Micromechanical Damage Research Articles

Mixed-modes (I/III) fracture of aluminum foam based on micromechanics of damage

Nowadays, foam structures have several applications in industries. Basically, discontinuities such as pores cause complications in evaluating the mechanical and fracture behavior. In other words, understanding the multi scale of porous cells in cracked foams can play a significant role in fracture prediction. The present research proposes an analytical approach based on the experimental concept of damage in which the behavior of cracked metal foam was investigated under mixed-mode I/III (tensile and out-of-plane) fracture. In this method, pores in foam are modeled as circular and elliptical defects. In this context, mechanical properties and crack inclination angle are obtained based on loading and configuration of pores. Dispersion of circular and elliptical discontinuities determines the overall modulus and crack inclination angle trajectory based on the experimental results, pure mode I SIF were occurred on (0°) and pure mode III were obtained at (62°) loading angle. The aluminum foams were produced and prepared according to the edge notch disc bend (ENDB) test method to obtain the onset of fracture initiation ( KIc and KIIIc) and trajectory of cracked foam body. The results demonstrated that analytical approach, based on circular pore distribution, had a good agreement with experimental results. Moreover, the results indicated that SIF in mode I and mode III had the most agreement with the experimental results at angles of 0° and 62°, respectively.

Stress-dependent matrix crack opening in SiC/SiC composites: theory and analytical prediction

ABSTRACT In the present work, stress-dependent matrix crack opening behavior of SiC/SiC composites was analyzed considering stochastic fragmentation in fibers. The stochastic matrix multiple fragmentation model was adopted to determine the fragmentation lengths of three different types, the accompanying interface debonding process was assessed by the fracture mechanics theory, and stochastic fiber fragmentation was determined by the global load sharing (GLS) criterion. Considering the aforementioned multiple micro-damage mechanisms, matrix crack opening displacements (CODs) in different matrix fragmentation states were determined using a micromechanical technique. Relationships between CODs and the constituent properties of SiC/SiC composites with fiber fragmentation were established. The crack opening behavior of SiC/SiC composites was also analytically predicted. Effects of fiber stochastic fragmentation on crack opening-related damage parameters were examined.

Mechanical Behavior of Titanium Alloys at Moderate Strain Rates Characterized by the Punch Test Technique

Material characterization at moderate strain rates is an important factor for improving the adequacy and accuracy of analysis of structures operating under extreme conditions. In this paper, the deformation and fracture of Ti-5Al-2.5Sn alloys were studied utilizing the punch test at strain rates up to several hundred per second. Loading velocities from 0.0003 to 15 m/s were realized during the spherical body penetration through a thin titanium plate. To describe the plastic flow and fracture of the Ti-5Al-2.5Sn alloy at strain rates ranging from 0.001 to 103 s-1, a micromechanical damage model was coupled with a viscoplastic constitutive model based on the dislocation dynamics. Numerical simulations of the punch test at 15 and 2 m/s were carried out to validate used constitutive relations. It was verified that the simulated fracture shape and deflections were similar to experimental ones. It was found that dynamic punch test is suitable for validation of damage kinetics under complex stress states.

A Review on Fatigue Mechanisms and Approaches for Hard metals

In this study, fatigue lifemechanisms and approaches for hard metals commonly known as cemented carbides that are supported by literature are reviewed on development of fatigue life technology methods. The hard metals materials are mainly used in engineering systems and failure due to fatigue is common phenomenon in engineering materials.The fatigue crack growth therefore plays very important role towards fatigue failure mechanisms and this remains a setback to engineering fraternity as far as usage of these hard metals are concerned. The fatigue process occurs when a material is subjected to fluctuating stress and strain, this may lead to failure when it accumulates loading due to cumulative damage, multiaxial and variable loading. The Crack initiation growth and micro-mechanisms of damage during fatigue growth with optimization of fatigue life technology approaches and fracture growth resistance are main focus of this review article. Thee review of this paper is based on fatigue life mechanisms adoption methodsfor hard metals with novelty of Crack Tip opening displacement technologyand its implementation in various systems will ought to assist technologists in extraordinary broad programs.In conclusion cumulative damage, fatigue mechanisms, multiaxial amplitude loading and fatigue crack growth mechanics techniques are important andan appropriate fatigue life technology mechanisms methods. Eventually these review article will give potential information on fatigue life technology adoption methods and this helps future researcher and technologists to consider the fatigue life models and these ideas will impact on future application of fatigue life failure mechanisms.

Coupled crystal plasticity and micromechanics damage model based on viscoplastic self-consistent theory and X-ray computed tomography

In recent years, coupled crystal plastic and micromechanics damage models have been applied to effectively deal with the complex interactions between the polycrystalline environments and void behaviors. The classical viscoplastic self-consistent (VPSC) model, as a widely used analytical model with excellent computational efficiency, has been extended to the dilatational-VPSC (DVPSC) model. However, as proposed by the model developers (Lebensohn et al., 2004), there are two main challenges hindering the further extension of the DVPSC model, including (1) the lack of the void nucleation and coalescence and (2) the effective experimental verification. Aiming at the above challenges, a modified-DVPSC model considering the void nucleation and coalescence was proposed and experimentally verified in this work. Accordingly, some improvements were made on the original DVPSC model including the linearization method, the self-consistent equations, and also the volume update equations. Then, with the support of the X-ray computed tomography (CT) technique, the void evolution and fracture mode were analyzed and revealed during the uniaxial tension deformation of 2055 aluminum alloy, through which the essential parameters of the modified model were calibrated. Compared to the previous VPSC and DVPSC models, this modified model accurately describes the stress-strain state, the porosity evolution, and fracture behaviors. Finally, the modified model was applied to investigate the effects of the different loading directions, exhibiting a substantial influence of texture on the void evolution. This research is thereby an innovative attempt for the improvement of the DVPSC model, which can provide inspirations for the future extensions of this theoretical system.

Micromechanics of rock damage and its recovery in cyclic loading conditions

SUMMARY Under compressive stress, rock ‘damage’ in the form of tensile microcracks is coupled to internal slip on microscopic interfaces, such as pre-existing cracks and grain boundaries. In order to characterize the contribution of slip to the overall damage process, we conduct triaxial cyclic loading experiments on Westerly granite, and monitor volumetric strain and elastic wave velocity and anisotropy. Cyclic loading tests show large hysteresis in axial stress–strain behaviour that can be explained entirely by slip. Elastic wave velocity variations are observed only past a yield point, and show hysteresis with incomplete reversibility upon unloading. Irrecoverable volumetric strain and elastic wave velocity drop and anisotropy increase with increasing maximum stress, are amplified during hydrostatic decompression, and decrease logarithmically with time during hydrostatic hold periods after deformation cycles. The mechanical data and change in elastic properties are used to determine the proportion of mechanical work required to generate tensile cracks, which increases as the rock approaches failure but remains small, up to around 10 per cent of the net dissipated work per cycle. The pre-rupture deformation behaviour of rocks is qualitatively compatible with the mechanics of wing cracks. While tensile cracks are the source of large changes in rock physical properties, they are not systematically associated with significant energy dissipation and their aperture and growth is primarily controlled by friction, which exerts a dominant control on rock rheology in the brittle regime. Time-dependent friction along pre-existing shear interfaces explains how tensile cracks can close under static conditions and produce recovery of elastic wave velocities over time.

Continuum damage micromechanics description of the compressive failure mechanisms in sustainable biocomposites and experimental validation

We investigate the compressive failure mechanisms in flax fiber composites, a promising eco-friendly alternative to synthetic composite materials, both numerically and experimentally, and explain their low compressive-compared-to-tensile strength, the compressive-to-tensile strength ratio being 0.28−0.6. We present a novel thermodynamically consistent continuum damage micromechanics model capturing events on the fiber-matrix scale. It describes the microstructure of a unidirectional composite and includes the instantaneous constitutive behavior of matrix and fibers. We show that flax fibers behave as elastic-plastic-damaged solids in compression. Furthermore, we show that fiber damage plays an utmost role in the compressive failure of flax fiber composites – it is a major determinant of the material's compressive stress-strain response. Using X-ray Computed Tomography (XCT) and Scanning Electron Microscopy (SEM), we identify the fiber damage as intra-technical fiber splitting and elementary fiber crushing. Due to microstructural similarities among natural fibers, the same micro-mechanisms are likely to appear in other bio-based fibers and their composites.

Investigation of the effect of nanoparticles on wood-bioplastic composite behavior using the acoustic emission method

Acoustic emission (AE) is a non-destructive method capable of detecting various fracture micromechanisms in composite materials. Discrimination of AE signals associated with various fracture modes is of interest when utilizing this technique. Thus, this paper investigates the identification of AE signal processing wavelet packet transform (WPT) failure mechanisms in wood-bioplastic nanocomposites. To this end, nanocomposites with different compositions were prepared and tested according to the ASTM D638 standard. AE signals were collected during the experiment, and analyzed by various methods. The fast Fourier transform method was used to analyze the fracture of polymers and wood and determine the frequency range of fracture. Additionally, a time-frequency analysis of the AE signal generated by the WPT was performed. The energy distribution criterion was employed to determine the main components of the various damage micromechanisms. The results reveal distinct types and percentages of failure micromechanisms in various composites. This method examined the effect of nanoparticles on the matrix’s tensile strength. The scanning electron microscopy method was used to study the morphology of composites and mechanism of failure. The phase composition of the produced materials was examined with X-ray energy dispersive spectroscopy. The results demonstrated the proposed techniques show a good performance in computational damage discrimination.

Prediction of mechanical behavior of rocks with strong strain-softening effects by a deep-learning approach

Rock materials exhibit various mechanical characteristics, and it is difficult to describe the strain–stress relation with strong strain-softening behavior by a single constitutive law. In the present study, a long short term memory (LSTM) deep learning method is proposed to predict the material’s deformation under different loading conditions. Unlike the traditional analysis method focusing on the determination of effective tangent stiffness tensor, the constructed LSTM-based procedure requires only the strain–stress values in certain cases to predict the future mechanical behaviors, even for rock materials with strong strain-softening, indicating that the loading history can be taken into account as time sequence data. In order to validate the accuracy, two applications are provided with the established LSTM model: predicting the deformation of granite and sandstone in conventional triaxial compression tests without introducing any elastoplastic parameters or constitutive laws, where the dataset for training the LSTM model is collected alternatively from an analytical micromechanical damage model and from laboratory experiments by considering a wide range of confining pressure. Comparisons of accuracy and convergence rate with different neural network structures are also carried out to check the best performance of the procedure. Implementation method of the trained LSTM model in Finite Element program as a constitutive relation is also provided and applied to the simulation of sandstone. Comparisons show that the LSTM-FEM method provides a good capacity to predict the mechanical behavior of rocks.

Micromechanisms and Characterization of Low-Velocity Impact Damage in 3D Woven Composites.

Low-velocity impact (LVI) damage of 3D woven composites were experimentally and numerically investigated, considering different off-axis angles and impact energies. The impact responses were examined by LVI tests, and the damage morphology inside the composites was observed by X-ray micro-computed tomography (μ-CT). Yarn-level damage evolution was revealed by developing a hybrid finite element analysis model. The results show that the impact damage has significant directionality determined by the weft/warp orientation of the composites. The damage originates at the bottom of the impacted area and then expands outwards and upwards simultaneously, accompanied by in-plane and out-of-plane stress transfers. The straight-line distributed weft/warp yarns play an important role in bearing loads at the beginning of loading, while the w-shape distributed binder warp yarns gradually absorb impact deformation and toughen the whole structure as the loading proceeds. The effect of directional impact damage on post-impact performance was explored by performing compressing-after-impact (CAI) tests. It is revealed that the CAI properties along principal directions are more sensitive to the low-velocity impact, and the damage mode is significantly affected by the loading direction.

Experimental and numerical study of micromechanical damage induced by MnS-based inclusions

The study is focused on the characterization of three microalloyed 38MnSiV6 steel variants in terms of microstructure, mechanical properties, and crack initiation. Conventional steel was used as reference material, while the two other variants with reduced sulfur content and Bi or Se addition were prepared. To determine the influence of the non-metallic inclusions present in the rolled round bars; hardness, tensile, impact toughness, and fatigue properties were examined longitudinally and transversally to the rolling direction. To describe the local strain and deformation around the inclusions during the microtensile tests, scanning electron microscope monitoring was adopted. Finite element analysis was applied to model strain/stress localization around inclusions during the microtensile tests. Modeling was performed in two solutions stages involving a macro-model (microtensile sample) and micro-model (inclusions). The comparison of the numerical analysis with the microstructural observations during microtensile tests showed a good agreement. The MnS and MnSeS inclusions were responsible for the intensive strain localization and subsequent crack initiation, while the Bi-MnS inclusions were not involved in this process.

High-temperature creep rupture behavior of dissimilar welded joints in martensitic heat resistant steels

The creep rupture behavior and microstructure changes of dissimilar welded joints of two types of martensitic heat resistant steel F92 and Co3W2, under a creep testing of 115–200 MPa and 873–893 K, were investigated. Fracture position and mode change from transgranular ductile fracture in parent F92 to intergranular brittle fracture in the fine grain heat affected zone (type IV cracking) near F92, with the drop of applied stress. Microstructural degradation resulted from creep with respect to dislocation density decrease, precipitates coarsening, dislocation cells, microvoids, macroscopic secondary cracks, and fracture surface morphology are researched based on detailed experimental results. The creep rupture data are analyzed employing Norton’s power law, Monkman–Grant relation, creep damage tolerance, and Larson–Miller parameter. Main creep damage micromechanisms are analyzed with the aid of calculated and experimental data. The extrapolating creep rupture stress for a creep life of 100,000 h are obtained and the influence of stress change and temperature change on creep life are compared in terms of calculated normalized creep life growth rate.

Micromechanics of Impact Damage in Glasses of the SiO2–TiO2 System

Micromechanics of Impact Damage in Glasses of the SiO2–TiO2 System

Analysis of damage and fracture behavior in ductile metal sheets undergoing compression and shear preloading

The present paper comprises the experimental and numerical analysis of damage and fracture behavior of ductile metals under non-proportional loading with compression and shear preloading. For this purpose, biaxial experiments with the H-specimen using a pneumatic downholder for compression loading followed by failure under a tension stress state and corresponding numerical simulations are performed and analyzed. A thermodynamically consistent anisotropic continuum model is presented. It takes the effects of micromechanical damage mechanisms on the macro scale behavior as a motivation for a phenomenological description of damage behavior as a function of stress state. The experimental findings are compared with the results of tests with proportional load paths and corresponding numerical simulations. Thereto, strain fields of the critical areas of the specimens are analyzed by means of digital image correlation (DIC) technique. Compression and shear preloading can lead to significantly reduced ductility of the material with initiation of failure processes in the small damage strain range. Evolution of numerically predicted plastic and damage equivalent strains illustrates their stress state dependence. The numerical results are also confirmed by images of fracture surfaces taken by scanning electron microscopy (SEM). This experimental-numerical methodology is therefore an efficient tool to develop and validate general modeling approaches, and it is characterized by almost arbitrarily definable load paths with changes in stress states occurring in forming processes.

Coupling of a phase-field method with a nonlocal micro-mechanical damage model for simulating ductile fracture

Coupling of a phase-field method with a nonlocal micro-mechanical damage model for simulating ductile fracture

Experimental investigation and micromechanics-based damage modeling of the stress relaxation mechanical properties in gray sandstone

The geomechanical behaviors of the gray sandstones under dry and fully saturated conditions are investigated by both of experimental and numerical studies in this paper. Conventional triaxial compression tests and single-stage stress relaxation tests are performed on cylindrical specimens under these two different states. The experimental results show that both peak strength and elastic modulus are reduced due to the presentence of the pore water which is contributed the weakening. While the typical incomplete attenuation characteristics of the two states are only having slight difference, the final stress relaxation extent under the statured condition is larger than the dry condition when the strain ratio is the same. Based on the pioneering work, a new micromechanical damage–friction coupling constitutive model with containing fewer parameters and each being physically meaningful for the stress relaxation is proposed here. Its chief novelty lies in considering both the instantaneous plastic internal variables and relaxation load to build the relationship between model’s parameter and experimental data. The single-stage relaxation tests of the sandstones are well-benchmarked with this model, which indicates that the proposed new model can capture the incomplete attenuation features of stress relaxation behavior of gray sandstones.

Damage Micromechanisms in Friction Stir-Welded DP600 Steel during Uniaxial Tensile Deformation

Damage Micromechanisms in Friction Stir-Welded DP600 Steel during Uniaxial Tensile Deformation

Effect of notch severity and crystallographic texture on local deformation and damage in commercially pure titanium

The evolution of local deformation and damage in commercially pure titanium was studied for flat un-notched, U-notch and V-notch tensile specimens with two distinct initial textures namely off-basal (BA) and prismatic-pyramidal (PP) orientations to establish the micro-mechanisms of anisotropic deformation and damage in titanium for different notch severity. BA orientation showed higher apparent yield stress, greater plastic deformation, better notch strength ratio (NSR) value than PP orientation for all the cases. Electron backscatter diffraction investigation showed an increase in the number of twins per grain and the thickness of twins with notch severity irrespective of orientation with BA samples showing higher twin activity than corresponding PP samples. In addition, extensive slip activity was observed within twins and parent grains in BA orientation unlike PP orientation that showed slip activity near grain boundaries. These microstructural observations correlated well with strain measurement from digital image correlation which also showed that the local maximum longitudinal strain for BA orientation increased with notch severity unlike PP orientation. Viscoplastic self-consistent crystal plasticity simulations at the mesoscale indicated that the activity of basal, pyramidal <c+a> slip systems and twinning increased at the expense of prismatic slip with an increase in notch severity for both the orientations. The experimental observations show extensive diffuse necking in BA orientation, while higher fraction of fluted voids with c-axis aligned along the major axis of the void are observed in the PP orientation. Prismatic slip promotes nucleation and growth of voids in the prism plane which is perpendicular to the tensile axis for PP orientation contributing to void growth along c-axis forming fluted voids, while higher twinning in BA orientation leads to fewer fluted voids, thus highlighting the significant role of crystallographic texture in controlling anisotropic deformation and damage mechanisms in titanium.

Mesomechanical damage of fiber mortar under temperature difference

The present study aims to establish a mesomechanical damage model for the description of anisotropic damage by mesocracking under temperature difference condition. With the basic solution of Eshelby theory and the Clapeyron equation, a mechanics framework is developed for cement materials, which are considered to be heterogeneous materials consisting of a cement stone matrix and imbedded penny-shaped microcracks. From the mesomechanical analysis, the critical conditions corresponding to the failure of fiber mortar under different initial crack density conditions are obtained. Furthermore, the Clapeyron equation is used to describe the relationship between the stress and the initiation or expansion of the fiber mortar crack and the damage evolution rule is expressed by the energy equation of the stiffness coefficient. The proposed model can accurately quantify the factors such as fiber distribution, microcrack density, temperature differences and the relationship between directional microcracks and crack strength in mortar. The results verified by laboratory tests and numerical simulations show that as the temperature difference increases from 0 °C to 75 °C, the cracking strength of the fiber mortar decreases and the number of cracks increases in the form of a power exponential function, which proves the micromechanical damage the validity of the model.

A micromechanical damage model for oxide/oxide ceramic matrix composites with hierarchical porosity under thermomechanical loading

This work presents a micromechanical damage model to describe the microstructural damage behaviors of ceramic matrix composites with hierarchical porosity during thermomechanical loading. The microstructure evolution may cause the nonlinear constitutive behavior, and a hierarchical porosity-based elasto-plastic constitutive model was developed. Damage mechanisms of matrix-crack, hierarchical pore nucleation and fiber-breaking are incorporated into the formulation of the damage model to describe various micromechanical damage modes of ceramic matrix composites accurately. Two damage variables are proposed for the damage evolution of matrix and fiber bundles. The main damage mechanisms in the matrix are matrix-cracking, and fibers breaking in the fiber bundles. The performance of the proposed damage model is verified by comparing with the existing experimental data. The proposed damage model outperforms the existing counterparts by capturing the microstructural damage mechanism and integrated into the damage model, and the contribution of different damage mechanisms can be quantified. The present work will provide a robust tool for describing the damage behaviors of matrix and fiber bundles in the ceramic matrix composites under thermomechanical loading, as well as allow a more accurate characterization of microstructural damage for a large extent of ceramic matrix composites.