare monterade på balken. Spännings och töjningfrån den simulerade modellen analyseras och utmattningslivslängden jämförs för olika metoder för att prediktera utmattning. Slutligen jämförs och veriferas dessa metoder.

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Xiang, Zhang. "Numerical simulation of fatigue crack propagation under variable amplitude loading." Thesis, Imperial College London, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.308604.

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Aguilar-Espinosa, Aaron Alejandro. "Effect of variable amplitude loading on fatigue crack growth rate." Thesis, Oxford Brookes University, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.496022.

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Fatigue crack growth (FCG) is a major cause of failure in many engineering components and structures that are subjected to dynamic loading conditions. Several models have been proposed for estimating crack growth rate da/dN under various conditions. The majority of work reported has focused on constant amplitude (CA) loading and some for variable amplitude (VA) loading. The estimation of da/dN under VA loading is complex due to effects of several factors such as plasticity, crack tip blunting, residual stresses, crack tip closure and crack tip branching which are associated with different levels of loading. These factors which cause acceleration or deceleration of the crack growth are known as interaction effects. Crack closure has been identified to be one of the main interaction factors, and finite element (FE) models have been developed to quantify it in terms of crack opening stresses. There are however still a number of issues regarding the modelling parameters such as mesh size, element type, number of loading increments and material hardening models that should be used and on whether crack closure represents the interaction effects sufficiently. Also modelling long crack lengths has been perceived to be too computationally intensive and therefore studies focus on short crack lengths only.

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Rodopoulos, C. A. "Fatigue studies under constant and variable amplitude loading in MMCs." Thesis, University of Sheffield, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.245575.

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Razmjoo, Gholam Reza. "A fracture mechanics approach to fatigue crack propagation under variable amplitude loading." Thesis, University of Manchester, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.259194.

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Massingham, M. "Fretting fatigue under variable amplitude loading and the role of contact geometry." Thesis, Cranfield University, 2005. http://dspace.lib.cranfield.ac.uk/handle/1826/11430.

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Problem Little research has taken place to investigate fretting fatigue under conditions of variable amplitude loading (VAL) and no research using a complex loading spectra representative of reality. The effect of cycles below the endurance limit has yet to be established for fretting fatigue within such a spectrum. These cycles are of particular concern owing to the large number of these cycles present in comparison to other cycles within the spectrum. Any effects of VAL on local conditions within the contact had also yet to be established. Solution This project attacked the problem of fretting fatigue under conditions of VAL on two fronts. Firstly a vigorous VAL testing program (reconstructed from in-service data) was employed to investigate the effect of VAL on life and damage in general. The relative importance of cycles within the spectrum, particularly those below the endurance limit, with regards to life was investigated. Secondly to establish the effect of VAL within the contact region finite element modelling (FEM) was performed. Single and three level loading histories were applied to the model in order to establish the effect of VAL locally within the contact and offer explanations to the experimental observations. A series of damage prediction parameters including Ruiz and strain life initiation parameters were assessed for their ability to predict such behaviour. A methodology for predicting fretting fatigue life and damage has also been developed during this project. Two contact geometries were tested: a cylindrical Hertzian contact and the rounded punch contact. Conclusions Cycles of amplitude below the constant amplitude fretting fatigue endurance limit are nondamaging within a VAL spectrum. This was primarily attributed to the cycles below the endurance limit having a unique location of damage that other cycles do not influence. More sites of crack initiation were observed in samples that had experienced VAL than those tested under conditions of constant amplitude loading (CAL). The multiple sites of initiation were attributed to the point of maximum damage changing location during VAL. The size of the slip region of a cycle was found to decrease post overload as was the magnitude of Ruiz predicted damage. Strain life parameters also predicted a beneficial effect of an overload on the predicted lives of following CAL cycles. Miner predictions of life were conservative due to the assumptions made by the parameter. Miner sums damage over the entire contact region, essentially attributing that damage to a single location. Miner therefore does not take into account the changing location of maximum damage and the effects of load order and interaction. An alternative methodology for predicting fretting fatigue life during VAL has been presented that has been shown to be more accurate than traditional Miner and can account for unique features within the VAL spectrum e.g. training flights. Both Miner and the VAL methodology have shown that it is the small amplitude cycles within a spectrum that are the most damaging.

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Massingham, Matthew. "Fretting fatigue under variable amplitude loading and the role of contact geometry." Thesis, Cranfield University, 2005. http://dspace.lib.cranfield.ac.uk/handle/1826/11430.

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Problem Little research has taken place to investigate fretting fatigue under conditions of variable amplitude loading (VAL) and no research using a complex loading spectra representative of reality. The effect of cycles below the endurance limit has yet to be established for fretting fatigue within such a spectrum. These cycles are of particular concern owing to the large number of these cycles present in comparison to other cycles within the spectrum. Any effects of VAL on local conditions within the contact had also yet to be established. Solution This project attacked the problem of fretting fatigue under conditions of VAL on two fronts. Firstly a vigorous VAL testing program (reconstructed from in-service data) was employed to investigate the effect of VAL on life and damage in general. The relative importance of cycles within the spectrum, particularly those below the endurance limit, with regards to life was investigated. Secondly to establish the effect of VAL within the contact region finite element modelling (FEM) was performed. Single and three level loading histories were applied to the model in order to establish the effect of VAL locally within the contact and offer explanations to the experimental observations. A series of damage prediction parameters including Ruiz and strain life initiation parameters were assessed for their ability to predict such behaviour. A methodology for predicting fretting fatigue life and damage has also been developed during this project. Two contact geometries were tested: a cylindrical Hertzian contact and the rounded punch contact. Conclusions Cycles of amplitude below the constant amplitude fretting fatigue endurance limit are nondamaging within a VAL spectrum. This was primarily attributed to the cycles below the endurance limit having a unique location of damage that other cycles do not influence. More sites of crack initiation were observed in samples that had experienced VAL than those tested under conditions of constant amplitude loading (CAL). The multiple sites of initiation were attributed to the point of maximum damage changing location during VAL. The size of the slip region of a cycle was found to decrease post overload as was the magnitude of Ruiz predicted damage. Strain life parameters also predicted a beneficial effect of an overload on the predicted lives of following CAL cycles. Miner predictions of life were conservative due to the assumptions made by the parameter. Miner sums damage over the entire contact region, essentially attributing that damage to a single location. Miner therefore does not take into account the changing location of maximum damage and the effects of load order and interaction. An alternative methodology for predicting fretting fatigue life during VAL has been presented that has been shown to be more accurate than traditional Miner and can account for unique features within the VAL spectrum e.g. training flights. Both Miner and the VAL methodology have shown that it is the small amplitude cycles within a spectrum that are the most damaging.

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Shamsaei, Nima. "Multiaxial Fatigue and Deformation Including Non-proportional Hardening and Variable Amplitude Loading Effects." University of Toledo / OhioLINK, 2010. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1279760342.

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Shenoy, Vikram. "Damage characterisation and lifetime prediction of bonded joints under variable amplitude fatigue loading." Thesis, Loughborough University, 2009. https://dspace.lboro.ac.uk/2134/5400.

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Adhesive bonding is one of the most attractive joining techniques for any structural application, including high profile examples in the aerospace, automotive, marine construction and electrical industries. Advantages of adhesive bonding include; superior fatigue performance, better stress distribution and higher stiffness than conventional joining techniques. When the design of bonded joints is considered, fatigue is of critical importance in most structural applications. There are two main issues that are of importance; a) in-service damage characterisation during fatigue loading and, b) lifetime prediction under both constant and variable amplitude fatigue loading. If fatigue damage characterisation is considered, there has been some work to characterise damage in-situ using the backface strain (BFS) measurement technique, however, there has been little investigation of the effects of different types of fatigue behaviour under different types of geometry and loading. Regarding fatigue lifetime prediction of bonded joints, most of the work in the literature is concentrated with constant amplitude fatigue, rather than variable amplitude fatigue. Fatigue design of a bonded structure based on constant amplitude fatigue, when the actual loading on the structure is of the variable amplitude fatigue, can result in erroneous lifetime prediction. This is because of load interaction effects caused by changes in load ratio, mean load etc., which can decrease the fatigue life considerably. Therefore, the project aims to a) provide a comprehensive study of the use of BFS measurements to characterise fatigue damage, b) develop novel techniques for predicting lifetime under constant amplitude fatigue and c) provide an insight into various types of load interaction effects. In this project, single lap joints (SLJ) and compound double cantilever beam geometries were used. Compound double cantilever beams were used mainly to determine the critical strain energy release rate and to obtain the relationship between strain energy release rate and fatigue crack growth rate. The fatigue life of SLJs was found to be dominated by crack initiation at lower fatigue loads. At higher fatigue loads, fatigue life was found to consist of three phases; initiation, stable crack propagation and fast crack growth. Using these results, a novel damage progression model was developed, which can be used to predict the remaining life of a bonded structure. A non-linear strength wearout model (NLSWM) was also proposed, based on strength wearout experiments, where a normalised strength wearout curve was found to be independent of the fatigue load applied. In this model, an empirical parameter determined from a small number of experiments, can be used to determine the residual strength and remaining life of a bonded structure. A fracture mechanics approach based on the Paris law was also used to predict the fatigue lifetime under constant amplitude fatigue. This latter method was found to under-predict the fatigue life, especially at lower fatigue loads, which was attributed to the absence of a crack initiation phase in the fracture mechanics based approach. A damage mechanics based approach, in which a damage evolution law was proposed based on plastic strain, was found to predict the fatigue life well at both lower and higher fatigue loads. This model was able to predict both initiation and propagation phases. Based on the same model, a unified fatigue methodology (UFM) was proposed, which can be used to not only predict the fatigue lifetime, but also various other fatigue parameters such as BFS, strength wearout and stiffness wearout. The final part of the project investigated variable amplitude fatigue. In this case, fatigue lifetime was found to decrease, owing to damage and crack growth acceleration in various types of variable amplitude fatigue loading spectra. A number of different strength wearout approaches were proposed to predict fatigue lifetime under variable amplitude fatigue loading. The NLSWM, where no interaction effects were considered was found to over-predict the fatigue life, especially at lower fatigue loads. However, approaches such as the modified cycle mix and normalised cycle mix approaches were found to predict the fatigue life well at all loads and for all types of variable amplitude fatigue spectra. Progressive damage models were also applied to predict fatigue lifetime under variable amplitude fatigue loading. In this case a fracture mechanics based approach was found to under-predict the fatigue life for all types of spectra at lower loads, which was established to the absence of a crack initiation phase in this method. Whereas, a damage mechanics based approach was found to over-predict the fatigue lifetime for all the types of variable amplitude fatigue spectra, however the over- prediction remained mostly within the scatter of the experimental fatigue life data. It was concluded that, the damage mechanics based approach has potential for further modification and should be tested on different types of geometry and spectra.

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Gates, Nicholas R. "Fatigue Behavior under Multiaxial Stress States Including Notch Effects and Variable Amplitude Loading." University of Toledo / OhioLINK, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1469637495.

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Zitounis, Vasilios. "Fatigue crack growth rates under variable amplitude load spectra containing tensile underloads." Thesis, Cranfield University, 2003. http://dspace.lib.cranfield.ac.uk/handle/1826/105.

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An extensive research program was performed to investigate the load interaction effect of the combined action of small amplitude high R ratio cycles and large amplitude low R ratio underloads on the crack growth of large cracks. The study was driven by the needs of the damage tolerance approach in the helicopter structures, which requires robust knowledge on the crack growth behaviour of the advance high strength alloys under the characteristic helicopter spectra loading. The study was conducted on three metallic alloys, Ti-10V-2Fe-3Al, Al8090 T852 and Al7010 T76351 using compact tension specimens (w=70mm, t=17mm). The potential drop technique was used for the measurements of the crack length. The crack opening loads were determined from the applied load versus crack opening mouth displacement curve using a curve fitting technique and crack opening displacement gauge. The experimental results show that cracks can grow faster than the life predictions with no load interaction effects under spectra containing tensile underloads. The acceleration effects are different depending on the number of the small cycles, the Kmax, the R ratio of the small cycles, the underload cycle and the material. Significant closure observations on the underloads and on the small cycles of variable amplitude loading spectra were made. Based on the test finding and on the studies of other researchers, it is suggested that the acceleration effects are mainly due to the reduction of crack opening point of the tensile underloads comparing with the Constant Amplitude Loading (CAL) data. An extensive evaluation of the ability of FASTRAN model to predict the fatigue lives under the tested loading spectra was carried out. The evaluation focuses on the influence of the constraint factor a and the ∆Keff curve inputs on the predictions. The model produces very good and consistent predictions for the three alloys, when the inputs represent adequately the actual fatigue mechanism. The model predicts the measured acceleration effects by reducing the closure level of the underloads.

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Din, Khafilah Binti. "Fatigue crack propagation under variable amplitude loading analysed by fracture mechanics and finite element analysis." Thesis, University of Manchester, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.334012.

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Post, Nathan L. "Reliability based design methodology incorporating residual strength prediction of structural fiber reinforced polymer composites under stochastic variable amplitude fatigue loading." Diss., Virginia Tech, 2008. http://hdl.handle.net/10919/26492.

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The research presented in this dissertation furthers the state of the art for reliability-based design of composite structures subjected to high cycle variable amplitude (spectrum) fatigue loads. The focus is on fatigue analyses for axially loaded fiber reinforced polymer (FRP) composites that contain a significant proportion of fibers in the loading direction and thus have fiber-direction dominated failure. The four papers presented in this dissertation describe the logical progression used to develop an improved reliability-based methodology for fatigue-critical design. Throughout the analysis extensive experimental fatigue data on several material systems was used to verify the assumptions and suggest the path forward. A comparison of 12 fatigue model approaches from the literature showed that a simple linear residual strength approach (Broutman and Sahu) provides an improvement in fatigue life prediction compared to the Palmgren-Miner rule, while more complex residual strength models did not consistently improve on Broutman and Sahu. Evaluation of the effect of load history randomness on fatigue life was made using experimental results for spectra in terms of the first order autocorrelation of the stress events. For approximately reversed Rayleigh distributed fatigue loading, load sequence was not critical in the material behavior. Based on observations of empirical data and evaluation of the micro-mechanics deterioration and failure phenomena of FRP composites under fatigue loading, a new residual strength model for the tension and compression under any load history was proposed. Then this model was implemented in a stochastic framework and a method was proposed to enable calculation of the load and resistance factor design (LRFD) parameters for realistic reliabilities with relatively few computations. The proposed approach has significant advantages over traditional lifetime-damage-sum-based reliability analysis and provides a significant step toward enabling more accurate reliability-based design with composite materials.
Ph. D.

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Pierron, Quentin. "Caractérisation de la fatigue des assemblages soudés soumis à des chargements à amplitude variable." Electronic Thesis or Diss., Université Paris-Saclay (ComUE), 2018. https://pastel.hal.science/tel-03920237.

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Dans le monde de l'automobile, les ingénieurs conçoivent des pièces de plus en plus légères pour répondre aux nouvelles normes de dépollution. En particulier, les pièces soudées à l’arc de la Liaison Au Sol (LAS) qui soutiennent une part du poids des véhicules doivent tenir à la fatigue malgré des épaisseurs de tôle relativement fines. Pour dimensionner les assemblages soudés, des chargements à amplitude constante sont utilisés : à la fois pour la caractérisation à la fatigue des matériaux et des assemblages mais aussi dans les calculs de champs mécaniques. Toutefois, les chargements réels vus par les pièces en service leurs sont différents. Les cahots sur la route, les virages et les freinages induisent des efforts dont l’évolution temporelle est complexe. Plusieurs modèles prédisent la durée de vie sous un chargement complexe à partir de chargement à amplitude constante. Parmi ces modèles, le modèle de cumul linéaire considère que l’endommagement créé par un cycle de chargement est indépendant du chargement qui le précède. Les résultats de la littérature contredisent cette hypothèse. Cette dernière peut induire des erreurs de dimensionnement importantes. Des modèles de cumul d’endommagement alternatifs de la littérature sont alors évalués. De plus, des essais sur structures soudées minces sont mis en œuvre pour compléter les résultats expérimentaux disponibles et ainsi contribuer à la compréhension du cumul d’endommagement sous chargements à amplitude variable. Un soin particulier a été pris pour concevoir, instrumenter, modéliser et comprendre ces essais. Les effets de déformée due au soudage et de maintien des éprouvettes sont analysés et pris en compte. Des mesures de champ de température sont utilisées pour identifier la dissipation qui permet un suivi indirect de l’endommagement. C’est une extension de la méthode de caractérisation de la limite d’endurance par auto-échauffement à l’endurance limitée des structures soudées sous chargement à amplitude variable. Les essais révèlent deux phénomènes de cumul d’endommagement non linéaire à effets contraires. D’une part, les surcharges créent des contraintes résiduelles de compression qui augmentent la durée de vie des assemblages soudés. D’autre part, la répétition des surcharges empêche l’adaptation locale. À la lumière de ces essais, des préconisations sont finalement données pour les chargements de fatigue et les modèles de cumul d’endommagement adaptés aux structures soudées
In the automotive field, engineers are designing lighter and more reliable parts to meet new emissions standards. Arc-welded parts of the chassis must be resistant to fatigue despite relatively thin steel sheet. To assess welded joints fatigue, constant amplitude loadings are used: for fatigue characterization of materials and assemblies but also for mechanical field calculations. However, in-service loadings are very different. Bumps on the road, cornering and braking induce complicated time evolutions of the loading. Several models translate the complex evolution of efforts into an equivalent constant amplitude loading. Among these models, linear cumulative damage model consider that the damage created by a loading cycle is independent on the previous loading. However, this fact is contradicted by most of the literature results. This hypothesis can lead to significant design errors. Alternative damage accumulation models from the literature are evaluated. Moreover, thin welded structures are tested to determine how the damage accumulates under variable amplitude loading. Care has been taken to design, instrument, model and understand these tests. The effects of residual deformation due to welding and specimens clamping are analyzed and considering. To follow indirectly the damage evolution, dissipation is identified from temperature field measurement. It is an extension of the method of characterization of the fatigue limit by self-heating to finite life domain for welded structures under variable amplitude loading. These tests reveal two opposing effects of nonlinear damage accumulation. On the one hand, overloads create residual compression stresses that increase the life of welded joints. On the other hand, the repetition of overloads prevents the occurrence of local elastic shakedown. Considering these tests, recommendations are finally given for the choice of relevant fatigue tests loadings and damage accumulation models suitable for welded structures

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Post, Nathan L. "Modeling the Residual Strength Distribution of Structural GFRP Composite Materials Subjected to Constant and Variable Amplitude Tension-Tension Fatigue Loading." Thesis, Virginia Tech, 2005. http://hdl.handle.net/10919/36196.

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One scheme for reliability-based design that is growing in popularity for civil and naval applications is the load and resistance factor design (LRFD). Our goal in this research is the development of a simulation to predict the remaining strength of structural composites subjected to variable fatigue loading and environmental exposure. The results of this simulation can then be used in LRFD to determine appropriate material factors of safety for engineering design applications. The work so far focuses on modeling the response of the material to fatigue damage only. A general phenomenological modeling approach is described and applied in two experimental studies using E-glass/vinyl ester composite materials. Strength distributions are modeled using Weibull statistics and residual strength is modeled using a strength-life equal rank assumption and a Monte-Carlo style simulation. The model provides good residual strength distribution fits to constant amplitude fatigue data and worked well for ordered block spectrum loading using a 735,641 cycle, 22 stress level spectrum. However, applying a randomized spectrum produced unexpected results with every specimen failing after 200,000 to 400,000 cycles while the model predicts identical residual strength when compared with the block loading case. This work points to a need for focus on developing a better understanding of load order impacts in design of composite structures based on constant amplitude fatigue tests. A future approach toward more detailed micro-mechanics fatigue damage modeling is suggested to enable better modeling of residual strength of laminates subjected to random loading fatigue.
Master of Science

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Kyriacou, S. A. "Experimental and theoretical studies into the fatigue crack propagation behaviour of the 7075 aluminium-zinc alloy under constant and variable amplitude loading." Thesis, Cranfield University, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.280937.

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Arcari, Attilio. "Enhanced strain-based fatigue methodology for high strength aluminum alloys." Diss., Virginia Tech, 2010. http://hdl.handle.net/10919/26178.

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The design of any mechanical components requires an understanding of the general statical, dynamical and environmental conditions where the components will be operating to give a satisfactory results in terms of performance and endurance. The premature failure of any components is undesirable and potentially catastrophic, therefore predictions on performances and endurances of components to proceed with repair or substitution is vital to the stability of the structure where the component is inserted. The capability of a component of withstanding fatigue loading conditions during service is called fatigue life and the designed predictions can be conservative or non conservative. Improvements to a strain based approach to fatigue were obtained in this study, studying the effects of mean stresses on fatigue life and investigating cyclic mean stress relaxation of two aluminum alloys, 7075-T6511 and 7249-T76511, used in structural aircraft applications. The two aluminum alloys were tested and their fatigue behavior characterized. The project, entirely funded by NAVAIR, Naval Air Systems Command, and jointly coordinated with TDA, Technical Data Analysis Inc., was aimed to obtain fatigue data for both aluminum alloys, with particular interest in 7249 alloy because of its enhanced corrosion resistance, and to give guidelines for improving the performances of FAMS, Fatigue Analysis of Metallic Structures, a life prediction software from the point of view of both mean stress effects and mean stress relaxation. The sensitivity of engineering materials to mean stresses is of high relevance in a strain based fatigue approach. The performance of the most common models used to calculate mean stress correction factors was studied for the two aluminum alloys 7075 and 7249 to give guidelines in the use of those for life predictions. Not only mean stresses have a high influence on fatigue life, but they are also subjected to transient cyclic behaviors. The following study considered both an empirical approach and a plasticity theory approach to simulate and include these transient effects in life calculations. Results will give valid directions to a successful modification of FAMS like any other life calculation software to include in the picture transient phenomena.
Ph. D.

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Miyaura, Edson Haruo. "Modélisation de fatigue et de mécanique de la rupture d'une structure éolienne soumise au chargement dynamique et aléatoire du vent." Thesis, Rouen, INSA, 2016. http://www.theses.fr/2016ISAM0005/document.

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L'objectif de cette thèse est de démontrer comment faire une analyse théorique de fatigue et de mécanique de la rupture d'une structure éolienne à l'axe horizontal. La chaîne des calculs nécessaires pour atteindre cet objectif s'avère être particulièrement longue pour deux raisons : d'abord, la vitesse du vent varie aléatoirement avec le temps ; deuxièmement, l'amplitude de vibration du mât est amplifié en raison des ses fréquences naturelles de vibration. Un chapitre entier est consacré à la modélisation de la vitesse du vent dans l'espace et dans le temps. Ce même chapitre démontre comment synthétiser un signal aléatoire à partir d'une fonction de densité spectrale de puissance (DSP). La force axiale du rotor est le chargement le plus important sur une structure éolienne à l'axe horizontal. Cette force a un rapport non linéaire avec la vitesse du vent. Cela implique la nécessité de déterminer la DSP de la force axiale à partir de son signal, en se servant d'une technique d'estimation spectrale. La méthode Thomson Multitaper s'est avéré la plus satisfaisante pour cette application. La DSP des déplacements du mât est déterminée en associant la réceptance du système structurel avec la DSP de la force qui représente tous les chargements. Un signal de contrainte peut finalement être synthétisé à partir de sa DSP. La technique de comptage de cycles de chargement connue sous le nom de rainflow est abordée et appliquée. Le fait que le signal de contraintes a une amplitude variable implique la nécessité d'employer une technique plus avancée de simulation de propagation de fissures. La technique choisie pour cette thèse est connue sous le nom de strip-yield (bande d'écoulement)
The objective of this thesis is to demonstrate how to do theoretical analyses of fatigue and fracture mechanics in a structure for horizontal axis wind turbine. The chain of calculations required to reach this objective is particularly long for two reasons : firstly, the wind speed varies randomly with time , secondly, the vibration amplitude of the mast is amplified due to its natural frequencies of vibration. A whole chapter is dedicated to modeling the wind speed in space and time. The same chapter shows how to synthesize a random signal by employing a power spectral density function (PSD). The axial force of the rotor is the most important loading on a structure for horizontal axis wind turbine. This force has a non linear relation with the wind speed. This implies the need to determine the PSD of the axial force from its signal, by employing a spectral estimation method. The Thomson Multitaper method revealed to be the most satisfactory for this application. The PSD of displacement of the mast is determined by associating the receptance of the structural system and the PSD of the force representing all loadings. Finally, a signal of stress can be synthesized from its PSD. The fatigue cycle counting method known as rainflow is discussed and employed. The fact that the signal of stress has a variable amplitude implies the need of a more sophisticated method to simulate a crack propagation. The method chosen in this thesis is called strip-yield

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Solomon, Daniel Maurice. "Effects of Mission Overloads on Fatigue Crack Growth in Ti-6Al-2Sn-4Zr-2Mo." University of Dayton / OhioLINK, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=dayton1532083676103601.

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Sharifimehr, Shahriar. "Multiaxial Fatigue Analysis under Complex Non-proportional Loading Conditions." University of Toledo / OhioLINK, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1544787705876488.

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Dimithe, Aboumou Loïc. "Etude expérimentale et simulation numérique de propagation de fissures dans un acier inoxydable martensitique durci par précipitation sous conditions représentatives en termes de température, spectre de chargement et vieillissement." Thesis, Chasseneuil-du-Poitou, Ecole nationale supérieure de mécanique et d'aérotechnique, 2017. http://www.theses.fr/2017ESMA0013/document.

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Reliant le moteur à turboréacteur à la voilure de l’avion, le mât-réacteur est une véritable « pièce maitresse » de l’avion. En effet, il transmet tous les efforts de l’avion au moteur. Il est soumis à des variations de températures allant de -40°C en croisière à290°C voir 400°C lors des phases de décollage. En plus, le mât-réacteur est à la merci de contraintes vibratoires très élevées qui,n’étant pas correctement maitrisées lors des analyses en tolérances aux dommages, peuvent conduire à la ruine de l’appareil. Entre290°C et 400°C, la martensite constitutive des composants en acier inoxydable martensitique durci par précipitation (15-5PH) du mât-réacteur, subit donc un vieillissement par transformation microstructurale. Ce vieillissement a un impact considérable sur les propriétés mécaniques, à savoir à une augmentation de la limite d’élasticité et de la contrainte à rupture aux dépens d’une réduction drastique de la ténacité et la ductilité. Afin de compléter la caractérisation des effets du vieillissement sur les propriétés mécaniques tout en considérant que ces structures sont dimensionnées suivant un principe de tolérance aux dommages, l’objectif de ce travail est d’étudier la résistance à la fissuration par fatigue de cet acier en fonction du vieillissement et de la température d’essai. La démarche adoptée repose sur une connaissance des comportements monotone et cyclique pour analyser les mécanismes en fissuration. Des essais de comportement cyclique ont ainsi été effectués à la température ambiante et à 300°C à différents niveaux de déformation imposés, sur l’acier 15-5PH dans son état de réception, puis pour des conditions vieillissement réalisées entre 300°C et 400°C et des temps d’exposition allant jusqu’à 10 000h. Les résultats obtenus mettent en évidence l’absence d’influence du vieillissement sur l’écrouissage cyclique de l’acier 15-5PH, aussi bien à température ambiant qu’à 300°C. Pour la plage de valeurs de ΔK balayée, le comportement en fissuration de l’acier 15-5PH sous amplitude de chargement constante n’est pas modifié par le vieillissement.Cependant, l’étendue du domaine stable de propagation est quant à elle réduite en fonction du degré de vieillissement à température ambiante. Cette réduction est due à la chute de ténacité du matériau avec le vieillissement. Les surfaces de rupture sont majoritairement transgranulaires pour toutes les conditions examinées. Toutefois, les régions proches de la rupture finale de certains états vieillis présentent des îlots de rupture statique à la température ambiante. Ces ilots sont inexistants à 300°C. Par ailleurs, sous l’effet de surcharges répétées, un effet retard sur la vitesse de fissuration a été mis en évidence. Ce retard est fonction à la fois du taux de surcharge, de la période de surcharge, du nombre de surcharge et du rapport de charge du chargement de base, mais est insensible au vieillissement. Par ailleurs, on dénote une fois de plus, une réduction de l’étendue du domaine de propagation stable à température ambiante. Des simulations de la propagation des fissures sous amplitude de chargement variable ont été effectuées à l’aide du modèle incrémental de prévisions de durées de vie en fissuration développé au LMT-Cachan. Les résultats issus de ce modèle ont ensuite fait l’objet d’une comparaison avec le modèle PREFFAS utilisé chez AIRBUS. Le modèle incrémental rend bien compte de certains effets de surcharges répétées. Il se révèle en outre moins conservatif que le modèle PREFFAS. Une méthodologie de prise en compte dans le modèle incrémental des effets de vieillissement fondée sur une équivalence temps/température de type Hollomon-Jaffe est enfin proposée
Connecting the turbofan engine to the wing of the aircraft, the engine pylon is a true « masterpiece » of the aircraft. Indeed, it transmits all the aircraft engine efforts. It is subject to temperature variations from -40°C in cruise to 290°C-400°C during take off and landing. In addition, the engine pylon is a prey to very high vibratory stresses, which should be properly taken into account during damage tolerances analysis to avoid the loss of the aircraft. Between 290°C and 400°C, the martensite of components in precipitation-hardenable stainless steel (15-5 PH) of engine pylon undergoes microstructural transformation (« ageing »). This ageing has a significant impact on the mechanical properties, characterized by an increase in yield strength and tensile stress and drastic reduction in toughness and ductility. To complete the characterization of the effects of ageing on the mechanical properties while considering that these structures are designed according to a principle of damage tolerance, the aim of this work is to study the fatigue crack growth behavior (FCGB) of this material according to the ageing conditions and the test temperature. The approach is based on knowledge of monotonous and cyclic behavior to analyze the fatigue crack mechanisms. The cyclic behavior tests have been carried out at room temperature and 300°C at different strains imposed levels, on the 15-5PH steel in its as-received and then to the ageing conditions realized, between 300°C and 400°C and exposure times of up to 10 000h. The results highlight the lack of influence of ageing on the cyclic hardening of 15-5 PH steel, both at room temperature to 300°C. For the range of ΔK values tested, the FCGB of the 15-5PH steel under constant load amplitude is not affected by ageing. However, the extent of the stable propagation domain is itself reduced according to the degree of aging at room temperature. This reduction is due to the fall of fracture toughness due to ageing. The fracture surfaces are mainly transgranular for all conditions examined. However, the areas close to the final rupture ofsome ageing statements present islands indicative of a static failure mode at room temperature. These islands are absent to 300°C.Under the effect of repeated loads, a delayed effect on the crack velocity has been demonstrated. This delay is a function of the overload rate, overload period, the number of overloads and the baseline load ratio, but insensitive to ageing. Furthermore, are duction in the extent of the area stable propagation is also noticed at room temperature. Fatigue crack growth simulations undervariable amplitude loading were made through the incremental model for damage tolerance analysis developed by LMT-Cachan. The model results were then subject to a comparison with the PREFFAS model used at AIRBUS. The incremental model is well aware ofsome of the effects of repeated overloads. It also proves less conservative than the model PREFFAS. For taking account the effects of ageing in the incremental model, simply report the hardening observed on old material, the cyclic hardening parameters are notaffected. A methodology based on time/temperature equivalence provided by Hollomon-Jaffe - and taking into account the effects of ageing in the incremental model is finally proposed

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Colin, Julie Anne. "Deformation History and Load Sequence Effects on Cumulative Fatigue Damage and Life Predictions." University of Toledo / OhioLINK, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1260390033.

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Nešpůrek, Lukáš. "STOCHASTIC CRACK PROPAGATION MODELLING USING THE EXTENDED FINITE ELEMENT METHOD." Doctoral thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2010. http://www.nusl.cz/ntk/nusl-233900.

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Tato disertační práce vychází z výzkumu v rámci francouzsko-českého programu doktorátu pod dvojím vedením na pracovišti Institut français de mécanique avancée v Clermont-Ferrand a na Ústavu fyziky materiálu AV v Brně. Úvodní výzkumný úkol na brněnském pracovišti se zabýval numerickou analýzou pole napětí v okolí čela trhliny v tenké kovové fólii. Zvláštní pozornost byla zaměřena na vliv speciálního typu singularity v průsečíku čela trhliny s volným povrchem. Těžiště disertační práce spočívá v numerickém modelování a stochastické analýze problémů šíření trhlin se složitou geometrií v dvojrozměrném prostoru. Při analýze těchto problémů se dříve zřídka používaly numerické metody, a to z důvodu vysoké náročnosti na výpočtový čas. V této disertaci je ukázáno, že aplikací moderních metod numerické mechaniky a vhodných technik v analýze spolehlivosti lze tyto problémy řešit s pomocí numerických metod i na PC. Ve spolehlivostní analýze byla využita lineární aproximační metoda FORM. Pro rychlost šíření trhlin se vycházelo z Parisova-Erdoganova vztahu. Pro parametry tohoto vztahu byl použit dvourozměrný statistický model, který postihuje vysokou citlivost na korelaci obou parametrů. Mechanická odezva byla počítána rozšířenou metodou konečných prvků (XFEM), která eliminuje výpočetní náročnost a numerický šum související se změnou sítě v klasické metodě konečných prvků. Prostřednictvím přímé diferenciace bylo odvozeno několik vztahů pro derivace funkce odezvy, čímž se dosáhlo lepší numerické stability a konvergence spolehlivostní analýzy a výrazného zkrácení doby výpočtu. Problém zatížení s proměnou amplitudou byl řešen aplikací transformace zatížení metodou PREFFAS. Využití distribuce výpočtů v síti PC umožnilo další zrychlení analýzy.

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Dallmeier, Johannes. "Experimental analysis and numerical fatigue modeling for magnesium sheet metals." Doctoral thesis, Technische Universitaet Bergakademie Freiberg Universitaetsbibliothek "Georgius Agricola", 2016. http://nbn-resolving.de/urn:nbn:de:bsz:105-qucosa-209124.

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The desire for energy and resource savings brings magnesium alloys as lightweight materials with high specific strength more and more into the focus. Most structural components are subjected to cyclic loading. In the course of computer aided product development, a numerical prediction of the fatigue life under these conditions must be provided. For this reason, the mechanical properties of the considered material must be examined in detail. Wrought magnesium semifinished products, e.g. magnesium sheet metals, typically reveal strong basal textures and thus, the mechanical behavior considerably differs from that of the well-established magnesium die castings. Magnesium sheet metals reveal a distinct difference in the tensile and compressive yield stress, leading to non-symmetric sigmoidal hysteresis loops within the elasto-plastic load range. These unusual hysteresis shapes are caused by cyclic twinning and detwinning. Furthermore, wrought magnesium alloys reveal pseudoelastic behavior, leading to nonlinear unloading curves. Another interesting effect is the formation of local twin bands during compressive loading. Nevertheless, only little information can be found on the numerical fatigue analysis of wrought magnesium alloys up to now. The aim of this thesis is the investigation of the mechanical properties of wrought magnesium alloys and the development of an appropriate fatigue model. For this purpose, twin roll cast AM50 as well as AZ31B sheet metals and extruded ME21 sheet metals were used. Mechanical tests were carried out to present a comprehensive overview of the quasi-static and cyclic material behavior. The microstructure was captured on sheet metals before and after loading to evaluate the correlation between the microstructure, the texture, and the mechanical properties. Stress- and strain-controlled loading ratios and strain-controlled experiments with variable amplitudes were performed. Tests were carried out along and transverse to the manufacturing direction to consider the influence of the anisotropy. Special focus was given to sigmoidal hysteresis loops and their influence on the fatigue life. A detailed numerical description of hysteresis loops is necessary for numerical fatigue analyses. For this, a one-dimensional phenomenological model was developed for elasto-plastic strain-controlled constant and variable amplitude loading. This model consists of a three-component equation, which considers elastic, plastic, and pseudoelastic strain components. Considering different magnesium alloys, good correlation is reached between numerically and experimentally determined hysteresis loops by means of different constant and variable amplitude load-time functions. For a numerical fatigue life analysis, an energy based fatigue parameter has been developed. It is denoted by “combined strain energy density per cycle” and consists of a summation of the plastic strain energy density per cycle and the 25 % weighted tensile elastic strain energy density per cycle. The weighting represents the material specific mean stress sensitivity. Applying the energy based fatigue parameter on modeled hysteresis loops, the fatigue life is predicted adequately for constant and variable amplitude loading including mean strain and mean stress effects. The combined strain energy density per cycle achieves significantly better results in comparison to conventional fatigue models such as the Smith-Watson-Topper model. The developed phenomenological model in combination with the combined strain energy density per cycle is able to carry out numerical fatigue life analyses on magnesium sheet metals.

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Ruchert, Cassius Olivio Figueiredo Terra. "Estudo da inferência de carregamento em histórias de vôos simulados na liga de Al aeronáutico SAE-AMS 7475 T7351." Universidade de São Paulo, 2007. http://www.teses.usp.br/teses/disponiveis/88/88131/tde-22092008-131854/.

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Este trabalho discutiu o estudo da inferência de carregamento e a reconstrução fratográfica da taxa de propagação da trinca por fadiga em histórias de vôos simulados do tipo Twist, mini-Twist, Falstaff e mini-Falstaff. O material estudado foi uma liga de alumínio de grau aeronáutico SAE-AMS 7475 T7351 na qual foram obtidas as propriedades mecânicas a partir dos ensaios de tração e dureza, análise química, metalográfica. Foram realizados ensaios de amplitude constante de carregamento em corpos de prova com geometria tipo C(T) extraídos de um bloco e ensaios de amplitude variável em corpos de prova com geometria tipo M(T) extraídos de uma placa. Corpos de prova do tipo C(T) foram extraídos da orientação L-T e T-L, à temperatura ambiente com diferentes espessuras para a realização de ensaios de propagação de trinca por fadiga (da/dN vs. ΔK) com distintos valores de razão de carregamento,R (0,1;0,3; 0,5; 0,7 e 0,8) de acordo com a norma ASTM-E647 (1993). Foram realizados dois tipos de ensaios de fadiga com amplitude constante de carregamento. Primeiro, o ensaio com ΔP constante para obtenção dos dados dos estágios II e III das curvas da/dN vs. ΔK para distintos valores de R e, posteriormente, o ensaio com ΔK constante e R crescente para ser utilizado no estudo da inferência de carregamento de amplitude constante a partir da relação entre a morfologia da estria, H/s e a razão de carregamento R. Adicionalmente foram realizados ensaios de tenacidade à fratura, nas direções L-T e T-L, conforme a norma ASTME1820 (1999). As taxas de propagação macroscópica e microscópica foram comparadas e boas correlações foram obtidas para um intervalo entre 0,1 a 1,0 μm/ciclo, mesmo com distintos R. Através da técnica proposta por Berkovitz (1995), o espectro de carregamento foi estimado para distintos valores de R, através da técnica de parametrização do fator K e os resultados foram comparados ao espectro de carregamento real imposto pela máquina de ensaio. Após os ensaios de amplitude constante de carregamento foram realizados ensaios de amplitude variável com a aplicação em corpos de prova M(T) de espectros de vôos simulados padronizados pela NLR do tipo Twist e Falstaff e seus vôos reduzidos, mini-Twist e mini- Falstaff, em uma máquina servohidráulica de ensaios dinâmicos. A partir destes ensaios notou-se que suprimir os vôos com os carregamentos menos severos influencia muito a vida da propagação da trinca de fadiga, quando vôos simulados do tipo Twist e mini-Twist, fato este não observado para o Falstaff e mini-Falstaff. Foi possível observar também o efeito da aceleração e desaceleração da trinca por fadiga observando os gráficos de d2a/dvôo vs. a obtidos dos ensaios de amplitude variável. Foi realizado um estudo da superfície de fratura do CP05 por meio da técnica fractográfica do reconhecimento de blocos marcadores sendo possível mostrar uma satisfatória reconstituição da taxa de propagação da trinca por fadiga de vôo simulado do tipo Falstaff por meio de medidas das marcações na superfície de fratura dos vôos mais severos, realizadas em imagens obtidas por MEV.
The loading inference and the fractographic reconstruction of fatigue crack propagation rate in simulated flight stories, as Twist, mini-Twist, Falstaff and mini-Falstaff were investigated in this work. The studied material was an aluminum alloy SAE-AMS 7475 T7351. Tensile and hardness tests were carried out in order to determine mechanical properties, as well as chemical and microstructure analysis of the material. C(T) specimens were extracted from a block of material for constant amplitude load testing and M(T) specimens were extracted from a plate and were tested in variable amplitude loading. The curves (da/dN vs. ΔK) for constant-amplitude loading were obtained from fatigue crack propagation tests performed in L-T and T-L directions in C(T) specimens for R (0.1; 0.3; 0.5; 0.7 and 0.8), at room temperature, according to ASTM-E647 (1993). Two different constant loading fatigue tests were performed. At constant ΔP and distinct values of R for obtaining data of stages II and III of da/dN - ΔK curves; and at constant ΔK and increasing the R ratio to be used in the inference of constant amplitude loading in the relation of H/s striation morphology (obtained from fractographic images) and the loading ratio, R. Additionally, fracture toughness tests were executed in T-L and L-T directions, according to ASTM-E1820 (1999). The macroscopic and microscopic fatigue crack propagations were compared and good correlation was obtained for the range 0.1-1.0 μm/cycle, for all R values. By using the K factor parametrization method, Berkovitz (1995), the loading spectrum was estimated for the distinct R-values and the results were compared to the actual spectrum imposed by the test machine. Variable amplitude tests were performed in M(T) specimens by applying simulating flight spectra normalized by NLR, as Twist, mini-Twist, Falstaff and mini-Falstaff, in a servohydraulic test machine MTS, in which a Flextest GT controller was coupled. The suppression of less severe loading flights showed a great influence in fatigue crack propagation life, in simulated flight tests Twist and mini-Twist, but not in Falstaff and mini-Falstaff. The acceleration and slowing down of fatigue crack were observed in the d2a/dflight - a curve obtained from variable loading tests. The fractured surfaces of some samples were examined by the fractographic technique of blocking marks recognition. A good reconstitution of fatigue crack propagation rate of a simulated flight like Falstaff was obtained by the measurements of marks in the fracture surfaces of the more severe flights, in MEV images.

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(12868682), Gary D. Kreiser. "Fatigue life of metals with particular reference to nonlinear damage accumulation under variable amplitude loading." Thesis, 2004. https://figshare.com/articles/thesis/Fatigue_life_of_metals_with_particular_reference_to_nonlinear_damage_accumulation_under_variable_amplitude_loading/20063894.

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Fatigue in engineering components and structures subjected to cyclic loading is characterised by the process of accumulation of damage over a period of time. This phenomenon of damage accumulation could potentially lead to catastrophic failures. Unfortunately the occurrence of such failures is difficult to predict due to a limited understanding of the fatigue process particularly under variable amplitude loading (VAL). Fatigue life of metals is usually determined from linear damage rule (LDR) that offers sufficient level of conservatism and accuracy, particularly for high cycle fatigue (HCF) loading blocks typified by magnitudes of stresses well within elastic limit and period exceeding millions of cycles. However, for low cycle fatigue (LCF) associated with a relatively small number of high amplitude load cycles causing bulk plastic strain, the damage process is affected by the amplitude and sequence of plastic strain. Past attempts to apply the LDR to predict fatigue life for LCF have been largely unsuccessful. Irrespective of the various modifications proposed, the LDR has failed to capture the nonlinear accumulation of damage observed for certain load sequences within VAL spectra, where at least a portion of the loads cause plastic strain. Amongst the variety of LCF life prediction models proposed in the literature, those based on the energy approach of continuum damage mechanics (CDM) provide significant scope to further the development as they conform to the principles of thermodynamics and hysteretic energy dissipation. This thesis presents an energy based nonlinear damage accumulation (NLDA) model that utilises the CDM concept and a cumulative damage parameter to account for nonlinear damage accumulation under VAL. The fatigue life predictions of the NLDA model for metals subjected to constant, high amplitude, strain controlled loading, compare well with the experimental data reported in the literature. The nonlinear capability of the NLDA model has been demonstrated using a well established two-step block -loading test.

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Iranpour, Mohammad. "FATIGUE CHARACTERIZATION OF RISERS AND PIPELINES UNDER REALISTIC VARIABLE AMPLITUDE LOADING AND THE INFLUENCE OF COMPRESSIVE STRESS CYCLES." Thesis, 2013. http://hdl.handle.net/10222/50472.

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One of the most prominent factors affecting the performance and longevity of risers is vortex induced vibration (VIV), which can cause severe fatigue damage, especially in risers used in deep waters. The available approaches for analyzing VIV induced fatigue in risers mainly focus on the VIV aspect of the problem; indeed less attention has been paid on the effect of VIV on a riser’s fatigue life and in prediction of fatigue life using various models. This dissertation first demonstrates how one can characterize fatigue of pipes and risers using an equivalent plate specimen as opposed to using a pipe specimen, thereby simplifying the task, yet obtaining good accuracy. Actual variable amplitude loadings (VAL) are used to study the fatigue crack growth in risers’ material with a focus on the various influencing parameters. Extensive experimental investigations are performed, followed by analytical and computational nonlinear finite element analyses. It is shown that the higher harmonics do cause significant fatigue damage, thus their influence should not be ignored. The influence of load interaction effects is also investigated, focusing on the fatigue crack growth retardation effects due to tension overloads, as well as the acceleration effects due to compression underloads. The crack closure concept is then used to explore into both the fatigue retardation and acceleration effects within a VAL scenario. An effective method for calculation of the stress intensity factor is proposed, which considers only the tensile portion of the stress range, while proposing another effective approach for accounting for the influence of compressive stress cycles. Moreover, a two-parameter approach is used in this dissertation, relating the fatigue crack growth rate (FCGR) to the crack tip opening displacement (CTOD). It is shown that the CTOD provides adequate information for calculating the FCGR under VAL, and it can be effectively used to account for the influence of the compressive stress cycles. The experimental investigation also considers the retardation effect resulting from the applied peak tensile overload cycles (TOLC) and the influence of various so-called “clipping” levels, demonstrating the significant influence of the TOLC on crack growth retardation in VAL.

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Ibrahim, Elfaitori. "Fatigue Life Assessment of 30CrNiMo8HH Steel Under Variable Amplitude Loading." Thesis, 2012. http://hdl.handle.net/10012/7216.

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The actual service loading histories of most engineering components are characterized by variable amplitudes and are sometimes rather complicated. The goal of this study was to estimate the fatigue life of nickel-chromium-molybdenum 30CrNiMo8HH steel alloy under axial and pure torsion variable amplitude loading (VAL) conditions. The investigation was directed at two primary factors that are believed to have an influence on fatigue life under such loading conditions: load sequence and mean stress. The experimental work for this research included two-step loading, non-zero mean strain loading, and VAL tests, the results of which were added to previously determined fully reversed strain-controlled fatigue data. The effect of load sequence on fatigue life was examined through the application of the commonly used linear damage accumulation rule along with the Manson and Marco–Starkey damage accumulation methods, the latter of which takes load sequence into account. Based on the two-step experimental results, both the Manson and Marco–Starkey methods were modified in order to eliminate the empirically determined constants normally required for these two methods. The effect of mean stress on fatigue life was investigated with the use of three life prediction models: Smith–Watson–Topper (SWT), Fatemi–Socie (FS), and Jahed–Varvani (JV). The cycles from the VAL histories were counted using a rainflow counting procedure that maintains the applied strain sequence, and a novel method was developed for the estimation of the total energy density required for the JV model. For two-step loading and for all three fatigue models employed, the modified damage accumulation methods provided superior fatigue life predictions. However, regardless of the damage accumulation method applied, the most satisfactory fatigue life correlation for VAL was obtained using the energy-based JV model.

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"Sub cycle Corrosion fatigue Crack Growth under Variable Amplitude Loading." Master's thesis, 2019. http://hdl.handle.net/2286/R.I.54990.

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abstract: Corrosion fatigue has been of prime concern in railways, aerospace, construction industries and so on. Even in the case of many medical equipment, corrosion fatigue is considered to be a major challenge. The fact that even high strength materials have lower resistance to corrosion fatigue makes it an interesting area for research. The analysis of propagation of fatigue crack growth under environmental interaction and the life prediction is significant to reduce the maintenance costs and assure structural integrity. Without proper investigation of the crack extension under corrosion fatigue, the scenario can lead to catastrophic disasters due to premature failure of a structure. An attempt has been made in this study to predict the corrosion fatigue crack growth with reasonable accuracy. Models that have been developed so far predict the crack propagation for constant amplitude loading (CAL). However, most of the industrial applications encounter random loading. Hence there is a need to develop models based on time scale. An existing time scale model that can predict the fatigue crack growth for constant and variable amplitude loading (VAL) in the Paris region is initially modified to extend the prediction to near threshold and unstable crack growth region. Extensive data collection was carried out to calibrate the model for corrosion fatigue crack growth (CFCG) based on the experimental data. The time scale model is improved to incorporate the effect of corrosive environments such as NaCl and dry hydrogen in the fatigue crack growth (FCG) by investigation of the trend in change of the crack growth. The time scale model gives the advantage of coupling the time phenomenon stress corrosion cracking which is suggested as a future work in this paper.
Dissertation/Thesis
Masters Thesis Aerospace Engineering 2019

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Mikheevskiy, Semen. "Elastic-Plastic Fatigue Crack Growth Analysis under Variable Amplitude Loading Spectra." Thesis, 2009. http://hdl.handle.net/10012/4402.

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Most components or structures experience in service a variety of cyclic stresses. In the case of cyclic constant amplitude loading the fatigue crack growth depends only on the crack, the component geometry and the applied loading. In the case of variable amplitude loading it also depends on the preceding cyclic loading history. Various types of load sequence (overloads, under-loads, or combination of them) may induce different load-interaction effects which can cause either acceleration or reduction of the fatigue crack growth rate. The previously developed UniGrow fatigue crack growth model for constant amplitude loading histories which was based on the analysis of the local stress-strain material behaviour at the crack tip has been improved, modified and extended to such a level of sophistication that it can be used for fatigue crack growth analyses of cracked bodies subjected to arbitrary variable amplitude loading spectra. It was shown that the UniGrow model enables to correctly predict the effect of the applied compressive stress and tensile overloads by accounting for the existence of the internal (residual) stresses induced by the reversed cyclic plasticity around the crack tip. This idea together with additional structural memory effect model has been formalized mathematically and coded into computer program convenient for predicting fatigue crack growth under arbitrary variable amplitude loading spectra. The experimental verification of the proposed model was performed using 7075-T6, 2024-T3, 2324-T7, 7010-T7, 7050-T7 aluminium alloys, Ti-17 titanium alloy, and 350WT steel. The good agreement between theoretical and experimental data proved the ability of the UniGrow model to predict fatigue crack growth and fatigue crack propagation life under a wide variety of real variable amplitude loading spectra.

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33

"Subcycle Fatigue Crack Growth Formulation for Constant and Variable Amplitude Loading." Master's thesis, 2016. http://hdl.handle.net/2286/R.I.40326.

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abstract: A previously developed small time scale fatigue crack growth model is improved, modified and extended with an emphasis on creating the simplest models that maintain the desired level of accuracy for a variety of materials. The model provides a means of estimating load sequence effects by continuously updating the crack opening stress every cycle, in a simplified manner. One of the significant phenomena of the crack opening stress under negative stress ratio is the residual tensile stress induced by the applied compressive stress. A modified coefficient is introduced to determine the extent to which residual stress impact the crack closure and is observed to vary for different materials. Several other literature models for crack closure under constant loading are also reviewed and compared with the proposed model. The modified model is then shown to predict several sets of published test results under constant loading for a variety of materials. The crack opening stress is formalized as a function of the plastic zone sizes at the crack tip and the current crack length, which provided a means of approximation, accounting for both acceleration and retardation effects in a simplified manner. A sensitivity parameter is introduced to modify the enlarged plastic zone due to overload, to better fit the delay cycles with the test data and is observed to vary for different materials. Furthermore, the interaction effect induced by the combination of overload and underload sequence is modeled by depleting the compressive plastic zone due to an overload with the tensile plastic zone due to an underload. A qualitative analysis showed the simulation capacity of the small time scale model under different load types. A good agreement between prediction and test data for several irregular load types proved the applicability of the small time scale model under variable amplitude loading.
Dissertation/Thesis
Masters Thesis Mechanical Engineering 2016

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Yang, Chia Han, and 楊佳翰. "Reliability and Fatigue Life Assessment of Rotational Shafts under Variable Amplitude Loading." Thesis, 2000. http://ndltd.ncl.edu.tw/handle/38739919439561100261.

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碩士
國立臺灣大學
機械工程學研究所
88
High-cycle fatigue is one of the major reasons that cause cracks at the shaft of the reactor coolant pump (RCP) of the domestic Maanshan nuclear power plant. In order to study the fatigue problem of the shaft, experimental work is carried out using a rotatory bending test machine in the present study. The experiment includes constant-amplitude, two-stress-level, three-stress-level and random-amplitude fatigue tests. Considerably large amounts of tests are performed for some particular cases with a view to studying the so-called ‘fatigue reliability’ of the tested material. After all tests are completed, analytical work is performed. The analytical work includes the construction of the P-S-N curve (Probabilistic S-N curve), the investigation of the probability distribution of the fatigue life, the examination of applicability of different damage accumulation rules, and the develop of simple formulas for the prediction of fatigue damage and fatigue life when random loading is concerned. After careful study, several conclusions are drawn in the present thesis for the tested material. First, the fatigue life follows a Weibull or normal probability distribution. Secondly, the Corten-Dolan, Marco-Starkey along with some fatigue damage accumulation rules provide better damage prediction than the others do. Thirdly, the proposed fatigue damage prediction algorithm can be extended from two-stress-level, three-stress-level to multi-stress-level as well as the case of random loading, although more experimental study is needed for the last case. It is believed that the about results are helpful for engineers working in the nuclear power plant to assess the integrity and safety of the RCP.

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Mehrzadi, Morteza. "FATIGUE CHARACTERIZATION OF AM60B MAGNESIUM ALLOY SUBJECTED TO CONSTANT AND VARIABLE AMPLITUDE LOADING WITH POSITIVE AND NEGATIVE STRESS RATIOS." 2013. http://hdl.handle.net/10222/21837.

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AM60B magnesium alloy is being increasingly used in auto industry in applications that usually involve various formats of cyclic loading scenarios. Therefore, the fatigue response of this alloy is investigated in this thesis. Our investigation is focused on characterization of the influence of compressive stress cycles within a given cyclic loading scenario on alloy’s crack propagation response. In the first part of this dissertation, fatigue crack growth rate (FCGR) of AM60B alloy subject to cyclic loadings with various stress ratios (both positive and negative) is investigated and a modified model is proposed to predict the FCGR under a wide range of stress ratios. Subsequently, using the modified model, the experimental results of the crack propagation tests are condensed into a single line in a logarithmic scale and the integrity of a proposed FCGR model is investigated. The investigation is continued by studying the influence of compressive stress cycle (CSC) on FCGR. Constant and random amplitude loadings with several magnitudes of CSCs are applied, leading to considerable acceleration in FCGR. The stress distribution ahead of the crack tip is also studied using the finite element method. The tensile residual stress and plastic zone are characterized upon the removal of the CSCs. The acceleration in the crack propagation is shown to be governed by the tensile zone ahead of the crack tip. Furthermore, application of an overload within an otherwise constant amplitude loading (CAL) has been known to retard the crack propagation, thus increase the fatigue life. This retardation would be a function of the affected zone and retardation magnitude. It is shown in this thesis that the affected zone would be influenced by the “sensitivity” of the material to overload. Moreover, it is also demonstrated that the nature of baseline CAL loading would also affect the retardation response and dimension of the affected zone. Therefore, modification to the Wheeler model is proposed, thereby enabling the model to account for material’s sensitivity and nature of the baseline loading. The integrity of the proposed model is verified by the experimental results obtained in this project, as well as those reported by other investigators for other alloys.

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El-Zeghayar, Maria. "The Introduction of Crack Opening Stress Modeling into Strain-Life and Small Crack Growth Fatigue Analysis." Thesis, 2011. http://hdl.handle.net/10012/5716.

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The work in this thesis is concerned with the mechanics of the initiation and growth of small fatigue cracks from notches under service load histories. Fatigue life estimates for components subjected to variable amplitude service loading are usually based on the same constant amplitude strain-life data used for constant amplitude fatigue life predictions. The resulting fatigue life estimates although they are accurate for constant amplitude fatigue, are always non conservative for variable amplitude load histories. Similarly fatigue life predictions based on small crack growth calculations for cracks growing from flaws in notches are non conservative when constant amplitude crack growth data are used. These non conservative predictions have, in both cases, been shown to be due to severe reductions in fatigue crack closure arising from large (overload or underload) cycles in a typical service load history. Smaller load cycles following a large near yield stress overload or underload cycle experience a much lower crack opening stress than that experienced by the same cycles in the reference constant amplitude fatigue tests used to produce design data. This reduced crack opening stress results in the crack remaining open for a larger fraction of the stress-strain cycle and thus an increase in the effective portion of the stress-strain cycle. The effective strain range is increased and the fatigue damage for the small cycles is greater than that calculated resulting in a non conservative fatigue life prediction. Previous work at Waterloo introduced parameters based on effective strain-life fatigue data and effective stress intensity versus crack growth rate data. Fatigue life calculations using these parameters combined with experimentally derived crack opening stress estimates give accurate fatigue life predictions for notched components subjected to variable amplitude service load histories. Information concerning steady state crack closure stresses, effective strain-life data, and effective stress intensity versus small crack growth rate data, are all obtained from relatively simple and inexpensive fatigue tests of smooth specimens in which periodic underloads are inserted into an otherwise constant amplitude load history. The data required to calibrate a variable amplitude fatigue crack closure model however, come from time consuming measurements of the return of crack closure levels for small cracks to a steady state level following an underload (large cracks for which crack closure measurements are easier to make cannot be used because at the high stress levels in notches under service loads a test specimen used would fracture). For low and moderately high hardness levels in metals crack growth and crack opening stress measurements have been made using a 900x optical microscope for the small crack length at which a test specimen can resist the high stress levels encountered when small cracks grow from notches. For very hard metals the crack sizes may be so small that the measurements must be made using a confocal scanning laser microscope. In this case the specimen must be removed from the test machine for each measurement and the time to acquire data is only practical for an extended research project. The parameters for the crack closure model relating to steady state crack closure levels vary with material cyclic deformation resistance which in turn increases with hardness. One previous investigation found that the steady state crack opening level was lower and the recovery to a steady state crack opening stress level after an underload was more rapid for a hard than for a soft metal. This observation can be explained by the dependence of the crack tip plastic zone size that determines crack tip deformation and closure level on metal hardness and yield strength. Further information regarding this hypothesis has been obtained in this thesis by testing three different steels of varying hardness levels (6 HRC, 35 HRC, and 60 HRC) including a very hard carburized steel having a hardness level (60 HRC) for which no crack opening stress data for small cracks had yet been obtained. This thesis introduced a new test procedure for obtaining data on the return of crack opening stress to a steady state level following an underload. Smooth specimens were tested under load histories with intermittent underload cycles. The frequency of occurrence of the underloads was varied and the changes in fatigue life observed. The changes in damage per block (the block consisted of an underload cycle followed by intermittent small cycles) were used to determine the value of the closure model parameter governing the recovery of the crack opening stress to its steady state level. Concurrent tests were carried out in which the crack opening stress recovery was measured directly on crack growth specimens using optical microscope measurements. These tests on metals ranging in hardness from soft to very hard were used to assess whether the new technique would produce good data for crack opening stress changes after underloads for all hardness levels. The results were also used to correlate crack closure model parameters with mechanical properties. This together with the steady state crack opening stress, effective strain-life data and the effective intensity versus crack growth rate data obtained from smooth specimen tests devised by previous researchers provided all the data required to calibrate the two models proposed in this investigation to perform strain-life and small crack growth fatigue analysis.

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Araújo, Francisco João Duarte de. "Influence and stability of deep rolling residual stresses in the steel AISI 1045 subjected to multi-level loads." Master's thesis, 2013. http://hdl.handle.net/10316/38811.

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Dissertação de Mestrado Integrado em Engenharia Mecânica apresentada à Faculdade de Ciências e Tecnologia da Universidade de Coimbra.
O objectivo deste trabalho é estudar a influência e estabilidade de tensões residuais no comportamento à fadiga no aço AISI 1045. O processo de laminagem profunda foi usado como tratamento mecânico para introduzir no material um estado de tensões residuais compressivas em zonas próximas da superfície. Testes à fadiga uniaxiais foram realizados de modo a avaliar o comportamento à fadiga do material tratado mecanicamente. Foram definidas três amplitudes de 300, 350 e 400 MPa aplicadas em ciclos de tensão constante e combinadas em ciclos de tensão variável. As tensões residuais e a largura do pico de difracção foram calculadas através de difracção de raio-X. O polimento electrolítico foi usado como processo para remover camadas de material. Medições de dureza foram realizadas de forma a fazer uma ligação entre o endurecimento causado pela laminagem profunda e o comportamento à fadiga. Os resultados mostram que o comportamento do material sujeito à fadiga é influenciado pela tensão aplicada. Elevadas amplitudes de tensão provocam elevadas deformações plásticas e menor resistência à fadiga. É observada uma melhoria do comportamento à fadiga do material tratado mecanicamente em relação ao não tratado. O material sujeito à fadiga exibe uma relaxação de tensões residuais que é mais pronunciada para amplitudes de tensão mais altas. Em fadiga de tensão variável, a deformação plástica aumenta ou diminui quando se varia a tensão aplicada. O número de ciclos aplicados em cada bloco de tensão influencia a estabilidade das tensões residuais induzidas. A relaxação das tensões residuais é bastante influenciada para ciclos de tensão constante seguidos de apenas meio ciclo com uma tensão mais alta ou mais baixa do que a tensão anterior.
The main objective of the work presented is the study of the influence and stability of near surface zone of deep rolled steel AISI 1045 subjected to cyclic loading. Deep rolling process was applied as a mechanical surface treatment to induce a compressive residual stress state in near surface zones of material. Tension/Compression fatigue tests were performed in order to evaluate cyclic deformation behaviour after the mechanical surface treatment. Three stress amplitudes of 300, 350 and 400 MPa were used in constant amplitude loading and combined in variable amplitude loading. Residual stress and FWHM (full width at half maximum) distribution measurement were carried out by X-ray diffraction regarding the influence of cyclic loading on near surface zone properties stability. Electropolishing was utilized as a removal material process. Microhardness measurements were carried out concerning determination of work hardened surface layers. Results show that cyclic deformation behaviour is influenced by stress amplitude. High stress amplitudes lead to high strain amplitudes and low fatigue lifetimes. An improvement of lifetime is achieved for deep rolled specimen showing the influence of mechanical surface treatment on fatigue lifetime. Residual stress distribution exhibits a relaxation due to cyclic loading, which is higher for higher stress amplitudes. In variable amplitude loading, a change of stress amplitude leads to an increase or decrease of plastic strain amplitude for higher or lower stress amplitudes, respectively. The number of cycles of each load block influences stability of residual stress state. A load block in constant amplitude loading followed by only half of one cycle of higher or lower stress amplitude than the previous block influences significantly residual stress relaxation.

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Dissertations / Theses on the topic 'Variable amplitude loading (VAL)'

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