Shear Damage Mechanism Research Articles

Shear damage mechanisms of jointed rock mass: a macroscopic and mesoscopic study.

The joints are existing throughout the underground rock mass. It is of great significance to investigate the shear performance of the rock mass to maintain the stability of the underground structure. In this study, we conducted orthogonal tests to determine the proportion of rock-like materials, and used JRC curves to make specimen molds and then prepare the specimens. We conducted straight shear tests and uniaxial compression tests to determine the various mechanical parameters of the rock-like materials. Next, we carried out the compression and shear tests to investigate the shear characteristics of the specimens, and study the damage pattern and shear strength of the jointed rock mass under different confining pressures and roughness levels. The mesoscopic displacements in the shear process of joints were analyzed by using ABAQUS. The test results show that the effect of the confining pressure on the shear strength of the joint plane is relatively obvious, and a larger confining pressure indicates a larger shear strength. The effects of different joint plane roughness and shear rated on the shear characteristics of the joint plane are also significant. The mesoscopic displacement difference inside the joint plane with higher roughness is relatively large, and the stress concentration phenomenon is obvious and lasts longer, which leads to the faster destruction of the specimen with higher roughness and the higher destruction degree. Therefore, we suggest that the priority should be given to the reinforcement of jointed rock mass with high roughness during the construction to prevent sudden destabilization and failure.

Failure mode analysis and prediction model of additively manufactured continuous carbon fiber‐reinforced polylactic acid

AbstractCarbon fiber‐reinforced composites are widely used in aerospace, rail transportation, and new energy vehicles due to their small specific gravity as well as good mechanical and chemical properties. Additive manufacturing of continuous carbon fiber‐reinforced polymer composites is an innovative composite manufacturing technique. In this paper, bending and shear specimens with different hatch spacing and layer thicknesses were prepared by fused filament fabrication (FFF) technique and subjected to three‐point bending experiments and interlayer shear experiments, respectively. The results showed that the maximum flexural strength and flexural modulus of the fabricated parts were 383.81 MPa and 41.23 GPa, respectively, and the maximum interlayer shear strength was 13.64 MPa. Finally, an optimized criterion for determining fiber kinking, fiber breakage, and matrix damage was proposed by considering in situ strength variations induced by process parameters. This new model was subsequently validated and could provide a method for fracture predicting of additive manufactured carbon fiber‐reinforced composites.Highlights CCFRC parts with different porosities were prepared by adjusting the layer thickness and hatch spacing. The bending and interlaminar shear damage mechanisms of CCFRC fabrications are revealed, and the effects of layer thickness and hatch spacing on their mechanical properties are investigated. Introduction of additive manufacturing factors to optimize failure models for CCFRC parts. Introduction of additive manufacturing coefficients to accurately predict the interlaminar shear strength of CCFRC parts by AM.

Study on the behavior of small cracks in PA38-T6 (6060-T6) aluminum alloy under multiaxial fatigue loadings

This study investigated the effects of multiaxial cyclic loading on the initiation and propagation of fatigue cracks in an industrial-grade aluminum alloy. Four different loading cases, including 90° out-of-phase and asynchronous loading, were used under the control of strain. The study used cellulose acetate film replication and SEM fractography techniques to increase the understanding of the evolution of fatigue damage on specimen surfaces. The material exhibited a shear damage mechanism with a significant amount of small cracks coalescence. It was explained why, despite a high agreement with the assumptions of the critical plane approach, the predicted fatigue life was overestimated at high loading levels. A correlation between the crack initiation period and overall fatigue life, regardless of loading case, was presented, which has a potential for practical applications.

A shear modified GTN model based on stress degradation method for predicting ductile fracture

The Gurson–Tvergaard–Needleman (GTN) model has provided a powerful description of the nucleation growth and coalescence of micro voids, but it has limitations in simulating shear fracture due to the absence of a description of shear localization behavior. A shear improvement method is proposed for simulating the ductile fracture of materials under different stress states. The modified model not only allows for strain hardening of the matrix material, but also accounts for the stress degradation caused by shear. The strength equation of the material is described by both the shear stress state function and a decay function, making it easier for materials under shear stress state to experience material softening and further inducing shear fracture. The modified GTN model is developed by incorporating the shear stress degradation factor into the yield function, while taking into account both void growth and shear failure mechanisms. By carefully calibrating the model’s parameters, the deformation and fracture processes of tensile, plane strain, notch tensile, and compression specimens in the 7A52 aluminum alloy are simulated. The damage evolution behavior of the material under different stress states is analyzed. The results indicate that the damage include void growth mechanism and void shear mechanism. The proportions of these two mechanisms vary under different levels of stress triaxiality. Upon localizing material deformation, the shear stress state intensifies, and the shear damage mechanism assumes a critical role in fracture. The modified GTN model accurately predicts the load-displacement response and fracture path of the 7A52 aluminum alloy under a wide range of stress states.

Spatio-Temporal Distribution of the Sources of Acoustic Events in Notched Fiber-Reinforced Concrete Beams under Three-Point Bending.

The acoustic activity, generated in notched, beam-shaped concrete specimens, loaded under three-point bending, is studied in terms of the position of the sources of acoustic events, and the frequency of their generation. Both plain specimens (without any internal reinforcement) and specimens reinforced with various types of short fibers were tested. The target of the study is to investigate the existence of indices that could be considered as pre-failure indicators of the upcoming fracture. In addition, an attempt is undertaken to classify the damage mechanisms activated to tensile or shear nature. Considering comparatively the spatio-temporal evolution of the position of the acoustic sources and the respective temporal evolution of the frequency of generation of acoustic events, it was concluded that for relatively low load levels the acoustic sources are rather randomly distributed all over the volume of the specimens. As the load increases toward its maximum value, the acoustic sources tend to accumulate in the immediate vicinity of the crown of the notch and the average distance between them approaches a minimum value. When this minimum value is attained, the load is maximized and the generation frequency of the acoustic events increases rapidly. The simultaneous fulfillment of these three conditions is observed a few seconds before the onset of propagation of the catastrophic macrocrack for all classes of specimens tested, providing a kind of warning signal about the upcoming fracture. Moreover, the classification of the damage mechanisms to tensile and shear ones revealed a crucial difference between the plain and the reinforced specimens after the maximization of the load applied. Indeed, while for the plain specimens, the prevailing damage mechanism is tensile microcracking, for the reinforced specimens a balance between tensile and shear damage mechanisms is observed after the load applied has attained its peak and starts decreasing.

Discrete Element Simulation on Macro-Meso Mechanical Characteristics of Natural Gas Hydrate-Bearing Sediments under Shearing

In order to study the macro-meso shear mechanical characteristics of natural gas hydrate-bearing sediments, the direct shear simulations of natural gas hydrate-bearing sediment specimens with different saturations under different normal stress boundary conditions were carried out using the discrete element simulation program of particle flow, and the macro-meso shear mechanical characteristics of the specimens and their evolution laws were obtained, and their shear damage mechanisms were revealed. The results show that the peak intensity of natural gas hydrate-bearing sediments increases with the increase in normal stress and hydrate saturation. Hydrate particles and sand particles jointly participate in the formation and evolution of the force chain, and sand particles account for the majority of the force chain particles and take the main shear resistance role. The number of cracks produced by shear increases with hydrate saturation and normal stress. The average porosity in the shear zone shows an evolutionary pattern of decreasing and then increasing during the shear process.

Deformation mechanisms and fracture in tension under cyclic bending plus compression, single point and double-sided incremental sheet forming processes

This study investigates the deformation and fracture mechanisms of two testing methods, tension under cyclic bending (TCB) and tension under cyclic bending plus compression (TCBC) and their relationship to single point (SPIF) and double-sided (DSIF) incremental sheet forming processes. Experimental tests were carried out by using a bespoke TCBC test rig and a DSIF machine with grade 1 pure Ti samples. The results show the elongation-to-fracture has a high relevance to the bending depth and compression, which leads to detailed investigation to the stress and strain evolutions in the local bending region using finite element (FE) method. A new Gurson-Tvergaard-Needleman (GTN) model is proposed with a modified shear damage mechanism utilising experimental fracture strain loci to calibrate the Lode angle effect under low stress triaxiality. It is found the bending and reverse-bending stages correspond to different stress states and significantly affect the fracture occurrence in TCB, TCBC and SPIF, DSIF processes. For the first time, the stress paths in the plane of stress triaxiality and Lode parameter are used to reveal the transition of deformation modes from equi-biaxial to plane strain tension in SPIF and DSIF, as compared to the plane stress tension in TCB and TCBC. Using the new GTN model, the simulation gives accurate predictions to the elongation-to-fracture in TCB and TCBC, and the fracture depth in SPIF and DSIF with an error of less than 8% in comparison to the experimental results. Although there is a distinction between the equi-biaxial and uniaxial tension deformations, the study concludes that the TCB and TCBC tests provide an insight into the formability improvement and represent intrinsic deformation mechanisms of SPIF and DSIF processes, an ongoing research question, which has drawn considerable attention in recent years.

Stochastic fatigue damage model for concrete under complex stress states

This paper presents a stochastic fatigue damage model for concrete subjected to complex stress states. A constitutive framework considering the tensile and shear damage mechanisms and elastic and plastic energy dissipations is employed. The damage law is established through a multiscale analysis of the damage evolution mechanism. Validation results against test data show that the model can effectively predict the static strength, stress-strain curve, fatigue life, and scattering range of concrete under various multidimensional loading scenarios.

Vehicular impacts on precast concrete bridge piers with grouted sleeve connections

The grouted sleeve connection has been demonstrated to be robust enough to maintain its integrity under earthquake loads. However, no research to date has examined the connection behaviour under direct shear, resulting in a high risk of failure of the pier subject to a vehicle collision. In this regard, this research first carried out static and impact loading tests on grouted sleeve specimens to characterize their direct shear damage mechanisms. Next, a simplified finite element (FE) modelling method was proposed to simulate connection behaviour, and its accuracy was verified using the test results. Based on the proposed connection modelling approach, truck-bridge collision analysis was performed on a prototype highway bridge that uses grouted sleeves in the pier. From the analysis results, increasing the number of column segments can lead to higher stiffness of the pier when resisting the vehicular impact. The friction property at the connection interface plays an essential role in avoiding unacceptably large rebar shear stress due to excessive sliding and developing the diagonal compression strut to resist the impact load. Finally, feasible retrofit techniques were explored for the precast concrete piers that showed inadequate impact resistance.

Numerical Study about the Influence of Superimposed Hydrostatic Pressure on Shear Damage Mechanism in Sheet Metals

It is generally accepted that the superimposed hydrostatic pressure increases fracture strain in sheet metal and mode of fracture changes with applying pressure. Void growth is delayed or completely eliminated under pressure and the shear damage mechanism becomes the dominant mode of fracture. In this study, the effect of superimposed hydrostatic pressure on the ductility of sheet metal under tension is investigated using the finite element (FE) method employing the modified Gurson–Tvergaard–Needleman (GTN) model. The shear damage mechanism is considered as an increment in the total void volume fraction and the model is implemented using the VUMAT subroutine in the ABAQUS/Explicit. It is shown that ductility and fracture strain increase significantly by imposing hydrostatic pressure as it suppresses the damage mechanisms of microvoid growth and shear damage. When hydrostatic pressure is applied, it is observed that although the shear damage mechanism is delayed, the shear damage mechanism is dominant over the growth of microvoids. These numerical findings are consistent with those experimental results published in the previous studies about the effect of superimposed hydrostatic pressure on fracture strain. The numerical results clearly show that the dominant mode of failure changes from microvoid growth to shear damage under pressure. Numerical studies in the literature explain the effect of pressure on fracture strain using the conventional GTN model available in the ABAQUS material behavior library when the mode of fracture does not change. However, in this study, the shear modified GTN model is used to understand the effect of pressure on the shear damage mechanism as one of the individual void volume fraction increments and change in mode of fracture is explained numerically.

Fatigue of four metallic materials under asynchronous loadings: Small cracks observation and fatigue life prediction

This paper is a continuation of previously published work. It presents the results of small cracks observation on the surfaces of fatigued specimens subjected to multiaxial loadings. Specimens were manufactured from four materials: PA38-T6 aluminum alloy, E235 and E355 non-alloy steels, and X5CrNi18-10 austenitic steel. Based on the cracking analysis, it was found that the aluminum alloy and low-alloy steels exhibited shear damage mechanism, while the austenitic steel showed the tensile damage mechanism. Fatigue life prediction was performed based on six popular damage parameters. A correlation between the observed damage mechanism and the fatigue life prediction quality was noted.

Shear Damage Mechanism and Early Warning Study of Shale Standard Layer Casing

Aiming at the problem of shear-induced casing damage caused by standard layer shale soaking water, the theory of rock mechanics is used to establish the fluid-solid coupling calculation model of the formation-cement ring-casing of the standard interval shale interbed, obtaining the strain and deformation changes of casing shear damage, and puts forward the casing logarithmic strain exacerbated area should prompt adoption of reasonable measures to slow down the casing damage. Three-level orthogonal numerical simulation experiments were carried out for the five factors affecting casing damage, the simulation results show the critical strain data of the casing damage and the true strain data of the casing under the influence of five factors. According to the statistical principle, the predictive mathematical model of the critical strain and true strain prediction of the casing is obtained by the multiple regression method, realize the prediction of the strain state of the casing under actual working conditions is realized, and then realize casing damage early warning.

Identification of fatigue damage mechanism in PA38-T6 aluminum alloy under multiaxial loadings – initial research

This paper presents results of the initial research which originated from previous work, which was aimed to study the effect of asynchronous loadings in elastic-plastic strain range. Among the others, a study of fatigue cracks on the fatigued specimens’ surfaces, was performed.The aim of the present work is to identify the mechanisms of initiation and propagation of small fatigue cracks in selected materials under multiaxial loadings, including asynchronous cases. To study the evolution of small cracks and main crack formation, the cellulose acetate thin foil replication technique was utilized. Fatigue tests were performed on thin-walled tubular specimens, which were mirror-polished after CNC machining, to avoid the influence of unwanted factors.This paper presents the first part of fatigue tests, performed on specimens manufactured from PA38-T6 (AW 6060-T6) aluminum alloy. Fully reversed axial, torsional and 90º out-of-phase loadings were applied in elastic-plastic strain regime, with strain control. For these loading cases a shear damage mechanism was identified, based on the observation of small cracks. The small cracks initiated ate grew on maximum shear strain planes. The main crack formed at the very end of fatigue life by coalescence of small cracks of high density, regardless the applied loading case and level. An interesting difference in cracking behavior was observed in case of out-of-phase loading, on the low loading level. Small cracks propagated, and the main crack formed by linking of a few propagating cracks.The research is in progress and will be extended to the lower levels of strain as well as other cases of loadings, including asynchronous loadings. It is planned to conduct the research using other materials, for example non-alloy steel and austenitic stainless steel.

Effects of z-pin’s porosity on shear properties of 2D C/SiC z-pinned joint

In this paper, 2D C/SiC z-pinned joint is prepared via chemical vapor infiltration. Effects of z-pin’s porosity on shear properties of z-pinned joint are investigated from the failure mode and the shear damage mechanisms. The results show that the average joint shear strength decreases asymptotically from 249.9MPa to 97.2MPa as the total porosity of 2D C/SiC z-pin increases from 14.4% to 27.3%. For each porosity, the joint shear strength is between the tensile strength and the in-plane shear strength of 2D C/SiC, which is rationalized by the rigid body sliding model of 2D C/SiC. Because high porosity leads to large rotational deformation of transverse fibers, a critical porosity about 17.7% is observed from shear-controlled failure to bending-controlled failure. A cohesive fracture model is developed to build the relation between the joint shear strength and the mechanical properties of 2D C/SiC. Very good accuracy is obtained.

Stochastic damage hysteretic model for concrete based on micromechanical approach

In the present paper, a kind of stochastic damage hysteretic model is proposed to describe the damage and hysteretic behaviors of concrete material. According to the model, a parallel system made up of micro-elements, which are developed based on the micro-tensile and shear damage mechanism respectively, is adopted to obtain the overall responses of concrete. The influence of plastic strain and hysteretic energy dissipation of the material are also considered in the model. To reflect the stochastic properties of concrete, the fracture strain of the micro-element is set as a random variable. Then the monotonic, loading and unloading curves of the parallel system are derived analytically by averaging the stochastic micro-elements and two hysteretic rules are combined to the proposed model to account for complicated loading conditions. Furthermore, a nonlocal process is introduced to the model to overcome the mesh dependence issues of softening materials. Finally, several numerical examples are conducted, demonstrating that the proposed model can provide reliable results reflecting the damage, plasticity and hysteretic behaviors of concrete material.

Effects of porosity on in-plane and interlaminar shear strengths of two-dimensional carbon fiber reinforced silicon carbide composites

Porosity effects on shear properties of 2D C/SiC were investigated based on the corresponding shear damage mechanisms. The results show that as the total porosity increases from 12.2% to 26.1%, the interlaminar shear strength decreases from 44.58MPa to 17.80MPa according to a power law, while the in-plane shear strength decreases linearly from 143.25MPa to 74.38MPa. Under interlaminar shear stress, delamination is resulted from matrix cracking and interface debonding/sliding mechanisms. Effects of porosity on interlaminar shear strength is controlled by the volume fraction of the delaminated matrix. Under in-plane shear stress, echelon matrix shear cracking and large scale fiber bridging mechanisms occur. The decreasing of in-plane shear strength depends on the spacing between the echelon cracks. Since the interface debonding/sliding mechanism occurs under both in-plane and interlaminar shear stresses, a relationship is obtained that the in-plane shear strength equals the sum of the interlaminar shear strength and the fiber bridging term under the minimum total porosity about 30.5%.

Oxidation effects on in-plane and interlaminar shear strengths of two-dimensional carbon fiber reinforced silicon carbide composites

Effects of oxidation on in-plane shear strength and interlaminar shear strength of two-dimensional carbon fiber reinforced silicon carbide composites (2D C/SiC) were studied based on the corresponding shear damage mechanisms. The results showed that oxidation had resulted in severe degradations of the two strengths, however, it had little influence on the shear failure mechanisms. Under in-plane shear loading, large-scale fiber bridging mechanism controlled the shear behaviors. A modified rigid body sliding model was proposed to characterize the in-plane shear strength. In Comparison, interface sliding mechanism controlled the interlaminar shear strength. Based on above failure mechanisms, the two shear strengths were quantitatively characterized by the constituent properties, such as diameter of carbon fibers, interface sliding stress, matrix fracture energy and matrix cracking spacing. Therefore, oxidation effects on them were attributed to the decreased constituent properties which mainly results from oxidation consumption of the carbon phases. Finally, for the conditions of this study, a relationship was proposed that the interlaminar shear strength equals the in-plane shear strength minus the fiber bridging stress.

Analysis of edge crack behavior of steel sheet in multi-pass cold rolling based on a shear modified GTN damage model

Edge cracks are commonly observed in steel strip in cold rolling, which may cause deterioration of quality and productivity of products. The edge defect is one of the important factors that lead to edge cracks in cold rolling. In this paper, the GTN damage model coupled with shear damage mechanism proposed by Nahshon and Hutchinson (2008) was employed to analyze the ductile damage and failure behavior of steel sheet with edge defects under multi-pass cold rolling. Cold rolling experiments of silicon steel with edge notches were performed on an experimental rolling mill to verify the accuracy of the numerical results. The simulation results show that two cracks appeared in the notch tip after five passes rolling and extended toward the direction with an angle around 45° and 135° to the rolling direction, which was in good agreement with the experimental data. The influences of notch shape and size on the onset of cracks around the notch tip are investigated and some significant conclusions are drawn.

Prediction of Edge Crack in Cold Rolling of Silicon Steel Strip Based on an Extended Gurson–Tvergaard–Needleman Damage Model

Edge cracking is commonly observed in cold rolling process. However, its failure mechanism is far from fully understanding due to the complex stresses and plastic flow conditions of steel strip under the rolling condition. In this paper, an extended Gurson–Tvergaard–Needleman (GTN) damage model coupled with Nahshon–Hutchinson shear damage mechanism was introduced to investigate the damage and fracture behavior of steel strip in cold rolling. The results show that extended GTN damage model is efficient in predicting the occurrence of edge crack in cold rolling, and the prediction is more accurate than that of the original GTN damage model. The edge cracking behavior under various cold rolling process parameters is investigated. It comes to the conclusion that edge crack extension increases with the increase of the reduction ratio, tension and the decrease of the roller radius and friction coefficient. The influence of shear damage becomes more significant in rolling condition with a larger reduction ratio, smaller roller radius, lower friction force, and tension.

On the Utilization of Shear Modified GTN Damage Model in Cold Rolling

Edge cracking is a commonly observed phenomenon in cold rolling process, but researchers appear to be far from fully understanding its failure mechanism due to the complex stress conditions of steel strip under the rolling condition. In this research, the shear modified GTN damage model coupled with Nahshon-Hutchinson shear damage mechanism was applied to investigate the damage and fracture behavior of steel strip in cold rolling. The results show that the shear modified GTN damage model is competent to predict the damage and fracture behavior of steel strip in cold rolling. By comparison to the cold rolling experiment, it presents that the prediction of edge crack occurrence of the shear modified GTN damage model is more accurate than that of the original GTN damage model.