Dissertations / Theses on the topic 'Tolerance to fatigue'

The challenging requirements set on new full composite aeronautical structures are mostly related to the demonstration of damage tolerance capability of their primary structures, required by the airworthiness bodies. And while composite-made structures inherently demonstrate exceptional fatigue properties, when put in real life working conditions, a number of external factors can lead to impact damages thus reducing drastically their fatigue resistance due to fiber delamination, disbonding or breaking. This PhD aims towards contributing to the better understanding of the behavior of the primary composite aeronautical structure after near-edge impacts which are inevitable during the service life of an aircraft. The behavior of CFRP structures after impacts in only one small piece of the big picture which is the certification of CFRP built aircraft, where several other parameters need to be evaluated in order to fulfill the airworthiness requirements. These parameters are also discussed in this PhD thesis in order to give a better understanding of the complex task of CFRP structure certification, in which behavior of the impacted structure plays an important role. An experimental and numerical campaign was carried out in order to determine the level of delamination damage in CFRP specimens after near-edge impacts. By calibrating the numerical model with experimental data, it was possible, for different configurations and energy levels, to predict the extension of a delamination in a CFRP structure and to estimate its residual static strength using a very simple but robust technique. The original contribution of this work to the analysis of CFRP structures is the creation of a model which could be applicable to wide range of thicknesses and stacking sequences of CFRP structures, thus potentially being suitable for industrial application, as well.

This thesis presents a new modeling framework and application methodology for the study of aircraft structures. The framework provides a ‘cradle-to-grave’ approach to structural analysis of a component, where structural integrity encompasses all phases of its lifespan. The methodology examines the holistic structural design of aircraft components by integrating fatigue and damage tolerance methodologies. It accomplishes this by marrying the load inputs from a fatigue analysis for new design, into a risk analysis for an existing design. The risk analysis incorporates the variability found from literature, including recorded defects, loadings, and material strength properties. The methodology is verified via formal conceptualization of the structures, which are demonstrated on an actual hydraulic accumulator and an engine nacelle inlet. The hydraulic accumulator is examined for structural integrity utilizing different base materials undergoing variable amplitude loading. Integrity is accomplished through a risk analysis by means of fault tree analysis. The engine nacelle inlet uses the damage tolerance philosophy for a sonic fatigue condition undergoing both constant amplitude loading and a theoretical flight design case. Residual strength changes are examined throughout crack growth, where structural integrity is accomplished through a risk analysis of component strength versus probability of failure. Both methodologies can be applied to nearly any structural application, not necessarily limited to aerospace.

Oxygen uptake ( O2) kinetics determine the magnitude of the O2 deficit and the degree of metabolic perturbation and is considered to be an important determinant of exercise tolerance; however, there is limited empirical evidence to demonstrate that O2 kinetics is a direct determinant of exercise tolerance. The purpose of this thesis was to investigate O2 kinetics as a determinant of exercise tolerance and to consider its potential interaction with the maximum O2 ( O2max) and the W′ (the curvature constant of the hyperbolic power-duration relationship) in setting the tolerable duration of exercise. Recreationally-active adult humans volunteered to participate in the investigations presented in this thesis. Pulmonary O2 kinetics was assessed on a breath-by-breath basis and exercise tolerance was assessed by a time-to-exhaustion trial, with exhaustion taken as the inability to maintain the required cadence. A period of repeated sprint training (RST) resulted in faster phase II O2 kinetics (Pre: 29 ± 5, Post: 23 ± 5 s), a reduced O2 slow component (Pre: 0.52 ± 0.19, Post: 0.40 ± 0.17 L•min-1), an increased O2max (Pre: 3.06 ± 0.62, Post: 3.29 ± 0.77 L•min-1) and a 53% improvement in severe exercise tolerance. A reduced O2 slow component and enhanced exercise tolerance was also observed following inspiratory muscle training (Pre: 0.60 ± 0.20, Post: 0.53 ± 0.24 L•min-1; Pre: 765 ± 249, Post: 1061 ± 304 s, respectively), L-arginine (ARG) administration (Placebo: 0.76 ± 0.29 L•min-1 vs. ARG: 0.58 ± 0.23; Placebo: 562 ± 145 s vs. ARG: 707 ± 232 s, respectively) and dietary nitrate supplementation administered as nitrate-rich beetroot juice (BR) (Placebo: 0.74 ± 0.24 vs. BR: 0.57 ± 0.20 L•min-1; Placebo: 583 ± 145 s vs. BR: 675 ± 203, respectively). However, compared to a control condition without prior exercise, the completion of a prior exercise bout at 70% Δ (70% of the difference between the work rate at the gas exchange threshold [GET] and the work rate at the O2max + the work rate at the GET) with 3 minutes recovery (70-3-80) speeded overall O2 kinetics by 41% (Control: 88 ± 22 s, 70-3-80: 52 ± 13 s), but impaired exercise tolerance by 16% (Control: 437 ± 79 s, 70-3-80: 368 ± 48 s) during a subsequent exercise bout. When the recovery duration was extended to 20 minutes (70-20-80) to allow a more complete replenishment of the W′, overall kinetics was speeded to a lesser extent (by 23%; 70-20-80: 68 ± 19 s) whereas exercise performance was enhanced by 15% (70-20-80: 567 ± 125 s) compared to the control condition. In addition, the faster O2 kinetics observed when exercise was initiated with a fast start (FS; 35 ± 6 s), compared to an even start (ES; 41 ± 10 s) and slow start (SS; 55 ± 14 s) pacing strategy, allowed the achievement of O2max in a 3 minute trial and exercise performance was enhanced. Exercise performance was unaffected in a 6 minute trial with a FS, despite faster O2 kinetics, as the O2max was attained in all the variously paced trials. Therefore, the results of this thesis demonstrate that changes in exercise performance cannot be accounted for, purely, by changes in O2 kinetics. Instead, enhanced exercise performance appears to be contingent on the interaction between the factors underpinning O2 kinetics, the O2max and the W′, in support of the proposed ‘triad model’ of exercise performance.

Master of Science
Department of Kinesiology
Thomas J. Barstow
Background and Aim: The power-duration relationship accurately predicts exercise tolerance for constant power exercise performed in the severe intensity domain. At intensities above critical power (CP), the power-duration relationship establishes a hyperbolic curve. However, the prediction of exercise tolerance is currently unclear for work rates within the extreme intensity domain (durations <2min). We hypothesized that the power-duration relationship deviates from a linear 1/time relationship for WRs within the extreme intensity domain. Methods: Six men completed nine bilateral knee-extension tests on non-consecutive days and then performed 3 exercise tests in the severe intensity domain (S1-S3; T[subscript lim]>2–15min) and 4 in the extreme domain at 60%, 70%, 80%, and 90%1RM (T[subscript lim]<2min), in random order. Twitch force (Q[subscript tw]), maximal voluntary contraction (MVC), and voluntary activation (VA) were measured on the right vastus lateralis before and after <80s) each test; EMG was measured on the right vastus lateralis throughout each test. T[subscript lim] were plotted as a function of 1/Time. T[subscript lim] for the extreme intensities were compared to the predicted T[subscript lim] of the slope of the S1-S3 regression. Results: The r² for the severe domain 1/time model was 0.99 ± 0.007. T[subscript lim] for exercise at 60%1RM was not different than the predicted T¬lim, however, T¬lim for exercise at 70–90%1RM was shorter than the predicted T[subscript lim] (p<0.05). Post hoc analysis of the extreme domain (70–90%1RM) revealed a significant linear relationship, suggesting a W’ within the extreme domain (W’ext). T[subscript lim] of exercise at 60% 1RM was not different from the predicted value of the 1/Time relationship of the extreme domain. Q[subscript tw] and MVC were significantly decreased following exercise at S1-S3 and 60% 1RM, while no changes existed in Q[subscript tw] or MVC following exercise at 80 and 90%1RM. Further, no changes were found in VA following any exercise intensity. Conclusion: These data suggest that exercise tolerance in the extreme domain is limited by different factors than in the severe domain. However, there is a separate but measurable W’ext. Further, the factors limiting exercise in the extreme domain must be those from can be recovered by the time post-exercise measurements were made.

The main aim of this research was to investigate the behaviour of adhesive joints exposed to repeated low-velocity impact i.e. impact fatigue (IF), and to compare this loading regime with standard fatigue (SF), i.e. non-impacting, constant amplitude, sinusoidal loading conditions. Two types of lap joint configuration using rubber toughened modified epoxy adhesives were used and exposed to various loading conditions in order to determine the fatigue behaviour of the joints for each load conditions. The fatigue life was investigated using bonded aluminium alloy (7075-T6) single lap joint (SLJ) specimens, where it was seen that IF is an extremely damaging load regime compared to SF. Different trends were visible in force-life plots for these two types of loading. In SF a gradual decrease in the fatigue life with increasing load was observed, whereas, in IF a significant decrease in life was seen at relatively modest levels of maximum force after relatively few cycles. Comparisons of the fatigue life show a considerably earlier failure in IF than in SF for comparable levels of force and energy. Additionally, it was demonstrated that the maximum force per cycle, loading time, stiffness and strength decreased as a result of damage generated in the sample during IF.

The prediction of fatigue life and evaluation of onset and growth of matrix cracks and delamination for general composite laminates are studied analytically using theories of damage tolerance, residual modulus degradation and residual strength degradation. Damage onset including matrix cracks and edge delamination are predicted by using a total strain energy release rate criterion which accounts for interactive effects of matrix cracks and delamination. The analytical models for modulus degradation, matrix crack density and delamination size growth as function of fatigue stress and fatigue cycles are proposed. The proposed approach provides four choices for predicting tension-tension fatigue life and for assessing fail-safety for structures made of composite laminates. The direct relation of physical damage to fatigue life and analytical equations for calculating residual elastic moduli E$ rm{ sb1, E sb2, v sb{12}}$ and G$ sb{12}$ in terms of fatigue load and fatigue cycles are proposed. The proposed approach enables prediction of fatigue behaviour of general laminates using experimental data of a basic lay-up such as unidirectional laminate. The finite element technique was utilized to model the fatigue failure process of notched laminates. A simple example of a laminate with a central hole under tension-tension fatigue loading was performed.

Barely Visible Impact Damage (BVID) can occur when laminated composite material is subject to impact, i.e. from runway debris or dropped tools, and may result in a significant reduction in the compressive strength of composite structures. A component containing BVID subjected to compression may fail via a number of mechanisms. However, it is assumed that the impact damage problems to be modelled will fail by delamination buckling leading to propagation of damage away from the original site. This precludes problems where the initial mechanism of failure is via kink banding or buckling of the full laminate. An analytical model is presented, for application to various composite structures, which predicts the level of compressive strain below which growth of BVID following local buckling of a delaminated sublaminate will not occur. The model is capable of predicting the critical through-thickness level for delamination, the stability of delamination growth, the sensitivity to experimental error in geometric measurements of the damage area and additionally establishes properties desirable for laminates optimised for damage tolerance. Problems treated with the model are split into two impact categories; ‘face’ (i.e. an out-of-plane skin impact) and ‘free edge’ (i.e. an in-plane stiffener edge impact) and two compressive loading regimes; ‘static’ and ‘fatigue’. Analytical results for static and fatigue compression of face impacted plates show an agreement of threshold strain to within 4% and 17% of experimental values respectively. In particular, for impacts to the skin under a stiffener subject to static loading the model is accurate to within 5%. An optimised laminate stacking sequence has shown an experimental increase of up to 29% in static strength can be achieved in comparison to a baseline configuration. Finally, compression testing has been undertaken on three coupons in order to validate an analysis of static free edge problems. Analytical results are, on average, within 10% of experimental results. An optimised laminate is theoretically predicted to increase static compression after free edge impact strength by at least 35%.

Master’s thesis deals with study of fatigue crack growth retardation models. This document presents description, evaluation, verification and finally application of retardation models demonstrated on the damage tolerant structure. Analysis was provided on the L 410 NG aircraft in the location of the rear spar lower flange joint. Output of this thesis is comparison of the damage tolerance analysis with the current method using in the Aircraft Industry a. s., i.e. linear model without using load interaction models. The comparisons are fatigue crack growth curves, threshold and intervals of inspection program. AFGROW software was used for verification and application of retardation models.

This Ph.D. thesis deals with methodology for determination of residual fatigue lifetime of railway axles based on damage tolerance approach. This approach accepts an existence of potential defect, which could lead to fatigue failure of whole axle. The behavior of crack in railway axle is described by approaches of linear elastic fracture mechanics. There are plenty of factors, which more or less influence determined residual fatigue lifetime. The aim of this thesis is to quantify effects of these factors. The first part of Ph.D. thesis represents overview of studied problems relating to fatigue damage of railway axles. This part is focused on parameters, which influence fatigue crack growth in railway axle materials. The second part of thesis shows procedure for determination of residual fatigue lifetime, which was developed at the Institute of Physics of Materials of the Academy of Sciences of the Czech Republic. The main aim of this thesis was to improve current procedure for more precise estimation of residual fatigue lifetime. Significant part of this work is determination of significance of studied factors, which influencing calculated residual fatigue lifetime of railway axles (e.g. effect of threshold value, load spectrum, retardation effects, residual stress, axle geometry, material of axle etc.). The procedures described and results obtained can be also used for determination of residual fatigue lifetime of general mechanical component (not only railway axles). Therefore, results obtained in this Ph.D. thesis can be used e.g. for assessment of regular inspection intervals of cyclically loaded general mechanical parts.

Doctor of Philosophy
Department of Anatomy and Physiology
Thomas J. Barstow
The physiological mechanisms determining the tolerable duration of exercise dictate human physical accomplishments across all spectrums of life. Despite extensive study, these specific mechanisms, and their dependence on oxygen delivery and oxygen utilization, remain, a certain extent, undefined. The purpose of this dissertation was to test the overarching hypothesis that muscle contraction characteristics (i.e., intensity of contraction, muscle contraction-relaxation duty cycle, etc.) alter oxygen delivery and oxygen utilization, which directly influence the power-duration relationship and fatigue development, and therefore, exercise tolerance. To accomplish this, specific interventions of altered muscle contraction-relaxation duty cycle and blood flow occlusion were utilized. In the first investigation (Chapter 2), we utilized low and high muscle contraction-relaxation duty cycles to alter blood flow to the active skeletal muscle, demonstrating that critical power (CP) was reduced with the high muscle contraction-relaxation duty cycle due to a reduction in blood flow, while the curvature constant (W’) was not altered. The second investigation (Chapter 3) utilized blood flow occlusion to show that CP was reduced and W’ increased for blood flow occlusion exercise conditions compared to control blood flow exercise conditions. The final investigation (Chapter 4) utilized periods of blood flow occlusion during and post-exercise to reveal greater magnitudes of peripheral and central fatigue development during blood flow occlusion exercise compared to control blood flow exercise. Moreover, this investigation demonstrated that W’ was significantly related to the magnitude of fatigue development. Collectively, alterations in oxygen delivery and oxygen utilization via muscle contraction characteristics and blood flow occlusion directly influence CP and the magnitude of fatigue development. However, W’ does not appear to be influenced by manipulations in oxygen delivery and oxygen utilization, per se. Rather, W’ may be determined by the magnitude of fatigue accrued during exercise, which is dependent upon oxygen delivery and oxygen utilization. The novel findings of the investigations presented in this dissertation highlight important physiological mechanisms that determine exercise tolerance and demonstrate the need for interventions that improve oxygen delivery and oxygen utilization in specific populations, such as those with chronic heart failure or chronic obstructive pulmonary disease, to improve exercise tolerance.

Hardmetal industry is continuously seeking for high-performance products at reduced costs. In addition, it is strongly struggled by the high and volatile prices of raw materials. At this juncture, producers and end-users are deeply concerned in increasing the performance and enhancing service-life and reliability of engineering products, and replacing current constituents by alternative and less critical materials. Premature and unexpected fracture, together with wear, is the main damage phenomenon limiting the life in most cemented carbide applications. In the vast majority of cases such ruptures stem from the combination of high monotonic and cyclic stresses, together with different damage-related features associated with harsh service conditions, such as corrosion, and thermal shock. Therefore, relevant consideration of fracture toughness and fatigue resistance is required if reliability and lifetime of hardmetals applications is to be increased. Following the above ideas, the purpose of this thesis is to improve the performance and increase the reliability of cemented carbides in rupture-limited applications on the basis of enhanced damage tolerance and reduced fatigue sensitivity through an optimal microstructural design. Within this framework, this investigation is composed of three main subjects covering different aspects related to the performance of hardmetals under service-like conditions. The first two sections are devoted to conduct a comprehensive study on the influence of the microstructure on fracture and fatigue behaviour of hardmetals. The aim of the third section is to evaluate microstructural effects on the tolerance of cemented carbides to service-like damage, induced either by localised corrosion or thermal shock. Main contribution to toughness in cemented carbides derives from plastic stretching of crack-bridging ductile enclaves at the crack wake, referred to as the multiligament zone. Hence, the development of a multiligament zone implies the existence of a rising crack growth resistance (R-curve) behaviour in cemented carbides. This effectiveness of this toughening mechanism is intimately related to the microstructural characteristics. Within this context, the first section of this thesis is dedicated to carry out a detailed investigation of fracture mechanics and mechanisms in cemented carbides, and to propose a relation to capture microstructural effects on the R-curve characteristics of these materials. Strength reduction of hardmetals under the application of cyclic stresses is related to the inhibition of the crack-tip bridging mechanism. For WC¿Co cemented carbides, the degradation of bridging ligaments is mainly associated with an accumulation of the fcc to hcp phase transformation. However, this mechanism does not apply for Ni binders; therefore, it remains unclear if effective fatigue susceptibility of Co-base hardmetals is comparable to that of cemented carbides consisting of alternative binders. Moreover, hardmetals exhibit crack-deflection as an additional toughening mechanism, but contrary to the case of crack-bridging, it is immune to fatigue loads. The effective action of this toughening mechanism is speculated to increase with rising carbide mean grain size. Hence, the second part of this thesis is devoted to study and understand the fatigue sensitivity of cemented carbides consisting of binders with deformation mechanisms beyond phase transformation as well as medium/coarse microstructures. Finally, the third section of this thesis consists of a systematic study on the influence of the microstructure on damage-related features induced by either thermal shock or corrosion, in order to set out guidelines for optimal microstructural design. In doing so, the structural integrity of damaged cemented carbides is assessed on the basis of residual strength, and microstructural effects on damage tolerance are captured by means of considering induced damage level as a critical parameter.
Por un lado, la industria del metal duro está sumergida en una búsqueda constante de materiales de altas prestaciones a un coste reducido. Por el otro lado, las materias primas tienen precios altos y volátiles, que comprometen la estabilidad del mercado. En esta coyuntura, los productores y los usuarios finales están muy interesados, tanto en aumentar el rendimiento, incrementar la vida útil y mejorar la fiabilidad de estos productos, como en su sustitución por materiales alternativos y considerados menos críticos. En este contexto, el desgaste y la ruptura prematura son los dos principales mecanismos que limitan la vida útil de las aplicaciones de metal duro. En la gran mayoría de los casos las rupturas prematuras derivan de la combinación de altas tensiones, tanto monótonas como cíclicas, con el daño inducido durante la vida en servicio, como la corrosión, y el choque térmico. Por lo tanto, con el fin de aumentar fiabilidad en estas aplicaciones, es necesario entender los mecanismos de daño y fallo en estos materiales. Así, el propósito de esta tesis es mejorar el rendimiento y aumentar la fiabilidad de los carburos cementados a partir del desarrollo de materiales con una mayor tolerancia al daño y una menor sensibilidad a fatiga, a través de un óptimo diseño microestructural. La presente investigación se compone de tres partes que abarcan diferentes aspectos relacionados con el desempeño de los metales duros en condiciones de servicio. Las dos primeras secciones están dedicadas a realizar un estudio general sobre la influencia de la microestructura en el comportamiento a fractura y fatiga del metal duro. El objetivo de la tercera sección es evaluar los efectos microestructurales en la tolerancia al daño de los carburos cementados, ya sea inducido por corrosión o por choque térmico. El principal mecanismo de tenacidad en los carburos cementados reside en el estiramiento plástico de ligamentos metálicos de puenteo que se forman detrás de la punta de la grieta, llamada la zona de multiligamentos. El desarrollo del mecanismo de puenteo implica un incremento de la resistencia a fractura a medida que aumenta la longitud de la grieta. Este mecanismo es conocido como curva-R y su eficacia está íntimamente relacionada con las características microestructurales del material. Así, la primera parte de esta tesis doctoral está dedicada a llevar a cabo una investigación detallada de los mecanismos de fractura en los carburos cementados, y a proponer una relación que permita captar los efectos microestructurales en las características de curva-R de estos materiales. Por otro lado, la segunda parte de la tesis está dedicada a estudiar la influencia de la microestructura, incluyendo tanto el tamaño de grano de la fase carburo como el contenido de la fase ligante y su naturaleza química, en la sensibilidad a fatiga de los carburos cementados. Así, se ha prestado una atención particular en estudiar el comportamiento a fatiga de los carburos cementados con base níquel y en su comparación con los de base cobalto. Por otro lado, también se ha estudiado la influencia del tamaño de grano en la deflexión de grieta como un mecanismo adicional de aumento de tenacidad, inmune a las solicitaciones cíclicas. Por último, la tercera sección de esta tesis consiste en un estudio sistemático de la influencia de la microestructura de los carburos cementados en su tolerancia al daño, inducido tanto por corrosión como por choque térmico, con el fin de establecer las directrices para un diseño microestructural óptimo. De este modo, la integridad estructural de carburos cementados se evalúa sobre la base de su resistencia residual a flexión después de la inducción de daño

Etude experimentale (essais de fatigue, essais de traction, de compression) sur des eprouvettes entaillees ou non, trouees ou non de stratifies resine epoxyde/carbone. Analyse mathematique avec des calculs par elements finis

Modern durability and damage tolerance predictions for composite material systems rely on accurate estimates of the local stress and material states for each of the constituents, as well as the manner in which the constituents interact. In this work, an number of approaches to estimating the stress states and interactions are developed. First, an elasticity solution is presented for the problem of a penny-shaped crack in an N-phase composite material system opened by a prescribed normal pressure. The stress state around such a crack is then used to estimate the stress concentrations due to adjacent fiber fractures in a composite materials. The resulting stress concentrations are then used to estimate the tensile strength of the composite. The predicted results are compared with experimental values. In addition, a cumulative damage model for fatigue is presented. Modifications to the model are made to include the effects of variable amplitude loading. These modifications are based upon the use of remaining strength as a damage metric and the definition of an equivalent generalized time. The model is initially validated using results from the literature. Also, experimental data from APC-2 laminates and IM7/K3B laminates are used in the model. The use of such data for notched laminates requires the use of an effective hole size, which is calculated based upon strain distribution measurements. Measured remaining strengths after fatigue loading are compared with the predicted values for specimens fatigued at room temperature and 350°F (177°C).
Ph. D.

Introduction: Acute mental fatigue is defined as a psychobiological state that may arise during or after prolonged cognitive activities. Despite several studies showed that mental fatigue appears to impair sport performance, the scientific comprehension of this topic is still limited. Therefore, the aim of this thesis is to broaden the knowledge on the effects of mental fatigue on the sport-related performance. Study 1: This study aimed to investigate the effect of mentally demanding cognitive tasks on rowing performance in prepubertal athletes. Seventeen young rowers completed three separate testing sessions during which they performed three different cognitive tasks before completing a 1500 m time trial on the rowing ergometer. In the two experimental conditions, one hour of a standard cognitive task (Stroop task) and an arithmetic school test were used to elicit mental effort; in the control condition a time-matched, not demanding activity was carried out (painting). The performance of the time trial did not differ between conditions; physiological and perceptual measures recorded during the physical task were not affected by the conditions. Study 2: This study investigated the effect of a mentally demanding response inhibitory task on time trial performance in sub-elite under 23 cyclists. Ten under 23 road cyclists completed two separate testing sessions during which they performed two different cognitive tasks before completing a 30-min time trial on the cycle ergometer. In the experimental condition, 30 min of a standard cognitive task (Stroop task) was used to elicit mental fatigue; in the control condition, a non-demanding activity was carried out. Mean power output and cadence were negatively affected by the Stroop task, while heart rate (HR), rating of perceived exertion (RPE), blood lactate concentration, and heart rate variability (HRV) did not differ between the two conditions. Study 3: This investigation examined the effects of mental fatigue on soccer-specific physical and technical performance in young players. Twelve under-14 (U14), twelve under-16 (U16) and twelve under-18 (U18) soccer players completed the two parts of the investigation. Part one assessed the soccer-specific physical performance using the Yo-Yo Intermittent Recovery Test, Level 1 (Yo-Yo IR1). Part two assessed the soccer-specific technical performance using the Loughborough Soccer Passing and Shooting Tests (LSPT, LSST). Each part was preceded by 30 min of Stroop task (mentally fatiguing task) or 15 min of reading magazines (control task) performed in a randomised and counterbalanced order. Mental fatigue significantly reduced Yo-Yo IR1 distance in the three age groups, alongside an increase in HR and RPE. Mental fatigue reduced soccer-specific physical performance in U14, U16 and U18 players, without alteration of technical performance, except for LSPT in U18. Study 4: This study investigated whether 4 weeks of endurance training could improve tolerance to mental exertion in untrained participants. Twenty participants completed a 4-week training protocol in a randomised and counterbalanced order. Baseline and follow-up assessment were conducted over three sessions in the week preceding and following the training period. During session 1, participants completed an incremental maximal ramp test. During sessions 2 and 3 participants completed a 15 min cycling time trial preceded by either a mental exertion or control task (counterbalanced). Following baseline assessments, participants were randomised into a physical training or placebo group that completed the training intervention thrice weekly over four weeks. The physical training resulted in increases in peak oxygen consumption (VO2peak) relative to the placebo group. Physical training group increased their time trial distance following the mental exertion task to a greater extent than the placebo group. RPE during the time trial and perceptual measures of mental exertion did not significantly change between groups. Conclusions: This thesis provides insight into the effects of mental fatigue on sport-specific physical and technical performance, focusing in broaden the knowledge on different age-groups and evaluating, for the first time, the effect of an endurance training protocol on the ability to tolerate mental fatigue.

Master's thesis deals with the damage tolerance analysis of wing-to-fuselage joint of L 410 NG airplane. Thesis includes determination of the load distribution to the individual attachments of wing-to-fuselage joint, residual strength analysis and residual fatigue life analysis of the most loaded attachment lugs, calculation of fatigue crack growth curves in the attachment solids and inspection program proposal. This analysis was performed using FE model of the wing and central part of the fuselage and AFGROW software.

The majority of aircraft structures are designed to be damage-tolerant such that safe operation can continue in the presence of minor damage. It is necessary to schedule inspections so that minor damage can be found and repaired. It is generally not possible to perform structural inspections prior to every flight. The scheduling is traditionally accomplished through a deterministic set of methods referred to as Damage Tolerance Analysis (DTA). DTA has proven to produce safe aircraft but does not provide estimates of the probability of failure of future flights or the probability of repair of future inspections. Without these estimates maintenance costs cannot be accurately predicted. Also, estimation of failure probabilities is now a regulatory requirement for some aircraft.

The set of methods concerned with the probabilistic formulation of this problem are collectively referred to as Probabilistic Damage Tolerance Analysis (PDTA). The goal of PDTA is to control the failure probability while holding maintenance costs to a reasonable level. This work focuses specifically on PDTA for fatigue cracking of metallic aircraft structures. The growth of a crack (or cracks) must be modeled using all available data and engineering knowledge. The length of a crack can be assessed only indirectly through evidence such as non-destructive inspection results, failures or lack of failures, and the observed severity of usage of the structure.

The current set of industry PDTA tools are lacking in several ways: they may in some cases yield poor estimates of failure probabilities, they cannot realistically represent the variety of possible failure and maintenance scenarios, and they do not allow for model updates which incorporate observed evidence. A PDTA modeling methodology must be flexible enough to estimate accurately the failure and repair probabilities under a variety of maintenance scenarios, and be capable of incorporating observed evidence as it becomes available.

This dissertation describes and develops new PDTA methodologies that directly address the deficiencies of the currently used tools. The new methods are implemented as a free, publicly licensed and open source R software package that can be downloaded from the Comprehensive R Archive Network. The tools consist of two main components. First, an explicit (and expensive) Monte Carlo approach is presented which simulates the life of an aircraft structural component flight-by-flight. This straightforward MC routine can be used to provide defensible estimates of the failure probabilities for future flights and repair probabilities for future inspections under a variety of failure and maintenance scenarios. This routine is intended to provide baseline estimates against which to compare the results of other, more efficient approaches.

Second, an original approach is described which models the fatigue process and future scheduled inspections as a hidden Markov model. This model is solved using a particle-based approximation and the sequential importance sampling algorithm, which provides an efficient solution to the PDTA problem. Sequential importance sampling is an extension of importance sampling to a Markov process, allowing for efficient Bayesian updating of model parameters. This model updating capability, the benefit of which is demonstrated, is lacking in other PDTA approaches. The results of this approach are shown to agree with the results of the explicit Monte Carlo routine for a number of PDTA problems.

Extensions to the typical PDTA problem, which cannot be solved using currently available tools, are presented and solved in this work. These extensions include incorporating observed evidence (such as non-destructive inspection results), more realistic treatment of possible future repairs, and the modeling of failure involving more than one crack (the so-called continuing damage problem).

The described hidden Markov model / sequential importance sampling approach to PDTA has the potential to improve aerospace structural safety and reduce maintenance costs by providing a more accurate assessment of the risk of failure and the likelihood of repairs throughout the life of an aircraft.

Les industriels du secteur aéronautique sont, de plus en plus, à la recherche de procédés de fabrication à forte valeur ajoutée sans modifier les paramètres d’infusabilité de la résine lorsque l’on change de tissu. Nous avons donc mis en œuvre le procédé d’infusion de résine liquide sur des composites carbone/époxyde de forte épaisseur (e>4 mm) en modifiant les cycles de polymérisation, les matériaux utilisés et les séquences d’empilement. Tous les tissus sont en carbone et la résine utilisée est la résine commerciale RTM6. Les structures aéronautiques sont sollicitées, en service, de différentes façons. Elles peuvent être accidentellement impactées par des engins de maintenance, des outils, de la grêle ou toute autre forme d’impact. Le problème pour les industriels est de pouvoir détecter l’endommagement créé et de comprendre les mécanismes mis en jeu lors de l’impact mais aussi leur évolution pendant un cyclage en fatigue. Nos travaux se sont donc inscrits dans cet objectif et différentes méthodes ont été mises en œuvre : détection de défauts d’impact et suivi en temps réel par thermographie infrarouge, détection de l’indentation résiduelle par numérisation par projection de franges. Parallèlement, le phénomène d’impact a aussi été traité par une étude statistique par plan d’expérience et une modélisation avancée a été créée avec l’utilisation de surfaces cohésives.

O presente trabalho, inicialmente, apresenta uma revisão a cerca de vários métodos de análise de fadiga e tolerância ao dano em materiais compósitos e, em particular, o conceito de elemento crítico e o método da resistência residual é estudado e aplicado à análise de materiais compósitos poliméricos, reforçados por fibras contínuas, sob cargas cíclicas. Casos de laminados com concentradores de tensão são também abordados, principalmente, devido ao crescente emprego de materiais compósitos. Diante do contexto apresentado, o presente trabalho buscou desenvolver uma ferramenta computacional, que fosse capaz de prever de forma consistente tanto a vida em fadiga de estruturas em material compósito como a resistência residual dessas estruturas sob a ação de carregamentos cíclicos. Dessa forma, um modelo baseado na resistência residual, que envolve integração numérica, é implementado em um programa computacional denominado SAFECom (Sistema de Análise de Fadiga de Estruturas em Compósito), o qual é aplicado a alguns estudos de casos. O SAFECom foi, então, avaliado, comparando resultados obtidos com dados da literatura. Verifica-se uma boa convergência entre os resultados nessa fase de avaliação. Por fim, são listadas possibilidades de aprimoramento do programa, que podem torná-lo aplicável a outros estudos de casos não abordados no presente trabalho, além da avaliação do mesmo através de ensaios experimentais.
This work firstly presents an overview about various methods of analysis of fatigue and damage tolerance of composite materials and, in particular, the concept of critical element and the method of residual strength are studied and applied to the analysis of polymer composites reinforced by continuous fibers under cyclic loading. Cases of laminates with stress concentrators are also addressed, mainly due to increasing use of composite materials. In this context, this work aimed to develop a computational tool that was able to consistently predict both the fatigue life of structures in composite material and the residual strength of these structures under the action of cyclic loads. Thus, a model based on residual strength, which involves numerical integration, is implemented in a computer program called SAFECom (System of Analysis of Fatigue for Composites), which is applied to some case studies. The SAFECom is evaluated by comparing results obtained from literature data. There is a good convergence between the results of this assessment. Finally, possibilities for improvement are listed on the program that can make it applicable to other case studies, which were not covered in this study, in addition to its evaluation by experimental tests.

This study addresses the effects of the changing workforce and the physiological and psychological benefits of fitness. The purpose of this experiment is to test the relationship between situational stressors, perceived fitness, exercise locus of control, self-control, and perceived stress.

Die Arbeit ruht auf zwei Säulen: Die eine besteht in der Aufbereitung, Erprobung und konsequenten Anwendung von Methoden der Skaleninvarianzanalyse, die andere in einem breiten Fundus an experimentellen Daten für aushärtbare Aluminiumknetlegierungen in der Form dünner Bleche, die unter gleichartigen, streng kontrollierten Bedingungen gewonnen worden sind. Als methodische Weiterentwicklungen sind die Fundierung des Umgangs mit der algebraischen Korrelation zwischen Vorfaktor und Exponent einer beliebigen Potenzgleichung, die Übertragung des Ansatzes der finiten Skaleninvarianz auf die Ermüdungsrißausbreitung sowie die Kombination der Idee eines geschwindigkeitsbestimmenden Schrittes mit der Dimensionsanalyse der umgebungsabhängigen Ermüdungsrißausbreitung bis hin zur Kartierung der geschwindigkeitsbestimmenden Schritte zu nennen. Auf experimenteller Seite wurde eine Datensammlung mit gemessenen Streubändern für die Ermüdungsrißausbreitung und das Verfestigungsverhalten von 39 Orientierungen bzw. Auslagerungszuständen von Aluminiumlegierungen aufgebaut. Diese Sammlung wird durch ausgewählte Messungen der Ermüdungsrißausbreitung im schwellenwertnahen Bereich, Restfestigkeitsversuche, Rißschließmessungen, Rauheitsmessungen an Bruchflächen, frequenzabhängige Messungen zum Umgebungseinfluß sowie Untersuchungen an drei Stählen und einer Magnesiumlegierung sinnvoll ergänzt. Auf der Basis der Meßdaten und der Analysemethoden wurde der Werkstoffeinfluß auf die Ermüdungsrißausbreitung in dünnen Blechen aus Aluminiumknetlegierungen bei Belastung mit konstanter Amplitude im Gültigkeitsbereich der linear-elastischen Bruchmechanik untersucht. Dabei wurden folgende Größen als wesentliche Einflußfaktoren identifiziert: - für die Gruppenzugehörigkeit: der Kohärenz- und Ordnungsgrad der festigkeitsbestimmenden Ausscheidungen und die resultierende Gleitverteilung, - für den gemeinsamen Vorfaktor der Legierungen der Gruppe 1: die elastischen Eigenschaften und das Spannungsverhältnis (Translation der Paris-Geraden), - für die Exponenten der Legierungen der Gruppe 1: 0,2%-Dehngrenze, athermischer Verfestigungsparameter, Probendicke und Kc-Wert als dimensionsloses Potenzprodukt (Rotation der Paris-Geraden), - für die Legierungen der Gruppe 2: das Ausmaß der Rißablenkung und eine bleibende Mode-II-Komponente der Rißöffnungsverschiebung, - für den Umgebungseinfluß der Legierung 6013 T6: Frequenz und Schwingbreite des Spannungsintensitätsfaktors. Die Diskussion umfaßt den wertenden Vergleich der experimentellen Ergebnisse mit Befunden und Modellen aus der Literatur, Erklärungsansätze für die Ursachen der Einflußnahme der wesentlichen Parameter sowie einen Modellansatz für die Legierungen der Gruppe 1 auf der Basis einer Mischungsregel. Dabei hatte sich erwiesen, daß keines der aus der Literatur bekannten Modelle alle Befunde richtig wiedergibt. Einige der ausgearbeiteten Erklärungsansätze bedürfen der zukünftigen Vertiefung
The work is based upon two essentials: the first one is the preparation and application of techniques of scale invariance analysis, the second one consists in a database of experimental results for heat-treatable thin-sheet wrought aluminium alloys obtained under uniform conditions. Progress with respect to methodology was achieved regarding, first, the algebraic correlation between sets of coefficients and exponents of any power law, second, the transfer of the concept of finite scale invariance to the phenomenon of fatigue crack growth (FCG), and third, the combination of the ideas of a rate-controlling step and dimensional analysis of environmental-assisted FCG including the mapping of rate-controlling steps. In the experimental part, a database containing both measured scatterbands of FCG and strengthening characteristics for several orientations and aging conditions of aluminium alloys amounting to a total of 39 different material conditions was established. This database was supplemented with results of selected measurements of near-threshold FCG rates, residual strength, crack closure, roughness of fatigue cracks, and frequency-dependent environmental-assisted FCG as well as investigations of three plain-carbon steels and a magnesium alloy. Based on these prerequisites, the influence of the material on the FCG behaviour of thin-sheet wrought aluminium alloys under constant-amplitude loading was investigated within the limits of validity of linear-elastic fracture mechanics. The following influence factors were identified to be essential: The assignment of alloys to one out of two groups is mainly determined by the degrees of coherency and order of the strength-controlling precipitates and the resulting type of slip distribution. The normalized-Paris-law coefficient for the first group is mainly dependent on the modulus of elasticity and the stress ratio. The Paris-law exponents for the first group are dominated by a dimensionless power monomial of the 0.2% proof stress, the athermal strengthening coefficient, sheet thickeness and the critical stress intensity factor. The retardation of the FCG rates of alloys of the second group relative to the first group is mainly determined by the amount of crack deflection and by a residual mode-II component of crack opening displacement. Finally, the environment-assisted FCG for aluminium alloy 6013 T6 reveals a coupled dependence on loading frequency and cyclic stress intensity factor. The discussion covers the evaluation of the results in relation to observations and models from the literature, the explanation of the modes of operation of the major influence factors and a model based on a mixing rule for the alloys of the first group. It turned out that there is not any model that reflects all of the observations simultaneously. Some of the ideas presented require to be worked out in more detail

The methodology proposed in this research extends the current full-scale test approach based on the life factor and the load enhancement factor, and provides information necessary to define inspection intervals for composite structures by studying the effects of extremely improbable, high-energy impact damage. This methodology further extend the current practice during damage-tolerance certification to focus on the most critical damage locations of the structure and interpret the structural and loads details into the most representative repeated load testing in element level to gain information on the residual strength, fatigue sensitivity, inspection methods and inspection intervals during full-scale test substantiation. A reliability approach to determine the inspection intervals to mitigate risks of unexpected failure during the damage tolerance phase, especially with large impact damages, was discussed. This methodology was validated with several full-scale test examples of the Beechcraft Starship forward wings with large impact damages on the front and aft spars. Procedures to generate reliable and economical scatter and load-enhancement factors necessary for a particular structural test by selecting the design details representing the critical areas of the structure is outlined with several examples and case studies. The effects of laminate stacking sequence, test environment, stress ratios, and several design features such as sandwich and bonded joints on the static-strength and fatigue-life shape parameters are discussed with detailed examples. Furthermore, several analytical techniques for obtaining these shape parameters are discussed with examples. Finally, the application of load enhancement factors and life factors for a full-scale test spectrum without adversely affecting the fatigue life and the damage mechanism of the composite structure is discussed. A methodology synthesizing the life factor, load enhancement factor, and damage in composites is proposed to determine the fatigue life of a damage-tolerant composite airframe. This methodology narrows the variability of different aspects of the damaged structure to determine the remaining fatigue life of the structure. In order to prevent unintentional failure of a damaged article during dadt testing, especially when investigating extremely improbable high-energy impact threats that reduce the residual strength of a composite structure to limit load, rigorous inspection intervals are required. The probability of failure of the damaged structure with the enhanced spectrum loads can be evaluated using the proposed cumulative fatigue unreliability model, which was validated through a full-scale test demonstration of a damaged article at the critical load path. Information from this model can be used also to allot economical and reliable inspection intervals during service based on a target reliability and a critical damage threshold. Full-scale dadt test conducted with a visual impact damage on the aft spar (secondary Load path) using the improved lefs based on the design details of Starship forward wing Structure demonstrated the repeated life requirements according the proposed load-life-damage Hybrid approach, and the post-dadt residual strength requirements. The forward-wing dadt test article with a large damage on the front spar (primary load path) demonstrated the capability of the cumulative fatigue unreliability model to predict the damage growth in terms of reliability and the capability of the model to determine the inspection levels. Although it is not a one-to one correlation for the damage propagation or its size, the cumulative fatigue unreliability model highlighted load segments that resulted in gradual progression of local damage, such as possible matrix cracks, and the global impact of high loads that resulted in evident damage growth
Wichita State University, College of Engineering, Dept. of Aerospace Engineering

Die Arbeit ruht auf zwei Säulen: Die eine besteht in der Aufbereitung, Erprobung und konsequenten Anwendung von Methoden der Skaleninvarianzanalyse, die andere in einem breiten Fundus an experimentellen Daten für aushärtbare Aluminiumknetlegierungen in der Form dünner Bleche, die unter gleichartigen, streng kontrollierten Bedingungen gewonnen worden sind. Als methodische Weiterentwicklungen sind die Fundierung des Umgangs mit der algebraischen Korrelation zwischen Vorfaktor und Exponent einer beliebigen Potenzgleichung, die Übertragung des Ansatzes der finiten Skaleninvarianz auf die Ermüdungsrißausbreitung sowie die Kombination der Idee eines geschwindigkeitsbestimmenden Schrittes mit der Dimensionsanalyse der umgebungsabhängigen Ermüdungsrißausbreitung bis hin zur Kartierung der geschwindigkeitsbestimmenden Schritte zu nennen. Auf experimenteller Seite wurde eine Datensammlung mit gemessenen Streubändern für die Ermüdungsrißausbreitung und das Verfestigungsverhalten von 39 Orientierungen bzw. Auslagerungszuständen von Aluminiumlegierungen aufgebaut. Diese Sammlung wird durch ausgewählte Messungen der Ermüdungsrißausbreitung im schwellenwertnahen Bereich, Restfestigkeitsversuche, Rißschließmessungen, Rauheitsmessungen an Bruchflächen, frequenzabhängige Messungen zum Umgebungseinfluß sowie Untersuchungen an drei Stählen und einer Magnesiumlegierung sinnvoll ergänzt. Auf der Basis der Meßdaten und der Analysemethoden wurde der Werkstoffeinfluß auf die Ermüdungsrißausbreitung in dünnen Blechen aus Aluminiumknetlegierungen bei Belastung mit konstanter Amplitude im Gültigkeitsbereich der linear-elastischen Bruchmechanik untersucht. Dabei wurden folgende Größen als wesentliche Einflußfaktoren identifiziert: - für die Gruppenzugehörigkeit: der Kohärenz- und Ordnungsgrad der festigkeitsbestimmenden Ausscheidungen und die resultierende Gleitverteilung, - für den gemeinsamen Vorfaktor der Legierungen der Gruppe 1: die elastischen Eigenschaften und das Spannungsverhältnis (Translation der Paris-Geraden), - für die Exponenten der Legierungen der Gruppe 1: 0,2%-Dehngrenze, athermischer Verfestigungsparameter, Probendicke und Kc-Wert als dimensionsloses Potenzprodukt (Rotation der Paris-Geraden), - für die Legierungen der Gruppe 2: das Ausmaß der Rißablenkung und eine bleibende Mode-II-Komponente der Rißöffnungsverschiebung, - für den Umgebungseinfluß der Legierung 6013 T6: Frequenz und Schwingbreite des Spannungsintensitätsfaktors. Die Diskussion umfaßt den wertenden Vergleich der experimentellen Ergebnisse mit Befunden und Modellen aus der Literatur, Erklärungsansätze für die Ursachen der Einflußnahme der wesentlichen Parameter sowie einen Modellansatz für die Legierungen der Gruppe 1 auf der Basis einer Mischungsregel. Dabei hatte sich erwiesen, daß keines der aus der Literatur bekannten Modelle alle Befunde richtig wiedergibt. Einige der ausgearbeiteten Erklärungsansätze bedürfen der zukünftigen Vertiefung.
The work is based upon two essentials: the first one is the preparation and application of techniques of scale invariance analysis, the second one consists in a database of experimental results for heat-treatable thin-sheet wrought aluminium alloys obtained under uniform conditions. Progress with respect to methodology was achieved regarding, first, the algebraic correlation between sets of coefficients and exponents of any power law, second, the transfer of the concept of finite scale invariance to the phenomenon of fatigue crack growth (FCG), and third, the combination of the ideas of a rate-controlling step and dimensional analysis of environmental-assisted FCG including the mapping of rate-controlling steps. In the experimental part, a database containing both measured scatterbands of FCG and strengthening characteristics for several orientations and aging conditions of aluminium alloys amounting to a total of 39 different material conditions was established. This database was supplemented with results of selected measurements of near-threshold FCG rates, residual strength, crack closure, roughness of fatigue cracks, and frequency-dependent environmental-assisted FCG as well as investigations of three plain-carbon steels and a magnesium alloy. Based on these prerequisites, the influence of the material on the FCG behaviour of thin-sheet wrought aluminium alloys under constant-amplitude loading was investigated within the limits of validity of linear-elastic fracture mechanics. The following influence factors were identified to be essential: The assignment of alloys to one out of two groups is mainly determined by the degrees of coherency and order of the strength-controlling precipitates and the resulting type of slip distribution. The normalized-Paris-law coefficient for the first group is mainly dependent on the modulus of elasticity and the stress ratio. The Paris-law exponents for the first group are dominated by a dimensionless power monomial of the 0.2% proof stress, the athermal strengthening coefficient, sheet thickeness and the critical stress intensity factor. The retardation of the FCG rates of alloys of the second group relative to the first group is mainly determined by the amount of crack deflection and by a residual mode-II component of crack opening displacement. Finally, the environment-assisted FCG for aluminium alloy 6013 T6 reveals a coupled dependence on loading frequency and cyclic stress intensity factor. The discussion covers the evaluation of the results in relation to observations and models from the literature, the explanation of the modes of operation of the major influence factors and a model based on a mixing rule for the alloys of the first group. It turned out that there is not any model that reflects all of the observations simultaneously. Some of the ideas presented require to be worked out in more detail.

The present thesis deals with probabilistic and defect tolerant fatigue assessment of wind turbine castings. To this end, two types of EN-GJS-400-18-LT ductile cast iron were investigated in this research, clean baseline material in the shape of casting blocks with different thicknesses and also defective material from a rejected wind turbine hub. To establish the required P–S–N diagrams for safe-life design of wind turbine castings, fatigue specimens with different dimensions machined from baseline casting blocks with different thicknesses. Constant amplitude axial fatigue tests were performed on these specimens at room temperature at R = 0 and R = −1. Geometrical size effect, wall-thickness effect (technological size effect) and mean stress effect on fatigue strength of baseline EN-GJS-400-18-LT material were evaluated and analyzed. Statistical analysis of fatigue data was done by means of the Weibull distribution, and P–S–N diagrams were established. The established P–S–N diagrams showed that the Weibull distribution is well fit to the scatter of the experimentally obtained fatigue life data. Weibull’s weakest-link method was used to evaluate the size effect. It made a satisfactory prediction of the fatigue strength for specimens with different dimensions. To study damage tolerant design of wind turbine castings, a rejected wind turbine hub was flame cut to several blanks and several defective fatigue specimens were machined from these blanks. Constant amplitude axial fatigue tests were performed on these specimens at room temperature at R = 0 and R = −1. Fatigue strength of baseline EN-GJS-400-18-LT was compared with that of defective material from the rejected wind turbine hub. The effect of graphite nodules and defects type, shape, size and position on fatigue strength of defective material was evaluated. The hypothesis that the endurance observed in an S−N test can be predicted based on the analysis of crack growth from casting defects through defect-free ‘base’ material was tested for the analyzed defective material in this research. It was shown that fatigue life of the analyzed defective cast component is controlled by fatigue crack growth and the slope of S − N curve for baseline EN-GJS-400-18-LT is different than the slope of S − N curve for defective EN-GJS-400-18-LT. To perform random defect analysis of wind turbine castings, establish the scatter of fatigue life and obtain the probability of failure of these components, 3D X-ray computed tomography was use to detect defects in defective specimens and find the defect size distribution and density of defects (number of defects per unit volume). The obtained defect size distribution and density for the defective material was used in random defect analysis to establish the scatter of fatigue life for defective specimens. Finally both safe-life and damage-tolerant design philosophies were used to evaluate the fatigue life of an EN-GJS-400-18-LT ductile cast iron block, representative of heavy-section wind turbine castings. The estimated S−N curves for the analyzed component based on these two methods were compared. It was shown that fatigue design of heavy section wind turbine cast iron components based on safe-life design philosophy may result in non-conservative design of these components

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Objective: To assess the six-minute step test (6MST) in terms of its ability to evaluate the aerobic functional capacity of COPD patients. In addition to compare, correlate and verify if there is an agreement between the metabolic (oxygen uptake-VO2), ventilatory (minute ventilation-VE) and cardiovascular variables (heart rate-HR), and perceived exertion of the 6MST and the incremental cardiopulmonary exercise test (ICPET). Methods: Metabolic and ventilatory variables, heart rate, dyspnea and lower limb (LL) fatigue were recorded from 18 COPD patients (five had mild COPD, five moderate, six severe and two very severe) performing the 6MST and the submaximal ICPET on cycle ergometer (work rate was increased by 5-10 watts) on different and not consecutive days. Results: There were no significant differences between VO2, HR, VE, dyspnea and LL fatigue mean values at the peak of both tests. Moderate correlations were found between the 6MST performance and ICPET s VO2 (r=0.49;p=0.05) and performance (r=0.63;p=0.005), high correlations were found between both VO2 (L/min and mL/kg/min) (r=0.76 and r=0.77;p=0.001), and moderate correlations were found between HR (r=0.68;p=0.002) and LL fatigue (r=0.59;p=0.011) between the tests. There was no agreement between VO2, HR and LL fatigue values between the tests. Conclusion: The 6MST has the ability to assess aerobic functional capacity and presents cardiorespiratory responses and perceived exertion similar to the ICPET in magnitude, however it cannot replace the ICPET, since the 6MST assesses and demands greater and more localized work from peripheral muscles, thus not reflecting the same ICPET cardiorespiratory capacity.
Objetivo: Verificar se o teste do degrau de seis minutos (TD6) permite avaliar a capacidade funcional aeróbia de pacientes com doença pulmonar obstrutiva crônica (DPOC) além de comparar, correlacionar e verificar se há concordância entre as variáveis metabólica (consumo de oxigênio-VO2), ventilatória (ventilação minuto-VE), cardiovascular (frequência cardíaca- FC) e de percepção de esforço entre o TD6 e o teste de exercício cardiopulmonar incremental (TECPI). Métodos: 18 pacientes com DPOC (cinco com obstrução leve, cinco moderada, seis grave e dois muito grave) executaram o TD6 e o TECPI submáximo em cicloergômetro (incrementos de 5 a 10W) em dias não coincidentes e não consecutivos. O TD6 foi realizado em um degrau cuja altura media 20cm e durante seis minutos os pacientes foram orientados a subir o mais rápido possível em cadência livre. Durante os testes foram avaliados o VO2, a VE, a FC, a sensação de dispneia (SD) e a sensação de fadiga de membros inferiores (SFMMII), por meio da escala de Borg. Resultados: Não houve diferenças significativas entre as médias dos valores de VO2, FC, VE, SD e SFMMII no pico de ambos os testes. Observaram-se correlações moderadas entre o desempenho no TD6 (número total de subidas no degrau) e o VO2 no TECPI (r=0,49;p=0,05) e entre o desempenho no TD6 com o desempenho no TECPI (r=0,63;p=0,005); correlações fortes entre os VO2 (L/min e mL/kg/min) (r=0,76 e r=0,77;p=0,001) e correlações moderadas das FC (r=0,68;p=0,002) e das SFMMII (r=0,59;p=0,011) entre os testes. Não se observou concordância entre os valores de VO2, FC e SFMMII entre os testes. Conclusão: O TD6 pode avaliar a capacidade funcional aeróbia e apresenta respostas cardiorrespiratórias e de percepção de esforço semelhantes ao TECPI em magnitude, porém não substitui o TECPI, uma vez que o TD6 não reflete a mesma capacidade cardiorrespiratória do TECPI por ser um teste que avalia e exige um trabalho maior e mais localizado da musculatura periférica.

Aeronautical products additively manufactured by Selective Laser Melting (SLM), are known to have fatigue properties which are negatively impacted by porosity defects, microstructural features and residual stresses. Little research is available studying these phenomena with respect to the short fatigue crack growth (FCG) inconsistency problem, the large focus being on the long FCG. This thesis seeks to add useful knowledge to the understanding of the mechanisms for short crack growth variability in SLM manufactured Ti-6Al-4V, with the two variables for the process conditions and build directions investigated. An in-situ FCG investigation using x-ray synchrotron computed micro-tomography (μXSCT) was used to visually observe and quantify the short crack path evolution. Crack growth, deflections and porosity interactions were noted and discussed in relation to microstructure, build layer thickness and build layer orientation. A novel use of in-situ energy dispersive x-ray diffraction (EDD) was able to show the lattice strains evolving as a propagating crack moved through a small region of interest. The results presented show the ability to reliably obtain all six elastic strain tensor components, and interpret useful knowledge from a small region of interest.

There are conflicting views in literature with respect to the damage tolerance behavior of as built SLM manufactured Ti-6Al-4V. In the 2018 review by Agius et al., the more prominent studies were considered with Leuders et al. showing the highest long FCG rates for cracks parallel to the build layer and Cain et al. showing cracks propagating through successive build layers as highest [1]–[3]. Cain et al. and Vilaro et al. report significant anisotropy in long FCG for different build orientations whereas Edwards and Ramulu present similar FCG behavior for three different build directions [2]–[5]. Kruth et al. concluded that for optimized build parameters without any (detectable) pores, the building direction does not play a significant role in the fracture toughness results [6]. All of the mentioned literature reported martensitic microstructures and the presence of prior grain structures for as built SLM Ti-6Al-4V.

No studies to the authors knowledge have considered the short FCG of SLM manufactured Ti‑6Al‑4V and its implications to the conflicting damage tolerance behaviors reported in literature [1]. In this work small cross-sectional area (1.5 x 1.5 ) samples in two different build conditions of as built SLM Ti-6Al‑4V are studied. The short FCG rate of three different build directions was considered with cracks parallel to the build layers shown to be the most damaging. The microstructure and build layer are shown to be the likely dominant factors in the short FCG rate of as built Ti-6Al-4V. In terms of porosity, little impact to the propagating short crack was seen although there is local elastoplastic behavior around these defects which could cause toughening in the non-optimized build parameter samples tested. The fracture surfaces were examined using a Scanning Electron Microscope (SEM) with the results showing significant differences in the behavior of the two build conditions. From the microindentation hardness testing undertaken, the smooth fracture surface of the optimized sample correlated with a higher Vickers Hardness (VH) result and therefore higher strength. The non-optimized samples had a ‘rough’ fracture surface, a lower VH result and therefore strength. Furthering the knowledge of short FCG in SLM manufactured Ti-6Al-4V will have positive implications to accurately life and therefore certify additive manufactured aeronautical products.

Laser cladding is a more recent approach to repair of aviation components within a damage tolerant framework, with its ability to restore not simply the geometric shape but the static and fatigue strength as well. This research analysed the fatigue performance of Ti-6Al-4V that has undergone a laser clad repair, comparing baseline specimens with laser clad repaired, and repaired and heat treated specimens. First an understanding of the microstructure was achieved by use of BSE imagery of the substrate, clad repaired region and post heat treated regions. The substrate of the material was identified with large grains which compared to a repaired clad region with a much finer grain structure that did not change with heat treatment. Next, performance of the specimens under tensile fatigue loading was conducted, with the clad specimens experiencing unexpectedly high fatigue performance when compared to baseline samples; the post heat treated specimen lasting significantly longer than all other specimens. It is theorised that the clad may have contributed to an increase in fatigue resilience due to its fine microstructure, when compared to the softer, more coarse substrate. The heat treatment is likely to have relaxed any residual stresses in the specimens leading to a reduction in any potential undesirable stresses, without impacting the microstructure. Residual stress analysis using EDD was unproductive due to the unexpected coarse microstructure and did not provide meaningful results. Fractography using the marker-band technique was explored with some success, proving a feesable method for measuring fatigue crack growth through a specimen post failure. Unfortunately fatigue crack growth throughout the entire fatigue life was not possible due to the tortuous fracture surface and potentially due to the fine micro-structure of the clad, resulting in interrupted marker-band formation. Future research shall expand on this work with a greater focus on residual stress analysis and its impact on fatigue.