Accumulated Plastic Strain Research Articles

Accumulated plastic strain behavior of granite residual soil under traffic loading

In this study, the accumulated plastic strain (APS) of granite residual soil (GRS) under traffic loading is investigated via cyclic hollow-cylinder and triaxial tests. GRS is a special type of soil that is found widely in the hot and rainy areas of southern China. Because of insufficient understanding of the cumulative plastic deformation of GRS, how the dynamic stress amplitude, degree of saturation, effective consolidation stress, and stress path affect the APS under loading cycles is explored and analyzed. The results show that the effective consolidation stress inhibits the development of APS, whereas the dynamic stress amplitude promotes it. The APS of GRS increases with the degree of soil saturation, and its noticeable water-softening characteristics become the least favorable factor for soil deformation. The combined action of water softening and principal stress rotation helps to increase the APS significantly, and it leads to the emergence of an APS developmental curve that differs from the stable curve. Accounting for the water-softening characteristics of GRS and various stress conditions, a simple and easily adopted APS model is developed to predict the evolution of APS with loading cycles. Although specific to GRS in China, the findings of this study are expected to apply more widely to other weathered geomaterials.

Experimental Analysis and Constitutive Modeling of Cyclic Behavior of 304 L Stainless Steel: Introduction of Isotropic Hardening Fading Effect

Abstract In this study, the mechanical behavior of austenitic stainless steel 304 L under low cycle fatigue was investigated under different uni-axial strain-controlled loadings of 0.5%, 0.8%, 1.0%, 1.2%, and 1.5%. The analysis of the experimentally determined strain versus stress hysteresis curves was carried out to achieve stress quantities such as amplitude stress, peak effective stress, and peak back stress. It was observed that in the early stage of cyclic loading, material underwent initial hardening, followed by softening phenomena which were more considerable in the lower strain range. Before the failure, the secondary hardening was observed at the final stage. In addition to accumulated plastic strain, it was shown that the peak back stress and peak effective stress which is associated with isotropic hardening and kinematic hardening behavior, respectively, are influenced by the strain range effect. Therefore, the coefficient of recall term that appeared in the Armstrong–Frederick nonlinear kinematic hardening model was considered to be dependent on the radius of the memory surface. Furthermore, to increase the ability of the plasticity constitutive model to show a smooth transition between various hardening stages, the radius of the yield surface which is associated with the isotropic hardening rule was equipped with the fading effect. Finally, by the comparison of numerical and experimental results, the capability of the rate-dependent constitutive model over classical rate-independent plasticity in the prediction of mechanical behavior of steel 304 L under strain-controlled cyclic loading was revealed.

An In Situ Resistance-Based Method for Tracking the Temporal Evolution of Recovery and Recrystallization in Ni-Base Single-Crystal Superalloy at Super-Solvus Temperatures

This study explores the influence of thermo-mechanical behavior and microstructure on recovery and recrystallization in single-crystal superalloys during casting and subsequent solution heat treatment, using miniature testing. Here, the temporal evolution of resistance was measured using in situ electrothermal mechanical testing (ETMT) to track the process of recovery and recrystallization (RX). It was found that recrystallization is dominant only when recovery is incomplete and is dependent on both the history dependence of the strain path as well as the magnitude of the accumulated plastic strain. A precursor to recrystallization is the occurrence of subgrains and deformation twins on the sample surface, where a characteristic butterfly-type morphology of γ′ precipitates is always observed in highly strained regions. The migration of RX grain boundaries is accompanied by the elimination of lattice curvature associated with the density of geometrically necessary dislocations. Homogenized samples provide the most reliable results, while interpreting resistivity changes with recovery and recrystallization is more challenging when inhomogeneity (microsegregation, local variation of mechanical properties) in as-cast material prevails.

Effect of Constraint and Crack Contact Closure on Fatigue Crack Mechanical Behavior of Specimen under Negative Loading Ratio by Finite Element Method

Mechanical behaviors at fatigue crack tips of cracked specimens under negative loading ratios are studied in detail by the finite element method in this paper. Three factors induced by specimen type and loading type on fatigue crack field are discussed, including constraint, compressive loading effect (CL effect) and crack contact closure. For mode I crack under negative loading ratios, the effects of the CL effect and crack contact closure on plastic strain accumulations are dominant, with the constraint effect being minor. The constraint effect has effects on the monotonous plastic zone, while the CL effect and contact closure both have effects on the reversed plastic zone (RPZ) and residual tensile plastic zone (RTPZ). That is, the higher the constraint, the smaller the size of the monotonous plastic zone; the greater the CL effect, or the smaller the contact degree, the larger the size of RPZ and RTPZ. For mode II crack, there is only CL effect on the crack tip field without the effect of constraint and contact closure, so plastic strain accumulation at the mode II crack tip is much greater than that at the mode I crack tip when they are under the same loading level.

Low cycle fatigue properties of refractory high-entropy HfNbTiZr alloy

Considerable effort has been applied toward developing refractory high-entropy alloys (RHEAs) with both high strength and good ductility. However, the deformation and fatigue failure mechanisms of these alloys under cyclic loading remain poorly understood. The present work addresses this issue by investigating the low-cycle deformation behavior and microstructural evolution of an HfNbTiZr RHEA with a nominal composition of 25 mol% each of Hf, Nb, and Ti, and Zr forming the balance under symmetric tension-compression conditions at room temperature. Analysis of the cyclic stress responses of the alloy reveals a variety of internal stress trends at different strain amplitudes (Δεa), which implies the operations of a variety of cyclic deformation mechanisms. Further analysis demonstrates that cyclic softening is mainly caused by a rapid decrease in frictional stress during the initial cycles representing roughly 10% of the total number of cycles to failure. In general, the microstructural characteristics of the alloy observed with an increasing number of cycles under various values of Δεa demonstrate that softening occurs due to the formation of slip bands (SBs) caused by high-density dislocations arising in conjunction with increasing plastic strain accumulation. Specifically, dislocation structures forming at a low value of Δεa = 0.8% mainly consist of planar SBs (PSBs), while low-density cross SBs (CSBs) form at Δεa = 1.1%, and the density of the CSBs increases with further increasing Δεa. In addition, a fatigue lifetime prediction model was presented that obtained good prediction accuracy for the HfNbTiZr RHEA investigated herein. Accordingly, the present study can be expected to provide a fundamental basis for understanding the deformation mechanisms of other RHEAs.

Numerical Evaluation of the Fracture Process of 41Cr4 Steel: Analysis of Cracks Grown in a Plane Strain State Domination Based on Experimental Results.

The paper presents the evaluation of the fracture process of 41Cr4 steel, based on experimental tests and numerical analysis carried out for growing cracks. Selected results of experimental studies presented in previous publications in 2015–2016 which were used in the construction of FEM models of SEN(B) specimens dominated by a plane strain state, are quoted. Using FEM simulations, the tensile curves and crack growth curves recorded during laboratory tests, several calculations were performed, among which can be distinguished the evaluation of plastic zones in the considered specimens, stress distributions, selected measures of geometric constraints, accumulated plastic strains and selected parameters of fracture mechanics. The numerical calculations presented in this paper allow to estimate the changes of the J-integral for the tested steel, the crack tip opening displacement, the size of the plastic zone and the stress distribution in front of the crack tip. Comprehensive numerical analysis with the use of the actual tensile curve clearly characterizes the behavior of the material under the influence of increasing external loads.

Deformation and critical dynamic stress for compacted volcanic ash subjected to monotonic and dynamic loads

Volcanic ash is widespread in volcanic areas and is considered as a potential construction material. Therefore, volcanic ash can be used as subgrade material when building highways and railways in volcanic areas. However, there is limited research on the critical dynamic stress as well as deformation characteristics of volcanic ash under dynamic load. The Global Digital System (GDS) monotonic triaxial tests and dynamic triaxial tests are presented in this paper to quantify the critical dynamic stress as well as deformation characteristics of volcanic ash subjected to monotonic and dynamic loads. The critical dynamic stress and deformation of volcanic ash are affected by stress level, water content and dynamic stress amplitude. Under monotonic loads, the strength of volcanic ash decreases with increasing water content and increases with increasing confining pressure. The deformation of volcanic ash exhibits strain softening behavior which is enhanced with increasing water content and weakened with increasing confining pressure. According to the shakedown theory, the patterns of accumulative plastic strain for the volcanic ash under different dynamic stress amplitudes are classified into three categories, named plastic shakedown, plastic creep, and incremental collapse. This study redefines the shakedown limits for volcanic ash based on the shakedown theory. The adoption of new shakedown limits provides more accurate descriptions of deformation characteristics at different stress levels, water contents and dynamic stress amplitudes. Based on the new shakedown limits, this paper presents empirical models for the calculation of critical dynamic stress for volcanic ash shakedown limits using regression analysis. As a contribution to the literature and potential engineering applications, the proposed models may provide theoretical significance for the subgrade state evaluation.

Influence of sloping ground conditions on cyclic liquefaction behavior of sand under simple shear loading

An experimental program of undrained cyclic simple shear tests was undertaken on Ticino sand, covering a broad range of initial relative densities and static stress levels, to investigate the effect of an initial static shear stress on failure mechanism, cyclic resistance, and pore water pressure generation. The results indicated that two types of failure mechanisms occurred, namely cyclic mobility and plastic strain accumulation, depending on the magnitude of the initial static shear stress with respect to the applied cyclic shear stress. Test results showed that the cyclic resistance of Ticino sand may either increase or decrease with an increasing initial static shear stress depending on the density state of the sand. The Kα correction factors measured under cyclic simple shear loading were compared with the values reported in the literature based on both cyclic triaxial and simple shear tests, suggesting that Kα could be dependent on the particle gradations and grain shape for a given depositional mechanism. Finally, it was verified that the initial static shear stress has a remarkable impact on the development of cyclic pore-water pressures (PWP) of the sands so that a modified PWP generation model has been effectively proposed in the present study.

ПЕРЕДУМОВИ ЗНЯТТЯ ОБМЕЖЕНЬ ДО РУХУ ВАНТАЖНИХ ТРАНСПОРТНИХ ЗАСОБІВ ЗА ВИСОКОЇ ТЕМПЕРАТУРИ ПОВІТРЯ

Introduction. Currently, in Ukraine, there are restrictions on the traffic of vehicles with an actual weight of more than 24 tons and an axle loading of 7 tons during the day at an air temperature above 28 °C, except for vehicles carrying dangerous, perishable goods, live animals and poultry, and as well as transportation related to the prevention or liquidation of the consequences of emergency situations. An air temperature of 28 °C is considered to be the temperature at which the road pavement structure under the influence of dynamic loadings can sharply lose its load-bearing capacity, which is one of the conditions under which a threat to traffic safety is created or roads can be damaged and destroyed. Problem statement. The analysis of the regulatory documentation showed that during designing of pavement structure, the climatic conditions of asphalt concrete performance should be taken into account and pavement must withstand the accumulation of plastic strains in summer, which requires measures to ensure its rutting resistance. At the same time, the algorithm for ensuring rut stability does not exist yet. It is quite likely that this issue can be resolved by performing appropriate research work and implementing harmonized European standards, respectively, by amending the building codes, etc. However, the question arises whether we do not perform the requirements of the regulatory documentation and design the pavement structure without considering the climatic conditions and ensuring the rut resistance of asphalt pavement, or still comply with these requirements, which should be clarified. Objective. Development of criteria for analyzing the performance conditions of asphalt concrete in the pavement structure and ensuring its rutting resistance during operation. Materials and methods. Prerequisites for the establishment of restrictions on the movement of freight vehicles at air temperatures above 28 ° C, analysis of operating conditions of asphalt concrete in the construction of pavement. Results. According to the results of the work, the influence of temperature, bitumen properties and loading time on the change of asphalt concrete properties has been established. Criteria for the analysis of climatic conditions of asphalt concrete have been developed and criteria for taking into account operating conditions have been established. Approximate bitumen grades for ensuring the asphalt concrete rutting resistance have been established on the basis of establishing the relationship between the standard properties of bitumen and operating properties. Conclusions. For evaluation of possible rut formation resistance of asphalt concrete, it is needed to perform an analysis by the bitumen grade in its composition, the total number of passes of vehicles with estimated loadings during the pavement operational term, the category of road, the type of asphalt concrete, the speed of traffic, as well as the year of pavement arrangement. In order to ensure the rut resistance of asphalt concrete, it is advisable to use the established criteria in the future during the pavement design.

Helical single-walled carbon nanotubes under mechanical and electrostatic loading

Here, we characterize the axial response of helical single-walled carbon nanotubes (HSWCNTs) to mechanical and electrostatic loads, separately. The study considers low-pitch (closed) and high-pitch (open) HSWCNTs of different number of turns in their natural states. The computations are carried out using molecular dynamics near 0 K and at 300 K. Under mechanical loading, the low-pitch HSWCNTs show a bilinear load ( F ) - strain ( ϵ ) relation up to their elastic limit by overcoming; (i) the inter-coil van der Waals and the elastic restoring forces, and (ii) only the elastic restoring forces, in that order. However, a tunable plastic-like region emerges between the two linear regimes which is dependent of the number of active coils. Unloading of low-pitch HSWCNTs within the elastic limit exhibits hysteresis in F − ϵ graph. However, unloading both low- and high-pitch HSWCNTs from a point beyond their elastic limits results in accumulation of plastic strain. Electrostatic loading of HSWCNTs causes inter-coil Coulombic repulsion which after overcoming the van der Waal’s attraction between them and the mechanical restoring forces, produces a net elongation. Interestingly, the charge per atom ( q ) – strain ( ϵ ) graphs for the high-pitch HSWCNTs up to their elastic limit are found to follow the power law: q ∝ ϵ 3 / 5 . Discharging the HSWCNTs completely from a point beyond their elastic limit shows plastic deformation. Mechanical and electrostatic loading, unloading, and reloading of the HSWCNTs up to the fracture point are also examined. The study shows a direction towards developing nano-electro-mechanical actuators and precise positioners based on HSWCNTs.

A revised Chaboche model from multiscale approach to predict the cyclic behavior of type 316 stainless steel at room temperature

In this work, we propose a revised Chaboche model based on a crystal plasticity multiscale approach. From our experiments, 316L steel exhibits two plasticity characteristics at room temperature during the cyclic loading: varied plastic modulus in strain-controlled cycling and continuous growing ratcheting strain in stress-controlled cycling. These characteristics cannot be predicted by using the classic Chaboche model. To modify this model, we used a crystal plasticity multiscale approach to see the microstructure effect on these characteristics. The result shows that the viscous effect, which is ignored in the classic model, plays an important role in stress-controlled cycling. With the multiscale simulation results, we revise the Chaboche model in two folds: a static recovery item is included in the kinematic hardening rule to consider the influence of viscosity on ratcheting; the dynamic recovery term is varied with the accumulated plastic strain for the change of plastic modulus during the strain-controlled cycling. With the revised model, we show that the strain- and stress-controlled cyclic behaviors from experiments can be well predicted. Also, the application of the revised model in integrated structural analysis is discussed.

Fatigue life prediction based on the hysteretic loop evolution of carbon steel under tensile cyclic loading

Accurately predicting the fatigue life of materials and components is of great significance for ensuring the safety of mechanical equipment. In most strain-controlled fatigue experiments, before fatigue fracture occurs, plastic strain accumulation and microstructure changes have a great impact on the cyclic stress–strain response of the material, which manifests as a hysteretic loop evolution. In this paper, the cyclic hardening/softening tendency under different strain amplitudes, the loss mechanism of the loading modulus and unloading modulus, as well as the evolution law of plastic strain energy of 45CrNiMoVA steel were investigated. A life prediction model based on the modified plastic strain energy was proposed according to the evolution law of plastic strain energy. The results showed that the prediction error of the modified plastic strain energy model was within the 2 times error band, which decreased significantly from 13.73% to 7.68% after parameters correction compared with the prediction error of the strain-based prediction model.

Plasto-elastohydrodynamic lubrication of heterogeneous materials in impact motion

The analysis of plastic evolution and material inhomogeneity is required for an accurate description of lubrication under impact loading. This work develops a semi-analytical model for heterogeneous materials in impact motion under plasto-elastohydrodynamic lubrication (PEHL) with limited inlet oil. A plasticity loop is proposed to obtain the accumulative plastic strain iteratively based on the analysis of the stress state. The inhomogeneous inclusion within the materials is homogenized with unknown eigenstrains according to the equivalent inclusion method. The surface displacements induced by the plastic strains and eigenstrains are introduced into the gap between the contact bodies to update the lubrication film thickness until the convergence is achieved. The consideration of plastic strains and eigenstrains makes the model more realistic for PEHL under impact loading. The responses including the pressure, film thickness, starvation parameter, residual deformation, temperature, and subsurface elastoplastic fields during the impact–rebound process are discussed. After the model validation against the reference data, the effects of material inhomogeneity and strain hardening on the lubrication response, as well as the typical features of elastic perfectly-plastic materials, are investigated to provide guidance for material reliability analysis.

Computation and experimental analysis of the resistance of superalloys to low cycle deformations

Diagrams of cyclic elastoplastic deformation in a general case characterize the resistance of a material to a low cycle loading and display a relation between current values of stresses and strains during deformation process. Those diagrams in the mechanics of deformation and fracture are described by rather complicated state equations with a nonlinear dependence on conditions and modes of loading including temperature, deformation rate, cycle form, type of the stress state, absolute size of sections, working medium, type of constructional material, etc. The entity of aforementioned factors affects a shape of the deformation curve (diagram) of the material and key parameters of the corresponding state equations, including basic characteristics of the mechanical properties, i.e., the elasticity module, yield point and ultimate stress, indicators of static and cyclic hardening. Experimental data on the kinetics of cyclic and one-way accumulated plastic strains in each loading half-cycle (cycle) obtained in static and low cycle tests of nickel superalloy specimens revealed that change in deformation characteristics exhibit a drastically nonlinear character in conditions of cyclic elastoplastic strain which is described on the basis of power equations and corresponding parameters of cyclic deformation diagrams. The parameters of those equations depend on the type of cyclically hardening/softening or stable material under deformation of the material in elastoplastic region. It is noted that resistance of materials to cyclic elastic-plastic strain can be described by a set of analytical expressions containing a kinetic function (which changes with number of loading half-cycles) in the form of power or exponential expressions depending on the cyclic properties and characteristics of the mechanical properties of the specific structural material. The obtained calculated and experimental data on the kinetics of strain of the alloy under study in conditions of cyclic elastoplastic loading, as well as on the parameters of the strain diagram (which are basic characteristics for the corresponding state equations) provide the possibility to take full advantage of using the deformation-kinetic criteria of damage accumulation under considered loading conditions to calculate the fatigue life of the structural elements manufactured from the alloy and operated, as a rule, under severe temperature and heavy-loaded conditions. The results of the experiments and calculations are given in the form of diagrams of cycle-by-cycle kinetics of the cyclic and accumulated plastic strains of the material for the soft and hard modes of loading in a wide range of test temperatures.

Failure mechanism of gas storage salt cavern cement sheath under the coupling effect of plasticity and fatigue damage

Due to their good air tightness and stability, gas storage salt caverns are widely used in rapid peak shaving of natural gas. The wellbore is used to connect the cavern to the surface pipeline, and its integrity can usually be guaranteed after cementing and can be verified by logging. In gas injection and production, the tightness of the cement sheath will deteriorate, which may lead to gas leakage accidents. To explore the wellbore integrity during operations, a theoretical model considering fatigue damage and plasticity is established, and constitutive equations are developed for numerical simulation. The model in this paper has been verified by mechanical tests and has good stability and accuracy. Through experiments and simulations, the following conclusions are drawn: (1) The coupling of plasticity and damage can rapidly lead to failure of the wellbore cement sheath interface. Only calculating plasticity or damage may overestimate the integrity of the cement sheath. (2) Fatigue damage and plastic strain under cyclic loading will be concentrated locally, mainly at the interface between the cement sheath and the casing, that is, the inner cemented surface is more likely to fail. (3) The plasticity and damage evolution of the cement sheath have different sensitivities to different cementing defects. Low-quality cementing such as lack of cement sheath will lead to rapid accumulation of wellbore plastic strain and damage. However, casing deviation does not increase plasticity and damage evolution.

Accumulative Plastic Deformation of the Improved Completely Weathered Granite Subgrade in High-Speed Railway

The improvement and reuse of completely weathered granite (CWG) with poor engineering properties for backfilling embankments can help protect the environment and bring great economic benefits. The embankment of high-speed railways demands an extremely low additional subgrade settlement under long-term dynamic loads. To analyze the applicability of the improved CWG used for subgrade filling, a series of laboratory dynamic triaxial and field cyclic loading tests were carried out. On the basis of the test results, a calculation model of accumulative plastic strain considering the dynamic stress ratio and the cement content was established. Using the proposed model and the numerically calculated dynamic stress, the accumulative plastic deformation of an embankment filled with improved CWG under various driving conditions was calculated and discussed. The variation of accumulative plastic deformation of the improved CWG with loading cycles could be described by a power function. The dynamic stress level had a significant influence on the accumulative plastic strain, and the strain in the bottom layer of the subgrade bed was greatly attenuated. Accordingly, most of the accumulative plastic deformation attenuation occurred within the depth range of the subgrade bed. The final total accumulative plastic deformations of the embankment were less than 2 mm (5 mm for the limit value) after 4 million times of high-speed train loadings, proving that the improved CWG can be used for subgrade filling. The influence of the axle load on accumulative plastic deformation was more significant than that of the train speed. The proposed calculation model of the accumulative plastic strain could provide valuable reference for similar railway engineering.

Densification of a polymer glass under high-pressure shear flow

The properties of glasses can change significantly as they evolve toward equilibrium. Mechanical deformation appears to influence this physical aging process in conflicting ways, with experiments and simulations showing both effects associated with rejuvenation away from and overaging toward the equilibrium state. Here we report a significant densification effect in a polymer undergoing shear flow under high pressure. We used the high-aspect ratio geometry of the layer compression test to measure the uniform and homogeneous accumulation of plastic strain during isothermal confined compression of a deeply quenched film of polystyrene glass. Combined scanning transmission x-ray microscopy (STXM) and atomic force microscopy confirmed defect-free deformation leaving up to 1.2% residual densification under conditions of confined uniaxial strain. At higher peak strain, plastic shear flow extruded glass from below the compressing punch under conditions of a high background pressure. A further density increase of 2% was observed by STXM for a highly thinned residual thickness of polymer that nevertheless showed no signs of crystallization or internal strain localization. While the confined uniaxial densification can be accounted for by a simple elastic--plastic constitutive model, the high-pressure extrusion densification cannot.

A new finite element procedure for simulation of flexural fatigue behaviours of hybrid engineered cementitious composite beams

This study proposes a new finite element (FE) analysis procedure for simulation of flexural fatigue behaviours of hybrid fibre reinforced engineered cementitious composite (hybrid ECC) beams. The proposed method simplifies the analysis of hybrid ECC beams under fatigue loading, which could have fatigue life up to millions of cycles, into a small number of static FE analyses corresponding to a set of designated load cycles with equivalent fatigue damages. An ECC material damage model is first developed to describe the degrading of material properties caused by fatigue loading. This material damage model defines the relationships of two critical damage properties of ECC, namely the degraded stiffness and the accumulated plastic strain with the number of load cycles applied. By using the developed material damage model, a simplified analysis procedure is then proposed for the fatigue analysis of hybrid ECC beams under bending. The accuracy and reliability of the proposed material damage model and analysis procedure are validated by comparing experimental results obtained from bending tests of three types of hybrid ECC beams under three different stress ranges with the modelling predictions. It is found that the proposed material damage model and analysis procedure provided good predictions of the fatigue responses of the hybrid ECC beams in terms of material damages, deformation and fatigue life.

Atomistic scale behaviors of intergranular crack propagation along twist grain boundary in iron under dynamic loading

Due to alternative exchange between single and double-teeth meshings, the upper part of gear tooth is subjected to dynamic tensile stress whose growth rate presents rectangular fluctuation. This work investigates the atomistic scale behaviors of intergranular crack propagation along twist grain boundary in body-centered cubic (bcc) iron under dynamic tensile stress. The effects of driving force and contact ratio are fully discussed. Results show that only stacking faults with face-centered cubic (fcc) atoms can be formed in lower monocrystal portion. Edge dislocations in upper monocrystal portion are suppressed by intergranular crack cleavage. Critical stresses for stacking fault nucleation and intergranular crack cleavage vary with driving force and contact ratio. By calculating actual stress intensity factor at crack tip, variations of critical stresses are found to be attributed to the variations of time-dependent factors. Although critical stresses vary with driving force and contact ratio, the effects of these factors on the growths of crack length and plastic zone are not obvious in the early stage of intergranular crack propagation. Accumulated plastic strain energy before intergranular crack cleavage is independent of driving force and contact ratio. Departing from the early stage, the growth rates of crack length and plastic zone increase significantly with an increase in driving force or a decrease in contact ratio. However, the final ductile level of intergranular crack propagation cannot vary with contact ratio and large driving force. By applying dynamic load, this work can be used to reveal the atomistic scale mechanism of gear failure. The results can provide a good reference for gear safety design.

Evaluation of Solder Joint Reliability in 3D Packaging Memory Devices under Thermal Shock

In 3D packaging memory devices, solder joints are critical links between the chip and the printed circuit board (PCB). Under severe working conditions, cracks inevitably occur due to thermal shock. If cracks grow in the solder joint, the chip will be disconnected with the PCB, causing its function failure. In this paper, the reliability of solder joints under thermal shock are evaluated for 3D packaging memory devices by means of the SEM and finite element analysis. As microscopically studied by the SEM, it is found out that the main failure mechanism of solder joints in such test is the thermal fatigue failure of solder joints. Finite element analysis shows that cracks are caused by the accumulation of plastic work and creep strain. The initiation and growth of cracks are mainly influenced by the inelastic strain accumulation. The trends of cracks are influenced by the difference between the coefficient of thermal expansion (CTE) of epoxy resin and that of the chip.