Shear Fracture Process Research Articles

Mechanism of chip segmentation transition from shear slip to shear fracture of Zirconium-based bulk metallic glass in mechanical machining

An in-depth understanding of material cutting deformation forms the foundation for manufacturing Zirconium-based bulk metallic glass (Zr-based BMG) parts. This study explores the transition in chip segmentation mode from shear slip to shear fracture in Zr-based BMG during mechanical machining. The investigation covers chip micromorphology, cyclic cutting force, machined surface quality, and the themo-mechanical properties under various chip segmentation modes linked to cutting conditions. A size-dependent transition in chip segmentation has been observed in the cutting of Zr-based BMG. At a specific cutting speed, a smaller uncut chip thickness favors chip formation via shear slip, resulting in a smooth primary shear zone (PSZ). Conversely, a larger uncut chip thickness leads to chip shear fracture, with the PSZ displaying fishbone-like and dimple patterns. The integration of the modified cutting kinematic model with the free volume-dependent shear transformation zone (STZ) model suggests that the transition from shear slip to fracture is due to an increase in uncut chip thickness, which causes a greater STZ volume in the PSZ. When the STZ volume surpasses a threshold, the intrinsic structural recovery of material from shearing cannot fully compensate for the free volume softening, leading to excessive free volume that ultimately triggers shear fracture. During the chip shear fracture process, the evolution from fishbone-like patterns to dimples in the PSZ is attributed to faster crack propagation at greater uncut chip thickness or higher cutting speeds. In practical applications, it is advisable to avoid chip shear fracture during machining to maintain high-quality machined surface quality.

Shear response and fracture processes of hybrid steel fiber reinforced concrete

Ten hybrid steel fiber reinforced concrete mixtures are designed to analyze the influence of the hybridization of three different steel fibers (long hooked-end, medium hooked-end, short wave) on the shear performance by Z push-off shear tests. The presence of steel fibers improves the shear strength and toughness of the concrete, which increase with the volume fraction of hybrid fibers. The hybrid steel fiber reinforcement affects the fracture processes. Micro cracks propagation, cracks onset, cracks propagation and cracks post-peak propagation are the four main fracture stages. The volume fraction above 1.0% changes the evolution trend of fracture process zone length. The hybridization of the long hooked-end steel fiber with the other steel fibers produces a synergistic effect on inhibiting the propagation of the fracture process zone (FPZ) compared with the long hooked-end steel fiber reinforced concrete. The hooked-end steel fiber produces a better effect on slowing down the damage degree factor than short wave shaped steel fiber. The damage degree factor at the same loading level is reduced with the increase of the volume fraction of hybrid steel fiber.

Phase field study on fracture behavior of crushable polymer foam

Polymer foam is a widely used material across various industries for its lightweight and exceptional energy absorption capability. Nevertheless, its porous structure and compaction properties present challenges in simulating crack initiation and propagation. The phase field method has attracted much attention for its advantages in predicting cracks. In this paper, we combine the phase field method with the “crushable foam” model and introduce the modified Drucker-Prager failure criterion to establish the phase field model of polymer foam materials. Material parameters are calibrated using experimental data from uniaxial compression, uniaxial tensile and shear punching experiments. The typical tensile, shear and compression instability fracture processes of polyurethane are then simulated using the proposed model, and the numerical results agree well with the experimental results. This indicates that the coupled phase-field crushable foam model not only exhibits the compaction properties of polymer foam materials but also accurately predicts its crack propagation under complex loading. Studying the fracture behavior of polymer foams under complex loads can enhance their engineering applications.

Hybrid phase-field modeling of mesoscopic failure in concrete combined with Fourier-Voronoi stochastic aggregate distribution modelling approach

Concrete crack propagation is significantly affected by its complex mesoscopic structure, tracking which is of great importance for revealing the fracture failure modes and crack propagation patterns of concrete. This study presents a stochastic aggregate distribution modeling approach combining the Fourier-Voronoi method and techniques with XCT images. it enables the generation of stochastic aggregate distribution models that mimic real concrete's morphological characteristics. Meanwhile, with the hybrid phase-field theory and finite element method, a multi-field coupling model is established to simulate the crack propagation. In contrast to the isotropic or anisotropic phase-field theory, with the hybrid phase-field theory the stiffness degradation of concrete material in all directions during crack propagation are taken into consideration. With the proposed methodology, the effects of volume fraction, shape, roughness and pre-notch depth on the fracture behavior and strength of concrete are investigated. Results show that both the tensile and shear strength of concrete grows with the increase of aggregate volume fraction. Benefited from the application of hybrid phase-field theory, two shear fracture failure modes of concrete with distinct shear strengths are revealed, influenced by aggregate interaction during the shear fracture process.

Finite Element Simulations of Dynamic Shear Fracture of Hollow Shear Pins

The shear pin structure is widely used in aeronautics and astronautics structures to deal with emergency structure separation problems. The shear pin design has a strict restriction on the precise failure load and definite failure mode. Previous research has conducted shear fracture tests and simulations of solid shear pins while there is a lack of detailed research on the shear fracture of hollow shear pins with large diameters. In this research, a 3-dimensional finite element model was built based on the actual shear pin installed on the aircraft engine pylon and the model was validated by the experiment. The influences of the inner diameter of hollow shear pins on the shear fracture process were investigated by conducting finite element simulations. The structural deformation, energy dissipation in the fracture process, and failure load of shear pins were evaluated. It is found that as the inner diameter increases, the failure mode of shear pins changed and would result in difficulties on the structure separation. To solve this problem, a new configuration of hollow shear pin was proposed for the purpose of obtaining both desired failure load and failure mode. The new configuration was verified by the fracture simulation and it is found that the new configuration is effective and can be used to improve the shear fracture performance.

Shear fracture performance of the interface between ultra-high toughness cementitious composites and reactive powder concrete

This study used unnotched compression tests and digital image correlation technology to analyze the influence of two different casting intervals and three different roughness conditions on the fracture toughness of the interfaces between ultra-high toughness cementitious composite (UHTCC) and reactive powder concrete (RPC). UHTCC exhibits pseudo-strain-hardening responses and multi-cracking behaviors, and the most notable characteristic of RPC is its dense microstructure. The results show that the complete interfacial shear fracture process included three stages: crack initiation, steady crack propagation and unsteady crack propagation. A double-K crack propagation criterion for interfacial shear fracture of composite structures was proposed by introducing two parameters: initial and unstable fracture toughness. After introducing interfacial roughness via the scratch method, the unstable fracture toughness decreased. The 24-h casting interval group had a lower initial fracture toughness and higher unstable fracture toughness than the 2-h group. The unstable fracture toughness of all the composite specimens exceeded that of C60 concrete, indicating that they had excellent resistance to interfacial shear fracture.

Shear fracture of longitudinally reinforced concrete beams under bending using Digital Image Correlation and FE simulations with concrete micro-structure based on X-ray micro-computed tomography images

The paper presents experimental and numerical investigations of the shear fracture in rectangular concrete beams longitudinally reinforced with steel or basalt bar under quasi-static three point bending. Shear fracture process zone formation and development on the surface of beams was investigated by Digital Image Correlation (DIC) whereas thorough analyses of 3D material micro-structure, air voids, width and curvature of shear cracking were carried out by X-ray micro-computed tomography (micro-CT). Moreover, the 2D shear fracture patterns in beams were numerically simulated with the finite element method (FEM) using isotropic coupled elasto-plastic-damage constitutive model for concrete enhanced by a characteristic length of micro-structure. Concrete meso-structure was modelled as a random heterogeneous four-phase material composed of aggregate particles, cement matrix, ITZ zones and air voids on the basis of X-ray micro-CT images. Experimental and numerical results revealed a satisfactory agreement regarding to the mechanism of failure, load-bearing capacity as well as cracking pattern.

Three-Dimensional Combined Finite-Discrete Element Modeling of Shear Fracture Process in Direct Shearing of Rough Concrete–Rock Joints

A three-dimensional combined finite-discrete element element method (FDEM), parallelized by a general-purpose graphic-processing-unit (GPGPU), was applied to identify the fracture process of rough concrete–rock joints under direct shearing. The development process of shear resistance under the complex interaction between the rough concrete–rock joint surfaces, i.e., asperity dilatation, sliding, and degradation, was numerically simulated in terms of various asperity roughness under constant normal confinement. It was found that joint roughness significantly affects the development of overall joint shear resistance. The main mechanism for the joint shear resistance was identified as asperity sliding in the case of smoother joint roughness and asperity degradation in the case of rougher joint asperity. Moreover, it was established that the bulk internal friction angle increased with asperity angle increments in the Mohr–Coulomb criterion, and these results follow Patton’s theoretical model. Finally, the friction coefficient in FDEM appears to be an important parameter for simulating the direct shear test because the friction coefficient affects the bulk shear strength as well as the bulk internal friction angle. In addition, the friction coefficient of the rock–concrete joints contributes to the variation of the internal friction angle at the smooth joint than the rough joint.

Experimental and analytical investigation on the interlaminar shear strength of carbon fiber composites reinforced with carbon nanofiber z-threads

This paper combines an experimental study of the interlaminar shear strength (ILSS) and fracture behavior of CFRPs reinforced with carbon nanofiber (CNF) z-threads (i.e., ZT-CFRPs) together with an analytical model to explain and predict the shear behaviors improved by the CNF z-threads. The ZT-CFRP prepreg was manufactured from resin film containing electrical-field-aligned CNFs by using a modified hot-melt flow-transfer process. The short-beam shear (SBS) test was conducted on the ILSS of ZT-CFRP laminates together with a microscopic morphology study to understand the roles of the CNF z-threads during the shear fracture process. The study revealed the significance of the alignment of the CNFs to increase the ILSS of CFRP laminates. It was shown that CNF z-threads statistically significantly increased the ILSS and the required shear fracture work of CFRPs when compared to the unmodified CFRP and the CFRPs reinforced with randomly dispersed CNFs due to energy transfer and mechanical interlocking from the CNF z-threads. Furthermore, calculations showed that CNF z-threads reduce unwanted stress concentrations, kIC during the double-notch shear (DNS) test. The analytical model helps to understand the interlaminar shear behavior of ZT-CFRPs further and is useful to predict additional experimental data and guide material design in the future.

Study of accelerated shear creep behavior and fracture process of micro-scale ball grid array (BGA) structure Cu/Sn–3.0Ag–0.5Cu/Cu joints under coupled electro-thermo-mechanical loads

Shear creep deformation and fracture behavior of micro-scale ball grid array (BGA) structure Cu/Sn–3.0Ag–0.5Cu/Cu joints under electro-thermo-mechanical coupled loads with high-density current stressing (6.0 × 103 A/cm2) at different temperatures (40, 60, 80, 100 and 120 °C) and various shear stress levels (12.5, 15, 17.5, 20 and 22.5 MPa) were studied intensively. The results show that electric current stressing can significantly accelerate the creep deformation and fracture process of BGA solder joints in terms of increasing steady-state creep stain rate and shortening creep lifetime of the joints. Garofalo hyperbolic-sine function has shown to be a suitable creep constitutive equation, which can be well used to formulate the creep deformation behavior of micro-scale Cu/Sn–3.0Ag–0.5Cu/Cu joints under electro-thermo-mechanical coupled loads, and the equation has the increased stress exponent of 8.6 and creep activation energy of 90.8 kJ/mol, respectively. Combined theoretical analysis and experimental characterization suggest that electric current stressing leads to the change of the dominant steady-state creep deformation mechanism from dislocation climb to lattice diffusion. The interruption creep tests demonstrate that crack initiates at the interface between the solder and intermetallic compounds (IMC) near the position of electrons entering or exiting the solder matrix when the steady-state creep stage approaches the limit, and then the crack first propagates along the solder/IMC interface and gradually turns to the solder matrix under combined shear and tensile stresses. The joint fractographies exhibit a mixed brittle-ductile fracture mode with one part in the interface and the other part in the solder matrix.

A phase-field model of frictional shear fracture in geologic materials

Geologic shear fractures such as faults and slip surfaces involve marked friction along the discontinuities as they are subjected to significant confining pressures. This friction plays a critical role in the growth of these shear fractures, as revealed by the fracture mechanics theory of Palmer and Rice decades ago. In this paper, we develop a novel phase-field model of shear fracture in pressure-sensitive geomaterials, honoring the role of friction in the fracture propagation mechanism. Building on a recently proposed phase-field method for frictional interfaces, we formulate a set of governing equations for different contact conditions (or lack thereof) in which frictional energy dissipation emerges in the crack driving force during slip. We then derive the degradation function and the threshold fracture energy of the phase-field model such that the stress–strain behavior is insensitive to the length parameter for phase-field regularization. This derivation procedure extends a methodology used in recent phase-field models of cohesive tensile fracture to shear fracture in frictional materials in which peak and residual strengths coexist and evolve by confining pressure. The resulting phase-field formulation is demonstrably consistent with the theory of Palmer and Rice. Numerical examples showcase that the proposed phase-field model is a physically sound and numerically efficient method for simulating shear fracture processes in geologic materials, such as faulting and slip surface growth.

Experimental and theoretical study on energy convergence characteristics of adiabatic shear fracture process in high-speed machining of hardened stainless steel

The formation of isolated segment chip due to the occurrence of adiabatic shear fracture (ASF) in adiabatic shear band (ASB) is a significant phenomenon under the high cutting speed. In the present work, the experimental and theoretical methods were adopted to further investigate the energy convergence characteristics in ASB during ASF process in high-speed machining. A hardened stainless steel used in turbine blade was selected as the workpiece. The chip morphology transformation from serrated chip to isolated segment chip was obtained through the high-speed machining experiment. The ductile crack propagation in ASB was observed microscopically. The relations of serrated segment geometry with the cutting conditions were revealed experimentally. According to the continuum governing equations and the deformation and energy analytical models, the distributions of shear velocity, shear strain rate, shear strain, and shear energy in ASB under various cutting speeds and feeds were analyzed combining with the constitutive and stress relations. The energy convergence characteristics in ASB during ASF process with the change of cutting conditions were analyzed and discussed. The results showed that the austenite blocks in the hardened stainless steel influenced the crack propagation in ASB. The larger shear strain induced thinner ASB would accelerate the thermal softening and strain localizing effects, resulting in severer energy convergence in ASB. The energy convergence was uniformly distributed and always kept a constant limit value in the whole ASB when ASF occurred.

Acoustic Emission Characterization of Hybrid Fiber Reinforced Cellular Concrete Under Direct Shear Loads

Acoustic emission (AE) is employed in this study to understand the failure process in fiber-reinforced cellular concrete (FRCLC) subjected to direct shear load. Synthetic polyolefin fibers are used as reinforcement in FRCLC blocks, and their influence on the shear-fracture process is investigated. AE parameters are investigated to assess the effectiveness of fibers in performance improvement of shear strength and toughness of CLC. In total, 24 CLC specimens are tested under direct shear. Effect of fiber dosage and combinations of macro and microfibers are considered as study parameters. Shear crack bridging effect of fibers during the process of shear-slip is investigated. Test results indicate that AE and shear fracture energy increase with increasing fiber dosage. It is observed that discontinuous structural synthetic fibers significantly improve the ultimate shear strength, residual strength, ductility, stiffness, and toughness of CLC. Also, the structural synthetic fibers reduced the shear crack width and shear-slip of CLC. AE parameters such as counts, energy, and 3D-crack source location are presented to illustrate the role of different fiber dosages on the direct shear response. Shear crack location was found to be consistent with the observed failure planes in the specimens. AE data analysis reveals that the number of mode II type events increased with increasing fiber dosage due to increased fiber pullout. Besides, mode I type events also increased as the failure mode changed from a single split crack to distributed cracking.

Effect of the cohesive law shape on the modelling of adhesive joints bonded with brittle and ductile adhesives

Cohesive Zone Models (CZMs) have been increasingly used for the fracture prediction of adhesively bonded joints. In order to better understand the relation between the fracture behaviors of the different geometry/adhesive combinations and the shape of CZMs, the butt-joint uniaxial tension, shear and the double cantilever beam (DCB) fracture experiments were conducted using two distinct adhesives, an epoxy-based brittle adhesive and a VHBTM tape ductile adhesive. Three shapes of CZMs were adopted, including the exponential, bilinear, and trapezoidal models, to numerically predict the experimental performance. The comparisons between the numerical predictions and the experiments results demonstrated that the bilinear CZM is suitable for modelling the brittle adhesive bonding butt-joints in the uniaxial tensile and shear fracture processes, and the exponential CZM is suitable for modelling the ductile adhesive bonding ones. The critical stresses, the fracture energy and the shape of the CZMs have significant effects on the numerical results of the butt-joints uniaxial tension and shear for the two types of adhesives. In addition, for the DCB specimen fracture processes, it is shown that the bilinear CZM and the trapezoidal CZM predicted the brittle adhesive bonding joints better, and the trapezoidal CZM fitted the experimental data best for the two types of adhesives bonding joints. The overall results demonstrate that the selection of the shapes of CZM depend on the properties of the adhesive to obtain an accurate fracture processes prediction.

Numerical investigation of fluid injection into poorly consolidated geomaterial considering shear dilation and compaction

We present a fully coupled poroelastoplastic finite element model to simulate micro-fracture evolution, compaction failure and tensile fracture propagation during fluid injection into poorly consolidated geomaterial. The model includes shear dilation, strain hardening and fracture process zone with permeability evolution. We investigated the effects of geomechanics (stress anisotropy) and fluid injection (fluid rheology) on rock deformation and fracture behavior. Results indicate that fluid rheology dominate the rock failure. Pore pressure increase and water wedge effect can cause two kinds of shear dilation which are prior to tensile failure. The effect of compaction is significant to tensile fracture geometries, which differ from those under elastic and poroelastic conditions. The difference between shear band and compaction zone is the transition of fracture regime from the leak-off dominated to the storage dominated governed by fluid injection.

A novel experimental method to characterise the shear strength of concrete based on pre-stressed samples. A comparison with existing techniques

Mode II shear fracture process can be observed in plain-concrete structures exposed to intensive loadings such as shock, blasting or projectile-impact. It is the reason why the quasi-static and dynamic shear fracturing properties of concrete need to be investigated. In previous works performed by Forquin and coauthors a passive confining cell was applied to PTS (Punch-Through-Shear) specimens in a similar way than in quasi-oedometric compression tests where metallic ring are used as passive confinement. However the change of confining level during the sample loading constitutes a main drawback of this technique. In the present work a novel testing method is proposed based on a pre-stressing of the sample. To do so, the concrete specimen is inserted into pre-stressed metallic cell. During the unloading stage confining stresses are transferred to the ligament of the sample. In a second stage, a differential displacement applied to the central part of the sample toward the side parts produces a shearing of the ligament. The experiments performed in static and dynamic conditions (SHPB testing) allow the shear response of concrete to be determined over a wide range of strain-rates. In the present communication, the testing technique and obtained results are compared to the obtained data with the previous experimental method.

Rate dependent of strength in metallic glasses at different temperatures

The correlation between the strength at the macroscale and the elastic deformation as well as shear cracking behavior at the microscale of bulk metallic glasses (BMGs) is investigated. The temperatures of 298 K and 77 K as well as the strain rate ranging from 10−6 s−1 to 10−2 s−1 are applied to the BMGs, in which the mechanical responses of the BMGs are profiled through the compression tests. The yield strength is associated with the activation of the elementary deformation unit, which is insensitive to the strain rate. The maximum compressive strength is linked to the crack propagation during shear fracture process, which is influenced by the strain rate. The cryogenic temperature of 77 K significantly improves the yield strength and the maximum compressive strength of the BMGs.

Experimental study on the process of adiabatic shear fracture in isolated segment formation in high-speed machining of hardened steel

Adiabatic shear fracture in serrated chip leads to isolated segment formation in high-speed machining with the further increase of cutting speed. An energy model of adiabatic shear band is built and investigated through high-speed machining tests and chip morphology examination. A quick-stop device designed to obtain chip roots in high-speed turning is applied. The adiabatic shear fracture process on considering the induced mechanism is investigated and the multi-stage physical model of adiabatic shear fracture is proposed through chip root morphology examination. The results show that adiabatic shear fracture in high-speed machining is a periodic process from energy convergence to release. The shear band collapses and results in isolated segments formation when the energy convergence in band exceeds the saturation limit. There are possibilities of utilizing the occurrence of adiabatic shear fracture to reduce the increased rate of stress, temperature, and energy and optimize the machinability in the chip formation aspect in high-speed machining.

Properties controlling the bend-assisted fracture of AHSS

Bend-assisted fracture, also commonly called shear fracture, is the splitting of metal sheets during forming in tight-bending regions. It has been shown to be predominantly a result of plastic localization for most advanced high strength steels (AHSS). Such fractures are poorly predicted by typical industrial methods involving finite element modeling (FEM) and forming limit diagrams (FLDs). In order to understand the source of the problem, the sensitivity of simulated shear-fracture formability to material and process parameters was determined using FEM in conjunction with a realistic range of constitutive models, element sizes, and friction coefficients. Two types of shear fracture process were simulated. (1) Draw-bend fracture (DBF) tests are laboratory analogs of industrial forming conditions producing shear fracture; they offer the opportunity of experimental validation but introduce complexity because of varying strain state and unavoidable transitions between shear fracture and tensile fracture. (2) Plane-strain (PS) draw-bend fracture simulations correspond more closely to industrial forming conditions; they simplify the modeling (fixed strain state, no transitions) but no corresponding full-scale laboratory experiments currently exist.The DBF test was found to be sensitive to every material and process parameter tested, with the largest factors being the form of 1-D hardening law and the yield function. Varying these quantities in ranges representing what practical measurements would produce showed variations in predicted formability of up to 80%. The PS simulations, which represent industrial practice more closely, showed large variations in predicted formability only for two variables: 1-D hardening law and friction coefficient. All other parameters were insignificant, except for thermo-mechanical effects, which were important for high-rate tests only.These results show why it is difficult or impossible to predict shear fracture using standard industrial techniques designed for traditional steels. They suggest ways to modify such techniques to accommodate advanced high strength steels. The results also give guidance to alloy designers in terms of which constitutive parameters are most important in inhibiting shear fracture, and which are relatively insignificant.

Experimental and numerical investigation of fracture behavior of asphalt mixture under direct shear loading

Experiments and heterogeneous simulations are combined to investigate cracking behavior of sheared asphalt mixture. A series of direct shear tests are performed at different temperatures with a self-manufactured direct shear test setup, and then a heterogeneous fracture modeling approach based on the random aggregate generation and packing algorithm and the bilinear cohesive crack model is utilized to simulate shear fracture processes. Two kinds of fracture, the single crack type for −20 and −10°C, and the double crack type for 0 and 5°C, are observed. Some internal mechanisms are analyzed based on numerical simulations.