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Different grain coarsening behaviors (i.e. abnormal and homogeneous) are prevalently observed in gradient nanograined (GNG) Cu under stress controlled high-cycle and strain controlled low-cycle fatigue tests, respectively. In this paper, to comprehensively understand the intrinsic fatigue mechanism of gradient nanograined structures, both high and low cycle fatigue behaviors of GNG Cu are investigated under strain-controlled fatigue tests with a wide strain amplitude ranges. Cyclic behavior transition from abnormal grain coarsening at small strain amplitude to homogeneous grain coarsening at large strain amplitude is observd in GNG Cu. Microstrucural analysis reveals that the grain coarsening behavior in either abnormal or normal (homogeneous) mode is closely related to the spatial distribution of the cyclic plastic strain in the GNG layer (localized or delocalized) under cyclic loading. Such unique cyclic strain amplitude-dependent fatigue behavior is inherent to the gradient nanostructure, which fundamentally differs from the conventional strain localizing mechanism in metals with homogeneous structures under cyclic loading.

The influence of stress rate ( σ ˙ ) on load/strain partitioning between different phases of a micro-alloyed 2.25Cr–Mo steel during stress-controlled fatigue deformation at room temperature (RT) wa...

This paper refers to cyclic shear strains (&gamma, c) and permanent shear strains (&gamma, p) of a soft cohesive soil, when both monotonic shear stresses (&tau, o) and cyclic shear stresses (&tau, c) are applied. The research is backed by an extensive experimental program with 139 cyclic simple shear tests that included identification and classification tests. These cyclic simple shear tests were conducted under different levels of stresses, &tau, o, before the cyclic phase. Laboratory tests were carried out on undisturbed samples from the Port of Barcelona, located in Spain on the Mediterranean coast, and characterized by a monotonic strength (&tau, max) approximately equal to 30% of the initial effective vertical stress (&sigma, &prime, ov). The samples were taken at depths between 29 and 52 m and correspond to an initial effective vertical stress between 277 and 413 kPa, respectively. In general, the results indicate that: (a) the combination of &tau, o and &tau, c controls the generation of &gamma, c and &gamma, p, (b) it is not always true that when &tau, o/&sigma, ov + &tau, c/&sigma, ov &asymp, &tau, max/&sigma, ov, the soil reaches failure cyclically, and (c) empirical relations useful for design can be established between &gamma, c, &gamma, p, and the number of cycles (N), for different relationships varying (&tau, ov) between 0% and 25%.

The microstructural stability during aging at 750 &deg, C of the &gamma, /&gamma, &prime, nickel-based superalloy AD730TM is investigated in this work. Strain-free aging and aging during fatigue tests are conducted, with a focus on the influence of the strain ratio, the maximum applied strain, and the cycle waveform (with or without dwell). Two classical mechanisms of &gamma, precipitates aging are identified at 750 &deg, C: the coarsening of small spherical &gamma, precipitates via the thermally-activated Ostwald ripening process and the coalescence of at least two precipitates into one. These mechanisms appeared to be concomitant during aging. It has been demonstrated that the coarsening kinetics of &gamma, precipitates can be described by a classical Lifshitz-Slyozow-Wagner (LSW) equation. The introduction of a cyclic strain during aging at 750 &deg, C increases the coarsening kinetics by means of changes in the volume diffusion of &gamma, forming elements and of constraint misfit effects. More precisely, it is shown that the higher the maximum applied strain and/or the strain rate, the higher the coarsening rates. Finally, dwell-fatigue promotes the activation of the &gamma, coalescence at 750 &deg, C.

This paper modified an Armstrong–Frederick-type plasticity model for investigating the stable cyclic deformation behavior of metallic materials with different sensitivity to nonproportional loadings. In the modified model, the nonproportionality factor and nonproportional cyclic hardening coefficient coupled with the Jiang–Sehitoglu incremental plasticity model were used to estimate the stable stress–strain responses of the two materials (1045HR steel and 304 stainless steel) under various tension–torsion strain paths. A new equation was proposed to calculate the nonproportionality factor on the basis of the minimum normal strain range. Procedures to determine the minimum normal strain range were presented for general multiaxial loadings. Then, the modified model requires only the cyclic strain hardening exponent and cyclic strength coefficient to determine the material constants. It is convenient for predicting the stable stress–strain responses of materials in engineering application. Comparisons showed that the modified model can reflect the effect of nonproportional cyclic hardening well.

Fatigue behaviors of the CoCrFeMnNi-based high-entropy alloy thin foils with different Al addition were investigated under dynamical bending fatigue loading. The results reveal that the Al addition can effectively decrease the cyclic strain localization and improve the fatigue resistance, which is associated with the reduced cyclic slip irreversibility. The physical reasons for such reduction can be attributed to the enhanced planar-slip ability induced by Al addition. Furthermore, the formation of BCC phases caused by 10 at.% Al addition also leads to a delay in the fatigue crack propagation.

This paper presents test results of the effect of thermomechanical aging conducted at 100 °C, 200 °C and 300 °C on the fatigue life of EN AW-2024 alloy at ambient temperature. The results of fatigue tests conducted on specimens with no preliminary thermal and mechanical loadings (as-received specimens) are discussed. These results were accepted as a point of reference for the results of fatigue life tests of material previously subjected to simultaneous, preliminary thermal and mechanical loads. Conducted tests made it possible to determine the effect of creep pre-deformation at elevated temperature on the cyclic properties of the tested alloy. The shape of the hysteresis loop obtained for material subjected to different thermomechanical loadings was analyzed in detail. The character of obtained fatigue life curves and cyclic strain curves is also discussed. The direct relationship between these curves and material properties shaped in the process of preliminary creep, at different temperatures and under different load values, was indicated. At the same time, comparative analysis was conducted for fatigue fractures in as-received specimens and fractures obtained in specimens burdened by pre-deformation-related effects.

Thermomechanical fatigue (TMF) behavior of nitrogen enhanced 316LN stainless steel (with 0.14 wt.% N) is investigated under in-phase (IP) and out-of-phase (OP) conditions at cyclic strain amplitudes of ±0.25 to ±0.8% and with a temperature interval of 623–873 K. The study elucidates the differences in cyclic stress-strain response, TMF lives and fracture behavior of the material under IP and OP-TMF in the light of dynamic strain aging, thermal recovery and creep. The manifestations of these factors/phenomena are found to vary along stress-strain hysteresis loops, thereby leading to differences in cyclic strain hardening exponent ( n′ ) and coefficient ( K′ ) between the tensile and compressive branches of hysteresis loops. Irrespective of the imposed cyclic strain, transgranular fatigue failure is observed under OP-cycling in contrast to mixed-mode fracture under IP-cycling. The IP-cycling led to considerably lower cyclic life (i.e. 34–49%) compared to those under OP-TMF, with the life reduction being more significant (∼48.5%) at intermediate mechanical strain amplitudes of ±0.4 and ±0.6%. A comparison of the fatigue design curves under TMF with the corresponding isothermal fatigue curves (as per the RCC-MR design code) revealed a lack of conservatism in the latter.

The real weld toe geometry is generally not mathematically perfect, resulting in obvious stress concentration effects, both on the weld section and along the longitudinal direction of the weld toe. The true stress-strain state at the local weld toe directly affects the fatigue performance and behavior of the welded structure. Therefore, a Fiber Bragg Grating (FBG) sensor based method for testing the cyclic strain at the weld toe was proposed. Cruciform welded joints were fabricated as specimens on which FBG sensors were arranged at several characteristic points along the weld toe curve. Strains at all the characteristic points under cyclic tensile load were measured and recorded, which showed the proposed measuring method could accurately obtain the complete local strain time histories along the weld toe. The strain time histories clearly reflected the cyclic hardening phenomenon in the early stage and the plastic yielding phenomenon in the final stage. Furthermore, based on the cyclic stress-strain constitutive model of the weld material, the stress-strain response curves of all the characteristic points were drawn. Combined with the fatigue fracture morphology, the mechanism of the unsynchronized initiation of the multiple cracks in the weld toe was investigated.

This article focuses on analysis of stretchable screen-printed conductive tracks onto a flexible substrate that can withstand high mechanical strain while maintaining good electrical conductivity. Straight track samples with measured thickness of 20 µm and various trace widths are printed using a stretchable Silver (Ag) ink. Four temperatures: 110 °C, 120 °C, 130 °C and 150 °C are chosen to cure the stretchable ink, where four curing times: 3, 10, 20 and 30 minutes, are carried out to each temperature. In total, 16 curing temperature-time combinations are used. Cyclic strain tests with 25 % and 50 % elongations are carried out to observe the variation in electrical resistance after being subjected to mechanical deformation and fatigue.

The fatigue performance has always been the key problem hindering the development of disruptive additive manufacturing (AM) technology. We recently found the stored energy induced by SLM process and cyclic strain during fatigue testing can trigger grain boundary migration and local recrystallization behavior. In addition, the junctions between the recrystallized grain tribes and the original coarse grains can lead to the plastic strain localization, promoting the fatigue crack initiation during cyclic deformation, seriously affecting the fatigue lifetime of SLMed 304L SS. Therefore, this study carried out a series of microstructural characterizations to investigate the formation mechanisms of local recrystallization and the role of local recrystallization on the fatigue performance of SLMed 304L SS, which can advance the current understanding of fatigue failure mechanisms of additively manufactured materials. The experimental results show that the detrimental effect of local recrystallization during cyclic deformation on fatigue properties of SLMed 304L SS exceeds that of small pores.

Additive Manufacturing (AM) has increasingly been used to fabricate parts in aerospace applications, which may require service lives beyond ten-million cycles due to the imposed high loading frequencies. Understanding the very high cycle fatigue (VHCF) behavior of these additive manufactured (AM) parts is an important step towards their design and qualification processes. This study focuses on the high cycle fatigue (HCF) and VHCF behaviors of both wrought and laser beam-powder bed fusion (LB-PBF) fabricated Inconel 718 in machined/polished surface condition, emphasizing on the influence of test frequency (i.e., cyclic strain rate). Uniaxial, fully-reversed force- and stress-controlled fatigue tests were conducted utilizing a servo-hydraulic and an ultrasonic test system operating at 5 Hz and 20 kHz, respectively, on wrought as well as LB-PBF vertically and diagonally built specimens. Fatigue cracks in the majority of the specimens were found to initiate from intra-granular slip bands near or at the surface, which gives rise to strong anisotropy in fatigue resistance in LB-PBF specimens due to the presence of columnar grains along the build directions. Longer fatigue lives were obtained at 20 kHz, which was ascribed to possibly lower-than-intended stresses applied in the ultrasonic tests. The corrected stress-life fatigue data at 20 kHz were found to converge to the one obtained from conventional testing at 5 Hz, implying no effect of cyclic strain rate on the fatigue behavior of Inconel 718 regardless of the fabrication process. The findings of this work confirm the use of ultrasonic fatigue testing to expedite generation of AM materials data to keep up with the current demand; however, the applied stress may need to be corrected.

A novel creep-fatigue life prediction model was deduced based on an expression of the strain energy density in this study. In order to obtain the expression of the strain energy density, the load-controlled creep-fatigue (CF) tests of P92 steel at 873 K were carried out. Cyclic strain of P92 steel under CF load was divided into elastic strain, applying and unloading plastic strain, creep strain, and anelastic strain. Analysis of cyclic strain indicates that the damage process of P92 steel under CF load consists of three stages, similar to pure creep. According to the characteristics of the strains above, an expression was defined to describe the strain energy density for each cycle. The strain energy density at stable stage is inversely proportional to the total strain energy density dissipated by P92 steel. However, the total strain energy densities under different test conditions are proportional to the fatigue life. Therefore, the expression of the strain energy density at stable stage was chosen to predict the fatigue life. The CF experimental data on P92 steel were employed to verify the rationality of the novel model. The model obtained from the load-controlled CF test of P92 steel with short holding time could predict the fatigue life of P92 steel with long holding time.

This study deals with the effect of the loading history on the cyclic behavior and the fatigue life of a 304L stainless steel at room temperature. The experiments have been performed using two specimens’ categories. The first one (virgin) has been submitted to only classical fatigue tests while in the second category, prior to the fatigue test; the specimen was subjected to a pre-hardening process under either monotonic or cyclic strain control. Cyclic softening followed by cyclic hardening are observed for the virgin specimens while only cyclic softening is exhibited by the pre-hardened specimens. The obtained results show that fatigue life is strongly influenced by the pre-hardening: the latter seems beneficial under stress control but detrimental under strain control, even in the presence of a compressive mean stress. The results are discussed regarding the cyclic evolution of the elastic modulus as well as the isotropic and kinematic parts of the strain hardening in different configurations: with or without pre-hardening, stress or strain control.

Particle breakage of carbonate sand under cyclic loading might be of interest in some circumstances, while few research results were reported. Drained and undrained cyclic triaxial tests terminated at different loading cycles were conducted on a carbonate sand to investigate the evolution characteristics of particle breakage during cyclic shearing. Considerable breakage was observed after successive cyclic triaxial shearing at moderate confining pressure levels (≤200 kPa). Abrasion of surface asperities and small sharp edges was found to be the primary pattern of particle breakage and the particle size distribution after cyclic shearing showed an increased mass content of fine particles. In drained cyclic tests with constant confining pressure and cyclic stress amplitude, particle breakage appeared to accumulate at a decreasing rate with increasing loading cycles, and the development of particle breakage with loading cycles can be fitted by a logarithmic equation. In undrained cyclic tests, particle breakage exhibited more complicated accumulation patterns because the effective confining pressure and cyclic strain amplitude varied greatly. A cycle-by-cycle incremental breakage evolution model was established, and the model revealed that the incremental breakage generated in subsequent loading cycles decreases with accumulated particle breakage and increases with increasing effective confining pressure and cyclic shearing intensity. The model was validated by comparing the model predictions with the particle breakage evolution observed during undrained cyclic triaxial tests.

Under the fully strain-controlled conditions, a closed-form multistage fatigue crack growth (MSFCG) life prediction model has been developed for discontinuous reinforced MMCs (DRMMCs) considering the effect of microstructural features. The model considers crack nucleation, small crack, and long crack life. The crack initiation is evaluated on the bases of slip steps deepening into a fatigue crack under the cyclic strain. Next, the small crack growth life is investigated within the microstructural threshold. Finally, for long fatigue crack propagation, previously developed model by Tevatia and Srivastava is modified and incorporated in the current model. Under the total strain-controlled conditions, reinforcement in the matrix results in an increase in both (i) the level of cyclic plastic deformation and (ii) cyclic plastic zone size near the vicinity of crack tip. The effect of initiation and small crack growth life is significant for low cycle fatigue applications.

This paper aims at studying the pre-strain effect on cyclic plastic behaviour of the 7050-T6 aluminium alloy. In order to meet this goal, different series of tests with various pre-strain histories are performed under fully-reversed strain-controlled conditions. In a second stage, fracture surfaces are examined by scanning electron microscopy to identify the main fracture damage micro-mechanisms associated with the pre-strain scenarios. Overall, the results show that higher tensile pre-strain levels lead to lower fatigue lives. This reduction of the fatigue life expectancy tends to increase for higher cyclic strain amplitudes.

International audience; The paper reports our interesting results gained during these last yearsabout the role of microstructure on strain localization of three different types ofmetallic alloys. The benefit of using advanced techniques (TEM, AFM,SEM-ECCI) is pointed out. Strain gradient and strain localization at microscopicscale and strain partition at mesoscopic scale are discussed and allow explanationsfor crack initiation site, precipitate dissolution or cyclic accommodation.

Changes of total cyclic strain energy at the notch for copper film specimen were analyzed before and after laser irradiation treatment. The results showed that laser irradiation can increase total cyclic strain energy and the effect of increase is more evident for the damaged copper specimen. Based on the damage-healing mechanism, an enhancement parameter and a healing parameter were defined by the local cyclic strain energy. A new model based on local strain energy was proposed to predict residual fatigue life for the damaged copper film specimen after laser irradiation. The predicted results by the proposed model agree well with the experimental lives.

Abstarct. The contribution of variable plastic strains (hysteresis loop), unilaterally accumulated plastic and elastic strains in the total accumulated damage in the limiting case (macro-crack formation) under quasi-static fracture is shown on the basis of the deformation-kinetic criterion of fatigue failure of cyclically stable steel, when the accumulated plastic strain is equal to or commensurable with the strain of a single (static) fracture. It is shown that in the case of low-cycle failure, the main damaging fraction is introduced by the static component (unilaterally accumulated strain), the value of which decreases with the increase of the number of cycles to failure (reduced load amplitude). This increases the contribution of variable cyclic strain. The influence of elastic strain in the area of quasi-static fracture is negligible, and the calculation of durability in practical applications can be carried out without taking into account the damaging role of elastic strains.

Small strain shear modulus G max is an essential parameter in soil dynamics, and it is usually estimated based on the Hardin and Richart equation. However, many previous researches on sands have indicated that the Hardin and Richart equation does not consider the influences of cyclic loading history on G max . In this paper, effects of cyclic loading history on G max of saturated clays under undrained conditions are studied using a combination device of piezoelectric-ceramic bender element system and cyclic triaxial apparatus. The dynamic pre-loading includes both relatively high amplitudes of cyclic stresses and cyclic strains. G max without cyclic loading history is also investigated for the comparison purpose. Test results show that, at the same effective stress, both cyclic strain history and cyclic stress history will induce reduction of G max compared to the corresponding G max values with non-cyclic loading effects. In strain-controlled tests, the reduction of G max is slight and relatively stable; while in stress-controlled tests, the reduction of G max increases suddenly and remarkably when the effective stresses degrade to a certain degree. The comparison between double amplitude axial strain and residual excess pore water pressure behaviors show that the remarkable reduction of G max can demonstrate the cyclic failure of saturated clays.

In the present work cyclic torsion test was used to simulate hot plate rolling process in order to study the effect of strain reversal on non-recrystallisation temperature using unalloyed and microalloyed austenite model alloys. It was found that the amount of strain reversal directly influences both static recrystallisation and strain-induced precipitation process significantly delaying their kinetics. The proper assessment of the interactions between strain reversal and microstructure evolution plays a crucial role during hot rolling process - as continuous changes in the deformation mode (strain reversal) affect the level of redundant strain (in the areas near the surface of the stock) and lead to strain inhomogeneity across the plate thickness. This complex strain path introduces microstructural inhomogeneity and makes its predictions very difficult. Proper understanding of the effects of strain reversal on microstructure evolution in the austenite will help to optimise the hot rolling process.

Resistance to low-cycle fatigue of hot-rolled plates manufactured from alloyage system Al-Mg-Si alloy 6082 is researched. Cyclic stability of alloy and cyclic proportional elastic limit were defined during tests of specimens without cracks loaded along the rolling direction using curves of cyclic strain and their generalization. Properties used for design of prefabricated structures from thick aluminum plates from alloy 6082 operating in the conditions of low-cycle fatigue are received.

Cyclic strain produces slip irregularities in the form of depressions and elevations on the surface of metals, often referred to as persistent slip bands, then surface slip irregularities grow as the number of cycles increases and result in roughening the surface. The surface roughening continues until the heights and depths of the extrusions and intrusions reach to a critical level leading to the initiation of a micro-crack. This study reports the implementation of a technique to increase the fatigue life of metals by polishing the specimens during cyclic loading. The critical values for the extrusions and intrusions are shown to be 4.8–5 μm and 5–5.5 μm, respectively. The results of an extensive set of experiments reveal that fatigue life can be extended significantly (from 45% to 100%) by polishing the surfaces at appropriate intervals. Results are also presented for determining an effective polishing period to substantially increase fatigue life performance.

International audience; Composite systems at nanoscale are promising to design new materials combining different interesting properties, like high strength and good plasticity. In the present work, we compare mechanical properties of gold-silicon core-shell and pure gold nanowires subjected to several tensile loadings and compressive unloadings at 300 K and 1 K, in order to evaluate the ability of a hard amorphous silicon shell to stabilize a crystalline gold core during cyclic loading. The hard shell appears involved in the quasi-reversible macroscopic plasticity of core-shell NW, by promoting twins, stacking fault and isolated full dislocations, due to a confinement effect of the core. In addition, our simulations revealed that the reversible behavior is progressively lost over cycles. The analysis of the amorphous shell shows that large strains combined with high strain rate prevent self-healing of the shell, leading to shell failure and to reversibility loss.

In the creation of engineered tissue constructs, the successful transport of nutrients and oxygen to the contained cells is a significant challenge. In highly porous scaffolds subject to cyclic strain, the mechanical deformations can induce substantial fluid pressure gradients, which affect the transport of solutes. In this article, we describe a poroelastic model to predict the solid and fluid mechanics of a highly porous hydrogel subject to cyclic strain. The model was validated by matching the predicted penetration of a bead into the hydrogel from the model with experimental observations and provides insight into nutrient transport. Additionally, the model provides estimates of the wall-shear stresses experienced by the cells embedded within the scaffold. These results provide insight into the mechanics of and convective nutrient transport within a cyclically strained hydrogel, which could lead to the improved design of engineered tissues.

A woven carbon/epoxy composite was subjected to fatigue crack growth under mixed Mode-I/Mode-II loading to obtain crack growth behavior at different cyclic strain energies. Owing to the woven structure of the material, pure Mode-II fracture is usually a difficult proposition because of friction, interference, and interlock of woven tows in adjacent plies at an interlaminar crack. These limitations were overcome by the use of a novel form of mixed Mode-I/Mode-II specimen, which imposes sufficient crack surface opening (Mode-I) to alleviate ply–ply interactions, but not so much as to obscure the sliding (Mode-II) response. Comparison with pure Mode-I fatigue crack growth data, in conjunction with a fracture interaction criterion, provided a means to extract the Mode-II behavior.

The first demonstration of a polymer optical fibre Bragg grating (POFBG) embedded in a 3-D printed structure is reported. Its cyclic strain performance and temperature characteristics are examined and discussed. The sensing patch has a repeatable strain sensitivity of 0.38 pm/mu epsilon. Its temperature behaviour is unstable, with temperature sensitivity values varying between 30-40 pm/degrees C.

The effect of displacement amplitude on fretting fatigue behavior of steel wires in low cycle fatigue at two cyclic strain levels was investigated. Evolutions of normalized tangential force and morphologies of contact scars were investigated to examine the wear mechanisms. Fretting regime and fretting fatigue life were explored. Acoustic emission technique was utilized to reveal wire failure mechanisms. The results show that an increase of displacement amplitude increases normalized tangential force but decreases the fretting fatigue life. Fretting contact conditions are all mixed fretting regimes. Fretting damage and crack nucleation and propagation accelerate with increasing displacement amplitude.

In order to clear the relationship between dislocation development and endurance limit in fatigued body-centered cubic (BCC) metals, the automotive grade interstitial-free steel (IF steel) was fatigued near the endurance limit in this study. When cycling just below the endurance limit, the dislocation structures are mainly composed of loop patches, moreover, a few large dislocation cells and dislocation walls can also be found, and thus these structures have no significant effect on fatigue failure. However, once cyclic strain slightly exceeds the endurance limit, the small dislocation cells tend to develop near grain boundaries and triple junction of the grains, and which provide a more appropriate structure for crack growth than do large dislocation cells.

The effects of cold dwell on titanium alloys have been widely studied in recent years, with many alloys showing detrimental mechanical properties when dwell periods are introduced at peak load. Failures in such situations are often characterised by the formation of quasi-cleavage facets, and models have been suggested to explain the mechanism by which they occur. This paper seeks to investigate how different mechanical test regimes influence facet formation. It is shown in the titanium alloy Ti834 that facet formation occurs readily under stress relaxation and creep loading, but is less influenced by cyclic strain control loading. The implications of this type of strain accumulation process on the mechanical properties of Ti834 are discussed, with close correlation between suggested models and experimental data.

Thermal fatigue properties of 200-nm-thick Au interconnects with a width of 2 lm were evaluated by applying alternating current. We found that the temperature distribution along the Au interconnect can be described by a simplified one-dimensional equation of heat conduction. The lifetime as a function of thermal cyclic strain range (De) for the narrow Au lines follows the conventional Coffin–Manson relationship when De 6 0.47%, while the number of cycles to failure extends to an engineering-defined high-cycle fatigue region.

Strain-controlled fatigue experiments were employed to evaluate automotive-grade interstitial-free ferrite steels under R = 0. Hundreds of grains were examined by scanning electron microscope (SEM) under electron channeling contrast image technique of backscattered electron image mode (BEI/ECCI) for comprehensive comparison of micrographs with those taken under transmission electron microscope (TEM). It is clearly revealed that cyclic hardening was virtually unobvious and dislocation cell structures were very rare when Δe/2 was controlled to within 0.1%. When Δe/2 was increased to 0.2%, the general dislocation structure exhibited a predominately dislocation wall structure prior to the secondary cyclic hardening, after which the formation of dislocation cells were observed. At Δe/2 = 1.0%, following an initial rapid-hardening stage, the dislocation cell structure of low-angle misorientation formed in the early stage was gradually converted into high-angle misorientation as the cyclic strain continued to be imposed.

In cyclic thermal tests of Si/solder/OFHC-Cu (silicon/solder/oxygen-free high conductivity copper)-layered plates, the authors observed either the cyclic growth or cyclic recovery of warpage to occur depending on the heat treatment of the copper before soldering. In this study, the test results are numerically analyzed by assuming three material models for the solder and two material models for the copper. It is shown that the test results are reproduced well if proper material models are used in finite element analysis. It is revealed that the so-called multiaxial ratcheting was induced in the solder, while the uniaxial type of ratcheting or cyclic strain recovery occurred in the copper. As a result, the Armstrong and Frederick model is suggested to be valid for the multiaxial ratcheting in the solder at such low strain rates as in the cyclic thermal tests, whereas the Ohno and Wang model is shown to be appropriate for the copper. To confirm this unexpected result for the solder, the Armstrong and Frederick model is applied to the multiaxial ratcheting of another solder at three strain rates.

The authors have analyzed the susceptibility of metallic materials to cyclic strain hardening depending on both their microphysical characteristics and macrophysical parameters represented by standard mechanical characteristics. At the microlevel, the degree of additional hardening is related to the material stacking fault energy. The materials with a high level of the stacking fault energy do not exhibit additional cyclic hardening under non-proportional loading, while those with a low energy are susceptible to a considerable strain hardening. For the macrolevel, on the basis of analysis of the mechanical properties of metallic materials, the authors have derived a correlation dependence between the hardening levels under static and cyclic loading. An approximate quantitative method to determine maximum strain hardening levels under non-proportional low-cycle strain-controlled loading is proposed. The lifetime under non-proportional loading has been shown to correlate well with the level of additional hardening.

Fatigue tests were conducted on S45C steel under fully reversed strain control conditions with axial/torsional (at) and torsional/axial (ta) loading sequences. The linear damage value (n 1 /N 1 + n 2 /N 2 ) was found to depend on the sequence of loading mode (at or ta), sequence of strain amplitude (low/high or high/low) and life fraction spent in the first loading. In general, at loading yields larger damage values than ta loading and the low-high sequence of equivalent strain leads to larger damage values than the high-low sequence. The material exhibits cyclic softening under axial cyclic strain. Cyclic hardening occurs in the torsion part of ta loading, which elevates the axial stress in the subsequent loading, causing more damage than in pure axial fatigue at the same strain amplitude. Fatigue life is predicted based on the linear damage rule, the double linear damage rule, the damage curve approach and the plastic work model of Morrow. Results show that overly conservative lives are obtained by these models for at loading while overestimation of life is more likely for ta loading. A modified damage curve method is proposed to account for the load sequence effect, for which predicted lives are found to lie in the factor-2 scatter band from experimental lives.

We demonstrate cyclic strain hardening and frequency-dependent fatigue life in electrodeposited nanocrystalline Ni subjected to tension–tension cyclic deformation. These observations are rationalized through mechanistic arguments based on the exhaustion of dislocation sources in the early stages of cyclic loading and on the domination of diffusive processes during later stages.

The tests of 63Sn-37Pb under cyclic strain and stress were conducted to investigate the material’s time-dependent behavior. A damage-coupled viscoplastic model and its failure criterion are proposed. The prediction of the proposed viscoplastic model implies that the model can describe the cyclic strain and cyclic stress ratcheting behavior, and the fatigue lives very well.

In this paper, a viscoplastic model incorporated with the hardening caused by the combined monotonic and cyclic strain loading is proposed. It is assumed that the isotropic and kinematic hardening rules are dependent on both the cyclic inelastic strain amplitude and the maximum inelastic strain. The current inelastic strain amplitude is described by a memory model in which the current inelastic strain amplitude can be approached very quickly. The fading memory on the previous maximum deformation resistance is taken into account to describe the fading memory on the previous loading history while the deformation resistance is presented by both the maximum inelastic strain and the current inelastic strain amplitude. The comparison between the predicted and experimental results indicated that the model can describe the ratcheting behavior under uniaxial cyclic stressing and the cyclic strain behavior under uniaxial cyclic straining in the unified viscoplastic model.

The improvement of metals formability by proper control of temperature, strain rate, and strain has been rather exhausted. The other way to obtain this goal is proper control of strain path. In the case of sheet metals the effect of strain path is widely known, but in massive metal forming processes that phenomena have been just under investigations. In the paper the effects of different cyclic strain paths on the mechanical properties and structures of CuAl10 aluminium bronze have been described. The strain paths contain various cyclic torsions. The amplitude was changed in the range of 0.01–0.6, temperature from 20 to 650 °C and strain rate from 0.01 to 1 s−1. The properties and structures obtained in cyclic strain paths are compared with those gained in monotonic torsion. It has been found that the flow stress in minor cyclic torsion is substantially dependent on cyclic conditions, mainly the strain amplitude used. The strain paths similar to those applied in the investigations are observed in some industrial processes.

The present study develops a model to correlate the self-excited panel motion with the cyclic elastoplastic behaviour of the material causing fatigue failure. The analysis takes into account the elastic and plastic responses of materials to estimate the fatigue life of the panels subjected to limit-cycle oscillations (LCO). A damage-dynamics analysis is performed to correlate the fatigue life of a two-dimensional panel as it is subjected to the dynamic bending-stretching loads due to air flowing above the panel. From dynamic time history of the panel motion, the cyclic strain/stress histories required for fatigue damage analysis have been obtained. To assess the fatigue life of panels, an energy-critical plane fatigue damage parameter has been applied. The total damage accumulation in the loading history is computed from the summation of the normal and shear energies on the basis of cycle-by-cycle analysis. The fatigue parameter used in this study has been used to successfully predict the life of thin aluminium 7075-T6 panel subjected to the dynamic bending-stretching loads due to air flowing above the panel.

Fatigue damage of 2.2Ni-1Cr-0.5Mo steel used fir high strength pressure tubes and vessels was evaluated using uniaxial specimens subjected to strain-controlled fatigue loading. Based on the fatigue test results from different strain ratios of -2. -i 0, 0.5, 0.75, the fatigue damage of the steel was represented by using a cyclic strain energy density. Mean stress relaxation depended on the magnitude of the applied strain amplitude. The high pressure vessel steel exhibited the cyclic softening behavior. Total strain energy density consisting of the plastic strain energy density and the elastic tensile strain energy density described fairly well the fatigue life of the steel, taking the mean stress effects into account. Compared to other fatigue damage parameters, fatigue life prediction by the cyclic strain energy density showed a good correlation with the experimental fatigue lift within a factor of 3.

The development of PSAs suitable for specific applications requires an optimization of the two most essential properties: sufficient bond strength and clean removability. The latter is conveniently obtained by controlling the mode of debonding to an interfacial failure, yet the issue of bond strength is challenging since the optimum bond strength must be available in the same failure mode. More complication arises from the fact that the methods, such as peel and creep tests, used to evaluate the bond strength are often qualitative and the results obtained may not accurately reveal the effects of various PSA design parameters. In the present study, a new method, named cyclic strain application (CSA) test, is devised to evaluate the durability of PSAs. A key feature of this test is that by applying a controlled cyclic strain on a PSA/substrate interface we can quantitatively monitor the temporal stress variation as well as the time for interfacial failure (t f ). As a result, the effects of various PSA design parameters, such as molar mass, blending with low molar mass fractions, adhesion promoters, and the level of crosslinking on PSA durability can be clearly seen. Other factors being equal, higher molar mass samples showed greater t f , while, blending with low molar mass fractions led to a reduced t f . The use of a strong adhesion promoter, as anticipated, increased t f . However, it should be noted that excessive bond strength may lead to a cohesive failure, which is detrimental in the design of good PSAs. From a comparison of t f with peel strength, it was clearly shown that t f , the key parameter obtained from the CSA test, had a strong correlation with peel strength. In addition, some other information, such as the detailed influences of the level of crosslinking and molar mass on PSA durability was also obtained from the CSA test. Lastly, some important PSA design concepts and other examples utilizing the CSA test are presented.

The ratcheting and strain cyclic characteristics of U71Mn rail steel were experimentally researched under uniaxial and non-proportionally multiaxial cyclic loading at room temperature. The effects of cyclic strain, stress and their histories on strain cyclic characteristics and ratcheting were studied, respectively. It is shown that: U71Mn rail steel exhibits a cyclic stabilization and non-memorization for previous loading history under strain cycling; however, the ratcheting of the material depends greatly not only on the current values of mean stress and stress amplitude, but also on their histories; the non-proportionality of multiaxial loading path only causes a negligible additional hardening for the material. Based on the Ohno–Wang non-linear kinematic hardening model [Int. J. Plast. 9 (1993) 375, 391], the uniaxial and multiaxial ratcheting behaviours of the material were simulated by a visco-plastic constitutive model. The simulated results are in good consistence with the experimental ones.

An experimental study was achieved for the cyclic properties of SS304 stainless steel subjected to uniaxial strain-controlled, uniaxial and nonproportionally multiaxial stress-controlled cyclic loading at room and high temperatures. The effects of cyclic strain amplitude, mean strain, temperature and their histories on the cyclic deformation behavior of the material were investigated under the uniaxial strain-controlled cyclic loading. The uniaxial and nonproportionally multiaxial ratcheting was researched under the asymmetrical stress-controlled cyclic loading with variable stress amplitudes, mean stresses, loading paths and their histories at room and high temperatures. It is shown that the uniaxial cyclic properties under strain-controlled cyclic loading and the ratcheting under asymmetric uniaxial and nonproportionally multiaxial stress-controlled cyclic loading depend not only on the current temperature and loading state, but also greatly on the previous loading history and the shape of loading path. The material presents much greater cyclic hardening and less ratcheting in the range of 400–600 °C than at room temperature, due to the strong dynamic strain aging taken place in this temperature range. Some significant results were obtained for the constitutive modeling of cyclic plastic deformation such as ratcheting.

Micromechanical change in packing structures can provide significant insights to better understand the cyclic mobility and post-liquefaction behaviors of granular soils. In this study, Discrete Element Method (DEM) is used to investigate the evolution of the packing structure under undrained cyclic loading. The coordination number is used to indicate the formation and destruction of a load-carrying structure in the post-liquefaction stage. A new index, termed as centroid distance, is proposed to quantify the effect of void and particle redistribution during cyclic loading. The new index is found to have a strong correlation with the mobilized maximum cyclic strain in post-liquefaction deformation.

The influence of mechanical strain on Ic is investigated for Bi-2223/pure-Ag superconducting tapes at 77 K. Taking into account of the pre-strain due to thermal contraction, all the results for normalized Ic vs. strain relation fall on a master curve. Normalized Ic values exceed 0.98 in the strain range from −0.2% to 0.2%. The strain test machine can be applicable also for fatigue tests. Application of cyclic strain (0%→x%→0%) is repeated. Strain cycles with maximum strain ranging from −0.18% to 0.25% cause no significant degradation. The results support the feasibility of HTS conductors for practical applications.

The objective of this study was to investigate the fatigue and fracture behavior of steel cord/rubber composites used in tire belts under constant cyclic strain loading. The material was a specially made tire belt layer in the form of rubber sheets reinforced with unidirectional cords consisting of two pairs of twisted steel wires. Failure mechanisms, damage development, and fatigue life were determined for single belt layers with different cord orientations. Tests were conducted at cord angles of 22°, 72°, and 90° degrees with a cyclic strain amplitude of 8.3% at a frequency of 10 Hz. Five different stages of damage development were observed: microcrack initiation, microcrack multiplication, macrocrack formation, slow macrocrack propagation, and fast macrocrack propagation leading to final failure. In the case of the 22° cord specimens, where the in-plane shear component was dominant, damage development consisted of microcrack initiation at the cord/rubber interface, the formation of more microcracks and macrocracks, and finally the formation of a major fatal macrocrack along the cord direction. In the case of 90° cord specimens, dominated by transverse tension, initial microcracks occurred within the cord, they propagated across the thickness of the specimen, and finally a major macrocrack propagated across the entire width of the specimen. The final crack propagated in part along the cord/rubber interface and in part within the cord. In the case of the 72° cord specimens, where both in-plane shear and transverse tension are critical, the initial microcracks occurred within the cord and the final macrocrack along the interface. For the same cyclic strain amplitude, the 90° specimens had the shortest fatigue life, and the 72° specimens had the longest. Additional tests were conducted at different strain amplitudes. The normalized modulus decreases slowly and nearly linearly with normalized fatigue lifetime up to a certain value of the latter, approximately 80% of the normalized logarithmic lifetime, and then it drops sharply. Cyclic strain amplitude also affects the failure mechanisms. High amplitudes produce localized damage, whereas low amplitudes produce dispersed damage. A residual life model was proposed based on stiffness degradation.