Evolution Of Microdefects Research Articles

Ductile damage and fracture characterizations in bi-cyclic biaxial experiments

This paper discusses novel extremely low-cycle bi-cyclic biaxial reverse experiments with large strains. Various tests with tension–compression–tension (TCT) shear cyclic loads, superimposed by either non-reverse or reverse cyclic loading patterns, are newly designed to investigate the plastic, damage, and fracture behavior. These experiments fill the gaps in carrying out complex reverse loading by utilizing 4mm thick biaxially loaded cruciform specimens. The deformed specimens in different loading stages are examined metallographically with micrographs to elucidate the influence of loading histories on the occurrence and evolution of micro-defects. In addition, scanning electron microscopy (SEM) is also used to analyze the fractured surfaces and verify the anisotropic damage mechanism. An advanced anisotropic cyclic plastic-damage continuum model is utilized to characterize the material behavior, including the Bauschinger effect, the stress-differential effect and the non-hardening effect after shear reverse loading as well as stress-state-induced damage mechanisms. Due to the utilization of the digital image correlation (DIC) technique, the numerical results can be compared with the experimental ones in both load–displacement curves and strain fields, respectively. The elastic strain triaxiality and strain Lode parameter are introduced, providing a novel perspective for analyzing the cyclic loading experiments.

Аспекты численного моделирования процессов разрушения упруго-хрупких тел

Understanding the nucleation and evolution of microdefects in solid bodies is important to ensure the reliability and safety of critical structures and to identify their strength and deformation resources. In numerical modeling, failure zones can be represented as areas with significantly underestimated rigid characteristics by analogy with the method of variable elastic parameters used in solving the boundary-value problems of the theory of plasticity. However, the formal application of numerical algorithms of plasticity does not always lead to an adequate description of failure processes especially in elastic-brittle bodies. This paper considers some aspects of the numerical simulation of failure processes, such as the calculation of a stress-strain state after reducing the rigidity of finite elements under constant boundary conditions by organizing an appropriate iteration procedure, and the selection of the maximum number of finite elements fractured per iteration, the value of a loading step, and the discretization degree of the computational domain. The influence of the above aspects on the results of failure simulation is illustrated by comparing the numerical solutions to the problem of deformation of the strip made of elastic-brittle material with the edge stress concentrator, which were obtained by different algorithms. The loading diagrams were plotted, and the implementation of the post-critical stage at the macro level was demonstrated. Failure kinetics was analyzed for different variants of implementation of the iterative procedure and at a variable number of elements fractured per iteration. It has been found that, in order to get an accurate description of deformation and failure processes, the automatic selection of a loading step seems to be more reasonable. Analysis has indicated that the discretization degree of the computational domain has a strong impact on the modeling results. This suggests that the finite element size should correspond to a certain strength constant of a material having the dimensions of length.

Failure analysis from microcracks to a dominant crack in MEMS thin films using combined damage and fracture mechanics

Reliability issues in MEMS devices are generally rooted in the evolution of microdefects in materials. Although the failure occurs at the top level of the device (macroscopic function), the root cause of the functional failure can be traced back to the bottom level (material micro defects). Therefore, it is crucial to consider the influence of material damage in the stress–strain constitutive equations to calculate the magnitude of crack propagation force. In this paper, a combined method of damage and fracture is considered for calculating the stress of crack tip of MEMS thin films. To calculate the initiations and growths of microcracks, continuum damage mechanics (CDM) is used. Based on the CDM method, a relationship between material degradation and the increase of electrical resistance is established. Through the established computational methods, we can perform a calculation analysis on the failure of MEMS caused by cracks. A multiscale quantification result establishes pairwise connections between material degradation characterization (damage D), crack propagation (tip stress K), and macroscopic performance (resistance R). This article provides a valuable theoretical basis for investigating the failure mechanisms of MEMS.

Determination of stress waves and their effect on the damage extension induced by surface defects of KDP crystals under intense laser irradiation

The residual crack defects on the surface of potassium dihydrogen phosphate (KDP) crystals are the bottleneck that limits the improvement of laser damage resistance in the application of high-power laser devices. The multiple stress waves introduced by these residual surface lateral cracks on crystals under laser irradiation are the main inducement for damage extension and reduction of laser damage resistance. However, the coupling of these stress waves complicates their propagation in the crystal, and the interaction mechanism between each stress wave and laser damage has not been quantitatively characterized. Herein, a laser damage dynamic model for surface lateral cracks is constructed to reproduce the dynamic behaviors of the evolution of micro-defects to sub-millimeter damage pits under laser irradiation. Combined with the time-resolved pump and probe technique, the distribution of stress waves induced by lateral cracks was detected in situ to determine the type of stress waves. Then, the initiation and extension of laser damage were analyzed quantitatively to establish the correlations between different stress waves and damage extension. It is found that the longitudinal, shear, and Rayleigh waves induced by lateral cracks lead to large crush zones on the surface of KDP crystals, as well as butterfly-like damage sites accompanied by a large number of cracks at the bottom in the longitudinal section. The scale of the damage site can reach up to approximately 150 µm for lateral crack defects with large surface widths. This study ultimately reveals the physical mechanism of damage evolution induced by lateral cracks, providing effective guidance for developing control standards of surface crack defects during optical ultra-precision machining processes. This is of great significance for the improvement of laser damage resistance of KDP crystals in high-power laser systems.

Fatigue life prediction for amorphous glassy polymers based on cumulative evolution of micro-defects

This study is focused on the fatigue life prediction model of the amorphous glassy polymers by alternating different parameters on the experimental results. The mechanism of fatigue failure was investigated through scanning electron microscopy. Respected to the different temperature conditions different regions of fracture were observed consisting of the thermal and mechanical zones. The proposed model is based on the cumulative evolution of micro-defects up to the rupture. In this regard, different constant parameters depending on the material, temperature, and frequency were considered. According to the results, the model showed a good ability to predict the fatigue life of the polymers.

Molecular dynamics study on the microscopic mechanism of in-service welding damage and failure

The risk of burn-through is a major concern when conducting in-service welding repair for oil-gas pipelines. The governing mechanism of burn-through has not hitherto been studied systematically. During in-service welding, different micro-zones of the pipe wall metal below the molten pool have different crack initiation mechanisms due to different temperature and stress conditions. This paper uses molecular dynamics to study the initiation and evolution of micro-defects in different micro-zones of the pipe wall under the in-service welding pool and reveals the microscopic mechanism of in-service welding damage and failure. The research found that the pipe wall under the in-service welding pool is divided into a fusion zone and a coarse grain zone. For the coarse grain zone, the grain boundary region is subjected to large deformations and causes high strain. The stress concentration at the grain boundary (especially the triple junctions) is caused by the arrest of grain boundary slip, which leads to the initiation and propagation of microcracks in the grain boundary region. The displacement vector of atoms at the grain boundaries is generally larger than that of the atoms in the crystal, further indicating that grain boundary slip is the primary mechanism of microcrack initiation in the coarse grain zone. Compared with the coarse grain zone, the grain boundary width of the fusion zone increases, and the grain boundary atoms show apparent disorder. The peak value of the radial distribution function decreases, and the peak becomes wider, indicating that the grain boundary pre-melting phenomenon occurs in the fusion zone. In addition, as the tensile strain increases, the instability of the pre-melting zone expands rapidly and advances into the crystal grain. During in-service welding, the high-temperature deformation of the coarse grain zone and the fusion zone of the tube wall are mainly controlled by the intergranular mechanism. Through molecular dynamics, the mechanism of grain boundary slip in coarse grain zone and grain boundary pre-melting in fusion zone were studied from the microscopic scale. Thereby the microscopic mechanism of in-service welding damage failure was further clarified.

Thermal evolution of microdefects in He ion irradiated W-Ni-Feheavy alloy

To investigate the thermal evolution of vacancy-type defects and the relationship between the evolution of vacancy-type defects and the formation of fuzz structure on the surface of W-Ni-Fe heavy alloys irradiated with He ions, isochronal annealing treatments at temperatures from 100 °C to 1000 °C were conducted on the irradiated 97W-2Ni-Fe alloy. The evolution of microdefects was characterized by positron annihilation spectroscopy (PAS). Thermal desorption spectroscopy (TDS), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) were used to supplement the positron annihilation method. A large number of vacancy-type defects were produced in the 97W-2Ni-Fe alloy after irradiation at room temperature and isochronal annealing, and the main defect types of positron capture changed at three stages: RT-600 °C, 600–800 °C, and 800–1000 °C. SEM found that a large number of nanoscale protrusions appeared on the surface of samples annealed at 800 °C, and these nanoscale protrusions were seeds of fuzz structures. The formation of fuzz structure is closely related to the movement of vacancy-type defects by PAS, TDS, SEM, and other characterization methods. In addition, we found that the high density Ni and Fe in the matrix phase of the 97W-2Ni-Fe alloy as He capture points would accelerate the nucleation and growth of He bubbles and promote the growth of the fuzz structure.

Damage model of unsaturated frozen soil while considering the influence of temperature rise under impact loading

Constitutive models and damage laws of frozen soil under high strain rates are crucial for the design of underground soil structures, and they lay an essential foundation for the research of instability phenomena in cold region engineering. The expansion and evolution of micro-defects and temperature damage are two basic mechanisms that control the macro and micro damage behavior of frozen soil. Considering frozen soil as a composite material, this study analyzes the damage mechanism of brittle materials subjected to impact loading. Finally, a dynamic model of unsaturated frozen soil under high pressure is established based on the complementary energy equivalence principle and Eshelby's equivalent inclusion theory. To analyze the effect of high-speed compression on the meso behavior of frozen soil, according to the Hugoniot energy theory, the evolution of temperature rise in the impact process is formulated. The results for frozen sandy soil confirm that the rate of temperature rise increases with the strain rate and has a significant weakening effect on the bearing capacity. Thus, the damage behavior of frozen soil changes according to different loading histories. Finally, based on the corresponding theoretical results, the mechanical response of frozen soil under different loading histories and the corresponding performance indicators are compared and analyzed.

Mechanical properties of rock–coal bi-material samples with different lithologies under uniaxial loading

In this paper, we performed uniaxial compression tests on fine sandstone–coal, coarse sandstone–coal, and oil shale–coal bi-material samples to investigate the lithology effects on their strength, macro-failure initiation (MFI), energy evolution, and failure characteristics, respectively. Uniaxial stress–strain characteristics of the samples primarily depended on the coal and were affected by coal–rock interactions. These interactions involved the following aspects: (1) The differences in the mechanical properties of rock and coal affected their stress states near the interface. They were in triaxial compression state with strength increasing or in triaxial compression-tension state with strength decreasing. (2) During the pre-peak stage, the rock deformation affected the values of input energy, elastic energy, and dissipated energy. Damage and energy accumulation in coal were also limited by rock deformation, resulting in higher strengths for the bi-material samples compared to pure coal samples. (3) During the post-peak stage, the rebound deformation of rock was accompanied by the elastic energy release, which promoted the failure of coal. MFI of the samples occurred within the coal. MFI models mainly included the tensile fracture of coal near the interface, the initiation and propagation of macro-cracks, and the development and evolution of micro-defects, accompanied by varying degrees of surface spalling or ejection failure. The coal showed the splitting ejection failure. The oil shale and coarse sandstone exhibited tensile failure or tensile–shear composite failure due to crack propagation in coal. Failure was not evident in the fine sandstone. The coal in the oil shale-coal bi-material samples became more broken due to the most serious rebound deformation of oil shale.

Physics of failure of die-attach joints in IGBTs under accelerated aging: Evolution of micro-defects in lead-free solder alloys

Fatigue of die-attach solder joints is quite critical to reliability of IGBT modules. In the present work, physics of failure of die-attach joints in IGBTs was discussed based on the evolution of micro-defects in materials under accelerated aging through experiments and Finite element (FE) simulations. It shows that voids, cracks of Sn-Ag-Cu (SAC) alloys and detachment of Si/SAC interfaces are three major micro-defects in the die-attach solder. Voids could accelerate evolution of other two micro-defects. Cracking of SAC alloys is the major failure mode of die-attach solder under power cycling and detachment of Si/SAC interfaces turns to be the dominant failure mode under transient electrical/thermal shocks. FE simulations indicate that the shear thermal stress τshear at Si/SAC interfaces is the decisive factor for switching between the two failure modes. When ΔTj > 105 °C, the average shear stress τshear_ave at Si/SAC interfaces exceeds the bonding strength and detachment of Si/SAC interfaces plays a major role in failure of die-attach solder. Based on physics of failure, two physical life models for die-attach joints under various accelerated aging conditions were proposed.

Modelling the Damage of Structural Components with Macrostructure Defects

The article presents a numerical analysis of the process of damage of structural steel, the scope of which encompassed the estimation of the time to failure of a structural component. The analysis was conducted using the Gurson–Tvergaard–Needleman material model, which takes into account the influence of microdefects on material strength. Considered was a plate element with a central hole modelling the material discontinuity that may arise in a structural component as a result of corrosion. The conducted simulation permitted an analysis of the phenomenon of nucleation and evolution of microdamage in S235JR steel, which allowed, for the analysed component, the detection of the initiation of microdamages and their development in the area susceptible to damage. Changes to the state of stress taking place during plastic deformation of structural steel due to the evolution of microdefects of the material structure were analysed. Presented are the results of this research, in which the stress state described by the stress triaxiality in relation to the changes in the volumetric fraction of voids determining the size of microdefects was given a detailed analysis.

The effect of residual stress on Σ3(111) twin boundaries fracture behavior in γ-TiAl with molecular dynamics simulation

In this paper, molecular dynamics (MD) simulations are performed to study the influence of residual stress on fracture behavior in γ-TiAl alloy with ∑3(111) twin boundaries. The tensile processes are simulated at 1, 300, 1050 K after the residual stress is introduced by prepressing. The evolution of microdefects during tensile fracture is observed, and the distribution of residual stress in three direction is characterized. Results show that the introduced residual stress is symmetrically distributed on both sides of ∑3(111) twin boundaries. The residual compressive stress can prevent the nucleation of micro-cracks and increase the yield strength. The fracture mode of the γ-TiAl alloy with ∑3(111) twin boundaries during tensile load at 1 K is the nucleation of microcracks at grain boundaries followed by intergranular fracture. The ∑3(111) twin boundaries have good thermal stability. The residual compressive stress is completely released and the amplitude of residual tensile stress increases at 1050 K, and the γ-TiAl alloy with ∑3(111) twin boundaries exhibits good ductility.

Failure Mechanism of Die-Attach Solder Joints in IGBT Modules Under Pulse High-Current Power Cycling

Applications under extreme conditions, such as solid circuit breakers and electromagnetic launching systems, are great challenges to semiconductor power devices. The die-attach solder joint is as one of the most vulnerable structures and critical to the reliability of insulated-gate bipolar transistor (IGBT) modules. In this paper, IGBT modules were cross sectioned and tested under pulse high-current power cycling. The failure mechanism of the die-attach solder in IGBTs at pulse high-current modes was investigated. Evolution of microdefects in the die-attach solder during power cycling was characterized and factors for the failure of die-attach solder joints was discussed. The results revealed that voids, cracks, and detachment of interface were the major microdefects in the die-attach solder layer. A detachment of the Si/Sn–Ag–Cu (SAC) interface is verified as the major failure mode under pulse high-current power cycling. Interface cracks between the Si-chip and die-attach solder layer were found to initiate first at the solder layer edges and then extended to the center of the solder layer with the increase of power cycles. The detachment of Si/SAC interface was more similar to the brittle fracture. The junction temperature swing and heating rate were the key factors for detachment of the Si/SAC interface.

Capabilities of X-ray diffuse scattering method for study of microdefects in semiconductor crystals

The capabilities of X-ray diffuse scattering (XRDS) method for the study of microdefects in semiconductor crystals have been overviewed. Analysis of the results has shown that the XRDS method is a highly sensitive and information valuable tool for studying early stages of solid solution decomposition in semiconductors. A review of the results relating to the methodological aspect has shown that the most consistent approach is a combination of XRDS with precision lattice parameter measurements. It allows one to detect decomposition stages that cannot be visualized using transmission electron microscopy (TEM) and moreover to draw conclusions as to microdefect formation mechanisms. TEM-invisible defects that are coherent with the matrix and have smeared boundaries with low displacement field gradients may form due to transmutation doping as a result of neutron irradiation and relaxation of disordered regions accompanied by redistribution of point defects and annihilation of interstitial defects and vacancies. For GaP and InP examples, a structural microdefect formation mechanism has been revealed associated with the interaction of defects forming during the decomposition and residual intrinsic defects. Analysis of XRDS intensity distribution around the reciprocal lattice site and the related evolution of lattice constant allows detecting different decomposition stages: first, the formation of a solution of Frenkel pairs in which concentration fluctuations develop, then the formation of matrix-coherent microdefects and finally coherency violation and the formation of defects with sharp boundaries. Fundamentally, the latter defects can be precipitating particles. Study of the evolution of diffuse scattering iso-intensity curves in GaP, GaAs(Si) and Si(O) has allowed tracing the evolution of microdefects from matrix-coherent ones to microdefects with smeared coherency resulting from microdefect growth during the decomposition of non-stoichiometric solid solutions heavily supersaturated with intrinsic (or impurity) components.

A unified model for coupling constitutive behavior and micro-defects evolution of aluminum alloys under high-strain-rate deformation

High strain rate forming (HSRF) is promising to break through the conventional forming limit of materials and thus to form hard-to-deform components. During HSRF process, long-lasting micro-defects before fracture, i.e. adiabatic shear bands (ASBs) in compression and voids in tension, are significant characteristics existing in a large strain range. These defects bring about the non-destructive softening of stress and continuous exertion of ductility, in return, the stress responses affect the evolution of micro-defects. However, the interaction between stress responses and defects evolution has not yet been reflected in the existing models, resulting in a limited prediction accuracy. Aiming at this issue, the competition between hardening caused by thermal activated dislocation movements and the flow softening brought by micro-defects evolution was well modeled in this work. During the modeling, the relation between the normal strain and effect zone of ASBs was established via the modification of Bai–Dodd model by considering the geometric features of ASBs. Moreover, by introducing a rate-dependent ASB trace angle, the half width of ASBs was expressed as a function of maximum shear strain and critical instability strain. The effect of strain and strain rate to the evolution of voids under tensile conditions was taken into account by combining Hollomon hardening law with Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation. Then, the interactions between micro-defects and structure-related athermal stress were characterized by connecting volume fraction of voids and effective bearing area in tension and the intensity of flow localization with ASBs width in compression. As a consequence, a unified model of constitutive behaviors coupled with micro-defects evolution was established with considering rate-dependent hardening and softening. Applied to aluminum alloys, this model predicts the stress responses, evolution of ASBs and voids, and low or negative strain rate sensitivity with high precision in large ranges of strain (0–0.6) and strain rate (0.001–5000 s−1). The proposed model is thus believed to be with a successful application in precise prediction and optimization of HSRF processes.

Effects of Implantation Sequence on the Micro-defects in H and O Implanted Silicon

Cz n-type Si (100) wafers are implanted with 190 keV O and 40 keV H ions in different implantation sequences. Cross sectional transmission electron microscopy and slow positron annihilation spectroscopy are used to study the formation and evolution of micro-defects. Our results clearly show that the defect morphology depends strongly on the implantation sequence. Large cavities are observed in O preimplanted samples, while only platelets are observed in H preimplanted samples. The change regularity of the S parameter is the same for the Si samples co-implanted at different sequences. But in H preimplanted samples, the S parameter is a little higher. The effects of the implantation sequence on the micro-defects have been discussed in combination with H and O implantation-induced defects as well as their interactions upon annealing.

The evolution of micro defects in He+ irradiated FeCrNi alloy during isochronal annealing

In order to study the microstructural evolution of the He+ irradiated FeCrNi model alloy, 140keV He+ ions were implanted into the specimen with the fluence of 1×1015ions/cm2 at room temperature. Positron annihilation spectroscopy was used to characterize the evolution of micro defects during isochronal annealing between 423 and 873K. The decrease of the S parameter between 573 and 623K has been associated to the annihilation of vacancy clusters, which were not trapped by helium atom. While the decline of the S parameter between 773 and 823K in the damage region might be caused by the dissociation of HenVm clusters. The dissociated vacancy clusters were unstable and annihilated rapidly, which decreases the concentration of vacancy defects.

Positron Annihilation Spectroscopy Investigation of Hot-Extruded Al-20Si-0.35RE Alloy

Positron annihilation spectroscopy is a nondestructive evaluation method which is sensitive to study the microdefects of materials. In this article, hypereutectic Al-20Si-0.35RE (wt.%) pre-alloyed powders was synthesized by using the water atomization method. The pre-alloyed powders were subsequently hot extruded with pressure of 250~500 MPa at 300°C. The effect of pressure of hot-extrusion on the evolution of microdefects in Al-20Si-0.35RE alloy was investigated by positron annihilation spectroscopy, electrical conductivity tests and hardness tests. It was shown that the accretion of pressure led to an apparent increase in conductivity and microhardness. For all samples, the hot-extrusion causes a pronounced change in positron annihilation related parameters signaling the variation of microdefects. The results of hardness tests and the electrical conductivity tests are varied following with the positron annihilation spectroscopy. Possible mechanisms of the influence of hot-extrusion on the behavior of microdefects are discussed.

Environmentally assisted “in-bulk” steel degradation of long term service gas trunkline

Comparison of mechanical (characteristics of strength, reduction of area and elongation, impact strength, hardness, fracture toughness), corrosion-mechanical (stress corrosion cracking in the artificial brine and hydrogen embrittlement) properties and parameters of hydrogen behaviour in the X52 steel in as-received state and after long term service are presented in the paper. The analysis of a change of the mentioned characteristics together with the results of hydrogen permeation and vacuum hydrogen extraction measurements indicate considerable “in-bulk” material degradation of trunk pipeline steel after long term service and essential role of hydrogen in these processes. The measurements of the internal friction have shown the enhancement of grain boundary relaxation, which has been associated with the formation of defects at the grain boundaries. Two stages of the “in-bulk” degradation of steel in the course of its long term exploitation in the pipe lines have been postulated: (1) the deformation aging and (2) the formation and evolution of microdefects.

Constitutive modeling of damage accumulation and fracture in multiphase materials

The dominant mechanisms of energy dissipation associated with the deformation processes preceding and accompanying failure in multiphase materials, the so-called pre-fracture processes, are described quantitatively as the evolution of microdefects randomly distributed in the volume of specimen. Damage kinetics in a composite material are studied in detail. It is demonstrated that a large number of secondary discontinuities present in multiphase materials, such as matrix cracks, debonding of particulates and/or fibers, fiber breaking, bridging and fiber pull-out, enhance the apparent material resistance to the continuing growth of damage. Constitutive modeling of these effects incorporates the basic concepts of Continuous Damage Mechanics and it leads to an integro-differential equation which governs the evolution of the microdefects, their coalescence and interaction with the dominant crack-like defect and finally, the ensuing time-dependent fracture of the component. The principal evolution equation is solved by finite difference and finite element methods. Results pertaining to various crack growth histories are then used as the data-base sufficient to suggest a closed form solution which defines the material resistance curve, or an ‘R-curve’ routinely used in Nonlinear Fracture Mechanics as a standard for evaluation of fracture response in nonelastic metals. In this way we are able to propose a unified theory explaining enhancement of the fracture toughness during the early stages of failure in all dissipative media.