Increasing Shear Strain Research Articles

Shear of granular materials composed of ellipses

Granular systems of disks (2D) and spheres (3D) under shear have been studied extensively. However, less attention has been paid to systems of non-spherical particles, i.e., ellipses, polygons, etc. Here we studied a quasi 2D granular system composed of ellipses under homogeneous simple shear. By tracking the positions and orientations of the ellipses, we have observed that they gradually rotate to align along a preferred direction as the shear strain increases. This preferred direction asymptotically approach a density-independent value at large strain. While first reported for disk systems, shear jamming is also observed for our ellipse system, as indicated by the increase of system pressure and coordination number with shear strain.

Energy dissipation in pH-sensitive hydrogels subjected to large amplitude oscillatory shear

Stimuli-responsive hydrogels belong to a class of shape-memory elastomers that have gained immense popularity due to their applications in the fields of drug delivery, tissue engineering, implants, bio-sensors, and many more. The pH-responsive hydrogels 2-hydroxyethyl methacrylate (2-dimethylamino) ethyl methacrylate (HEMA-DMAEMA) and 2-hydroxyethyl methacrylate acrylic acid (HEMA-AA) exhibit swelling behavior in response to changes in pH. The present study characterizes the energy dissipation of HEMA-DMAEMA and HEMA-AA subjected to large amplitude oscillatory shear. The experiments were conducted at strain levels 0.5%, 1.0%, 1.5%, 2%, and 5% maximum amplitude strain at a constant frequency of 1 Hz. The results showed an increase in energy dissipation with an increase in shear strain for both buffer solutions. The energy dissipation obtained for the HEMA-DMAEMA samples was greater in 11.0 pH buffer solution as compared to 3.0 pH buffer solution. On the contrary, the energy dissipation for HEMA-AA samples was found to be greater in the 3.0 pH buffer solution as compared to the 11.0 pH buffer solution.

Deformation and Failure Characteristics of Soil-Rock Mixture considering Material Composition and Random Structure

The soil-rock mixture is a cohesive-frictional geomaterial subjected to impacts of composition and structure seriously. When it suffers from gravity or other kinds of loadings, the loss of its bearing capacity always appears a progressive failure. In this study, the ultimate criterion of the frictional material changing from the deformation stage to the failure stage is analyzed first and then the deformation and failure characteristics of the soil-rock mixture with different compositions and structures are discussed by the discrete element method. The results indicate that the deformation and failure of the soil-rock mixture under axial pressure appear a significant phenomenon of detouring around rock blocks. The bond failure zones and the ultimate shear strain increase with the increase of rock block proportion. The distribution of the bond failure zones always has a good uniformity with the inclination of rock block inclinations. The increase of cementation degree between particles expands the distribution of the bond failure zones but minifies the ultimate shear strain.

Effect of Combined Torsion and Tension on the Microstructure and Fracture Behavior of 316L Austenitic Stainless Steel

In this study, the microstructure evolution and fracture behavior of 316L austenitic stainless steel (ASS) deformed by torsion–tension at room temperature was systematically investigated. During combined tension and torsion, the grain size was refined with increasing shear strain. In addition, the microtexture exhibited a preferred orientation with increasing shear stress. The metastable austenite underwent phase transition when the shear stress increased. The shear stress affected the fracture morphology, and dimples with different sizes and depths were observed for different pre-torsions. The dislocation density increased significantly owing to severe shear deformation. In addition, the dislocation structure evolved and subgrains appeared with the accumulation of shear strain during this combined deformation. The result showed that pre-torsion deformation plays an important role in improving the comprehensive performance and controlling the microstructure evolution of the sample subjected to tension deformation.

Effect of Surface Finish on the Shear Properties of SnAgCu-Based Solder Alloys

The reliability of an electronic assembly is typically limited by the failure of one of the solder interconnections. One of the key factors that define the quality of solder interconnections is the strength of the solder joint attachment to the printed circuit board (PCB). The ball shear testing is commonly used as a quantitative approach to evaluate the integrity of the solder. Several factors control the shear strength and failure mechanism of solder joints, including the surface finish, solder alloy composition, and speed of shearing (shear strain rate). In this paper, the effect of surface finish on the shear properties of various “microalloyed” SnAgCu-based solder materials was investigated. Individual solder joints were fabricated on a PCB with a solder mask defined configuration. A series of shear testing was conducted at four different strain rates of 0.001, 0.01, 0.1, and 1 s−1. Shear stress–strain curves were recorded for each test, and both shear strength and shear energy were measured. Following the shear testing, fractured top surfaces and cross sections were inspected using SEM/EDS microscopy to characterize the failure mechanism. The results showed that both shear strength and shear energy increase with increasing shear strain rate due to the viscoplasticity of the solder materials. Failure mode analysis indicated the existence of three failure mechanisms, including ductile failure, brittle failure, and mixed failure. Higher occurrence of brittle failure was observed when the shear strain rate is high regardless of surface finish. It was also found that the combination of solder alloy with high silver (Ag) and bismuth (Bi) content and electroless Ni/immersion Au (ENIG) surface finish is more susceptible to brittle failure compared to the rest of solder alloy-surface finish combinations.

Weaker Than Weakest: On the Strength of Shear Zones

AbstractThin, laterally continuous ultramylonites within kilometer‐scale ductile shear zones may control middle to lower crustal strength where deformation is localized. Interconnected phyllosilicate networks are commonly suggested to be the weakest geometry a shear zone can reach, yet fine‐grained polyphase mixtures are commonly found in the cores of high‐strain zones. We study a continental strike‐slip shear zone which deformed granulite facies quartzofeldspathic migmatitic gneisses at retrograde amphibolite to greenschist facies conditions. A brittle feldspar framework and interconnected phyllosilicate networks control the strength of the lower strain protomylonites and mylonites, respectively, whereas the ultramylonites comprise a fine‐grained mixture of the host rock minerals. The localization of strain in ultramylonites demonstrates how fine‐grained polyphase mixtures can be weaker than, and supersede, interconnected phyllosilicate networks with increasing shear strain. This contradicts the common assumption that interconnected layers of phyllosilicates is the weakest state a shear zone can reach.

Evaluation of Non-uniformity of Specimen During Hollow Torsional Test

Using the proposed evaluating method, the variation of non-uniformity of the mean effective stress and the stress ratio and their distribution along the radius direction are discussed and explained in detail. It is found that there are several peak for the non-uniformity of mean effective stress but only one peak for the stress ratio. As the apparent shear strain increases, the distribution of mean effective stress and stress ratio along the radius also varies and the outer is always faster than the inner.

Dilatancy-triggering surface for advanced constitutive modelling of sand

This paper presents, for sand, a formulation for a surface in stress space defining the onset of dilatancy based on the volumetric threshold strain concept, the expression for the small-strain shear modulus in terms of soil state and an expression for secant shear modulus degradation with increasing shear strain. The size of the surface depends on both density (void ratio) and mean effective stress. The use of the proposed surface is demonstrated for Toyoura sand. The paper also presents a multiaxial formulation for the surface that can include kinematic hardening. The proposed surface can be useful in constitutive modelling, especially to capture seismic or cyclic loading conditions.

Flat Internal Buckling Modes of Fibrous Composite Elements under Tension and Compression at the Mini- and Microscale

This paper proposes a refined formulation of linearized problems of internal nonuniformly scaled flat buckling modes of a rigid lamina consisting of fibers and fiber bundles with allowance for their interaction with the surrounding matrix. Fibers are the structural elements of fibrous composites and in a prebuckling (unperturbed) state under the action of shear stresses and tensile (compression) stresses in the transverse direction. The problems are formulated using equations constructed by reducing the version of geometrically nonlinear equations of the elasticity theory to one-dimensional equations of the theory of rectilinear rods. These equations are based on the use of the refined Timoshenko shear model with allowance for tension-compression strains in the transverse direction for the rigid lamina and the transverse-soft layer model with immobile boundary planes in a perturbed state for the epoxy layers. It is shown that loading samples with a structure is accompanied by constant changes in the composite structure due to implementation and alternation of the internal buckling modes with a varying wave formation parameter. This particularly allows explaining the changing of the effective shear modulus of the fibrous composite with increasing shear strains.

Obtaining hexagon-shaped billets of copper with gradient structure by twist extrusion

Processing by twist extrusion was used to produce copper billets with a hexagonal cross-section. A special feature of the billets is their gradient microstructure owing to a specific strain pattern: an undeformed core and a deformed outer sheath. The initial sample with a 40 mm diameter was processed at room temperature by four twist extrusion passes through a twist die having a hexagonal cross-section and a small twist-line slope angle β = 40° with a back-pressure of 100 MPa. The results showed that the processed sample had an undeformed core 20 mm in diameter and a deformed outer sheath with a gradient microstructure in which the average grain size decreased gradually from 50 μm to 0.5 μm towards the periphery along a radius direction. The microstructure gradient represented the various stages of the grain refinement process corresponding to increasing shear strain. The texture revealed two different shear deformation modes whose shear planes were perpendicular and parallel to the extrusion axis. The measured Vickers hardness showed good agreement with the microstructure. In the deformation zone, the classical Hall-Petch relation between the hardness and the grain size of the various characteristic areas formed by twist extrusion was found.

Plastic behavior and failure mechanism of Ti-6Al-4V under quasi-static and dynamic shear loading

A new kind of double-shear specimen (DSS) is used to study quasi-static and dynamic behaviors of Ti-6Al-4V under simple shear conditions. With different loading techniques, a wide range of shear strain rates are covered from 0.002 s−1 to 60,000 s−1. The flow stress curves are determined under different conditions. It's observed that the work-hardening effect on the flow stress is weakened gradually with the increasing strain rates. Both the yield stress and the failure initiation stress show an increasing tendency with the strain rates. On the contrary, the failure initiation strain and the fracture strain both decrease with strain rates. From numerical simulation, it's seen that a shear dominated stress/strain state is formed in the shear zone, where the stress triaxiality and the Lode angle parameter basically keep constant for different strain rates. Based on the fracture morphology, it's concluded that with the increasing shear strain rates the failure mechanism changes from a ductile fracture to an ASB dominated process. The macro regularities of the failure properties can be explained well by the different micro-mechanisms.

Effects of fault-weakening processes on oblique intracontinental rifting and subsequent tectonic inversion

In many mountain belts, deformation concentrates along mechanically weak fault zones inherited from earlier tectonic events. This work investigates the effects of two modes of structural weakening on the orientation of rifting and later tectonic inversion with respect to the imposed divergence/convergence direction in a high-resolution 3D finite difference model with a viscous-frictional rheology. In the first set of experiments, weakening consists in a decrease in frictional strength with increasing shear strain. The generated normal faults strike orthogonal to the imposed divergence direction. These faults are reactivated during tectonic inversion and absorb 50 to 70 percent of accumulated strain. In the second set of experiments, frictional strength is a decreasing function of shear strain rate. The generated faults are oblique to the divergence direction, implying oblique fault slip. Fault reactivation depends on the obliquity of the inverted rift to convergence direction, where larger obliquity leads to more intense fault reactivation. These new numerical results are compared to previous analogue and numerical models on the one hand, and natural examples of intracontinental mountain ranges due to tectonic inversion on the other hand. These comparisons demonstrate that both modes of frictional weakening should be taken into account when seeking to understand large-scale rifting and inversion tectonics.

Experimental Study on the Small-strain Stiffness and the Damping Properties of Strong Structural Clay

The small strain parameters of soil are important indexes for correct evaluation of the soil deformation. Based on the Stokoe resonant column system, the small strain dynamic characteristics of Zhanjiang strong structured clay under different consolidation stresses are investigated, including the dynamic shear modulus and damping ratio. Results show that under the same stress level, the maximum dynamic shear modulus of undisturbed soil is slightly larger than that of the remolded, and both of them decrease with the increase of shear strain. And the higher stress level is, the higher the maximum dynamic shear modulus would be, and the more significant the attenuation would be. As the shear strain increases to a certain extent, the damping ratio of the undisturbed soil increases sharply, but that of the remolded increases slowly. The maximum dynamic shear modulus of undisturbed soil shows a singular feature that it increases at the beginning and then decreases with the increase of effective confining pressure; and the inflection point corresponding to the effective confining pressure is larger than the structural yield stress of soil. The maximum dynamic shear modulus of the undisturbed soil is not only affected by the positive effect of the consolidation pressure and void ratio, but also affected by the negative effect of the structural damage induced by the stress level.

International investigation of shear displacement capacity of various elastomeric seismic-protection isolators for buildings

In recent years, various types of elastomeric isolators are used for seismic isolated buildings globally. International standards for elastomeric seismic-protection isolators, ISO 22762 were first developed in 2005, and this standard has been revised through a systematic review to issue the 3rd edition now (3rd edition was issued in October 2018). This paper describes breaking test results of elastomeric isolators to investigate the ultimate property (breaking limit) of isolators used for buildings around the world. Shear displacement capacity is one of the most important parameters of elastomeric isolators, and these tests results are useful for our proposal of the introduction of a classification system (classification of isolators according to their performance) to ISO 22762 in the near future. Breaking tests were carried out on 11 isolators produced in five countries. The isolators having diameters of around 800 mm, and the total rubber heights of around 160 mm were tested with increasing shear strain under constant compressive stress using a multi-axial testing system. The breaking shear strain was distributed between 300 and 450% or more.

Crystallographic preferred orientations of ice deformed in direct-shear experiments at low temperatures

Abstract. Synthetic polycrystalline ice was sheared at temperatures of −5, −20 and −30 ∘C, to different shear strains, up to γ=2.6, equivalent to a maximum stretch of 2.94 (final line length is 2.94 times the original length). Cryo-electron backscatter diffraction (EBSD) analysis shows that basal intracrystalline slip planes become preferentially oriented parallel to the shear plane in all experiments, with a primary cluster of crystal c axes (the c axis is perpendicular to the basal plane) perpendicular to the shear plane. In all except the two highest-strain experiments at −30 ∘C, a secondary cluster of c axes is observed, at an angle to the primary cluster. With increasing strain, the primary c-axis cluster strengthens. With increasing temperature, both clusters strengthen. In the −5 ∘C experiments, the angle between the two clusters reduces with strain. The c-axis clusters are elongated perpendicular to the shear direction. This elongation increases with increasing shear strain and with decreasing temperature. Highly curved grain boundaries are more prevalent in samples sheared at higher temperatures. At each temperature, the proportion of curved boundaries decreases with increasing shear strain. Subgrains are observed in all samples. Microstructural interpretations and comparisons of the data from experimentally sheared samples with numerical models suggest that the observed crystallographic orientation patterns result from a balance of the rates of lattice rotation (during dislocation creep) and growth of grains by strain-induced grain boundary migration (GBM). GBM is faster at higher temperatures and becomes less important as shear strain increases. These observations and interpretations provide a hypothesis to be tested in further experiments and using numerical models, with the ultimate goal of aiding the interpretation of crystallographic preferred orientations in naturally deformed ice.

Loading rate on the mechanical behaviour of structured clays

ABSTRACTWith the objective to study the effect of viscous property on the mechanical behaviour of structured clays, a series of compression and consolidation-undrained shear tests in different loading rates, using oedometer and triaxial instruments respectively, were carried out on undisturbed samples of clays. The compression curves in different load incremental ratios shows that the load incremental ratio has an effect on structural yield stresses, while it has little influence on compression indices of clays. The stress–strain–pore pressure curves show that shear strength increases, while pore water pressure decreases with increasing shear strain rate, and the critical state stress ratio is constant in reasonable shear strain rate range. When the shear strain rate exceeds the boundary of shear strain rate, the pore water pressure drops sharply and the failure mode changes from plastic failure to brittle failure. The test results illustrate that loading rate has an effect on the structural yield stress and shear strength, while it has little influence on compression index and critical state stress ratio of structured clays.

Functional assessment of strains around a full-thickness and critical sized articular cartilage defect under compressive loading using MRI

The objective of this study was to evaluate the effect of full-thickness chondral defects on intratissue deformation patterns and matrix constituents in an experimental model mimicking invivo cartilage-on-cartilage contact conditions. Pairs of bovine osteochondral explants, in a unique cartilage-on-cartilage model system, were compressed uniaxially by 350N during 2s loading and 1.4s unloading cycles (≈1700 repetitions). Tissue deformations under quasi-steady state load deformation response were measured with displacement encoded imaging with stimulated echoes (DENSE) in a 9.4T magnetic resonance imaging (MRI) scanner. Pre- and post-loading, T1, T2 and T1ρ relaxation time maps were measured. We analyzed differences in strain patterns and relaxation times between intact cartilage (n=8) and cartilage in which a full-thickness and critical sized defect was created (n=8). Under compressive loading, strain magnitudes were elevated at the defect rim, with elevated tensile and compressive principal strains (Δϵmax=4.2%, P=0.02; Δϵmin=-4.3%, P=0.02) and maximum shear strain at the defect rim (Δγmax=4.4%, P=0.007). The opposing cartilage showed minimal increase in strain patterns at contact with the defect rim but decreased strains opposing the defect. After defect creation, T1, T2 and T1ρ relaxation times were elevated at the defect rim only. Following loading, the overall relaxations times of the defect tissue and especially at the rim, increased compared to intact cartilage. This study demonstrates that the local biomechanical changes occurring after defect creation may induce tissue damage by increasing shear strains and depletion of cartilage constituents at the defect rim under compressive loading.

Triblock Copolymer Toughening of a Carbon Fibre-Reinforced Epoxy Composite for Bonded Repair.

Epoxy resins are the most widely used systems for structural composite applications; however, they lack fracture toughness, impact strength and peel strength due to high cross-linking densities. Use of conventional toughening agents to combat this can lead to reductions in mechanical, thermal and processability properties desirable for bonded composite applications. In this work, an asymmetric triblock copolymer of poly(styrene)–b–poly(butadiene)–b–poly(methylmethacrylate) was used to modify an epoxy resin system, with the materials processed using both vacuum bag and positive pressure curing techniques. Interlaminar fracture toughness testing showed improvements in initiation fracture toughness of up to 88%, accompanied by a 6 °C increase in glass transition temperature and manageable reductions in gel-time. Shear testing resulted in a 121% increase in ultimate shear strain with only an 8% reduction in shear strength. Performance improvements were attributed to nano-structuring within the toughened resin system, giving rise to matrix cavitation and dissipation of crack front strain energy upon loading.

Measurement of Amorphous Peat Shear Strength in the Direct Shear Box at High Displacement Rates

The ASTM standard provides guidelines for the drained direct shear test (DST) and requires the samples to be sheared at rates estimated from time to failure, $$t_{f} \ge 50t_{50}$$ (where $$t_{50}$$ is the time required to achieve 50% consolidation). This paper investigates the potential of estimating the undrained strength of peat in the DST owing to its ease of accessibility and simplicity over other laboratory tests. In this experimental study, peat samples were sheared at various displacement rates at $$t_{f} /t_{50}$$ values of 0.065–70. The samples tested at the ASTM specified rate of $$t_{f} \ge 50t_{50}$$ exhibit continuous increase and decrease in shear stress and volume respectively with increasing shear strain. Hence, determining the drained strength properties of peat in the DST apparatus is complicated. However, it is concluded that the shear stress ratio, $$\tau /\sigma {^\prime }_{v}$$ of the DST test samples sheared at rates corresponding to $$t_{f} \le 0.2t_{50}$$ , reasonably corresponds to that measured in undrained direct simple shear (DSS) tests on peat, found in the literature. Therefore, DST conducted at high displacement rates will be adequate for preliminary evaluation of the shear strength of peat at close to undrained conditions, when constant volume DSS apparatus is not available.

An effective way to estimate the Poisson's ratio of silty clay in seasonal frozen regions

This paper advances the present understanding of the nonlinear characteristic of Poisson's ratio in seasonal frozen regions. The influence of freeze-thaw cycles, confining pressure and shear strain is investigated on the basis of extensive resonant column tests. It is demonstrated that the Poisson's ratio increases with increasing shear strain and decreases with increasing confining pressure. A new empirical equation is formulated to describe the strain-dependency of Poisson's ratio. With the fitting parameters, the influence of freeze-thaw cycles on Poisson's ratio is discussed. Based on the Hardin-Drnevich model, an effective method for estimating the Poisson's ratio in seasonal frozen regions is developed. It is found that the comprehensive effects of the parameters are adequately represented by maximum shear modulus, normalized shear modulus and confining pressure, which makes the method simple and facile for use.