Distribution Of Microcracks Research Articles

Deep learning method for predicting the strengths of microcracked brittle materials

The strengths of brittle or quasi-brittle materials strongly depend on the interaction of distributed microcracks. Traditional micromechanics methods are difficult to exactly predict the strengths of materials containing a large number of microcracks. In this paper, a micromechanics-based deep learning method is proposed to predict the strengths of two-dimensional microcracked brittle materials. Utilizing a numerical method based on Kachanov’s theory of microcrack interaction, we generate a data set containing a large number of images of two-dimensional microcracked specimens and their load-bearing capacity under various in-plane loading. A deep neural network is formulated based on this data set to establish the implicit mapping between the load-bearing capacity of the specimens and the spatial distribution of microcracks. Numerical experiments demonstrate that the trained deep neural network can accurately and efficiently predict the load-bearing capacity of microcracked brittle materials.

Investigation of the spatial distribution pattern of 3D microcracks in single-cracked breakage

The implementation of multi-crack recognition requires a theoretical basis from single-crack failure results. This study investigates the spatial distribution pattern of microcracks in single-crack-damaged coal specimens subjected to uniaxial compression. The distribution mechanism of the induced microcracks is analysed in terms of macrocracks and stress states. Gaussian and Weibull distributions are selected to explore the spatial distribution of microcracks, and quantile–quantile plots are used to study the probability density plots. The results show that the updated data better demonstrate the distribution characteristics of the ellipsoid shape visually, and the microcracks are observed to converge near the envelope. Linear distribution features are found in the scattered points of the quantile–quantile plots. The goodness-of-fit tests indicate that both Gaussian and Weibull distributions can be assigned to describe the microcrack distribution on the new axis. The probability density distribution results show that the shape is preferred to obey Gaussian distribution , whereas Weibull distribution exhibits a greater possibility of shape variation. The study results can enhance the existing knowledge on crack spatial distribution of acoustic emission positioning results associated with rock materials. In addition, the statistical results can interpret the cracks by recognising them with a specified distribution pattern.

Crack resistance, strength and dynamic fatigue of quartz fibers with copper coatings

Metallized coatings can significantly improve the operational properties of quartz fibers. The research was conducted to determine the crack resistance, strength and dynamic fatigue of optical fibers without any coating and with copper coatings. The microhardness of quartz fibers was measured by the diamond indentation of end surfaces. The stress intensity parameter K1cwas found from the A. Niihara semi-empirical dependence. The geometry of indentation and radial cracks was studied using a scanning electron microscope. The crack resistance of uncoated quartz turned out to be almost 3 times less as compared to the copper coating fiber, which is presumably due to the additive contribution of compressive stresses on fiber surfaces and quartz wetting with copper. Copper-coated optical fiber drawing increases the tensile strength, crack resistance and dynamic fatigue parameter, and it is the main resource for maintaining operation in the conditions of a statistical approach to structural strength. Comparative tests were conducted to check the optical fiber strength by two-point bending and axial tension methods. Experimental tests conducted to check the ultimate mechanical strength of quartz optical fibers showed a significant spread of data, which indicates the presence of cracks of various sizes in a brittle solid and is a characteristic feature of brittle fracture as suggested by the A. Griffiths theory. In addition, it was assumed that the chaotic distribution of defects and microcracks extends along the entire length of a brittle solid, a quartz optical fiber in this case. A statistical model based on the Weibull distribution was used to describe surface microcracks depending on the fiber length. As a result, Weibull graphs were plotted in coordinates connecting the probability of failure with the strength, fiber length and parameter describing the ultimate strength.

Characterization of Microstructural Damage and Failure Mechanisms in C45E Structural Steel under Compressive Load

In this paper, the microstructural damage evolution of a steel with a ferrite–pearlite microstructure (C45E) was investigated during the process of cold upsetting. The development and the accumulation of microstructural damage were analyzed in different areas of samples that were deformed at different strain levels. The scanning electron microscopy (SEM) results showed that various mechanisms of nucleation of microcavities occurred during the upsetting process. In quantitative terms, microcavities were predominantly generated in pearlite colonies due to the fracture of cementite lamellae. In addition, the mechanism of decohesion had a significant influence on the development of a macroscopic crack, since a high level of microcracks, especially at higher degrees of deformation, was observed at the ferrite/pearlite or ferrite/ferrite interfaces. It was found that the distribution of microcavities along the equatorial plane of the sample was not uniform, as the density of microcavities increased with increasing strain level. The influence of stress state, i.e., stress triaxiality, on the nucleation and distribution of microcracks, was also analyzed.

Mechanical property and microstructure of cemented tailings backfill containing fly ash activated by calcium formate.

Cemented tailings backfill (CTB) is the most economical and environmental method to recycle tailings and fly ash (FA) for filling mining, but the high content of FA will weaken its strength property. This paper aims to use calcium formate (CF) as an activator to stimulate the activity of FA, thereby enhancing the mechanical property of CTB. The influence of FA and CF content on the stress-strain behavior, dilatancy deformation, and compressive strength of CTB was investigated using uniaxial compression test and scanning electron microscope. The coupling effect mechanism of FA and CF content on the compressive strength of CTB was revealed. The results show that increasing the content of FA and CF can enhance the bearing capacity of CTB during the dilatancy deformation stage, but the excessive content of FA and CF will lead to the attenuation of peak stress. The relations between FA content, CF content, and the compressive strength of CTB can be characterized by quadratic polynomial. Adding CF can stimulate the activity of insoluble FA, increasing the utilization of FA in CTB and producing rich hydration products to fill the internal defects of CTB. The microstructure of CTB is effectively improved by adding CF, including the size and distribution of microcracks and micropores, so that the strength property of CTB is optimized. However, too much CF will make the microstructure of CTB loose and porous, resulting in more microcracks and micropores. Microcracks propagate and connect with micropores to form defects, which deteriorate the microstructure of CTB, thus weakening the strength parameters of CTB. This study provides a method to increase the utilization of FA in CTB, which is of great significance for strengthening the mechanical properties of CTB and improving engineering economic benefits.

Phase stability, thermo-physical property and thermal cycling durability of Yb2O3 doped Gd2Zr2O7 novel thermal barrier coatings

Novel double-ceramic-layer (DCL) thermal barrier coatings (TBCs) composed of (Yb 0.1 Gd 0.9 ) 2 Zr 2 O 7 (YbGZO) top layer and YSZ bottom layer were deposited via EB-PVD technology. The phase stabilities and thermo-physical properties of the YbGZO bulk ceramics as well as the diffraction peak constituent, elemental content, microstructure and thermal exposure behavior of DCL thermal barrier coatings were evaluated. After calcination from room temperature up to 1573 K, the YbGZO ceramic powders do not form new phases and still maintain excellent thermal stability while the thermal diffusivity of YbGZO bulk reaches the minimum value of 0.207 ± 0.004 mm 2 /s at 1273 K. Meanwhile, the thermal conductivity of YbGZO ceramics is in the range of 0.88–1.04 W/(m⋅K), which is correspondingly lower than that of YSZ bulk material ((1.20–1.46) W/(m⋅K)) due to the fact that the ceramic top layer of YbGZO has more irregular columns distribution and a larger intercolumnar gap. After spallation failure, DCL coating is mainly composed of YbGZO with fluorite phase and Y 0.08 Zr 0.92 O 1.06 of tetragonal phase which the pyramidal morphologies on top have disappeared with an obvious densification of ceramic coatings. In comparison, the sintering behavior of YSZ coating is more serious than that of YbGZO coating. Moreover, the penetration of vertical microcracks through the whole ceramic coatings could promote the formation of oxygen diffusion channels and further accelerate the internal oxidation behavior of grain boundaries and voids within the bond coat. In addition, the irregular distribution of transverse microcracks has led to the transgranular fracture of columnar grains.

Research on the establishment of gas channeling barrier for preventing SCP caused by cyclic loading-unloading in shale gas horizontal wells

Sustained casing pressure (SCP) was an urgent problem to be solved during shale gas exploration and development. To cope with the issue of SCP, a method of establishing gas channeling barrier was proposed, and the location of the barrier was optimized, considering engineering and geological factors. A series of tests, containing uniaxial/triaxial compression tests and cyclic loading-unloading tests, were carried out to evaluate the accumulated plastic strain. Also, a full-scale cement sheath sealing integrity evaluation device was employed to verify the appearance of the micro-annulus. Computed tomography scan and nuclear magnetic resonance tests were performed to measure and analyze the distribution of microcracks and pores in the cement sheath after cyclic loading-unloadings. Numerical models of the wellbore assembly were established, which considered wellbore structures of different intervals of an actual deep shale gas well. Research findings indicated that after loading and unloading a certain number of times, the accumulation of plastic strain showed an increasing trend with the increase of the measured depth in the vertical section, but decreased with the rise of the measured depth in the horizontal section. The method of setting gas channeling barrier was proposed and could be used to avoid SCP by using the cement slurry with low elastic modulus, which could significantly reduce the cost of cementing operation. Many factors, including the non-uniform in-situ stress, wellbore structure, fracturing stage number, casing internal pressure, and formation mechanical properties were considered to establish the optimization method of barrier locations, and this method was verified by using the logging data of an actual well as well as the engineering and geological data. • An optimization method of establishing gas channeling barrier was proposed. • Loading-unloading of inner pressure leads to porosity increase of cement sheath. • First barrier should be established far away from the heel. Fig. 8 displays the gas leakage measurement results. As observed, gas leakage was detectable in the 22nd cycle at the injected gas pressure of 5 MPa. The gas channeling rate also increased as the injection pressure elevated. In order to observe which interface failed, the annulus was photographed, as presented in Fig. 9 . Obviously, micro-annulus appears at the inner surface of the cement sheath without internal cracks following repeated loading/unloading cycles. The obtained results show that the micro annulus mainly occurs in the inner interface of cement sheath.

Structural levels of process modeling the formation and wear of high-strength thermal spray coatings

Modeling of physical processes at different structural levels provides an effective solution to a number of physicochemical problems connected with production and application of thermal spray coatings. The use of structure simulation methods allows one to optimize the structure of coatings for particular service conditions. A bidirectional scale of structural levels for modeling the properties of strengthening thermal spray coatings on machine parts is considered. Moving along the scale in a positive direction, we can distinguish the following structural levels: groups of several sprayed particles that form separate microareas of a coating, extended reinforcing elements in the form of wires, rods or grid, transitional interlayer zones, zones between the coating and the substrate, individual layers obtained in one pass, thick transition zones in gradient coatings with a variable composition across the thickness, blocks of several monolayers in multilayer coatings, island elements in discrete coatings, macroelements of the block structure of coatings with controlled distribution of microcracks, individual macrostripes of the coating, continuous single-layer or multi-layer coating, structural coating. Moving from the zero level in the opposite direction, we have the following structural elements: composite particles of conglomerated grains, cladding shells, zones of different phase composition, individual domains of sprayed particles, individual elements sprayed particles and groups of grains, grains, dendrites, shear zones and slip systems, fragments of grains, packets of martensite laths, disclination cells, loops and dipoles, bands in the substructure, kink bands, microtwins, disclination clusters, dislocation pile-ups and tangles, shear bands, dislocation walls, slip systems, linear defects in the form of dislocations, ledges at grain boundary and crowdions, kinks and jogs formed due to intersection of dislocations, point defects in the real structures of solids, crystal structure, electronic structure of matter, electronic structure of individual atoms.

Failure mechanisms and damage evolution of hard rock joints under high stress: Insights from PFC2D modeling

Rock joints can be subjected to high normal stress in deep rock projects such as deep mining, tunnelling, and oil and gas exploitation. The spatial and temporal damage evolution and distribution of rock joints cannot be directly observed due to the opaque nature of rocks. Discrete element modeling provides a good alternative solution to visualize the development of micro-cracks under complex geological conditions. In this study, 2D Particle Flow Code (PFC) modelling is conducted to shed light on the failure mechanism and damage evolution of hard rock joints, especially under high stress. The influence of normal stress, initial joint roughness and grain size on the shear behavior and micro-damage are examined, and the spatial and temporal distribution of micro-cracks are also compared and quantified. Results indicate that more cracks develop for a joint sheared under a higher normal stress, and a rougher joint also tends to generate more cracks. On the other hand, the spatial distribution of shear-induced micro-cracks becomes more unevenly distributed with increasing normal stress and roughness. Three stages, namely slow increase, fast increase and slow increase can be divided on the temporal cumulative crack development curve. The micro-cracks grow faster in the fast growth stage for a rougher joint sheared under a higher normal stress, which is consistent with the acoustic emission monitoring in the laboratory direct shear tests. Both peak shear strength and damage zone size are affected by the particle size, which is closely associated with the more interlocked effect of large grains on the two sides of the shear interface. The maximum length of the crack is dependent on the magnitude of contact force, rather than the shear displacement, and the maximum contact force tends to be larger for the rougher joint.

Oxidation behaviour of C/C–SiC brake discs tested on full-scale bench rig

C/C–SiC composites are considered to be strong candidates for the new generation of high-speed train brake discs. To achieve a better application, it is necessary to improve understanding of the oxidation behaviour of C/C–SiC brake discs after a full-scale bench test rig. In this study, full-scale braking bench tests for C/C–SiC self-mated brake pairs were conducted under a braking speed of 350–420 km/h and a braking pressure of 17–28 kN. Moreover, the oxidation behaviour and mechanisms of the C/C–SiC brake discs during the practical braking process were investigated. The results indicate that the oxidation behaviour is highly dependent on the friction surface region of the C/C–SiC brake disc owing to the distribution of microcracks, the formation of friction films, the difference in temperature, and the contact content with O 2 . Specifically, the oxidation depths of the friction layer on the inner circumferential surface, middle friction surface, and outer circumferential surface were 278.3, 252.1, and 359.9 μm, respectively. Furthermore, the oxidation reaction preferentially occurs in the active area of the C fibre and pyrolytic carbon (PyC) during the braking process.

Effect of calcium formate as an accelerator on dilatancy deformation, strength and microstructure of cemented tailings backfill

Recycling mining wastes to produce cemented tailings backfill (CTB) is the optimal approach to eliminate the environmental pollution caused by their accumulation. However, its low strength limits its application. Using calcium formate (CF) as an accelerator for improving its mechanical properties is of great significance to promote sustainable development. The effects of CF dosage and curing time on dilatancy deformation, compressive strength and microstructure of CTB were investigated through mechanical compression, scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) tests. The strengthening and deterioration mechanisms of CF dosage on CTB were revealed, and its engineering practicability was systematically evaluated. The results show that the variation of volumetric strain in the dilatancy deformation stage firstly increase and then decrease with the increases of CF dosage and curing time. The relationship between CF dosage and compressive strength can be characterized by quadratic polynomial, and the optimal CF dosage characterizing the superior mechanical property of CTB is between 1.60 and 1.84. The supplement of CF reduces the size and distribution of microcracks and micropores, thereby optimizing the microstructure of CTB. Nevertheless, the excessive dosages of CF deteriorate the microstructure of CTB and produce serious defects, which cannot be effectively filled by hydration products, thus weakening the strength property of CTB. This study provides an effective accelerator for improving the mechanical properties of CTB, which is of great significance to promote the recycling of tailings.

A multi-scale prediction model of elastic modulus for ceramic matrix composites considering oxidation damage

The oxidation damage evolution law of ceramic matrix composites prepared by chemical vapor infiltration (CVI) process was analyzed. Considering the effects of temperature and oxidation time, prediction models of elastic modulus about fiber and cell were established based on the distribution of microcracks in matrix and the oxidation process of interface, fiber and matrix. The prediction results show that the tensile elastic moduli of carbon fiber (Cf)/SiC and SiC fiber (SiCf)/SiC composites decrease more obviously with the increase of temperature and oxidation time. The elastic prediction model was verified by the tensile tests of the composites after high temperature oxidation. The error between the prediction results and the test results of SiCf/SiC composites with BN interphase after oxidation at 1000 ℃ for different time is no more than 2%, and the error between the prediction results and the test results of Cf/SiC composites with PyC interphase after oxidation at 700℃ for different time is no more than 7%.

Directional hydraulic fracturing (DHF) using oriented perforations: The role of micro-crack heterogeneity

This study presents numerical investigations about the role of micro-crack heterogeneity in hydraulic fracture (HF) development and directional hydraulic fracturing (DHF) creation. By comparing the results of heterogeneous models (named DFN models) with the associated homogeneous models, the behavior is categorized into three modes: DFN models, which consume more (A), comparable (B), and much lower (C) energy than the associated homogeneous models. Variation of micro-crack distributions, which determines HFs interaction in the stress shadow zone, contributes to the difference in HF morphologies and injection energy consumption. The connection process, where one HF connects the other, predominates the energy cost in the DHF. Offset-related interactions usually occur between HFs and DFN in case of favorable distribution of micro-cracks, which guides the HFs to propagate through the interaction zone, thus favoring the connection process and improving the DHF efficiency. Adverse distribution of local cracks (even a single crack) with dilation-related interactions near the connection zone can produce extremely high stress due to the opening of the cracks, which significantly deteriorates the connection process and reduces the DHF efficiency. Therefore, a limited variation of crack distribution near the connection zone could cause a pronounced effect on HF connection and the associated energy consumption. Due to the limited sizes and shear slips of the micro-cracks in these simulations, deformation energy and fracture energy dissipation dominate the energy conversion during the injection, while frictional energy consumption plays a secondary role. The two-stage injection strategy can be a promising method to weaken the stress shadow effect, i.e., subsequent HFs extend directly to the previous one by taking advantage of the stress state produced in the first stage. The frictional resistance of micro-cracks also plays a significant role in DHF creation. Weak cracks allow the HFs to go through the interaction zone and avert the strict connection process, consequently improving the DHF efficiency.

Study of the fracturing behaviour of an electrohydraulic shock wave in sandstone under static pressure

To understand the application of electrohydraulic shock waves (EHS) in deep rock formations, the fracturing behaviour of repetitive EHS under a static pressure environment is studied in this paper. The static pressure environment of sandstone is simulated and a load model of the transmission wave of EHS at the liquid–rock interface is proposed. Analysis of the spatial distribution, dense zone and fracture zone of microcracks shows that static pressure has a significant inhibitory effect on the formation, distribution and growth of microcracks under repetitive EHS. Compared with normal pressure, cracks in a static pressure environment are mainly concentrated around the well hole, the microcracks are denser and the extension length is shorter. At the same time, the degree of damage is established to evaluate the overall cumulative damage to the sandstone. The coupling relationship between degree of damage, static pressure, discharge energy and discharge times is fitted. The fitting model shows a behaviour of continuous decelerating growth of microcracks, which can be divided into two phases, an initial growth phase and a slow-growth phase. The increase in static pressure mainly suppresses the initial growth phase, and the discharge energy needs to be increased to promote the initial degree of damage and the rate of accumulation of microcracks in the slow-growth phase. The rate of accumulation of microcracks in the slow-growth phase can be controlled through multi-level adjustment of the discharge energy. This paper provides guidance and simulation methods for rock fracturing applications of EHS in static pressure environments.

Effect of Binder Coatings on the Fracture Behavior of Polymer–Crystal Composite Particles Using the Discrete Element Method

Polymer–crystal composite particles formed by crystals coated with binders are widely used in the fields of medicine, energy, the chemical industry, and civil engineering. Binder content is an important factor in determining the mechanical behavior of composite particles. Therefore, this study aimed to investigate the underlying effect of binder coatings in the fracture micromechanics of polymer–crystal composite particles using the discrete element method (DEM). To achieve this objective, realistic particle and crystal shapes were first obtained and reconstructed based on X-ray micro-computed tomography (μCT) scanning and scanning electron microscope (SEM) images. A series of single particle crushing tests and DEM simulations were conducted on real and reconstructed polymer–crystal composite particles, respectively. Based on the experimental and DEM results, the effect of binder coatings on the crushing strength and crushing patterns of polymer–crystal composite particles was measured. Moreover, the micromechanics of the development and distribution of microcracks was further investigated to reveal the mechanism by which binder coatings affect polymer–crystal composite particles.

Elastic anisotropic behaviour related to crack distributions in two gneisses

Stress-strain response of foliated rocks shows that mechanical behaviour and degree of anisotropy are influenced by the spatial arrangement of phyllosilicates. But the anisotropy of these rocks is essentially due to the characteristics and distribution of cracks aligned with mica beds. At the laboratory scale, the elastic symmetry can be represented by the transverse isotropy, with the lowest elastic modulus perpendicular to the plane of aligned microcracks. These issues are discussed with reference to experimental data obtained for two gneisses of the same geological formation. The gneisses show a quite similar strength behaviour, but very different deformabilities. The measures of dynamic and static deformabilities under loading prove the influence of the progressive closure of open cracks on the compliance tensors of both gneisses. The relationship between the elastic parameters and the characteristics of the crack distributions is discussed in the framework of non-interacting crack models. Assuming the presence of two different sets of cracks, crack densities have been estimated. The different deformabilities of the two gneisses can be ascribed to their different microcrack distributions. The degree of anisotropy due to cracks reduces as stresses increase, differently for the two gneisses.

Experimental and numerical simulation study of crack coalescence modes and microcrack propagation law of fissured sandstone under uniaxial compression

Fissured rock mass is the most common construction object in geotechnical engineering and its mechanical properties are closely related to the stability of geotechnical engineering. A laboratory uniaxial compression test and a numerical simulation test were separately carried out on the fissured sandstone samples. Based on parameters, such as crack coalescence modes, the ratio of rise time to amplitude (RA) and the ratio of acoustic emission (AE) counts to duration (AF) of AE signals and spatial distribution of microcracks, failure modes and microcrack propagation laws of fissured sandstone were studied. The results demonstrate that the failure mode of fissured sandstone changes from being dominated by tensile failure into by shear failure with the increase of the dip angle of prefabricated fissures. The results of RA and AF of AE signals illustrate that it is difficult to produce tensile microcracks, but easy to generate shear microcracks in the samples with the rise of the dip angle of prefabricated fissures. In the numerical simulation test, when the dip angle of prefabricated fissures is small (θ ≤ 60°), microcracks initiate near the tip of prefabricated fissures. However, if the dip angle is large (θ ≥ 75°), the initiation location of microcrack is slightly affected by prefabricated fissures. In addition, the method for determining tensile and shear cracks was proposed based on the velocity field of particles. The results show that as the dip angle of prefabricated fissures increases, there are increasingly less tensile macrocracks, but increasingly more shear cracks at failure of the samples, which is basically consistent with conclusions obtained in the laboratory test of rock mechanics.

Fracture of Porous Ceramics: Application to the Mechanical Degradation of Solid Oxide Cell During Redox Cycling

Solid Oxide Cell (SOC) is a high-temperature electrochemical device that can be operated in both fuel and electrolysis modes. It is constituted of a dense electrolyte in Yttria Stabilized Zirconia (YSZ) sandwiched between two porous electrodes. The hydrogen electrode is classically made of a cermet composed of Nickel and YSZ (Ni-YSZ). Because of system failures in operation (e.g. a fuel shortage or air re-introduction during the system shutdown), the Nickel is liable to be re-oxidized inducing its volume expansion. The Ni swelling within the cermet leads to a mechanical damage in the YSZ backbone [1] resulting in a degradation of the overall cell performances [2]. Therefore, it is still required to enhance the hydrogen electrode robustness for a higher redox tolerance. For this purpose, it is essential to predict accurately the formation and distribution of micro-cracks in the YSZ network during the re-oxidation. Nevertheless, the modeling of the initiation and propagation of cracks in complex microstructures of porous ceramics is still a subject of investigation. Moreover, there is also a lack of data on fracture properties for the porous YSZ material. In this frame, the present study was focused to propose a relevant model for the initiation and the growth of cracks in porous ceramics in order to simulate the fracture induced by Ni re-oxidation. For this purpose, a coupled experimental and modeling approach has been adopted (Fig.1). For the experimental part, the mechanical characterizations have been performed on thin YSZ membranes of different porosities representative of SOC microstructures. The compression test has been chosen as it generates a quasi-uniform stress field in the tested samples. To this end, micro-pillars of few tens of micro-meters in section have been milled in the YSZ membrane using a xenon plasma focused ion beam. For each investigated porosity, the compressive fracture strength has been measured on several pillars. The tested samples have been carefully characterized to assess the damage in the microstructure. As expected, the compressive strength was found to decrease with increasing the porosity. Moreover, a transition has been observed from a brittle behaviour at low porosity towards a local damage at high porosity [3]. All these experimental data have been used to validate a numerical tool to simulate the fracture in porous microstructure. In this study, a model based on the Phase Field Method (PFM) has been developed. In this approach, the crack is modeled implicitly through a smooth scalar damage variable and its width is controlled by a length scale parameter. Therefore, this method is well adapted to solve the fracture problem in complex and heterogeneous materials since it is mesh independent. The implemented model corresponds to the one proposed by C. Miehe et al. [4] who proposed a staggered scheme for the numerical resolution. A special attention has been paid in this work to evaluate the model capability to accurately predict the crack initiation for porous electrode material. For this purpose, the results of the PFM simulations have been compared to the mixed criterion proposed by D. Leguillon [5]. The study has been conducted on notched samples with different opening angles and for macro-cracks blunted by different cavity sizes. It has been highlighted that the two approaches provide consistent predictions for the crack initiation. Moreover, it has been checked that the PFM model is able to reproduce accurately the expected decrease of the fracture toughness when increasing the material porosity. Once these studies completed, the PFM model has been used to simulate the micro-compression test using a multiscale approach. It has been found that the model is able to reproduce correctly the decrease of the measured compressive fracture strength with the sample porosity. Moreover, the distribution of cracks in the simulations has enabled analysing the transition from a brittle to a local damage detected during the experiments. The validated model has been applied to simulate the fracture induced by the Ni re-oxidation. The repartition and density of micro-cracks in the YSZ backbone has been computed during the Ni reoxidation and the results have been confronted with an available tomographic data set. Finally, the impact on the cermet redox tolerance of the Yttrium content in YSZ has been discussed thanks to the modelling. [1] Faes, A. et al. J. Power Sources 193, 55–64. [2] Laurencin, J. et al. J. Power Sources 192, 344–352. [3] Meille, S. Et al . J. Eur. Ceram. Soc. 32, 3959–3967. [4] Miehe, C. et al. Comput. Methods Appl. Mech. Eng. 199, 2765–2778. [5] Leguillon, D. J. Mech. – A/Solids 21, 61–72. Figure 1

DEM investigation on the mechanical behavior of mudstone in the hollow cylinder torsional shear test

The discrete element method (DEM) has been used to investigate the mechanical and failure behavior and the associated micro-cracking process of mudstone under the hollow cylinder torsional test by using a commercial DEM code PFC3D. Micro-parameters of the numerical model were first calibrated to the experimental results of the uniaxial compression tests and the uniaxial tensile tests. Based on the calibration results, numerical hollow cylindrical torsional shear tests were carried out under three typical stress conditions, i.e. triaxial compression, torsional compression and pure torsional shear. The numerical results show that the mechanical behavior of mudstone, in terms of rock strength, micro-crack initiation and propagation, and failure mode, can be significantly affected by principal stress rotation, which is in line with previous experimental results. The analysis of the micro-crack process confirms that the principal stress rotation plays an important role in the initiation and propagation of micro-cracks. It is shown that the microcracks induced by tensile stress occur in a preferential direction parallel to the major principal stress σ1, while the orientation distribution of the shear micro-cracks is related to the shear bands. Furthermore, the initial micro-crack stress is found to decrease after introducing the principal stress rotation, which provides a new interpretation of degradation of strength and permeability of mudstone under principal stress rotation. Three typical failure modes of specimens in different loading states are reproduced in this study, and the microscopic mechanism is also discussed. Based on this study, it is found that ignoring the principal stress rotation may overrate the stability and tightness of underground storage in bedded rock salt caverns.

Effect of Micro Solidification Crack on Mechanical Performance of Remote Laser Welded AA6063-T6 Fillet Lap Joint in Automotive Battery Tray Construction

Remote laser welding (RLW) has shown a number of benefits of joining 6xxx aluminium alloys such as high processing speed and process flexibility. However, the crack susceptibility of 6xxx aluminium alloys during RLW process is still an open problem. This paper experimentally assesses the impact of transverse micro cracks on joint strength and fatigue durability in remote laser welding of extruded AA6063-T6 fillet lap joints. Distribution and morphology of transverse micro cracks were acquired by scanning electron microscope (SEM) on cross-sections. Grain morphology in the weld zone was determined by electron backscatter diffraction (EBSD) while static tensile and dynamic fatigue tests were carried out to evaluate weld mechanical performance. Results revealed that increasing welding speed from 2 m/min to 6 m/min did not introduce additional transverse micro cracks. Additionally, welding at 2 m/min resulted in tensile strength improvement by 30% compared to 6 m/min due to the expansion of fusion zone, measured by the throat thickness, and refinement of columnar grains near fusion lines. Furthermore, the weld fatigue durability is significantly higher when fracture occurs in weld root instead of fusion zone. This can be achieved by increasing weld root angle with optimum weld fatigue durability at around 55°.