Showing papers on "Hydrostatic stress published in 2020"

Analytical Solution for Coupled Diffusion Induced Stress Model for Lithium-Ion Battery

Abstract: Electric cycling is one of the major damage sources in lithium-ion batteries and extensive work has been produced to understand and to slow down this phenomenon. The damage is related to the insertion and extraction of lithium ions in the active material. These processes cause mechanical stresses which in turn generate crack propagation, material loss and pulverization of the active material. In this work, the principles of diffusion induced stress theory are applied to predict concentration and stress field in the active material particles. Coupled and uncoupled models are derived, depending on whether the effect of hydrostatic stress on concentration is considered or neglected. The analytical solution of the coupled model is proposed in this work, in addition to the analytical solution of the uncoupled model already described in the literature. The analytical solution is a faster and simpler way to deal with the problem which otherwise should be solved in a numerical way with finite difference method or a finite element model. The results of the coupled and uncoupled models for three different state of charge levels are compared assuming the physical parameters of anode and cathode active material. Finally, the effects of tensile and compressive stress are analysed.

Creep behavior due to interface diffusion in unidirectional fiber-reinforced metal matrix composites under general loading conditions: a micromechanics analysis

Abstract: In this paper, the creep behavior due to interface diffusion in unidirectional fiber-reinforced metal matrix composites under general transverse loading conditions is studied. A micromechanics model is adopted to estimate the average stresses in fibers, which is demonstrated to be of reasonable accuracy by comparing with finite element analysis. The creep deformation due to interface diffusion is characterized by a tensor-form creep strain rate, which is related to the components of average stress in fibers and possesses incompressibility. The derived creep strain tensor is then introduced into fibers as eigenstrain to estimate the effect of creep strain on stress redistribution in composites. Eventually, the macroscopic creep strain of composites under constant external loads and the stress relaxation at fixed displacements due to interface diffusion are calculated by the incremental creep analysis procedures based on the average field theory. The effects of loading conditions on the overall creep behavior are examined in detail. Results of this study show that under the fixed strain condition, the initial biaxial stress ratio actually varies rather than remains unchanged during the stress relaxation process. In both constant stress and constant strain conditions, stresses in fibers have a propensity to approach hydrostatic stress state, thus suppressing the interface diffusion. These findings were not reported in previous researches and may bring new understanding on diffusion-induced creep deformation in metal matrix composite materials.

A stress triaxiality based modified Liu–Murakami creep damage model for creep crack growth life prediction in different specimens

Abstract: Liu–Murakami creep damage model is improved to predict the creep life of various cracked specimens. The modified creep damage law is implemented in the framework of extended finite element method (XFEM) for performing elasto-plastic creep crack growth simulations. Experiments show that the crack tip constraints vary from component to component which leads to variation in crack growth rates. A stress triaxiality function is introduced in the modified Liu–Murakami damage model to address the variation in crack growth rates. Moreover, a new definition of stress triaxiality (ratio of a linear combination of maximum principal stress and hydrostatic stress to von Mises stress) is proposed based on Leckie and Hayhurst failure criterion. The new definition of stress triaxiality is a key parameter in the prediction of time to failure. The modified Liu–Murakami creep damage model is used for the creep crack growth (CCG) simulations of several specimens under different loading conditions. Parametric studies are also performed to study the influence of various parameters on the CCG. Moreover, a combined framework of continuum damage mechanics and XFEM is used to predict the CCG life of a turbine blade. This work establishes that the modified Liu–Murakami creep damage law accurately predicts the creep life of cracked components under different constraint conditions.

Constitutive Modeling of Porous Shape Memory Alloys Using Gurson-Tvergaard-Needleman Model Under Isothermal Conditions

Abstract: Based on the Gurson–Tvergaard–Needleman (GTN) model, a constitutive relationship considering both the effects of strain hardening and hydrostatic stress for porous shape memory alloys (SMAs) is pro...

In-Situ Grain Resolved Stress Characterization During Damage Initiation in Cu-10%W Alloy

Abstract: The evolution of stress during damage initiation and accumulation in a two-phase alloy consisting of a ductile copper (Cu) matrix with a randomly dispersed brittle tungsten (W) phase was studied using multiple non-destructive experimental probes. Neutron diffraction measurements were performed to examine the macroscopic strain partitioning between the two phases during a uniaxial tension test. The same material was then examined with high-energy x-ray diffraction microscopy (HEDM) and micro-computed tomography ($$\mu \hbox {-CT}$$) measurements to monitor micromechanical field evolution. The neutron diffraction data indicated a redistribution of load between the Cu and W phases as deformation proceeds. Using HEDM to monitor individual grain micromechanical behavior, an increase followed by decrease in hydrostatic stress and a similar stress triaxiality behavior were found to occur in a subset of W grains. These same W grains were found to be in close proximity to voids observed via tomography at later stages of deformation. From these observations, we conclude that high stress triaxiality development in the W particles leads to decohesion of the interface between the Cu and W phases. The debonded regions eventually grew and coalesced with neighboring voids leading to material failure.

Process-induced matrix defects: Post-gelation

Abstract: Formation of resin matrix defects is common during composites manufacturing The mechanisms behind these defects depend on the state of the resin at the time of their formation Process-induced matrix defects are commonly studied in the form of porosity before gelation, or matrix cracking after vitrification The present study investigates defect formation when the thermoset resin is between gelation and vitrification After gelation, the resin is in a rubbery state and does not flow At this state, cure in a constrained geometry leads to internal stress development and potential formation of defects An experimental setup was used which allowed direct visual observation of the resin behavior after gelation It was found that post-gelation defect formation is driven by hydrostatic stress and the defect morphology is cure rate-dependent An analytical approach to avoid post-gelation defects based on comprehensive material characterization and internal stress evolution calculations is proposed, and experimentally validated

Analysis of Rock Load for Tunnel Lining Design

Abstract: Rock load relaxed by surrounding ground due to a tunnel excavation is one of the most important parameters in support system design of underground structures. There are three different methods for estimating rock load, namely, empirical solutions, convergence-confinement method (CCM), and numerical techniques. In the present study, rock loads acting on the final support system of Bazi-Deraz water conveyance tunnel at its different sections are estimated using aforementioned methods. Results show that, in a hydrostatic stress field, empirical and plastic zone-based numerical methods provide maximum and minimum rock load estimates, respectively, and numerical technique which is based on local safety factor obtains a more accurate rock load than the method based on plastic zone. Comparing empirical and analytical methods, rock load estimation using Goel-Jethwa’s method obtains the closest values to analytical results in hydrostatic stress field. It is, however, observed that these methods cannot be used in non-hydrostatic stress fields. Therefore, it is proposed to utilize a coupled numerical-CCM method to estimate rock load when stress coefficient K is larger than 1.

Stress Distribution in Silicon Subjected to Atomic Scale Grinding with a Curved Tool Path

Abstract: Molecular dynamics (MD) simulations were applied to study the fundamental mechanism of nanoscale grinding with a modeled tool trajectory of straight lines. Nevertheless, these models ignore curvature changes of actual tool paths, which need optimization to facilitate understanding of the underlying science of the machining processes. In this work, a three-dimensional MD model considering the effect of tool paths was employed to investigate distributions of stresses including hydrostatic stress, von Mises stress, normal and shear stresses during atomic grinding. Simulation results showed that average values of the stresses are greatly influenced by the radius of the tool trajectory and the grinding depth. Besides the averaged stresses, plane stress distribution was also analyzed, which was obtained by intercepting stresses on the internal planes of the workpiece. For the case of a grinding depth of 25 A and an arc radius 40 A, snapshots of the stresses on the X–Y, X–Z and Y–Z planes showed internal stress concentration. The results show that phase transformation occurred from α- silicon to β- silicon in the region with hydrostatic stress over 8 GPa. Moreover, lateral snapshots of the three-dimensional stress distribution are comprehensively discussed. It can be deduced from MD simulations of stress distribution in monocrystalline silicon with the designed new model that a curved tool trajectory leads to asymmetric distribution and concentration of stress during atomic-scale grinding. The analysis of stress distribution with varying curve geometries and cutting depths can aid fundamental mechanism development in nanomanufacturing and provide theoretical support for ultraprecision grinding.

Unraveling the origin of stress-dependent glass transition temperature in metallic glasses

Abstract: The effect of externally applied stress on glass transition temperature Tg, whose underlying mechanism remains a fascinating and open question in materials science and condense-matter physics, has been explored in a range of metallic glasses (MGs) in the last decades. Recent studies have found that hydrostatic pressure leads to a slight increase in Tg, while uniaxial compression causes notable decrement. However, the origin of stress-dependent Tg variation is largely unknown. Here we report new results of tensile stress effect on Tg in a Ni60Nb40 (at.%) MG through in situ tensile loading and high temperature synchrotron radiation X-ray diffraction measurements. Combining our experimental data with available reports on hydrostatic pressure and uniaxial compression, a universal picture of stress-induced variation in Tg of MGs under various stress states was obtained. We elucidate the physics underlying the stress-induced Tg variation by decomposing the uniaxial tensile stress into a hydrostatic stress combined with two pure shear stresses, and investigating the individual contributions of the decomposed stress states on Tg by atomistic simulations. Our results reveal an enhanced stress effect on Tg through a combination of different stress components. The variation in Tg was linked with stress-aided changes in local atomic configurations. Our theoretical predictions are shown to be in remarkable agreement with experiments, and fill the gap in the current understanding of stress-dependent Tg in MGs.

Numerical appraisal of rock mass anisotropy effect on elastic deformations of a circular tunnel

Abstract: The paper describes effects of anisotropic mechanical properties of rock masses on elastic behaviour of a circular tunnel under both hydrostatic and non-hydrostatic in situ stress states This study is based on field data obtained from two actual case studies In both cases, the rock masses have transversely isotropic structures Hence, a 2D finite element modelling based on the equivalent continuum approach is used for the analysis The tunnel deformation behaviour has been investigated for both isotropic and transversely isotropic conditions To evaluate the degree of anisotropy of rock mass, an “anisotropy index” and a “normalized displacement ratio” have been defined The effect of orientation of the isotropic planes is further investigated The results show that in a hydrostatic stress state, the maximum displacement always occurs in a direction perpendicular to the isotropic planes In this case, three empirical equations have been developed to compute the normalized displacement ratio, the deviation, and the direction of displacement vector at any arbitrary point on the tunnel periphery The results further show that if the anisotropy index increases, the displacement difference (the difference between the maximum and the minimum displacements) on the tunnel walls increases too For the non-hydrostatic stress state, simultaneous effects of stress ratio, anisotropy index, and orientation of isotropic planes on normalized displacements have been investigated In this case, the location of maximum displacement inclines towards the direction of major principal stress This effect is more noticeable when the isotropic planes are oriented at an angle of 90° relative to the direction of the major principal stress The paper also provides an empirical equation to determine the location of maximum displacement on the tunnel walls Finally, the practical application of the results is further illustrated by an actual case study

Transport property evolution during hydrostatic and triaxial compression of a high porosity sandstone

Abstract: Permeability and porosity are two crucial properties related to underground gas storage. This research investigates the permeability and porosity evolution of a Vosges sandstone under different str...

Residual stress effect governing electromigration-based free-standing metallic micro/nanowire growth behavior

Abstract: In this study, the effect of residual stress in a film on the growth behavior of a free-standing metallic micro/nanowire due to electromigration (EM) is examined. The growth of a wire is accompanied by atomic diffusion, accumulation of atoms, and release of compressive EM-induced localized hydrostatic stress due to the accumulation of atoms. Hence, the growth of the wire dominantly depends on the EM-induced localized stress caused by the accumulation of atoms. Because rigid passivation generates a strong localized stress field in the metallic interconnect, with greater accumulation of atoms, the EM-induced localized stress state for wire growth is influenced by passivation conditions, including the thickness and residual stress associated with passivation. Two samples with different passivation thicknesses, resulting in different levels of residual stress, were used to elucidate the influence of passivation conditions on the growth performance of Al microwires. The growth rate was experimentally measured. An x-ray diffraction system was used to obtain the value of residual stress in passivation, demonstrating that a higher absolute value of compressive residual stress results in a lower growth rate. In contrast, a lower absolute value increases the growth rate of the wire and can decrease the delamination risk of the topmost passivation, deposited by sputtering. Contrarily, a passivation that is too thin, resulting in a lower absolute value of compressive stress, increases the risk of passivation crack due to the accumulation of atoms by EM. A suitable passivation thickness for a desired wire growth must be determined based on this finding.

Multiaxial static and fatigue behaviour of elastic and structural adhesives for railway applications

Abstract: Adhesive bonding has become a fundamental manufacturing technology in the railway industry by allowing lightweight design of structures, which is beneficial in terms of energy, environmental and cost efficiency. Due to the polymeric nature of adhesive materials, the presence of stress multiaxiality has a strong effect on their mechanical behaviour under static and fatigue conditions. The present work deals with an investigation of the multiaxial behaviour of an elastic (polyurethane-based) and a structural (epoxy-based) adhesive for railway applications. Samples with different stress multiaxialities (butt joint, scarf joint, and thick adherend shear test joint) were tested under static and fatigue conditions. The stress multiaxiality was defined as the ratio between the hydrostatic stress (p) and the von Mises equivalent stress (q). Finite Element Analysis showed that the mechanical properties of adhesives have a strong influence of the stress multiaxiality distribution of joints with elastic adhesive reaching higher levels of stress multiaxiality. Static tests revealed that elastic adhesive joints are more sensitive to multiaxiality (i.e. higher hydrostatic stresses) than their structural counterparts, especially in samples with larger hindering of lateral contraction. From fatigue test results of both adhesives, namely SN curves, it was demonstrated how multiaxial p-q fatigue diagrams can be constructed. One of the main advantages of this approach is the possibility of predicting the fatigue lifetime of joints regardless of their multiaxial stress state.