Shear Fracture Angle Research Articles

Soil Pressure and Deformation Analysis of Small Size Deep Foundation Pit

In this paper, the influence of space effect on soil pressure and deformation of deep foundation pit was considered, and the finite soil pressure calculation model was established. The soil pressure of deep foundation pit was calculated by assuming the slip surface and using the finite soil limit equilibrium theory. Then, PLAXIS 3D finite element software was used to establish finite element models of different plane sizes and depths. The distribution regulation of side wall soil pressure and deformation of deep foundation pit was calculated. Finally, the results of finite soil pressure calculation was compared with finite element method. The results shown that: The soil pressure of small deep foundation pit was affected by space effect, and the soil pressure and deformation decrease significantly along the foundation pit depth. Shear fracture Angle was related to the ratio of width to depth of foundation pit, and it was no longer a constant value of 45°+φ/2. Therefore, the spatial effect should be considered in the calculation of soil pressure of small deep foundation pit. The research results can provide some guidance for the design and calculation of similar small size deep foundation pit.

Revealing grain coarsening and detwinning in bimodal Cu under tension

Abstract Metals with a bimodal grain size distribution have been found to have both high strength and good ductility. However, the coordinated deformation mechanisms underneath the ultrafine-grains (UFGs) and coarse grains (CGs) still remain undiscovered yet. In present work, a bimodal Cu with 80% volume fraction of recrystallized micro-grains was prepared by the annealing of equal-channel angular pressing (ECAP) processed ultrafine grained Cu at 473 K for 40 min. The bimodal Cu has an optimal strength-ductility combination (yield strength of 220 MPa and ductility of 34%), a larger shear fracture angle of 83° and a larger area reduction of 78% compared with the as-ECAPed UFG Cu (yield strength of 410 MPa, ductility of 16%, shear fracture angle of 70°, area reduction of 69%). Grain refinement of recrystallized micro-grains and detwinning of annealing growth twins were observed in the fractured bimodal Cu tensile specimen. The underlying deformation mechanisms for grain refinement and detwinning were analyzed and discussed.

Generalized Mohr-Coulomb strain criterion for bulk metallic glasses under complex compressive loading

The Mohr-Coulomb (M-C) stress criterion is widely applied to describe the pressure sensitivity of bulk metallic glasses (BMGs). However, this criterion is incapable of predicting the variation in fracture angles under different loading modes. Moreover, the M-C criterion cannot describe the plastic fracture of BMGs under compressive loading because the nominal stress of most BMGs remains unchanged after the materials yield. Based on these limitations, we propose a new generalized M-C strain criterion and apply it to analyze the fracture behaviors of two typical Zr-based BMG round bar specimens under complex compressive loading. In this case, the predicted initial yielding stress is in good agreement with the experimental results. The theoretical results can also describe the critical shear strain and fracture angle of BMGs that are associated with the deformation mode.

Microstructure and compressive/tensile characteristic of large size Zr-based bulk metallic glass prepared by laser solid forming

The large size, crack-free Zr55Cu30Al10Ni5 bulk metallic glass (BMGs) with the diameter of 54 mm and the height of 15 mm was built by laser solid forming additive manufacturing technology, whose size is larger than the critical diameter by casting. The microstructure, tensile and compressive deformation behaviors and fracture morphology of laser solid formed Zr55Cu30Al10Ni5 BMGs were investigated. It is found that the crystallization mainly occurs in the heat-affected zones of deposition layers, which consist of Al5Ni3Zr2, NiZr2, ZrCu, CuZr2 phases. The content of amorphous phase in the deposit is about 63%. Under the compressive loading, the deposit presents no plasticity before fracture occurs. The fracture process is mainly controlled by the shear stress and the compressive shear fracture angles of about 39°. The compressive strength reaches 1452 MPa, which is equivalent to that of as-Cast Zr55Cu30Al10Ni5 BMGs, and there exist vein-like patterns, river-like patterns and smooth regions at the compressive fractography. Under the tensile loading, the deposit presents the brittle fracture pattern without plastic deformation. The fracture process exhibits normal fracture model, and the tensile shear fracture angle of about 90°. The tensile strength is only about 609 MPa, and the tensile fractography mainly consists of micro-scaled cores and vein-like patterns, dimple-like patterns, chocolate-like patterns and smooth regions. The results further verified the feasibility and large potential of laser additive manufacturing on fabrication and industrial application of large-scale BMGs parts.

Evaluating the material strength from fracture angle under uniaxial loading

The most common experimental methods of measuring material strength are the uniaxial compressive and tensile tests. Generally, shearing fracture model occurs in both the tests. Compressive strength is higher than tensile strength for a material. Shearing fracture angle is smaller than 45° under uniaxial compression and greater than 45° under uniaxial tension. In this work, a unified relation of material strength under uniaxial compression and tension is developed by correlating the shearing fracture angle in theory. This constitutive relation is quantitatively illustrated by a function for analyzing the material strength from shear fracture angle. A computational simulation is conducted to validate this theoretical function. It is full of interest to give a scientific illustration for designing the high-strength materials and engineering structures.

Tensile and Compressive Properties of Mg-Li Alloys Subjected to Different Heat Treatment

Tensile and compressive tests were performed on Mg-Li alloy samples subjected to annealing and aging treatment respectively. The results show that the compressive strength of the alloy is obviously higher than the tensile strength. Meanwhile, it is found that the shear fracture angle of the compressive specimens is smaller than that of the tensile specimens.

Fracture behaviors under pure shear loading in bulk metallic glasses.

Pure shear fracture test, as a special mechanical means, had been carried out extensively to obtain the critical information for traditional metallic crystalline materials and rocks, such as the intrinsic deformation behavior and fracture mechanism. However, for bulk metallic glasses (BMGs), the pure shear fracture behaviors have not been investigated systematically due to the lack of a suitable test method. Here, we specially introduce a unique antisymmetrical four-point bend shear test method to realize a uniform pure shear stress field and study the pure shear fracture behaviors of two kinds of BMGs, Zr-based and La-based BMGs. All kinds of fracture behaviors, the pure shear fracture strength, fracture angle and fracture surface morphology, are systematically analyzed and compared with those of the conventional compressive and tensile fracture. Our results indicate that both the Zr-based and La-based BMGs follow the same fracture mechanism under pure shear loading, which is significantly different from the situation of some previous research results. Our results might offer new enlightenment on the intrinsic deformation and fracture mechanism of BMGs and other amorphous materials.

Mechanical properties and fracture mechanisms of ultrafine grained Cu-9.6% Zn alloy processed by multiaxial cryoforging

Aim of the present study is to investigate mechanical properties and fracture mechanisms of low stacking fault energy ultrafine grained Cu-9.6wt% Zn brass processed by multiaxial forging (MAF) at cryogenic temperature. The alloy was developed by metal mold casting followed by homogenized annealing at 800°C for 4h before cryoforging. The forging was carried out for 5, 9 and 12 cycles which are equivalent to cumulative strains of 3.0, 5.4 and 7.2, respectively. The brass deformed with a cumulative strain of 7.2 showed more than 9 times improvement in the yield strength (YS=680MPa) compared to that of the homogenized annealed sample (70MPa). However, the ductility decreased to a very low range after deforming to 12 cycles because of extreme work hardening at the cryogenic temperature due to suppression of dynamic recovery. Cryoforging followed by short-annealing (225–300°C for 20min) was found to be effective to enhance the ductility without much affecting its YS. Formation of ultrafine grains along with nanotwins (confirmed by TEM) results a simultaneous improvement in the YS and ductility in the cryoforged+short-annealed samples. Macroscopic tensile fracture mechanisms and fractography analysis were carried out for the cryoforged and cryoforged+short-annealed samples. The unified tensile fracture criterion (Ellipse criterion) in terms of modification of the stress state in the fracture zone suggested that reduction of necking angle is resulted from the geometrical hardening effect due to the change of stress state. As number of forging cycles increases, which correspondingly increases critical normal stress (σo) and shear stress (τo), the tensile shear fracture angle decreases indicating in the increase of shear mode fracture. This is ascertained to the grain size refinement, which increases the hardness of the material. Fractography analysis also highly corroborated with the analysis of the unified tensile fracture criterion and %elongation. The dimple size as well as %elongation was found to gradually decrease with decrease in the shear fracture angle, which is equivalent to increase in the cumulative strain.

Tensile fracture of ultrafine grained aluminum 6061 sheets by asymmetric cryorolling for microforming

The size effect on the mechanism of fracture in ultrafine grained sheets is an unsolved problem in microforming. This paper describes a tensile test carried out to study the fracture behavior and the shear fracture angles of both rolled and aged ultrafine grained aluminum 6061 sheets produced by asymmetric cryorolling. A scanning electron microscope was used to observe the fracture surface. The finite element method was used to simulate the tensile test using the uncoupled Cockcroft–Latham and Tresca criteria and the coupled Gurson–Tvergaard–Needleman damage criterion. It was found that the shear fracture angle decreases gradually from 90° to 64° with an increasing number of passes. The results of simulations using the Gurson–Tvergaard–Needleman criterion show trends similar to the experimental ones. The paper also presents a discussion on the fracture mechanism and the size effect during the tensile test.

Varying tensile fracture mechanisms of Cu and Cu–Zn alloys with reduced grain size: From necking to shearing instability

In ultrafine-grained (UFG) or nanocrystalline (NC) materials, achieving high strength often induces loss of ductility due to the formation of shear fracture without obvious necking feature. To investigate mechanical properties and fracture mechanism with reduced grain size, the macroscopic tensile fracture behaviors of UFG or NC Cu and Cu–Zn alloys were systematically investigated. It is found that the tensile strength and uniform elongation of UFG or NC Cu and Cu–Zn alloys display simultaneously increasing trend. The limitation of ductility can be attributed to the occurrence of shear bands in these materials; their main characteristic, the shear fracture angle, decreases with decreasing grain size as well as the degree of necking. The macroscopic fracture mechanisms are explained using an Ellipse criterion in terms of modification of the stress state in the fracture zone. It is suggested that the reduction of necking degree results from the geometrical hardening effect due to the change of the stress state. As the degree of necking decreases with decreasing grain size, the tensile shear fracture angle decreases too in the UFG/NC materials. Thus improving the geometrical hardening ability may effectively inhibit the formation of shear fracture, as the nucleation of shear bands becomes difficult.

Strain rate sensitivity and fracture behavior of severely deformed Al–Mn alloy sheets

► Aluminum alloy 3003 was processed by constrained groove pressing (CGP) technique. ► Strain rate sensitivity values increase with increasing imposed strain by CGP. ► Fracture behavior changes with imposed strain. ► Shear fracture angle increases with increasing of CGP pass number. Wrought sheets of an Al–Mn alloy are subjected to severe plastic deformation using constrained groove pressing (CGP) method. After applying CGP passes on the sheets, tensile tests at different strain rates are carried out and the strain rate sensitivity values are calculated. Results show that strain rate sensitivity is increased from 0.0081 in annealed sheets to 0.019 after three passes of constrained groove pressing. This arises from intrinsic decrease in grain size due to severe straining. Experimental results reveal that after three CGP passes, the rate of increasing in strain rate sensitivity is decreased. Fracture surfaces of the tensile samples are observed using scanning electron microscopy. It is found that with increasing pass number, shear fracture angle is increased from 45° for annealed sample to 60° after three passes of CGP. This is due to the change in the mechanism of fracture from tensile fracture to shear fracture which is detectable from the formation of parabolic dimple instead of hemispherical equiaxed ones.

Discussions on Fracture Factors of Brittle Materials under the Shear-Compression Condition

Based on fundamental ideas in tribology and basic concept of stress state in solid mechanics, the existence of frictional force on shear plane is discussed under uniaxial compression of brittle materials. On account of macroscopic fracture forms and mesoscopic fracture mechanisms, the key factors influencing shear fracture angle are analyzed. The results show that, when brittle materials are compressed and shear fracture occurs, shear fracture surface at the crack initiation point is consistent with the maximum shear stress. But the reason of shear fracture angle examined in experiment greater than 45º lies in that, the existence of frictional force between endface of specimen and pressure head of testing machine, and additional tensile stress produced in the materials when harder crystalline grain wedge in softer medium have changed original uniaxial compression stress state and the direction of maximum shear stress on next fracture path.

Compressive mechanical behavior of carbon/carbon composites under dynamic loading

Both quasi-static and dynamic compressive properties of 2-D carbon/carbon (C/C) composites in their thickness direction were determined using a universal test machine and Split Hopkinson Pressure Bar with pulse shapers. The compressive failure of the composite was photographed to demonstrate the failure mode at different strain rates,and the effects of strain rate on fracture mode are discussed. Results show that the compressive stiffness and strength of the C/C composites increase with strain rate. The compressive stiffness and strength of the C/C composites under dynamic loading with a strain rate of 5×102/s are increased by about 66% and 55% respectively compared with quasi-static results with a strain rate of 10-4/s. Under quasi-static conditions,the samples failed by shear with a shear fracture angle of approximately 40o,and carbon fiber bundles were sheared and crushed. Under dynamic conditions,the samples were broken into many fragments with different sizes,and carbon fiber bundles were fractured by splitting. The different failure modes may be related to the strain rate sensitivity of the carbon matrix and the interface strength.

Tensile properties and fracture mechanism of Al–Mg alloy subjected to equal channel angular pressing

Casting Al–2.77 wt.% Mg alloy was subjected to equal channel angular pressing (ECAP) and subsequent low-temperature annealing treatment. Tensile properties and fracture modes of as-ECAPed and annealed samples were investigated. It is found that the strength of the Al–Mg alloy increases with increasing the number of ECAP passes, while its elongation decreases. After annealing, the elongation is recovered to a large extent, consequently, the static toughness of the alloy is enhanced by a combination of ECAP and annealing treatment. Vickers hardness HV of the Al–Mg alloy monotonously increases with increasing the number of ECAP passes and follows a relationship of HV/ σ b ≈ 2.5, independent of the number of ECAP passes. Meanwhile, it is noted that the casting alloy only exhibits necking before failure, while the alloy subjected to ECAP fails in shear mode, with an increasing shear fracture angle at high number of ECAP passes. Besides, the low temperature annealing treatment hardly affects the tensile fracture mode of the ECAPed alloy. Based on the experimental results above, the tensile failure mechanisms of the ECAPed Al–Mg alloy are discussed.

Effects of tungsten fiber on failure mode of zr-based bulk metallic glassy composite

The authors systematically investigated the effects of tungsten fiber on failure mode as well as deformation and fracture mechanisms in tungsten fiber-reinforced Zr41.25Ti13.75Ni10Cu12.5Be22.5 bulk metallic glassy composite under uniaxial compression at room and high temperatures. At room temperature, the failure mode of the composite changes from shear fracture to longitudinal splitting failure with increasing fiber volume fraction. Similar to the observations in monolithic metallic glasses, the shear fracture angle of the composite is approximately equal to 39∼40 deg, indicating that the Mohr-Coulomb criterion is suitable to give the critical shear fracture condition of the composite. When the compression tests were performed below the glass transition temperature of Zr41.25Ti13.75Ni10Cu12.5Be22.5 metallic glassT g, the deformation behavior of the composite strongly depends on the strain rates and the test temperature, which is quite similar to the deformation behavior of monolithic metallic glasses in the supercooled liquid region. The corresponding failure mode of the composite changes from shear or splitting fracture to bending failure with decreasing strain rate or increasing test temperature. The failure modes at the temperature nearT g are mainly controlled by the metallic glass matrix due to the decrease in its viscosity at high temperature. Based on these multiple failure modes, the effects of test temperature and tungsten fiber volume fraction on deformation and fracture mechanisms are summarized.

Rotation mechanism of shear fracture induced by high plasticity in Ti-based nano-structured composites containing ductile dendrites

Ti-based nano-structured composites with ductile dendrites often fail under a shear fracture angle larger than 45° with the compression stress axis. This can be explained by a rotation mechanism of the shear plane and bending of the shear bands, indicating a good mechanical performance of the nano-structured composites.