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An implicit integration scheme was developed for simulating the viscoplastic constitutive behavior of Sn3.0Ag0.5Cu solder and programmed into a user material subroutine of the finite element software ANSYS. The numerical procedure first solves the essential state variables by using a three-level iterative procedure, and updates the remaining stress and state variables accordingly. The numerical implementation was applied to consider the responses of solder joints in an electronic assembly under temperature cycling condition. The viscoplastic strain energy density accumulation over one temperature cycle was identified as a feasible parameter for evaluating the thermomechanical reliability of the solder joints. [ABSTRACT FROM AUTHOR]

This study experimentally investigated the whole-life ratcheting behavior of sintered copper joints with consideration of the effects of stress rate, stress amplitude, mean stress, maximum stress, and stress ratio. Deformation of joints under cyclic load was measured by a noncontact displacement detecting system. In these joints, ratcheting strain and ratcheting strain rate were found to be sensitive to loading factors. For example, strain decreased as mean stress and stress amplitude decreased, and also as stress ratio and stress rate increased. Meanwhile, larger stress amplitude or mean stress reduced ratcheting fatigue life, while increasing stress ratio and stress rate increased fatigue life. The results indicate the modified Goodman model accounting for the effects of mean stress can usefully predict the ratcheting fatigue life of sintered copper joints at room temperature. [ABSTRACT FROM AUTHOR]

This paper provides an overview of analysis methodologies that can be employed to overcome many of the issues associated with the structural integrity assessment of plasma facing components. These issues arise from the multiple materials construction of the plasma facing components (tungsten armour, CuCrZr heat sink and copper interlayer) making direct application of standard (elastic) structural integrity assessment methods problematic. Example analysis results are used to illustrate these issues, from which it is concluded that PFC structural integrity assessment requires the use of elastoplastic analysis methods, and that this should include: a manufacturing simulation cycle to account for residual stress (and manufacturing strains); pre and post irradiation simulation cycles (with appropriate material models) to correctly assess accumulated ductility usage, and the study of plastic strain in tungsten recrystalised layers to anticipate deep cracking. The issues raised by stress/strain singularities (caused by dissimilar material joints) and multiple ratcheting mechanisms (global, local, material and bi-material ratcheting) are also discussed and proposals made. [ABSTRACT FROM AUTHOR]

Stainless steels with harmonic-structure design have a great balance of high strength and high ductility. Therefore, it is imperative to investigate their fatigue properties for engineering applications. In the present work, the harmonic-structured SUS316L stainless steels were fabricated by mechanical milling (MM) and subsequent hot isostatic pressing (HIP) process. A series of ratcheting-fatigue tests were performed on the harmonic-structured SUS316L steels under stress-control mode at room temperature. Effects of grain structure and stress-loading conditions on ratcheting behavior and fatigue life were investigated. Results showed that grain size and applied mean stress had a significant influence on ratcheting-strain accumulation and fatigue life. Owing to the ultrafine grained structure, tensile strength of the harmonic-structured SUS316L steels could be enhanced, which restrained the ratcheting-strain accumulation, resulting in a prolonged fatigue life. A higher mean stress caused a faster ratcheting-strain accumulation, which led to the deterioration of fatigue life. Moreover, a modified model based on Smith–Watson–Topper (SWT) criterion predicted the ratcheting-fatigue life of the harmonic-structured SUS316L steels well. Most of the fatigue-life points were located in the 5 times error band.

This work provides an experimental and computational analysis of low cycle fatigue of a tantalum polycrystalline aggregate. The experimental results include strain field and lattice rotation field measurements at the free surface of a tension–compression test sample after 100, 1000, 2000, and 3000 cycles at ±0.2% overall strain. They reveal the development of strong heterogeneites of strain, plastic slip activity, and surface roughness during cycling. Intergranular and transgranular cracks are observed after 5000 cycles. The Crystal Plasticity Finite Element simulation recording more than 1000 cycles confirms the large strain dispersion at the free surface and shows evidence of strong local ratcheting phenomena occurring in particular at some grain boundaries. The amount of ratcheting plastic strain at each cycle is used as the main ingredient of a new local fatigue crack initiation criterion.

Highlights • Ratcheting is a rate-independent elastoplastic phenomenon. • Ratcheting should not be confused with creep, cyclic softening and fatigue damage. • Within the small strain assumption conditions, ratcheting is not significant for 304L SS. • Outside the small strain assumption conditions, ratcheting is observed in 304L SS under large stress amplitudes. Abstract In this work our goal is to better understand the origin of the cyclic accumulation of the inelastic strain (often called ratcheting) observed in 304L SS subjected to uniaxial cyclic stress control at room temperature. Recent works performed in the frame of small strain assumption attribute this phenomenon essentially to creep (Taleb, 2013). However, outside this frame, it seems that creep is not the only contributor in this phenomenon (Facheris, 2014). New experiments are performed here in order to investigate the role played by creep, cyclic softening, fatigue damage, the mode of control (engineering or true stress) and ratcheting in this observation. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

Low cycle fatigue and ratcheting deformation of 25Cr2MoVA steel under cyclic tension were tested at 550°C. The effects of stress rates for 0·1, 0·5, 2·5, 5, 10, and 40 MPa s-1 on ratcheting effect and fracture appearance were discussed systematically. Results indicate that the steady static creep rate very approaches to the ratcheting strain rates for 0·1 MPa s-1, but it is obviously greater than those of greater stress rates tested. Moreover, the steady ratcheting strain rate reduces rapidly with increasing the frequency when the frequency is less than 36·8 h-1, and then changes slightly for the greater frequency. This implies the steady ratcheting rate of 25Cr2MoVA is approximately rate-independent when the stress rate is greater than 10 MPa s-1. Additionally, there are some microvoids in the central fibrous regions of specimens under static creep and cyclic tension with 0·1, 0·5, and 5 MPa s-1, but very few microvoids for the specimen under cyclic tension with 10 MPa s-1 can be observed. Furthermore, the low cycle fatigue life is relatively short when the stress rate is less than 10 MPa s-1, but it enhances exponentially subsequently. [ABSTRACT FROM AUTHOR]

Abstract: The uniaxial fatigue and ratcheting behavior of commercial pure titanium (CP‐Ti) was investigated by asymmetric cyclic stress‐controlled experiments at room temperature. The effects of mean stress, stress amplitude, stress ratio, and peak stress on ratcheting behavior and fatigue life were discussed. It was found that increasing mean stress, stress amplitude, and peak stress or decreasing stress ratio reduced fatigue life and promoted ratcheting behavior. The applicability of different fatigue life models was analyzed, and a new stress ratio‐related failure model was proposed based on the exponential increase of fatigue life with stress ratio. Among all the models investigated in this study, the exponential stress ratio‐related model has more advantage in fatigue life predictions for CP‐Ti under ratcheting‐fatigue interaction. [ABSTRACT FROM AUTHOR]

The 3rd Edition of API 579-1/ASME FFS-1 2016 Fitness-For-Service includes a new Part 14 dedicated to fatigue assessment. An important section in this part covers the fatigue assessment of welded joints. In this paper, an overview of the fatigue methods for welded joints is provided and extensions are recommended. First, an overview is given of the classical fatigue method used in the ASME B&PV Code based on smooth bar fatigue curves in conjunction with a fatigue strength reduction factor. In addition, the mesh insensitive structural stress method is outlined using an equivalent stress parameter based on fracture mechanics considerations in conjunction with a master S-N curve based on the analysis of over 2000 high and low cycle S-N test data. The resulting master S-N curve approach is applicable to high cycle fatigue and low cycle fatigue if a Neuber correction is introduced. In this paper, a new structural strain method is presented to extend the early structural stress based master S-N curve method to the low cycle fatigue regime in which plastic deformations can be significant while an elastic core is present. With this new method, some of the inconsistencies of the pseudo-elastic structural stress procedure can be eliminated, such as its use of Neuber’s rule in approximating structural strain beyond yield. The earlier mesh-insensitive structural stress based master S-N curve method can now be viewed as an application of the structural strain method in the high cycle regime, in which structural strains are linearly related to traction-based structural stresses according to Hooke’s law. Thus, both low cycle and high cycle fatigue behavior can now be treated in a unified manner. In the low-cycle regime, the structural strain method characterizes fatigue damage directly in terms of structural strains that satisfy a linear through-thickness deformation gradient assumption, material nonlinear behavior, and equilibrium conditions. A PVRC Joint Industry Project is currently sponsoring work on the structural strain method that will lead to its incorporation in the next edition of API 579-1/ASME FFS-1. [ABSTRACT FROM AUTHOR]

Ratcheting occurs easily because of the presence of mean stress during the stress-control fatigue of engineering components. For ductility exhaustion dominated fatigue failure, a new fatigue life prediction model is developed by introducing the mean ratcheting strain rate to incorporate the effects of ratcheting and mean stress on fatigue life. The prediction accuracy of the proposed model was compared with that of the generalised damage parameter, Xia-Kujawski-Ellyin, Walker and Goswami models. Specifically, the model predictions and tested lives were compared using nine sets of experimental data from the literature. In the statistical analysis of these five models, the proposed model provides the highest accuracy and robust life predictions with the lowest model prediction errors. [ABSTRACT FROM AUTHOR]

This paper presents the latest research results of ratcheting behavior of aluminum alloy 2124-T851 under uniaxial loading. All fatigue tests were carried out at the Technical University of Ostrava mainly under block loading in order to determine the effect of stress amplitude and mean stress on the strain response of the material. Experimental results show that the critical value of the axial deformation is close to the ductility of the material investigated. The aim of this paper also is to draw important conclusions in terms of ratcheting modelling for the material investigated. [ABSTRACT FROM AUTHOR]

Reflow voids created by solder oxidation reduce the reliability of lap joints. In situ visualization of reflow voids in Sn-3Ag-0.5Cu (SAC305) lap-shear solder joints under cyclic stressing was realized by X-ray computed tomography (CT), while the ratcheting deformation of the solder joints was monitored by a non-contact displacement detecting system (NDDS). The results revealed that the shape evolution of reflow voids in solder joints, as characterized by the sphericity of the voids, can be divided into three stages: i.e., the initial stage with a sharp drop, a stable stage, and a rapidly declining stage. A new evolution law for describing the progress of sphericity was proposed, and was further introduced into a viscoplastic constitutive model based on the OW-AF nonlinear kinematic hardening rule. The damage-coupled OW-AF model yielded an accurate estimation of the whole-life ratcheting behavior of Sn-3Ag-0.5Cu (SAC305) lap-shear solder joints. [ABSTRACT FROM AUTHOR]

A damage-coupled constitutive model has been built to predict the ratcheting effect and fatigue life of Cu-Nb micro-composite conductor under different loading conditions. The model is rate-independent and has no isotropic hardening effect according to the experiments reported in Part I. The Abdel-Karim-Ohno model (Abdel-Karim and Ohno, 2000) is improved for a better description of the ratcheting effect. Two damage factors D t and D a are respectively defined to be related to the ratcheting strain and accumulative plastic strain. The damage factor D t is coupled to the model to reflect the stress reduction. On the contrary, the damage factor D a is not coupled to the model but still calculated to decide the failure of the conductor. Accordingly, there are three failure criteria for different peak stress levels without considering the ratcheting strain above 5%. When the peak stress is above 758.5 MPa, the failure is decided by the peak stress being equal to the tensile stress corresponding to the ratcheting strain. When the peak stress is between 650 MPa and 758.5 MPa, the failure is caused by the competition between the ratcheting strain and D a whose critical values are respectively 5% and 0.065. When the peak stress is below 650 MPa, failure happens due to D a > 0.065. As a result, the simulated ratcheting effect is more or less overestimated for some loading conditions, which leads to certain deviation between the predicted and experimental fatigue life. Nevertheless, the precision is still reasonable with the fatigue life ranging from 10 2 to 10 5 which is large enough for a further analysis of the fatigue behavior of pulsed magnets where the conductor is applied. [ABSTRACT FROM AUTHOR]

The uniaxial fatigue behavior of the Cu–Nb micro-composite conductor has been studied considering the effect of peak stress and stress ratio (ratio of the minimum stress to peak stress). With the uniaxial tensile test and the three-point bending test, it is found that the conductor is cyclic stable without hardening or softening and its time dependence is slight below the loading frequency of 67 Hz. The ratcheting deformation in the uniaxial tension–tension fatigue test is found to increase with the peak stress while being restrained at higher stress ratio. SEM observations show that there are mainly three fracture modes in the Cu–Nb conductor at different loading conditions: the fracture by necking, the shear fracture and the nominal fracture. The fracture by necking at higher peak stress is caused by microvoid coalescence while the nominal fracture at lower peak stress is due to the progressive interfacial cracks. The shear fracture at intermediate peak stress can be seen as a mixture of the other two modes with visible tortuous dimples and fatigue striations. Given the same peak stress, a higher stress ratio would result in a more brittle fracture as a mixture of the neighboring two of the above three modes. In a macroscopic point of view, the above differences in fracture modes are related with the ratcheting deformation and accumulative cyclic deformation. The ratcheting deformation is responsible for the fracture by necking at higher peak stress with lower stress ratio while the accumulative cyclic deformation leads to the nominal fracture at lower peak stress with higher stress ratio. The shear fracture can still be seen as a combined effect of deformation types. At last, the experimental S–N relations of the conductor are nonlinear in log–log scale in a broad range of peak stress due to different fracture modes. They fit well with a modified Basquin’s Equation proposed in this paper. [ABSTRACT FROM AUTHOR]

• Monotonic and cyclic softening modeling in viscoplasticity. • Softening carried by back stress. • Monotonic and cyclic softening kinetics dependent on strain rates. • Cyclic softening modeling sensitive to the strain ratio. • Qualitative analysis of ratcheting with cyclic softening. A phenomenological model is proposed to predict both monotonic and cyclic softening in Inconel 718 superalloy at 650 °C. Contrary to the current cyclic softening models consisting of a modified isotropic hardening variable, an internal state variable acting on the amplitude of the back stress is proposed and implemented in a crystal-plasticity model. Crystal-plasticity finite-element simulations are performed to validate quantitatively the model on uniaxial strain-controlled monotonic and cyclic loading for several plastic strain rates and strain amplitudes as well as to gain insights on the ratcheting phenomenon depending on the mean stress and stress amplitude values when cyclic softening occurs. [Display omitted] [ABSTRACT FROM AUTHOR]

This article proposes a model to predict uniaxial and multiaxial ratcheting life by addressing the three primary parameters that influence failure life: fatigue damage, ratcheting damage and the multiaxial loading path. These three factors are addressed in the present model by (a) the stress amplitude for fatigue damage, (b) mean stress-dependent Goodman equation for ratcheting damage and (c) an inherent weight factor based on average equivalent stress to account for the multiaxial loading. The proposed model requires only two material constants which can be easily determined from uniaxial symmetric stress-controlled fatigue tests. Experimental ratcheting life data collected from the literature for 1025 and 42CrMo steel under multiaxial proportional and nonproportional constant amplitude loading ratcheting with triangular sinusoidal and trapezoidal waveform (i.e. linear, rhombic, circular, elliptical and square stress paths) have shown good agreement with the proposed model. [ABSTRACT FROM AUTHOR]

Fatigue failure, ratcheting behaviour and influence of pre-fatigue on fatigue behaviour were investigated under uniaxial cyclic loading for CK45 steel at room temperature. The fatigue life was recorded for various stress ratios, and then, three mean stress models were considered. The Walker model showed an acceptable accuracy in comparison with Smith-Watson-Topper and Park et al. models. The ratcheting strains were measured for various loading conditions in order to evaluate the impact of mean stress, stress amplitude and stress ratio on ratcheting behaviour. The experimental results showed that the ratcheting strain increased with increasing mean stress, stress amplitude and stress ratio. In addition, the results of the post-ratcheting-fatigue tests showed that although the fatigue life decreased with increasing pre-ratcheting strain (the ratcheting strain that is accumulated in pre-fatigue), the loading condition that pre-fatigue experiments were conducted has a significant effect on subsequent fatigue behaviour. [ABSTRACT FROM AUTHOR]

Highlights: [•] Strain controlled pull-pull fatigue tests have been carried out. [•] Large ratcheting strains have been observed from the first mechanical cycles. [•] A marked evolution of the functional properties of the alloy has been observed. [•] Fatigue data were analyzed according to a modified Coffin-Manson approach. [Copyright &y& Elsevier]

Abstract: Ratcheting behavior of sintered nanosilver joints was examined by cyclic shearing tests at different temperatures. Fatigue lives of sintered nanosilver joints were compared with those of soldered SAC305 joints. An improved non-contact displacement detecting system combining infrared heating was developed to measure the deformation of the joints. Effects of temperature, mean stress, and stress amplitude on the ratcheting behavior of both kinds of the joints were analyzed. Shear strength of the sintered nanosilver joints is larger than that of the soldered SAC305 ones. The ratcheting displacement and its rate in both kinds of the joints increase with increase in temperature, mean stress, and stress amplitude. The fatigue life of sintered nanosilver joint is much longer than that of soldered SAC305 one under the same loading conditions. It is concluded that sintered nonosilver joint has demonstrated a longer fatigue life and better response to shearing and cyclic loading than SAC305 joint, especially at high temperatures. An improved temperature-dependent modified Goodman model is proposed and is proved to be able to accurately predict the fatigue life of the sintered nanosilver joints and the soldered SAC305 joints by introducing the temperature-dependent fatigue strength exponent and the fatigue strength coefficient. [Copyright &y& Elsevier]

Abstract: The evolutions of uniaxial tension properties and the ratcheting fatigue behavior of AZ31 magnesium alloy sheet pre-corroded in the phosphate buffered solution (PBS) stimulated physiological environment with increasing immersion time were investigated in this study. The corrosion behavior of AZ31 sheet was also studied. It was found that the corrosion rate decreased exponentially with immersion time until stabilized at about 0.05g/m2 h after 28 days immersion. The corrosion behavior was characterized by pitting corrosion and progressive pitting corrosion in the direction of pits depth was gradually inhibited. The Young's modulus and the elongation of AZ31 sheet decreased exponentially with increasing immersion time and became low levels with reductions of around 10% and 20% respectively at 28 days immersion. The ultimate strength, however, decreased no more than 5%. With increasing immersion time, both the ratcheting strain and ratcheting strain rate in the stable stage of ratcheting strain evolution tended to increase. 28 days degradation decreased the fatigue lives by a factor of about 2.5–5. The pre-corroded damage defined as life reduction after degradation on the basis of the Miner's linear damage rule (LDR) was found to increase exponentially with immersion time regardless of the loading conditions. A ratcheting fatigue life prediction model based on the Miner LDR, the Smith–Watson–Topper (SWT) parameter and the Basquin's equation was proposed and yielded good prediction for the pre-corroded AZ31 sheet. [Copyright &y& Elsevier]

Abstract: Based on the careful analyses of the stress controlled fatigue behavior of commercially pure titanium (CP-Ti) at room temperature, two stress regions can be evidently identified by cycle strain range, ratcheting strain, min ratcheting strain rate, ratcheting failure strain and fracture morphologies. In the high stress region, the ratcheting behavior is significant, and the failure is caused by the large ratcheting deformation. However, in the low stress region, the ratcheting behavior is weak, and the failure is the typical fatigue fracture, with fatigue crack initiation, fatigue extension, and instant rupture. In the high stress region, for both ratcheting and fatigue behaviors, the influence of max stress is greater than stress amplitude. On the contrary, in the low stress region, the influence of stress amplitude on the fatigue behavior is greater than max stress. But the ratcheting behavior in the low stress region is still max stress controlled. Based on the discussion on the fatigue life prediction model, traditional Basquin model and energy based model cannot supply precise fatigue life in the wide stress range, since the ratcheting behavior is not considered. The coupled fatigue damage and ratcheting damage model is proved to make a robust prediction. [Copyright &y& Elsevier]

Abstract: This paper investigates the ratcheting and fatigue behaviour of 90° single unreinforced mitred pipe bends subjected to a cyclic in-plane closing moment with a non-zero mean value and constant internal pressure. An experiment was conducted to induce ratcheting and low cycle failure of the mitred pipe bend. Material and structural response is considered both locally and globally using strain gauges at the locations of highest strain and also by measuring the displacement of the mitre end. These results along with the number of cycles to failure are compared with those produced from nonlinear finite element analysis. The predicted crosshead displacement from the multi linear model showed a good agreement with the test results. However, the finite element model failed to accurately replicate the strain level or trend from the tests, indicating the weakness of the material model used in simulating the cyclic hardening effect. It was also found that the FE models proposed were not able to model the final failure mode of the mitre due to the exclusion of crack simulation in the analysis, i.e. interaction between ratcheting and low cycle fatigue cracking was not considered in the idealised numerical model. [Copyright &y& Elsevier]

Abstract: This paper assesses the sensitivity of cyclic plasticity to microstructure morphology by examining and comparing the microplastic ratcheting behavior of different idealized microstructures (square, hexagonal, tessellated, and digitized from experimental data). This analysis demonstrates the sensitivity of computational accuracy to the various approximations in microstructural representation. The methodology used to perform this study relies on a coupling between microstructural characterization, mechanical testing and numerical simulations to investigate the influence of the microstructure on the purely tensile uniaxial microplastic ratcheting behavior of pure nickel polycrystals. The morphology and deformation behavior of polycrystals were characterized using electron back-scatter diffraction (EBSD), while a finite element model (FEM) of crystal plasticity was used in a computational framework. The predicted cyclic behavior is compared to experimental results both at the macroscopic and microstructural scales. The stress–strain response is less sensitive to the details of the microstructural representation than might be expected with all representations displaying similar macroscopic constitutive response. However, the details of the plastic strain distribution at the microstructural scale and the related estimations of damage mechanics vary substantially from one microstructural representation to another. [Copyright &y& Elsevier]

Abstract: To compensate the drawback of most kinematic hardening laws who exhibit hardening saturation, a solution is proposed by replacing the accumulated plastic strain rate in the springback term by a rate related to the kinematic hardening variable itself. The proposed approach defines a power-law counterpart to the linear (Prager) and exponential (Armstrong–Frederick) laws. [Copyright &y& Elsevier]

The cyclic stress-strain behaviour of two microalloyed steels with different microstructures has been characterised at room temperature under strain controlled low cycle fatigue. The cyclic stress-strain curve in the double logarithmic plot shows a linear relation for both steels. A transition of the cyclic stress-strain curve from softening to hardening with increasing strain amplitude has been observed with respect to the corresponding tensile curve. The strain amplitude for the onset of cyclic softening to hardening transition has been found to be dependent on grain size. The strain lifetime behaviour, estimated from modified universal slopes equation, shows similar trends as Nb or V bearing microalloyed steels. The cyclic characteristics of the two microalloyed steels have been compared with corresponding predeformed state carried out under stress controlled conditions. While, cyclic saturation was observed in case where the extent of predeformation was within the Lüders strain, cyclic softening occurred when it exceeded the Lüders strain. It has been attempted to provide a mechanistic understanding of the differences in the cyclic behaviour of the two steels owing to the microstructure and predeformation. [ABSTRACT FROM AUTHOR]

In Part 1, a novel two-dimensional model was presented for multi-axial thermal stresses, elastic strains, creep strains, and creep energy density at the interfaces of solder joints in leadless chip resistor (LCR) assemblies. In this paper, the model is used to characterize the creep performance of SnPb and SAC lead-free solder joints in LCRs. For both the SAC lead-free and eutectic tin-lead solder joints the predicted cyclic stresses and strains exhibit ratcheting behavior, in good agreement with finite-element predictions. The model is also employed to assess the role of ramp rate and temperature variations in accelerated thermal cycling (ATC) tests. The predictions of the present model correlate well with the experimentally measured number of cycles-to-failure using the Coffin-Manson strain-based model for the SnPb solder and energy-based life prediction model for the SAC solder joints. [ABSTRACT FROM AUTHOR]

The effects of combined thermal and hygroscopic cycling on the adhesion performance of an epoxy coating were measured using a novel electrode sensor. The sensor is uniquely designed, consisting of a series of independent interdigitated electrode traces which are arranged parallel to the sensor edges. Coupled with single-frequency capacitance measurements, the sensor detects changes in capacitance in the adhered coating–sensor interfacial region as a function of the distance from the edge of the sensor, x. Recently, this sensor was utilized by O'Brien and co-workers to measure interfacial diffusion and the concentration profile of fluid in an adhesive joint (Int. J. Adhesion Adhesives 23, 335–338 (2003)). In the present work, large capacitance changes due to debonding and displacement of the coating by fluids at the sensor surface were used to monitor coating delamination. The apparent debond growth rate and number of cycles until failure were determined as a function of coating thickness, fluid environment and sensor surface chemistry. The results show that the coating becomes more durable as the thickness is reduced; and also that thermal and hygroscopic cycling of coatings produces different results than conventional continuous adhesion tests. This study suggests that this novel sensor or a similar design is applicable for the study of adhesion loss and interfacial diffusion processes, and could be extended to other coatings or adhesives in a variety of environments. General trends about coating durability are also discussed. [ABSTRACT FROM AUTHOR]

Fatigue life of fibrous metal matrix composites is limited by the distribution of fibre strengths, the fibre-matrix interfacial strength, and the fatigue resistance of the matrix. The aim of this work is to provide fatigue results for a beta titanium alloy over a range of temperatures and stresses that can be used as input for predicting fatigue life of a titanium matrix composite. Stress controlled tests having fatigue ratios between −1 and −0.2 were conducted on a limited number of samples machined from unreinforced laminated Ti-15Mo-3Al-2.7Nb-0.2Si (TIMETAL®21S) sheets to represent as closely as possible the in situ matrix material. Stress control was used to enable quantification of strain ratcheting for tensile mean stresses and a fast loading rate was used to minimize time-dependent (creep) deformation. Stress amplitude-life data at 20, 482 and 648 °C for fully reversed loading are well fit by a power law. Normalizing the stress amplitude with respect to the power law coefficient appears to account for the temperature dependence of the S–N curves. As the tests had large strains and lives were in the low-cycle fatigue range, strain range at the half-life was also correlated to life. For tensile mean stress cycling at 482 and 648 °C, the rate of strain ratcheting per cycle increased to failure; shakedown was not observed. [ABSTRACT FROM AUTHOR]

Stainless steels with harmonic-structure design have a great balance of high strength and high ductility. Therefore, it is imperative to investigate their fatigue properties for engineering applications. In the present work, the harmonic-structured SUS316L stainless steels were fabricated by mechanical milling (MM) and subsequent hot isostatic pressing (HIP) process. A series of ratcheting-fatigue tests were performed on the harmonic-structured SUS316L steels under stress-control mode at room temperature. Effects of grain structure and stress-loading conditions on ratcheting behavior and fatigue life were investigated. Results showed that grain size and applied mean stress had a significant influence on ratcheting-strain accumulation and fatigue life. Owing to the ultrafine grained structure, tensile strength of the harmonic-structured SUS316L steels could be enhanced, which restrained the ratcheting-strain accumulation, resulting in a prolonged fatigue life. A higher mean stress caused a faster ratcheting-strain accumulation, which led to the deterioration of fatigue life. Moreover, a modified model based on Smith–Watson–Topper (SWT) criterion predicted the ratcheting-fatigue life of the harmonic-structured SUS316L steels well. Most of the fatigue-life points were located in the 5 times error band. [ABSTRACT FROM AUTHOR]

This work provides an experimental and computational analysis of low cycle fatigue of a tantalum polycrystalline aggregate. The experimental results include strain field and lattice rotation field measurements at the free surface of a tension–compression test sample after 100, 1000, 2000, and 3000 cycles at ± 0.2 % overall strain. They reveal the development of strong heterogeneites of strain, plastic slip activity, and surface roughness during cycling. Intergranular and transgranular cracks are observed after 5000 cycles. The Crystal Plasticity Finite Element simulation recording more than 1000 cycles confirms the large strain dispersion at the free surface and shows evidence of strong local ratcheting phenomena occurring in particular at some grain boundaries. The amount of ratcheting plastic strain at each cycle is used as the main ingredient of a new local fatigue crack initiation criterion. [ABSTRACT FROM AUTHOR]

• Fatigue damage behaviour of angle ply GFRP laminates is monitored by TLP. • Debond zones and size are critical to fatigue damage. • Global damage in (0°/90°), (±15°) while localized damage in (±30°), (±45°) laminate. • Samples exhibited different damage pattern at different stress level. • Ratcheting strain and dynamic stiffness remain a function of fiber orientation. In this study, fatigue behaviour of angle plies woven glass fiber reinforced polymer (GFRP) composite laminates has been investigated. Tension-tension fatigue tests were carried out under different maximum applied stresses that varied in the range of 50-90% of the respective ultimate strength of the laminates. Damage development in the form of debond zones and their growth was monitored. The results of the study are presented in terms of stress–strain curve, Wӧhler curve, delamination growth, total dissipated energy, dynamic modulus, and creep strain, respectively. (0°/90°) and ±15° laminate witnessed global damage while localized damage occurred in (±30°)8, (±45°)8 laminate. [ABSTRACT FROM AUTHOR]

The ratcheting behavior of magnesium alloy AZ31B was investigated under controlled cyclic stress at frequencies of 0.5 Hz, 1.0 Hz and 2.0 Hz. The result showed that cyclic loading accelerates strain accumulation as compared with static creep at room temperature, and the effect of cyclic frequency on strain rate dε r / dt , represented by strain in a unit time, is of multiplicity due to the diversification of deformation mechanisms. However, strain rate dε r / d N, represented by strain in one cycle, decreases evidently with increasing frequency at steady strain accumulative stage. High frequency promotes twinning-detwinning, and is favorable for prolonging the fatigue life in terms of cyclic number to failure. The fatigue life increases linearly with decrease of plastic strain energy density under all frequency conditions. This leads to conclude that frequency affects the fatigue life via changing plastic strain energy stored in the deformed microstructure. [ABSTRACT FROM AUTHOR]