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1
Alkoles, Omar M. S. "Mechanical behaviour and fracture toughness of unfilled and short fibre filled polypropylene both drawn and undrawn. Experimental investigation the effect of fibre content and draw ratio on the mechanical properties of unfilled and short glass fibre filled polypropylene." Thesis, University of Bradford, 2011. http://hdl.handle.net/10454/5510.
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The goal of this research is to investigate the combined effects of glass fibre reinforcement and molecular orientation in polypropylene-short glass fibre composites. Specimens have been fabricated using the injection moulding process and drawn using a small die drawing rig. The effects of die drawing on the fibre composites are complex, with the drawing process orienting both the polymer molecules and the glass fibres. This may be accompanied by the creation of voids in the polymer matrix and their destruction in the compressive stress field thus restoring the interfacial contact area between fibre and matrix. Unfilled and short glass fibre filled polypropylene specimens, with fibre content 7% wt, 13%wt, 27%wt, and 55%wt, were injection moulded prior to the die drawing process. An experimental program of die drawing within an oven at elevated temperature was conducted for polypropylene filled to various levels and at different strain rates. The specimens drew to draw ratios in the range ¿=1.41 to ¿=5.6. Mechanical characterization of the test materials has been conducted by examining the tensile stress strain and fracture behaviour under uniaxial conditions. The influence of glass fibre content and drawing conditions (draw ratio) on the fracture toughness and crack propagation was investigated using the double edge notched fracture test. The notch lengths ranged from 1.5 to 2.5 mm for 10 mm wide specimens. The critical stress intensity factor increased as the fibre content increased up to a limiting filler level. The fracture toughness of both unfilled and fibre filled polypropylene were found to be highly dependent on draw ratio. The results were analysed to find out the optimal draw ratio and fibre content that yielded the maximum modulus, strength and fracture toughness. Data showed that, at a given draw ratio, modulus, strength and fracture toughness increased with increasing fibre content to a maximum and then decreased. The optimum material was obtained at a draw ratio of 2.5 and filler loading 13wt%.
2
Alkoles, Omar M. "Mechanical behaviour and fracture toughness of unfilled and short fibre filled polypropylene both drawn and undrawn : experimental investigation of the effect of fibre content and draw ratio on the mechanical properties of unfilled and short glass fibre filled polypropylene." Thesis, University of Bradford, 2011. http://hdl.handle.net/10454/5510.
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The goal of this research is to investigate the combined effects of glass fibre reinforcement and molecular orientation in polypropylene-short glass fibre composites. Specimens have been fabricated using the injection moulding process and drawn using a small die drawing rig. The effects of die drawing on the fibre composites are complex, with the drawing process orienting both the polymer molecules and the glass fibres. This may be accompanied by the creation of voids in the polymer matrix and their destruction in the compressive stress field thus restoring the interfacial contact area between fibre and matrix. Unfilled and short glass fibre filled polypropylene specimens, with fibre content 7% wt, 13%wt, 27%wt, and 55%wt, were injection moulded prior to the die drawing process. An experimental program of die drawing within an oven at elevated temperature was conducted for polypropylene filled to various levels and at different strain rates. The specimens drew to draw ratios in the range γ=1.41 to γ=5.6. Mechanical characterization of the test materials has been conducted by examining the tensile stress strain and fracture behaviour under uniaxial conditions. The influence of glass fibre content and drawing conditions (draw ratio) on the fracture toughness and crack propagation was investigated using the double edge notched fracture test. The notch lengths ranged from 1.5 to 2.5 mm for 10 mm wide specimens. The critical stress intensity factor increased as the fibre content increased up to a limiting filler level. The fracture toughness of both unfilled and fibre filled polypropylene were found to be highly dependent on draw ratio. The results were analysed to find out the optimal draw ratio and fibre content that yielded the maximum modulus, strength and fracture toughness. Data showed that, at a given draw ratio, modulus, strength and fracture toughness increased with increasing fibre content to a maximum and then decreased. The optimum material was obtained at a draw ratio of 2.5 and filler loading 13wt%.
3
Lammens, Bastien. "Caractérisation de la décohésion dynamique des matériaux composites à matrice organique (CMO)." Electronic Thesis or Diss., Ecole centrale de Nantes, 2024. http://www.theses.fr/2024ECDN0007.
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Les matériaux composites stratifiés à matrice organique sont utilisés dans le domaine de l'aéronautique pour alléger la masse des structures. Cependant, lors d'un impact sur ce type de matériaux, différents mécanismes d'endommagements peuvent apparaître comme le délaminage. C'est un processus de décohésion macroscopique du milieu interlaminaire qui peut être caractérisé par GIC (ou KIC). La littérature montre une grande disparité dans les mesures du fait d’un découplageincomplet des effets du confinement de la résine par les fibres, des non linéarités de comportement et/ou des effets de vitesse. Ce travail propose d’élaborer un protocole expérimental de caractérisation de résine pure via mesures de champs pour étudier méthodiquement ces couplages. L’objectif est d’élucider l’impact de la vitesse de propagation de fissure et des effets de structure sur le comportement en fissuration et ainsi étendre l’approche de Griffith aux stratifiés. Différentes géométries d’éprouvette sont utilisées pour reproduire certains effets de structure. Des vitesses de fissuration allant du quasi-statique à la dynamique sont étudiées et l’ensemble des essais sont interprétés au travers de la mécanique élastique linéaire de la rupture et de l’étude des facies. Ce travail propose finalement une modélisation décrivant l'évolution de KIC, pour la résine HexplyM21 utilisée dans l'aéronautique, à partir des termes non-singuliers du champ de contraintes, le Tstress,B-stress et aussi de la vitesse ȧ dans les gammes [0 - 15] MPa, [-200 - 10] MPa.m-0.5 et [10-6, 600] m.s-1 respectivementOrganic matrix laminated composites are increasingly used in the aeronautical field to reduce the weight of structures. However, during an impact on this type of material, various damage mechanism can occur, such as delamination. This is a process of macroscopic decohesion of the interlaminar environment, which can be characterised by GIC (or KIC ). The literature shows a wide disparity in measurements due to incomplete decoupling of the effects of resin confinement by fibers, nonlinearitiesbehaviour and/or velocity effects. This work proposes to develop an experimental protocol to characterise pure resin usingfullfields measurements to methodically study these couplings. The goal is to evaluate the impact of the crack propagation speed and the structural effects on the fracture behaviour and in particular to extend Griffith's theory to laminated composites. Different specimen geometries are used to reproduce structural effects. Crack propagation speeds ranging from quasi-static to dynamic are studied and all the tests are analysed using linear elastic fracture mechanics and the fracture surfaces. Finally, this work proposes a model to describe the evolution of KIC for the resin HexplyM21 used in aeronautics field, from the non-singularterms of the stress field T-stress, B-stress and also the speed ȧ in the ranges [0 - 15] MPa, [- 200 - 10] MPa.m-0.5 et [10-6, 600] m.s-1 respectively
4
Keller, Scott. "Stress Intensity Factor Dependence of HG-AL Liquid Metal Embrittlement." Master's thesis, University of Central Florida, 2009. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/2220.
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When high strength aluminum alloys are subjected to liquid metals, physical and chemical reactions ensue resulting in what is known as liquid metal embrittlement (LME). A subset of environmentally-assisted cracking, LME is exhibited when a liquid metal, e.g. Hg or Ga, comes into intimate contact with a solid metal having significant susceptibility. As mechanical loads are applied, the interaction between the two metals results in a reduction in the flow properties of the solid metal. Several theories have been proposed to identify the underlying microstructural failure mechanism; however, none have been widely accepted, as failures can typically incorporate features common to several failure theories. In an effort to confirm, extend or replace the physically-based theories, fracture mechanics experiments on Al 7075-T651 in liquid mercury have been conducted. Experiments were conducted in a custom environmental chamber capable of exposing specimens to liquid environments while applying a mechanical load. Through both plane-strain fracture and stress intensity factor-dependent (SIF) tests, fracture toughness values along with incubation periods were analyzed and provided data for a load-based theory of LME. These mechanical test data, along with metallographic analysis, show that the phenomena of LME is both strongly time- and SIF-dependent.M.S.M.E.
Department of Mechanical, Materials and Aerospace Engineering;
Engineering and Computer Science
Mechanical Engineering MSME
5
Teh, Lay Seong. "Library of geometric influences for stress intensity factor weight functions." Thesis, University College London (University of London), 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.566060.
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This research thesis reports the development of a novel concept for Linear Elastic Fracture Mechanics (LEFM) analysis - Composition Theory of Stress Intensity Factor Weight Functions (CToWF). A generic closed form composition model has been derived to generate Mode I SIFs for an edge crack propagating in a symmetrically loaded two-dimensional body. The CToWF concept has demonstrated, by verification with published solutions and Finite Element Analysis (FEA), that the SIF weight function for a new cracked body can be evaluated by isolating and combining appropriate constituent geometries. Being a unique property of crack and component geometry, the newly determined weight function enables rapid generation of SIFs for the same cracked component under different stress systems. Over two thousand Finite Element (FE) models were analysed to provide constituent geometrical configurations and to validate the SIFs calculated from the CToWF model where published solutions were not available. These are Mode I SIFs for edge cracks emanating from two-dimensional notches i. e. semi-elliptical, U- and V-notches in semi-infinite bodies along with their associated stress distributions. Hence, a comprehensive database has been established. Using the versatile composition model with the database, a large number of new SIF solutions for edge cracks from equivalent notches in finite bodies have been obtained. This `Library' of geometric influences, which are presented as weight function coefficients in tabular form, can now be composed by the CToWF approach to generate SIFs for modelling crack propagation through residual stress fields and other complex stress systems. In general, this universal approach, which is easy-to-implement yet maintaining high accuracy, has tremendous potential in allowing rapid assessment of defects prone to linear elastic fracture behaviour via the evaluation of SIFs. Further work to enhance the understanding of this novel concept is proposed to develop a broader practical use in real engineering applications.
6
Arli, Sirisha Divya. "An Investigation on the Stress Intensity Factor of Surface Micro-cracks." Wright State University / OhioLINK, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=wright1495620917553525.
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Finlayson, Eric F. "Stress Intensity Factor Distributions in Bimaterial Systems - A Three Dimensional Photoelastic Investigation." Thesis, Virginia Tech, 1998. http://hdl.handle.net/10919/36504.
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Stress-freezing photoelastic experiments are conducted using two different sets of photoelastic materials to investigate stress intensity behavior near to and coincident with bimaterial interfaces. Homogeneous, bonded homogeneous, and bonded bimaterial single edge-cracked tension specimens are utilized throughout the investigation for comparative purposes. The first series of tests involves machined cracks obliquely inclined to the direction of far field tensile loading. Mixed-mode stress intensity factors are observed and quantified using a simplified analytical algorithm which makes use of experimentally measured data. In this series of tests, the bimaterial specimens consist of a photoelastic material bonded to the same material containing a moderate quantity of aluminum powder (for elastic stiffening purposes). Moderate yet similar increases in stress intensity factors are observed in bonded homogeneous and bonded bimaterial specimens, suggesting the presence of bondline residual stresses (rather than elastic modulus mismatch) as the primary contributing factor. The second series of tests involves the bonding of mutually translucent photoelastic materials whose elastic module differ by a ratio of approximately four to one. Cracks are placed both near and coincident to the bimaterial interfaces. Mode-mixity and increases in stress intensity are found only in bimaterial specimens whose cracks are placed close to the bondline. Using the materials from the first series of tests it is shown that the increases in these near-bondline experiments are due to thermal mismatch properties (incurred during the stress freezing cycles) rather than mechanical mismatch properties.Master of Science
8
Ventura, Antunes Fernando Jorge. "Influence of frequency, stress ratio and stress state on fatigue crack growth in nickel base superalloys at elevated temperature." Thesis, University of Portsmouth, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.285929.
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Garrido, F. A. DiÌaz. "Development of a methodology for thermoelastic investigation of the effective stress intensity factor." Thesis, University of Sheffield, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.412241.
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Azeez, Ahmed. "Effect of dwell time on stress intensity factor of ferritic steel for steam turbine applications." Thesis, Linköpings universitet, Mekanik och hållfasthetslära, 2018. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-148283.
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In the transition from conventional to green energy production resources, steam turbines are used to satisfy the lack of energy during peaks in the demand times and the limited access of renewable resources. This type of usage for steam turbines makes them operate on a flexible schedule, which leads to unpredictable issues related to shorter component life and faster crack propagation. Thus, the steam turbine components must be examined to determine their specific life period. This will help set proper maintenance intervals and prevent unexpected failures. For that, thermo-mechanical fatigue (TMF) testing is used, where a specimen made of the same material as the turbine component is subjected to both temperature and load variation. The specimen is pre-cracked to investigate the crack propagation behavior, which is the focus of this study. This thesis work concentrates on simulating the TMF cycle for the steam turbine casing component. The material is 9%-10%Cr ferritic steel. The aim is to understand the material behavior during crack propagation and to predict a useful testing parameter. The method provided in this work discusses two cases, both are out-of-phase (OP) TMF tests with strain control. The maximum and minimum temperatures for the cycle are 600 ˚C and 400 ˚C respectively, while the maximum and minimum strain levels are 0 and respectively. The study will investigate different , which is the maximum compressive strain level. Case 1 has a dwell time at the maximum temperature only, while case 2 has dwell times at both maximum and minimum temperatures. The method utilizes the stress intensity factor (SIF) to characterize the crack tip conditions. Also, it uses Paris' law to estimate the duration of the tests. For simplification, only the elastic behavior of the material is considered. The results obtained show no effect of using different pre-crack lengths due to the strain control condition. Minor effects can be observed by using different dwell times, however very short dwell times must be avoided to produce reliable results. A recommended dwell time of 5 minutes could be used, since longer dwell times will make the test prohibitively time-consuming. The compressive strain levels used in the work shows large effects on the results. Using low compressive strain values will produce a very long time for the tests, while very high compressive strains produce large plasticity. Thus, high compressive strains must be avoided since the SIF describes cracks for only elastic or near elastic cases. Also, small compressive strain levels in case 2 should not be used since it will lead to results like case 1. This is due to the small creep effect at the minimum temperature. Finally, compressive strain levels of 0.6 %, 0.5 % and 0.4 % are recommended for case 1, while only 0.6 % compressive strain level is recommended for case 2. This thesis contributes to the fields of solid mechanics, fracture mechanics and the use of TMF testing, where a recommended set of testing parameters are provided.
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Demircivi, Bengi. "Thermal Stress Intensity Factor Evaluation For Inclined Cracks In Functionally Graded Materials Using Jk-integral Method." Master's thesis, METU, 2006. http://etd.lib.metu.edu.tr/upload/12607844/index.pdf.
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The main objective of this study is to evaluate mixed mode stress intensity factors
for inclined embedded cracks in functionally graded materials. Fracture analysis
of inclined cracks requires the calculation of both Mode I and Mode II stress
intensity factors ( I K , II K ). In this study, k J -integral is used to calculate I K
and II K . Equivalent domain integral approach is utilized to evaluate the k J -
integral around the crack tip. The present study aims at developing a finite
element model to study inclined crack problems in graded media under
thermomechanical loading. A two dimensional finite element model is developed
for inclined cracks located in a functionally graded medium. Structural and
thermal problems are solved using two dimensional finite elements namely 8-
noded triangles. Material properties are sampled directly at the integration points
of the elements, as required by the numerical integral evaluation. The main results
of the study are the stress intensity factors at the crack tip for functionally graded
materials subjected to thermomechanical loading.
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Gloss, Kevin T. "A Photoelastic Investigation into the Effects of Cracks and Boundary Conditions on Stress Intensity Factors in Bonded Specimens." Thesis, Virginia Tech, 2000. http://hdl.handle.net/10919/32786.
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An investigation into the influence of cracks in bonded specimens is conducted.
Photoelastic specimens containing a bondline are subjected to a constant displacement
boundary condition created by bonded end grips. Specimens containing various crack
orientations are analyzed to determine stress intensity factors at the induced crack tips.
Specimens containing interface and sub-interface cracks were investigated. Two global
geometries were used in this investigation, square and rectangular. The constant
displacement boundary condition was induced on the specimen through dead weights
hung from bonded aluminum end grips. Stress intensity factors were determined using
photoelastic techniques. The stress intensity factors were examined to determine trends
in the results as a function of changes in geometry. The effects of the induced boundary
condition, the specimen geometry, and the bondline were investigated. The results from
this investigation were compared to known solutions with a similar specimen geometry.
These tests exhibited influences from the bondline, the boundary conditions, and the
specimen geometry. The bondline tended to decrease the stress intensity factor for
specimens with small crack lengths and tended to increase the stress intensity factor for
specimens containing long crack lengths. As the crack length increased so too did the
stress intensity factor. A reduction in the bondline to crack distance with sub-interface
crack specimens caused a reduction in the stress intensity factor. A reduction in the
global height of the specimen caused a reduction in the stress intensity factor also. The
results from this investigation will aid in the understanding of the influence of interface
and sub-interface cracks in bonded specimens.Master of Science
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Someshwara, Korachar Eshwari. "A Numerical Based Determination of Stress Intensity Factors for Partially Cracked Flexural I-shaped Cross-sections." Thesis, Virginia Tech, 2019. http://hdl.handle.net/10919/89068.
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The AASHTO LRFD design specifications and the AASHTO manual for bridge evaluation are consistently revised using knowledge of previous bridge failures. Although modern steel structures are designed to resist fatigue cracking from service loads, cracks in the tension flanges of steel bridge girders have been observed as a result of stress concentrations, design errors, welding quality control, and vehicular impacts. Cracks can grow in size with time and active cyclic live loads and may result in a member fracture. Fracture is a dangerous limit state which occurs with little to no warning. One method to quantify the stress field in the vicinity of a crack tip is by calculating the Stress Intensity Factor (SIF) around the crack tip. Finding SIFs for a cracked geometry may help an engineer to determine the fracture potential based on crack dimensions found during the inspection. Rolled I-beam and steel plate girders are extensively used as bridge superstructure members to efficiently carry live loads. This research was focused on determining Stress Intensity Factors (SIFs) of partially cracked I-sections using Finite Element Analysis. Two different tension flange crack profiles were studied: edge cracks, and full-width cracks. The SIF solutions were further used to study the fracture behavior and stress redistribution in the partially cracked flexural I-shaped members.Master of Science
Steel is one of the fundamental materials used in the construction of bridge structures, and steel girder bridges are one of the most common types of bridge structures seen in the United States. Past bridge failures have helped engineers to understand shortcomings in design specifications, and AASHTO codes have been developed and revised over the years to reflect an improved understanding and evolution of engineering behavior. Engineers must make sure that a design is robust enough for functional use of the component during its service life. It is also equally important to understand the potential chances of failure and make the structure strong enough to overcome any failure mechanisms. Fracture is one structural failure mode which occurs with little to no warning and hence is very dangerous. One efficient way to quantify the stress field in the vicinity of a crack tip is by calculating the Stress Intensity Factor (SIF) around a crack tip. Fracture literature is available which describes different methods of determining SIFs for cracked members. However, there are no solutions available to find a SIF of a partially cracked flexural I-shaped members. This research was focused on determining Stress Intensity Factors and studying the fracture behavior of partially cracked I-sections using Finite Element Analysis. The resulting SIF solutions were further used to study the fracture behavior and stress redistribution in partially cracked flexural I-shaped members.
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Boussattine, Zaid. "Consequences of the thermal effects generated during fatigue crack growth on the mode one stress intensity factor." Thesis, Paris, ENSAM, 2018. http://www.theses.fr/2018ENAM0047/document.
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Lors du chargement cyclique d’une pièce métallique fissurée, un champ de température hétérogène est créé à la pointe de la fissure. Ce champ de température est dû à trois types de sources de chaleur : (i) la première source est une source de couplage thermoélastique liée à la partie hydrostatique du tenseur des contraintes résultant de la sollicitation mécanique cyclique. Elle fluctue périodiquement dans le temps et l’énergie qui lui est associée est nulle à la fin de chaque cycle de chargement ; (ii) la deuxième source de chaleur est une source dissipative et intrinsèque au comportement du matériau. Elle est reliée au phénomène de l’auto-échauffement dû à la microplasticité dissipée en chaleur dans le matériau à l’échelle microscopique. Elle est positive et s’accumule dans le temps ; (iii) enfin, la troisième source de chaleur a les mêmes origines et propriétés que la deuxième source, mais elle est associée à la plasticité, à l’échelle macroscopique, qui se dissipe en chaleur dans la zone de plasticité cyclique à la pointe de la fissure. En présence de ces trois sources de chaleur, le champ de température résultant génère un champ de contrainte dû au phénomène de la dilatation thermique. Ce nouveau champ des contraintes s’ajoute au champ des contraintes dû au chargement mécanique cyclique, et donc l’état des contraintes sur la fissure est modifié. En conséquence, le facteur d’intensité des contraintes, qui est un paramètre clé dans la modélisation de la propagation des fissures, est modifié. D’où l’objectif de cette thèse qui vise à quantifier les conséquences de ces trois sources de chaleur sur le facteur d’intensité des contraintes, et ce dans le cas d’une fissure longue de fatigueBy subjecting a cracked specimen to a cyclic loading, thermal effects take place and create a heterogeneous temperature field around the crack tip. Those thermal effects are associated with coupling and dissipative heat sources, namely: (i) the heat source due to thermoelastic coupling generated by the hydrostatic part of the stress tensor related to cyclic mechanical loading; (ii) the heat source due to intrinsic dissipation associated with the self-heating phenomena originating from plasticity at the microscopic scale; (iii) and the heat source due to cyclic plasticity, at the macroscopic scale, which occurs in the reverse cyclic plastic zone ahead of the crack tip, and dissipates into heat. The overall heterogeneous temperature field resulting from the heat sources induces a heterogeneous stress field due to thermal expansion phenomena. As a consequence, the stress state over the crack is modified and leads to modify the stress intensity factor, which is a key parameter in modeling fatigue crack growth. Therefore, the aim of this PhD thesis is to quantify the consequences of the heat sources on the stress intensity factor, in the case of a long propagating fatigue crack
15
Sayar, Mehmet Burak. "Determination Of Stress Intensity Factors In Cracked Panels Reinforced With Riveted Stiffeners." Master's thesis, METU, 2011. http://etd.lib.metu.edu.tr/upload/12614060/index.pdf.
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This thesis presents a study about the determination of the stress intensity factors in cracked sheets with riveted stiffeners. Stress intensity factors are determined with both analytical method and finite element method for different combination of rivet/stringer spacing and stringer to sheet stiffness ratio. Analytical part of the thesis is a replication of the original study of Poe which assumes rigid rivet connections with no stringer offset. In the analytical part, the whole systems of equations of Poe are re-derived, and it is shown that there are two typographical errors in the expressions for the calculation of the influence coefficients of the cracked sheet and the stringer. Major objective of the analytical part is to develop a computer code which calculates the variation of the normalized stress intensity factor with the crack length for any combination of rivet/stringer spacing and stringer to sheet stiffness ratio. Analytical part of the study also covers the effect of broken stiffener on the stress intensity factor of the cracked sheet. The stress intensity factors of stiffened cracked sheets are calculated by the finite element method by incorporating fastener flexibility and stringer offset. Finite element solutions are performed by Franc2D/L and Abaqus, and comparisons are made. The effect of geometry, fastener flexibility, and stringer offset on the stress intensity factors are studied by presenting normalized stress intensity factor versus crack length curves. Finally, as a case study a sample damage tolerant stiffened panel is designed according to FAR 25 safety criteria. Experiments are performed for determining mechanical and crack growth properties of Al 2124 which is used as the material in the case study. Present study showed that the most significant effect on the stress intensity factor is seen when stringer-cracked sheet offset is included in the analysis model.
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Pierola, Javier. "Three dimensional stress intensity factor for large arrays of radial internal surface cracks in a cylindrical pressure vessel." FIU Digital Commons, 1993. http://digitalcommons.fiu.edu/etd/2514.
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The objective of this study is to present the mode I stress intensity factor distribution (SIF) along the crack-front for a wide array of semicircular and semi-elliptical surface cracks inside of a pressurized thick-walled cylinder. A three-dimensional finite element package ANSYS is used to evaluate the SIF for multicracked cylinder with number of cracks from n=1 to 128, the ratio of crack-depth to the wall thickness a/t=0.05 to 0.6, the ellipticity of the crack (the crack-depth to the semi-crack length) a/c=0.2 to 1.5, the ratio of the outer to the inner radius ro/ ri=2.
A substructuring technique is introduced which solved a coarse model meshed with ten-node isoparametric elements and applied the resulting displacements in the boundary surface of a submodel which is built employing singular elements along the crack-front to produce the 1/√r singularity . The SIF is evaluated using nodal-displacement method.
To validate the modeling and analysis procedure of the present results various configurations were solved using this method and compared to other finite element solutions. The present results were in very good agreement: less than 5 % comparing with Raju and Newman's results and within 8 % of Kirkhope's results.
An empirical equation to calculate the maximum SIF, was developed in this study. The equation was obtained by nonlinear fitting of the finite element results and the error was within ± 5.7 %.
17
Lee, Christopher T. "Critical Heat Stress Evaluation In Two Ebola Ensembles." Scholar Commons, 2016. http://scholarcommons.usf.edu/etd/6299.
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Ebola, a type of filovirus that causes hemorrhagic fevers, dominated global headlines in 2014 when the largest Ebola epidemic in history took place in West Africa. Healthcare practitioners were at particular risk of contracting Ebola while taking care of patients with the disease because they were easily exposed to bodily fluids such as blood, urine, saliva, and feces, quite often in the intensive care unit (ICU). While personal protective equipment (PPE) protects the healthcare practitioner by providing an effective barrier against the virus, users were also at risk for heat stress because of the type of protective clothing. In this study, coveralls made of monolithic barriers, which prevent water vapor from escaping the suit, were compared to coveralls made of micro- porous material, which allows evaporated sweat to escape the suit. The Microgard® 2000 TS Plus, made of micro-porous barrier material and the monolithic barrier Microgard® 2300 Plus were compared against a control ensemble of work clothes consisting of a long-sleeve shirt and trouser.
A progressive heat stress protocol was used to determine the critical environment at the upper limit of compensable heat stress. The critical condition was the point at which the heat gain caused by wearing the protective ensemble as well as dry heat exchange was balanced by the maximum heat loss due to evaporative cooling. Wet bulb globe temperature at the critical condition (WBGTcrit ) ,total evaporative resistance (Re,T,a), and clothing adjustable factor (CAF) were calculated for each ensemble based on data at the critical point. Also at the critical condition, participant rectal temperature vi (Tre) , heart rate (HR), skin temperature (Tsk), and physiological strain index (PSI) were noted and compared for each ensemble.
A two-way ANOVA (ensemble x participant) for WBGTcrit and Re,T,a as dependent variables was used to determine whether or not there were differences among ensembles. Tukey’s honest significance test was used to determine where significant differences occurred. WBGTcrit was 33.8, 26.3, and 22.9 °C-WBGT for Work Clothes, M2000, and M2300 respectively. Re,T,a was 0.012, 0.031, and 0.054 kPa m2 W-1 for WC, M2000, and M2300 respectively. The higher the WBGTcrit for an ensemble, the more it can support evaporative cooling and hence the better it is at ameliorating heat stress. Based on this trial, the micro-porous ensemble Microgard® 2000 TS Plus has better heat stress performance than vapor-barrier Microgard® 2300 Plus. As expected, there were no differences for any of the physiological metrics at the critical conditions.
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Hassan, Tasnim. "NUMERICAL STUDY OF TRANSIENT RESPONSE OF AN INTERFACE-CRACK IN A TWO LAYERED PLATE (ANTI-PLANE, STRESS INTENSITY FACTOR)." Thesis, The University of Arizona, 1985. http://hdl.handle.net/10150/275474.
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Jagtap, Nimish V. "Application of the Hypersingular Boundary Integral Equation in Evaluating Stress Intensity Factors for 2D Elastostatic Fracture Mechanics Problems." University of Cincinnati / OhioLINK, 2006. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1163788461.
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Riggenbach, Kane Ryan. "Finite Geometry Correction Factors for the Stress Field and Stress Intensities at Transverse Fillet Welds." Case Western Reserve University School of Graduate Studies / OhioLINK, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=case1339121761.
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Jimenez, Acosta Carlos Efren. "Assessment of weld residual stress effects on fatigue crack propagation in ferritic pressure vessel steels." Thesis, University of Manchester, 2016. https://www.research.manchester.ac.uk/portal/en/theses/assessment-of-weld-residual-stress-effects-on-fatigue-crack-propagation-in-ferritic-pressure-vessel-steels(ef8f4e14-3b9c-4f45-876b-dca5d5e9c121).html.
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This project aims to characterise the fatigue behaviour of a crack propagating in a residual stress field changing from tension to compression in the welded zone of a ferritic pressure vessel steel. The fatigue tests were carried out keeping the applied stress intensity factor range constant to determine the role of residual stresses on fatigue crack growth. The residual stresses prior to crack growth were evaluated by X-ray diffraction. The weight function method was used to infer the expected influence of the residual stress on the crack tip in terms of the residual stress intensity factor. Two metrics were used to quantify the crack driving force local to the fatigue crack. Firstly the stress intensity amplitude expressed in terms of the change in the J-integral between maximum and minimum load and secondly the change in the crack opening displacement COD to estimate closure stress intensity factor. The displacement fields local to a fatigue crack were obtained by Digital Image Correlation (DIC) and then analysed by JMAN, an in-house developed algorithm to extract the J-integral based on finite element method and implemented using MATLAB. The difference between the applied stress intensity factor range and the effective crack driving force at the crack tip was determined in order to understand the interaction between the prior residual stresses and crack closure phenomena. Three different R-ratios were evaluated during the experiment (R=0.1, R=0.3 and R=0.5) in order to quantify the effect of residual stress on crack tip stress intensity and crack opening displacement. R-ratio plays a very important role on the fatigue crack growth rate (FCGR): as R increases, FCGR also increases. The COD was assessed by means of the displacements obtained by DIC local to the crack faces. The COD method turned out to be more insightful than the JMAN method for characterising the crack propagation, this is due to the presence of plasticity in the ligament which breaks the non-linear elastic conditions, causing the path-dependence on the J-integral. The FCGR is influenced to a greater degree by the R-ratio and to a lesser degree by the residual stress effect. There is a direct relationship between R and FCGR: as R increases, FCGR also increases, irrespective of the presence of tensile or compressive residual stresses, with the crack closure showing more tendency to occur at low R (i.e. R=0.1) than at high R (i.e. R=0.5). The relationship between R and the residual stress effects on FCGR is inversely proportional: as R increases, the effect of RS decreases.
22
Kecelioglu, Galip. "Stress And Fracture Analysis Of Riveted Joints." Master's thesis, METU, 2008. http://etd.lib.metu.edu.tr/upload/12610132/index.pdf.
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The objective of this study is to model and analyze a three dimensional single riveted lap joint (with and without a crack). By using finite element method, stress and fracture analyses are carried out under both the residual stress field and external tensile loading. Using a two step simulation, riveting process and subsequent tensile loading of the lap joint are simulated to determine the residual and overall stress state. Residual stress state due to riveting is obtained by interference and clamping misfit method. By employing different interference and clamping misfit values, the effects of riveting process parameters on stress state are examined. Two cracks namely the semi elliptical surface crack at faying surfaces of plates and the quarter elliptical corner crack at rivet hole are the most widely observed crack types in riveted joints. Fracture analysis of cracked riveted joints is carried out by introducing these two crack types to the outer plate at a plane perpendicular to the loading. The mixed mode stress intensity factors (SIFs) and energy release rates (G) around the crack front are obtained by using displacement correlation technique (DCT). Effects riveting process parameters (interference and clamping ratios) and geometrical parameters (crack shape and size) on fracture parameters are studied. The stress intensity factor solutions presented herein could be useful for correlating fatigue crack growth rates, fracture toughness computation, and multiple site damage (MSD) analysis in aircraft bodies.
23
田中, 啓介, Keisuke TANAKA, 義明 秋庭, Yoshiaki AKINIWA, 晶広 高橋, Akihiro TAKAHASHI, 照明 御厨, and Teruaki MIKURIYA. "ねじり - 軸力負荷における鉄鋼薄肉円管試験片における円孔からの疲労き裂の伝ぱ挙動." 日本機械学会, 2003. http://hdl.handle.net/2237/9149.
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Lekesiz, Huseyin. "Stress Intensity Factors and Effective Spring Stiffness for Interfaces with Two and Three Dimensional Cracks at the Interface between Two Dissimilar Materials." The Ohio State University, 2010. http://rave.ohiolink.edu/etdc/view?acc_num=osu1293574318.
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25
Shi, Zhijun. "Predicting fatigue crack growth life in integral metallic skin-stringer panels." Thesis, Cranfield University, 2012. http://dspace.lib.cranfield.ac.uk/handle/1826/7281.
Full textAbstract:
During the past few years, in comparison to traditional riveted structures, integral metallic skin stringer structures have played more and more important roles in aircraft design due to the fact they are economical and also have the ability to reduce weight. Their wide application in aircraft, especially large integral structures is limited because of the fact that they have shortcomings in damage tolerance performance. Hence, calculating the crack growth lives and improving the damage tolerance performance of integral structures by selecting appropriate materials or choosing rational structures is a critical work. Therefore the purpose of this thesis is to find effective analysis methods of integral metallic skin-stringer panels for the use in engineering. Cont/d.
26
Kose, Ayse. "Thermal Stress Problem For An Fgm Strip Containing Periodic Cracks." Master's thesis, METU, 2013. http://etd.lib.metu.edu.tr/upload/12615697/index.pdf.
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In this study the plane linear elastic problem of a functionally graded layer which contains periodic cracks is considered. The main objective of this study is to determine the thermal stress intensity factors for edge cracks. In order to find an analytic solution, Young&rsquos modulus and thermal conductivity are assumed to be varying exponentially across the thickness, whereas Poisson ratio and thermal diffusivity are taken as constant. First, one dimensional transient and steady state conduction problems are solved (heat flux being across the thickness) to determine the temperature distribution and the thermal stresses in a crack free layer. Then, the thermal stress distributions at the locations of the cracks are applied as crack surface tractions in the elasticity problem to find the stress intensity factors. By defining an appropriate auxiliary variable, elasticity problem is reduced to a singular integral equation, which is solved numerically. The influence of such parameters as the grading, crack length and crack period on the stress intensity factors is investigated.
27
Ščotka, Martin. "Vliv koncentrace napětí ve vrubu na napjatost a deformaci na čele trhlin zatížených ve smykových zátěžných módech." Master's thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2017. http://www.nusl.cz/ntk/nusl-318836.
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This diploma thesis deals with influence of a notch on stress-strain states at the front of cracks by shear modes. Starting with fracture mechanics and its division, followed by stress intensity factor and calculate its by finite element method. Calculation is solved for two types of notches, U-notched and V-notched, both notches were modeled parametrically so their geometry was changeable and stress intensity factor were calculated for all configurations. Subsequently was solved next calculation of stress intensity factor but for shaft without notch. Finally, was evaluated influence of notch on stress intensity factor. Software for finite element method has been used ANSYS. Others calculation was provided in software MATLAB
28
Fletcher, Oclla Michele. "Critical Heat Stress Evaluation of Two-Layer Clothing Ensembles and the Contributionof a Full-Face Negative Pressure Respirator." Scholar Commons, 2012. http://scholarcommons.usf.edu/etd/4040.
Full textAbstract:
Protective clothing ensembles are worn by workers as a barrier to chemical and physical hazards, but can restrict heat loss and increase worker heat stress. The question of whether a respirator adds to heat stress or strain burden is a continuing concern among occupational health professionals. The purpose of this study was to determine if there are differences in heat stress or strain among the current Toxicological Agent Protective (TAP) ensemble and two ensemble variations used in demilitarization of chemical weapons. Four acclimatized adult males wore five ensembles in a balanced design while walking in a climatic chamber at a metabolic rate of about 170 W m-2. Heat stress (critical wet bulb globe temperature-WBGTcrit, evaporative resistance-Re,T,a, Clothing Adjustment Factor [CAF]) and heat strain (physiological strain index [PSI]) were compared against work clothes (WC) without respirator (a baseline ensemble); the current TAP apron over cloth coveralls with respirator (TAP+CA); the current TAP apron over cloth coveralls with respirator plus Tychem F® chemical barrier pants (TAP+CA+P); and Tychem F® Coveralls over cloth coveralls with respirator (VB+CA). A no-respirator comparison with the Tychem F coveralls (VB+CA-noR) was added to evaluate the contribution of a full-face negative pressure air-purifying respirator to heat stress. A progressive heat stress protocol was used to determine WBGTcrit, Re,T,a, CAF, and PSI. The results (WBGTcrit [°C-WBGT], Re,T,a [kPa m2 W-1], and PSI) were WC (35.5, 0.0112, 2.0), TAP (31.6, 0.0175, 1.8), TAP+P (27.7, 0.0240, 1.9), VB+CA (25.9, 0.0287, 1.8), and VB+CA-noR (26.2, 0.0293, 1.8). Mixed effects ANOVA was used to assess ensemble effects. Tukey's test was used to determine where significant differences occurred. WBGTcrit was the WBGT at the upper limit of thermal balance. Re,T,a increased while WBGTcrit progressively decreased going from WC to TAP+CA to TAP+CA+P to VB+CA. WBGTcrit was different between Work Clothes and TAP+CA and between WC and TAP+CA and the other ensembles. Re,T,a was different among all ensembles, except no differences in WBGTcrit and Re,T,a were observed between the presence and absence of a respirator with VB+CA. There were no differences among all ensembles for rectal temperature, heart rate, and PSI. Based on both WBGTcrit and Re,T,a, there were significant increases in heat stress going from WC to TAP+CA to TAP+CA+P to VB+CA. No differences in WBGTcrit, Re,T,a, and PSI were found for the presence or absence of a respirator, indicating no additional heat stress or strain burden. CAF is the WC WBGTcrit minus the ensemble WBGTcrit.. The recommended clothing adjustment factors (CAFs) are 0°C-WBGT for WC, 4 °C-WBGT for TAP+CA, 8 °C-WBGT for TAP+CA+P and 10 °C-WBGT for VB+CA. As vapor-barrier ensembles are sensitive to humidity, adding 2 °C-WBGT to VA+CA for a CAF of 12 °C-WBGT is recommended. This implicates the type of protective clothing ensemble worn will play a much bigger role in workplace heat stress effects and risk than the wear of a respirator.
29
Lozano, Minguez Estivaliz. "Fatigue and fracture mechanics of offshore wind turbine support structures." Thesis, Cranfield University, 2015. http://dspace.lib.cranfield.ac.uk/handle/1826/9756.
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Wind power, especially offshore, is considered to be one of the most promising sources of ‘clean’ energy towards meeting the EU targets for 2020 and 2050. However, its popularity has always fluctuated with the price of fossil fuels since nowadays wind electricity production cannot compete with nuclear or coal electricity production. Support structures are thought to be one of the main drivers for reducing costs in order to make the wind industry more economically efficient. Foundations and towers should be fit for purpose, extending their effective service life but avoiding costs of oversizing. An exhaustive review of the background and state of the art of the Fatigue-Life assessment approaches has been carried out, combining analysis of the gathered experimental data and the development of Finite Element models based on contemporary 3D solid models with diverse Regression Analyses, in order to identify their weakness and evaluate their accuracy. This research shows that the guides and practices currently employed in the design and during the operation of the offshore wind turbine support structures are obsolete and not useful for optimisation, which generally leads to conservationism and an unnecessary increase in costs. The basis for a comprehensive update of the Girth Weld and Tubular Joint S-N curves and the Stress Concentration Factors of Tubular Joints has been set out. Furthermore, a reliable methodology for deriving the Stress Intensity Factor at the deepest point of a semi-elliptical surface saddle crack in a tubular welded T-joint has been proposed.
30
Voskoboinikov, Roman E. "Analytical Description of Brittle-to-Ductile Transition in bcc Metals. Nucleation of dislocation loop at the crack tip." Forschungszentrum Dresden, 2010. http://nbn-resolving.de/urn:nbn:de:bsz:d120-qucosa-29522.
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Nucleation of dislocation loop at the crack tip in a material subjected to uniaxial loading is investigated. Analytical expression for the total energy of rectangular dislocation loop at the crack tip is found. Depencence of the nucleation energy barrier on dislocation loop shape and stress intensity factor at the crack tip is determined. It is established that the energetic barrier for nucleation of dislocation loop strongly depends on the stress intensity factor. Nucleation of dislocation loop is very sensitive to stress field modifiers (forest dislocations, precipitates, clusters of point defects, etc.) in the crack tip vicinity.
31
Atay, Mehmet Tarik. "Fracture Of A Three Layer Elastic Panel." Phd thesis, METU, 2005. http://etd.lib.metu.edu.tr/upload/3/12606360/index.pdf.
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The panel is symmetrical about both x- and y- axes. The central strip (strip1) of width 2h1 contains a central transverse crack of width 2a on x-axis. The two strips (strip2) contain transverse cracks of width c-b also on x-axis. The panel is subjected to axial loads with uniform intensities p1 and p2 in strip1 and strip2 , respectively at . Materials of all strips are assumed to be linearly elastic and isotropic. Due to double symmetry, only one quarter of the problem and will be considered.
The solutions are obtained by using Fourier transforms both in x and y-directions. Summing several solutions is due to the necessity for sufficient number of unknowns in general expressions in order to be able to satisfy all boundary conditions of the problem. The conditions at the edges of the strips and at the interfaces are satisfied and the general expressions for a three layer panel become expressions for the panel with free edges. Use of remaining boundary conditions leads the formulation to a system of two singular integral equations. These equations are converted to a system of linear algebraic equations which is solved numerically
32
Hinneh, Perry. "The Finite Block Method : a meshless study of interface cracks in bi-materials." Thesis, Queen Mary, University of London, 2018. http://qmro.qmul.ac.uk/xmlui/handle/123456789/39754.
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The ability to extract accurately the stress intensity factor and the T-Stress for fractured engineering materials is very significant in the decision-making process for in-service engineering components, mainly for their functionality and operating limit. The subject of computational fracture mechanics in engineering make this possible without resulting to expensive experimental processes. In this thesis, the Finite Block Method (FBM) has been developed for the meshless study of interface stationary crack under both static and dynamic loading in bi-materials. The finite block method based on the Lagrangian interpolation is introduced and the various mathematical constructs are examined. This includes the use of the mapping technique. In a one-dimensional and a two-dimensional case, numerical studies were performed in order to determine the interpolation error. The finite block method in both the Cartesian coordinate and the polar coordinate systems is developed to evaluate the stress intensity factors and the T-stress for interface cracks between bi-materials. Using the William's series for bi-material, an expression for approximating the stress and displacement at the interface crack tip is established. In order to capture accurately the stress intensity factors and the T-stress at the crack tip, the asymptotic expansions of the stress and displacement around the crack tip are introduced with a singular core technique. The accuracy and capability of the finite block method in evaluating interface cracks is demonstrated by several numerical assessments. In all cases, comparisons have been made with numerical solutions by using the boundary collocation method, the finite element method and the boundary element method, etc.
33
Voskoboinikov, Roman E. "Analytical Description of Brittle-to-Ductile Transition in bcc Metals. Nucleation of dislocation loop at the crack tip." Forschungszentrum Rossendorf, 2002. https://hzdr.qucosa.de/id/qucosa%3A21779.
Full textAbstract:
Nucleation of dislocation loop at the crack tip in a material subjected to uniaxial loading is investigated. Analytical expression for the total energy of rectangular dislocation loop at the crack tip is found. Depencence of the nucleation energy barrier on dislocation loop shape and stress intensity factor at the crack tip is determined. It is established that the energetic barrier for nucleation of dislocation loop strongly depends on the stress intensity factor. Nucleation of dislocation loop is very sensitive to stress field modifiers (forest dislocations, precipitates, clusters of point defects, etc.) in the crack tip vicinity.
34
Bhamare, Sagar D. "Determination of Optimal Material Combination for Multilayer Thin Films to Improve Performance against surface Crack Propagation." University of Cincinnati / OhioLINK, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1258479408.
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35
Karlsson, David. "Analysis of Three-Dimensional Cracks in Submodels." Thesis, Linköping University, Department of Management and Engineering, 2007. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-8105.
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A common technique to evaluate load paths in complex structures is to perform FE-calculations with relative large elements. This procedure gives no information regarding stress concentrations at e.g. holes or radius but this phenomenon can later on be investigated in details with local individual submodels. Displacements is taken from the global model and used to analyse stress concentrations and crack driving parameters in the submodel.
Today, the crack controlling stress intensity factors are in general cases obtained from handbook solutions of elementary cases. This method requires engineering judgements in a conservative manner and one way to improve the solution is to model the crack in its correct surroundings in a local three-dimensional submodel.
This master thesis is focused on the development of an automated support for analysing three-dimensional cracks in submodels. The results from a global Nastran model can be imported to Trinitas and used for a more accurate stress and fatigue life analysis in a local model. Here a three-dimensional crack tip subdomain can be generated inside an eight point brick volume. The crack tip subdomain is specially designed and adjusted for accurate determination of stress intensity factors along the crack front. For example, all points are adjusted with respect to the brick volume and the crack size, triangular wedge elements are applied around the crack tip, the midpoints for these elements are moved to quarter points and the crack front is curved. The crack tip subdomain is validated against several reference cases and shows sufficiently good results with respect to the stress intensity factor.
Finally, the automated crack tip subdomain generation is applied to a geometrically complex part of a main wing carry-through bulkhead of a fighter aircraft in order to show the applicability of the procedure in an industrial environment.
36
Yilmaz, Engin. "Cracked Elastic Annulus Bonded To Rigid Cylinder." Master's thesis, METU, 2005. http://etd.lib.metu.edu.tr/upload/2/12606782/index.pdf.
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In this study, a long annulus bonded to a rigid cylinder containing an axisymmetric circumferential crack of width (d-c) at the midplane is considered. The material of the annulus is assumed to be linearly elastic and isotropic. The external surface of the annulus is free of stress. Surfaces of the crack are subject to distributed compressive loads.
The Fourier and Hankel transform techniques are used to solve the governing equations which are reduced to a singular integral equation for crack surface displacement derivative. This integral equation is converted to a system of linear algebraic equations which are solved numerically by using Gauss-Lobatto and Gauss-Jacobi quadrature formulas. Then, the stress intensity factors at the edges of the crack are calculated. Results are presented in graphical form.
37
Erozkan, Deniz. "Finite Strip With Rigid Ends And Edge Notches." Master's thesis, METU, 2009. http://etd.lib.metu.edu.tr/upload/2/12610880/index.pdf.
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This study considers a symmetrical finite strip with a length of 2L and a width of 2h containing two collinear edge cracks located at the center of the strip. Each edge crack has a width h–
a. Two ends of the finite strip are bonded to two rigid plates through which uniformly distributed axial tensile loads of intensity p0 are applied. The finite strip is assumed to be made of a linearly elastic and isotropic material. For the solution of the finite strip problem, an infinite strip of width 2h containing two internal cracks of width b&
#8211
a at y=0 and two rigid inclusions of width 2c at y=±
L is considered. When the width of rigid inclusions approach the width of the strip, the portion of the infinite strip between the inclusions becomes identical with the finite strip problem. When the outer edges of the internal cracks approach the edge of the strip, they become edge cracks (notches). Governing equations are solved by using Fourier transform technique and these equations are reduced to a system of three singular integral equations. By using Gauss-Lobatto and Gauss-Jacobi integration formulas, these three singular integral equations are converted to a system of linear algebraic equations which is solved numerically.
38
Durucan, Ayse Rusen. "Axisymmetric Finite Cylinder With Rigid Ends And A Circumferential Edge Crack." Master's thesis, METU, 2010. http://etd.lib.metu.edu.tr/upload/12612287/index.pdf.
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An axisymmetric finite cylinder with rigid ends and a circumferential edge crack is considered in this study. The finite cylinder is under the action of uniformly distributed loads at two rigid ends. Material of the finite cylinder is assumed to be linearly elastic and isotropic. This finite cylinder problem is solved by considering an infinite cylinder containing an internal ring-shaped crack located at z=0 plane and two penny-shaped rigid inclusions located at z=±L planes. General expressions of the infinite cylinder problem are obtained by solving Navier equations with Fourier and Hankel transforms. This infinite cylinder problem is then converted to the target problem by letting the radius of the rigid inclusions approach the radius of the cylinder and letting the outer edge of the crack approach the surface of the cylinder. Consequently, these rigid inclusions form the rigid ends and internal crack form the circumferential edge crack resulting in the problem of a finite cylinder with rigid ends having an edge crack. The problem is reduced to a set of three singular integral equations. These singular integral equations are converted to a system of linear algebraic equations with the aid of Gauss-Lobatto and Gauss-Jacobi integration formulas and are solved numerically.
39
Louvros, Dimitrios. "Investigation of the reliability deterioration of ageing marine structures." Thesis, Cranfield University, 2013. http://dspace.lib.cranfield.ac.uk/handle/1826/8470.
Full textAbstract:
In the present work, an investigation of the fatigue life benefits emerging from
fillet weld geometries optimization has been carried out.
At first, an introduction to ageing mechanisms, corrosion and especially fatigue,
acting on operating marine structures has been made. Residual stresses at
weld toes, stress modes, and types, geometrical factors (weld angle, toe radius,
leg length), welding techniques selected, post-welding treatment and plate‟s
material are some of the principal factors affecting the fatigue life of a fillet weld
joint.
Especially, the accuracy of various approaches in fatigue life estimation of
specific geometries under pre-set types and levels of stress is studied. It is
evident so far that even the notch stress concept is the most accurate method
based on S-N curves, the Fracture Mechanics approach can offer more
accurate solutions of a crack development through the material. Towards this, a
literature review on crack evolution aspects in welded and non-welded plates
under bending and tension was performed; substantial parameters were
determined and finally implemented in the LEFM model which was used for the
simulation purposes of Chapter 6.
As far as the crack aspect ratio evolution is concerned, an extensive reference
is available in literature since many researchers have investigated its
contribution to the determination of geometrical paths, commonly known as
“Preferred Propagation Paths”. Their significance is related with our ability to
determine accurate SIF solutions leading to precise fatigue life estimations.
A typical fillet weld joint 2-D model has been developed in CAE Abaqus
software and a Finite Element Analysis of subject T-profile has been carried out.
Through this analysis, the fillet weld angle, the weld leg length, the weld toe
curvature radio ρ and the carrying load plate thickness are examined for their
impacts on the maximum surface stress. Finally, a number of stress mitigating
measures are proposed and their effects are analyzed.
Undoubtedly, the notch stress concept today is gradually gaining more and
more acceptance among other fatigue analysis practices, hence the need for an
estimation of the actual surface stresses along fillet weld toes, has become
imperative. Towards this, different 2-D geometries are tested against stress
concentration factors developed at weld toes, which are calculated on the basis
of maximum in-plane principal stresses over nominal stresses in mode I pure
bending and pure tension respectively. Moreover, validation with corresponding
results from literature is provided. Finally, three different concepts for reducing
the maximum surface stresses are presented. The first one proposes grinding
of the weld toe area and formulation of an artificial U-notch or a part- circular
profile. The second one applies to non-penetrating welds and assumes the
existence of a root gap of a specific geometry which is related to the fatigue life
and stress concentration factor of the fillet weld joint. Last but not least, the
relatively recent concept of the variable radius notch is discussed, even though
it is applicable mostly to notched bodies, not weld joints.
Afterwards, a Linear Elastic Fracture Mechanics analysis of reference 2D fillet
weld model is demonstrated. A number of geometrical parameters considered
at previous stage for their impact on surface Stress Concentration levels at the
weld toe region, have been correlated to fatigue life benefits in terms of
increased number of stress cycles till failure.
An extensive analysis of 9 different T-butt weld joint geometries has been
provided in order to investigate how positively a possible SCF reduction can
affect the fatigue life of a weld joint. Essential geometric variations (weld angle,
length, toe radius, root slot) were considered in the 2D model. All calculated
benefits both in pure bending and pure tension cases have been reported
accordingly.
Based on a linear interpolation of the points scatter (SCF, N-cycles) both in
banding and tension, it was observed that a surface stress mitigation of 1%
could lead to 1,33 up to 2,5% fatigue life benefit in the range of SCF=2 – 2,5. It
is evident so far that the geometrical optimization of a weld joint in respect of
notch stress mitigation can be a powerful tool both in shipbuilding and
maintenance practice in the future. However, technically wise their application
may incur high initial costs of improved tools of welding and post welding
treatment and robots even though it would consist a cost effective solution in a
medium/long term basis.
Finally, the above process is followed by a reliability analysis of the most critical
geometrical parameters affecting the fatigue life of a fillet weld joint. Reliability
assessment results concerning medium, high and low cycle fatigue are provided
and a comparative analysis of each factor‟s impact on fatigue life has been
carried out.
40
Ševčík, Martin. "Výpočtové modelování deformačně-napěťových stavů čelního soukolí pomocí MKP." Master's thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2008. http://www.nusl.cz/ntk/nusl-227992.
Full textAbstract:
This diploma thesis deals with computation modeling of spur gearing using FEM. The methodology of checking calculation was suggested for gears which are not possible to check only by using standard ČSN 01 4686. The analysis of influence of vicinity of stress concentration on stress distribution in tooth root was performed. It is possibility to use the gears with thinner rim than the standard ČSN 01 4686 recommends. The main part of this work is devoted to fatigue crack propagation study in gears with thin rim. Aiding T-stress the influence of constraint and stress multiaxiality on crack propagation was taking into account. The predicted fatigue crack propagation paths were compared with certain experimental data. It is demonstrated that the considering of influence of constraint improves estimation of consequent crack propagation direction and in some cases the influence of constraint can significantly change the estimated crack propagation path.
41
Ševeček, Oldřich. "Solution of General Stress Concentrators in Anisotropic Media by Combination of FEM and the Complex Potential Theory." Doctoral thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2009. http://www.nusl.cz/ntk/nusl-233837.
Full textAbstract:
Disertační práce se věnuje problematice obecných koncentrátorů napětí v anisotropních prostředích. Zejména se jedná o problém trhlin končících na rozhraní dvou různých materiálů, či problém obecného více-materiálového klínu. Cílem práce je vytvořit komplexní nástroj pro posuzování obecných koncetrátorů napětí tj, popis pole napětí v jeho okolí, zahrnutí případného vlivu přemostění trhliny do výsledného pole napětí a definici lomových kritérií pro obecný koncentrátor v anisotropním prostředí. U popisu pole napětí je využit tzv. Lechnického-Strohův formalismus a technika spojitě rozložených dislokací využívající teorii komplexních potenciálů. V práci je rovněž široce uplatněn tzv. dvoustavový "psí"-integrál (pro výpočet různých součinitelů asymptotického rozvoje pro napětí), založený na Bettiho recipročním teorému v kombinaci s metodou konečných prvků. Pro formulaci lomových kritérií je použita teorie tzv. „konečné lomové mechaniky“ a teorie sdružených asymptotických rozvojů. Studován je především vztah mezi ohybem trhliny podél rozhraní a její případnou pentrací do základního materiálu. Veškeré potřebné výpočty jsou prováděny v matematických softwarech MAPLE 10.0, MATLAB 7.1 a konečnoprvkovém systému ANSYS 10.0.
42
Leon, Armando. "Non-Linear Vibration and Dynamic Fracture Mechanics of Bridge Cables." Licentiate thesis, Karlskrona : Blekinge Institute of Technology, 2011. http://urn.kb.se/resolve?urn=urn:nbn:se:bth-00488.
Full textAbstract:
In the present work, the non-linear vibrations and the corresponding dynamic fracture mechanics of cables of cable-stayed bridges are studied. The cables are among the most critical components in cable-stayed bridges and there are different damage sources such as corrosion, vibration, fatigue and fretting fatigue that can significantly affect them, thereby reducing the cable’s service life and even producing their failure. Cable-Parametric Resonance is the specific non-linear vibration studied in this research. This type of vibration occurs due to displacements presented at the cable supports. These displacements are induced by the wind and traffic loads acting on the pylon and deck of the bridge. Under certain conditions, unstable cable-vibration of significant amplitude can be registered. Therefore, numerical and experimental analyses are carried out in order to describe this phenomenon and to determine the corresponding instability conditions. Two non-linear models of cable-parametric resonance are studied to predict the cable response. In the simulation method, the non-linear components are treated as external forces acting on the linear systems, which are represented by Single Degree of Freedom systems and described by digital filters. A clear non-linear relationship between the excitation and the cable response is observed in the simulations and the experiments. The corresponding experimental analysis is based on a scaled model (1:200) of the Öresund bridge and a good agreement between the numerical and experimental results is found. After obtaining the relationship between the cable response and the excitation, the cable instability conditions are determined. This is done by finding the minimum displacement required at the cable supports in order to induce nonlinear cable vibration of considerable amplitude. The instability conditions are determined within a wide range of excitation frequencies and conveniently expressed in a simplified and practical way by a curve. The determination process is rather fast and offers the possibility to evaluate all bridge cable stays in a rather short time. Finally, the dynamic fracture mechanics of the cable is considered by studying the fracture toughness characteristics of the material under dynamic conditions. Finite Element simulations on a pre-cracked three-point bending specimen under impact loading are performed. The observed cable instability is equivalently considered as the associated response to impact load conditions, and a crack as a defect on the wires of a cable stay. The simulations are based on an experimental work by using the Split Hopkinson pressure bar (Jiang et al). The dynamic stress intensity factor KI(t) up to crack initiation is then obtained by different methods. The numerical estimations based on the specimen’s crack tip opening displacement (CTOD) and mid-span displacement were closest to the experimental results. It is observed that a better estimation of the dynamic stress intensity factor relies on a proper formulation of the specimen’s stiffness.Lic March 2011
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Afshar, Hosseinabadi Reza. "Stress analysis of periodic notches by using the strain energy density approach." Doctoral thesis, Università degli studi di Padova, 2014. http://hdl.handle.net/11577/3423526.
Full textAbstract:
This research focuses on the stress analysis of periodic notches by using the strain energy density approach. Bolts, screws and rotary-shouldered connections, as examples of periodic notched components, play an important role in the performance of the machinery. The contents are related to two-dimensional (2D), as well as three-dimensional (3D) modeling of periodic notches both in the case of round and sharp notches. The analyses are based on the numerical modeling of periodic notches with linear elastic assumption of the material. The simple analytical expressions for the notch stress intensity factors (NSIFs) of periodic sharp notches, as well as theoretical stress concentration factors (SCFs) of periodic blunt notches are obtained. Using the strain energy density (SED) approach, the coarse mesh in the finite element models is used and compared with the results obtained from the fine meshing. In fact, using SED approach, the averaged strain energy in a control volume allows using the coarse meshes in order to determine the NSIFs and SCFs of notched components precisely.
In the case of 3D analysis, the thickness effects with particular attention on coupling modes, which due to Poisson effect are automatically generated, are studied. These modes can have a significant effect on the structural integrity of mechanical components.
In addition, two collaborative industry projects with: Officine Meccaniche Zanetti s.r.l. and Omera s.r.l. are successfully implemented.Questa ricerca si concentra su "Analisi delle sollecitazioni di intagliati periodici utilizzando l'approccio di densità di energia di deformazione", si è occupato di problematiche relative alla modellazione bidimensionale e tridimensionale di intagli periodici raccordati e a spigolo vivo. Bulloni, viti e connessioni rotanti spalle, come esempi di componenti intagliati periodiche, svolgono un ruolo importante nelle performance delle macchine. L'attività ha coinvolto prevalentemente la modellazione numerica in campo elastico ed ha permesso di ottenere delle semplici espressioni per la stima dei fattori d’intensificazione delle tensioni (NSIFs) e dei fattori teorici di concentrazione delle tensioni (SCFs) in funzione di tutti i parametri geometrici considerati. Le analisi numeriche sono state effettuate in prima battuta con mesh fitte e successivamente con mesh molto rade. Nel secondo caso l’energia di deformazione mediata in un volume di controllo ha permesso di determinare con precisione i fattori tensionali di riferimento e alcune espressioni per l’applicazione diretta a problematiche simili. Nel caso tridimensionale sono stati studiati e analizzati gli effetti legati allo spessore con particolare riferimento ai modi accoppiati che vengono automaticamente generati per effetto Poisson e che possono incidere in modo rilevante sull’integrità strutturale di componenti meccanici. I risultati raggiunti sono stati applicati a casi aziendali con due collaborazioni tutt’ora in atto con Officine Meccaniche Zanetti e Omera formalizzate in progetti di ricerca in cui il dottorando è stato il principale protagonista.
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Brabenec, Ladislav. "Topologicko-geometrický návrh a deformačně-napjatostní analýza tvaru disku železničního kola pro různé provozní podmínky na základě analýz LELM." Master's thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2011. http://www.nusl.cz/ntk/nusl-229933.
Full textAbstract:
The thesis deals with the behaviour of a cracked rail wheel. The aim was to perform the strain analysis of intact wheel as well as the fracture analysis of the primary direct cracked wheel. Solution includes an analysis of operating conditions, assessment of the substantiality of articular components of load, stiffness of the wheel, a comprehensive analysis of fracture of the selected railway wheel profile and optimization of the wheel shape depending on the matching fracture properties.
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Yilmaz, Suphi. "Buckling Driven Delamination Of Orthotropic Functionally Graded Materials." Master's thesis, METU, 2006. http://etd.lib.metu.edu.tr/upload/3/12607836/index.pdf.
Full textAbstract:
In today's technology severe working conditions increase demands on structural materials. A class of materials which are developed to meet these increased demands is Functionally Graded Materials (FGMs). These are inhomogeneous structural materials which are able to withstand large temperature gradients and corrosive environment. Application areas of FGMs are in aerospace industry, nuclear reactors, chemical plants and turbine systems. FGMs have gradual compositional variation from metal to ceramic which give them mechanical strength, toughness and heat resistance. However under high temperature gradients, cracking problems may arise due to thermal stresses. In layered structures the final stage of failure may be delamination due to crack extension. The objective of this study is to model a particular type of crack problem in a layered structure consisting of a substrate, a bond coat and an orthotropic FGM coating. There is an internal crack in the orthotropic layer and it is perpendicular to material gradation of coating. The position of the crack inside the coating is kept as a variable. The steady-state temperature distribution between the substrate and the coating causes a buckled shape along crack face. The critical temperature change, temperature distribution, mixed mode stress intensity values and energy release rates are calculated by using Displacement Correlation Technique. Results of this study present the effects of geometric parameters such as crack length, crack position, etc as well as the effects of the type of gradation on buckling behavior and mixed mode stress intensity factors.
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Cetin, Suat. "Analytical Solution Of A Crack Problem In A Radially Graded Fgm." Master's thesis, METU, 2007. http://etd.lib.metu.edu.tr/upload/3/12609120/index.pdf.
Full textAbstract:
The objective of this study is to determine stress intensity factors (SIFs) for a crack in a radially graded FGM layer on a substrate. Functionally graded coating with an edge crack perpendicular to the interface and a homogeneous substrate are bonded together. In order to make the problem analytically tractable, geometry is modeled as an FGM strip attached to a homogeneous layer. Introducing the elastic foundation underneath the homogeneous layer, an FGM coating on a thin walled cylinder can be modeled. At first, governing equations are obtained from stress displacement and equilibrium equations. Then using an assumed form of solution in terms of Fourier Transforms for displacements and applying the boundary conditions, a singular integral equation is obtained for the mode-I problem. Solving this singular integral equation numerically, stress intensity factors are obtained as functions of crack length, strip thicknesses and inhomogeneity parameter.
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(unal), Kutlu Ozge. "Computational 3d Fracture Analysis In Axisymmetric Media." Master's thesis, METU, 2008. http://etd.lib.metu.edu.tr/upload/12609872/index.pdf.
Full textAbstract:
In this study finite element modeling of three dimensional elliptic and semielliptic
cracks in a hollow cylinder is considered. Three dimensional crack and
cylinder are modeled by using finite element analysis program ANSYS.
The main objectives of this study are as follows. First, Ansys Parametric
Design Language (APDL) codes are developed to facilitate modeling of different
types of cracks in cylinders. Second, by using these codes the effect of some
parameters of the problem like crack location, cylinder&rsquos radius to thickness ratio (R/t), the crack geometry ratio (a/c) and crack minor axis to cylinder thickness ratio (a/t) on stress intensity factors for surface and internal cracks are examined. Mechanical and thermal loading cases are considered. Displacement Correlation Technique (DCT) is used to obtain Stress Intensity Factors.
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Het, Kivanc. "Effects Of Geometrical Factors On Fracture Toughness Using Semi-circular Bending Type Specimens." Master's thesis, METU, 2008. http://etd.lib.metu.edu.tr/upload/12609320/index.pdf.
Full textAbstract:
Semi-circular specimens (SCB) under three point-bending which are commonly used for fracture testing of rocks were used here for fracture mechanics tests. A total of 65 specimens were tested by using Ankara andesite rock.
Investigations including the effects of initial notch thickness, different loading span ratios (S/R), flattened loading end, and little dimensional variations when preparing the specimens were carried out.
Stress intensity factors for specimens with different geometries were computed individually by using a 3D finite element program ABAQUS.
Specimens with a preliminary notch thickness varying from 0.84 to 3.66 mm were tested under three point bending.
For a second group of specimens loading span was changed and fracture toughness variation was studied. Another change in the specimen geometry was made by machining a flat loading end at the upper load application point. Fracture toughness values were computed using the stress intensity values computed from numerical modeling and failure loads from the experiments.
It was found that up to 2 mm fracture toughness was not affected by variations in the thickness of preliminary notches. Fracture toughness was not affected by changing the loading span. For specimens with flat loading ends, fracture toughness was about 16% lower than the value found from regular SCB type specimens loaded at a point at the top by a steel roller.
As a result of about 46 experiments average fracture toughness of Ankara Gölbasi andesite was found as 1.36 MPa .
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來海, 博央, Hirohisa KIMACHI, 拓. 田中, Hiroshi TANAKA, 啓介 田中, Keisuke TANAKA, 康一 吉田, and Koichi YOSHIDA. "長繊維強化プラスチックスにおける巨視的モードⅠ負荷を受ける層間き裂の進展経路." 日本機械学会, 2000. http://hdl.handle.net/2237/9168.
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Ševeček, Oldřich. "Řešení obecných koncentrátorů napětí v anisotropních prostředích pomocí kombinace MKP a teorie komplexních potenciálů." Doctoral thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2009. http://www.nusl.cz/ntk/nusl-233849.
Full textAbstract:
Disertační práce se věnuje problematice obecných koncentrátorů napětí v anisotropních prostředích. Zejména se jedná o problém trhlin končících na rozhraní dvou různých materiálů, či problém obecného více-materiálového klínu. Cílem práce je vytvořit komplexní nástroj pro posuzování obecných koncetrátorů napětí tj, popis pole napětí v jeho okolí, zahrnutí případného vlivu přemostění trhliny do výsledného pole napětí a definici lomových kritérií pro obecný koncentrátor v anisotropním prostředí. U popisu pole napětí je využit tzv. Lechnického-Strohův formalismus a technika spojitě rozložených dislokací využívající teorii komplexních potenciálů. V práci je rovněž široce uplatněn tzv. dvoustavový "psí"-integrál (pro výpočet různých součinitelů asymptotického rozvoje pro napětí), založený na Bettiho recipročním teorému v kombinaci s metodou konečných prvků. Pro formulaci lomových kritérií je použita teorie tzv. „konečné lomové mechaniky“ a teorie sdružených asymptotických rozvojů. Studován je především vztah mezi ohybem trhliny podél rozhraní a její případnou pentrací do základního materiálu. Veškeré potřebné výpočty jsou prováděny v matematických softwarech MAPLE 10.0, MATLAB 7.1 a konečnoprvkovém systému ANSYS 10.0.