Showing papers on "Hydrostatic stress published in 2017"

Strength Behavior, Creep Failure and Permeability Change of a Tight Marble Under Triaxial Compression

Abstract: The coupled hydro-mechanical behaviors of a tight marble are investigated by a series of laboratory tests with continuous gas injection during the hydrostatic compression, triaxial compression and compressive creep tests. Hydrostatic compression tests are firstly carried out in three steps to identify the viscous effect of hydrostatic stress on deformation and permeability of the marble. Coupled triaxial tests are then conducted at a constant axial strain rate under five different confining pressures (P c) with continuous gas injection. Coupled creep behaviors of the marble are also characterized by a constant deviatoric stress test under P c = 30 MPa with gas flowing at a constant injection pressure. The high-stress unloading failure behavior of the marble is finally investigated by an unloading test with a previous multi-step creep phase to realize a high-stress state as well as to investigate the time-dependent deformation of marble under different deviatoric stresses. Experimental results reveal that gas permeability of the marble shows an evident rate-dependent effect in hydrostatic compression. Mechanical behaviors of the tight marble are closely depended on the applied P c in triaxial tests, and its permeability exhibits a decrease phase at initial deviatoric loading and turns to increase at a critical stress corresponding to the initial yield stress. Marble can withstand more important plastic deformation under high P c than under lower ones. Gas flow seems to be more sensitive than the strains to characterize the creep behaviors of the marble. No time-dependent strains are observed when deviatoric creep stress is lower than 50% of its peak strength under P c = 30 MPa.

Ballistic impact response of high strength aluminium alloy 2014-T652 subjected to rigid and deformable projectiles

Abstract: Experimental and numerical studies were conducted to determine the impact response of 15 mm thick AA2014-T652 forged plates in the velocity region from 800 m/s to 1300 m/s Spherical projectiles (10 mm diameter) of hardened steel and soft iron were launched from propellant gun of 30 mm bore diameter and their impact and residual velocities were measured by capturing the impact phenomena with a high speed camera Residual velocities of projectiles were in good agreement with Recht-Ipson analytical model, when kinetic energy of the fragments ejected from the target was accounted for in the energy balance Failure in target plates occurred due to a combination of failure mechanisms such as hydrodynamic flow, spalling, ductile hole growth and scabbing A comprehensive material characterization program was executed to study the plastic flow and failure of the material Tensile tests were carried out on target and projectile materials at different stress triaxialities, strain rates and temperatures The experimental data of stress-strain curves were used to calibrate the material parameters of Johnson-Cook constitutive model, which relates the flow stress of the material to effective plastic strain, strain rate and temperature Fracture strain values were used to calibrate the material parameters of Johnson-Cook failure model, which relates the fracture strain of a material to stress triaxiality, strain rate and temperature Finite element analyses of all the impact experiments were carried out using a two dimensional axisymmetric model Numerical results overestimated the ballistic limit velocities as the quasi-brittle fracture of target could not be captured using Johnson-Cook failure model Limitations of Johnson-Cook failure model were analyzed and numerical simulations were repeated using hydrostatic tensile stress failure model A non-linear equation of state was also introduced in the model for more accurate calculations of hydrostatic stress These modifications resulted in an excellent correlation between experimental and numerical results

Influence of laser nanostructured diamond tools on the cutting behavior of silicon by molecular dynamics simulation

Abstract: In this study, a series of large-scale molecular dynamics simulations have been performed to study the nanometric cutting of single crystal silicon with a laser-fabricated nanostructured diamond tool. The material removal behavior of the workpiece using a structured diamond tool cutting is studied. The effects of groove direction, depth, width, factor, and shape on material deformation are carefully investigated by analyzing normal stresses, shear stress, von Mises stress, hydrostatic stress, phase transformation, cutting temperature, cutting force and friction coefficients. Simulation results show that a cutting tool groove orientation of 60° produces a smaller cutting force, less cutting heat, more beta-silicon phase, and less von Mises stress and hydrostatic stress. Moreover, tools with a smaller groove orientation, groove depth and groove width, and larger groove factor lead to more ductile cutting and an increased material removal rate. However, a cutting tool with a smaller groove width results in more heat during the nanoscale cutting process. In addition, the average temperature of the subsurface increases as groove factor increases, showing that a tool groove accelerates heat dissipation to the subsurface atoms. Furthermore, this V-shape groove cutting is shown to improve material removal ability in nanoscale cutting.

Nature Inspired Strategy to Enhance Mechanical Properties via Liquid Reinforcement

Abstract: Solid-solid interface mechanism understanding of composite inclusions, when extended to solid-liquid interface design of composite using Eshelby theory, indicates a possibility of decreasing effective stiffness with increasing liquid inclusion in a solid matrix. In contrast, experimental evidence in the current paper suggests high stiffness and enhanced dynamic energy absorption in a soft polymer (polydimethylsiloxane) with high bulk modulus liquid inclusions (gallium). The basic deformation mechanism is governed by hydrostatic stress causing shape change of the liquid inclusion in large deformation regime and strain hardening of a soft polymer matrix. In addition, dynamic viscoelasticity and fluid motion also play a significant role. These understandings are developed here based on analytical modeling and a detailed finite element with smooth particle hydrodynamic simulations. The large deformation with viscoelasticity of gallium composite shows higher energy absorption and dissipation. Similar strategies of liquid reinforcement to compliant solid matrices are abundant in nature, for example, the intervertebral discs in the spinal cord and deep sea animal skin and lungs.

A multi-surface plasticity model for ductile fracture simulations

Abstract: The growth and coalescence of micro-voids in a material undergoing ductile fracture depends strongly on the loading path. Void growth occurs by diffuse plasticity in the material and is sensitive to the hydrostatic stress, while void coalescence occurs by the localization of plastic deformation in the inter-void ligaments under a combination of normal and shear stresses on the localization plane. In this paper, a micromechanics-based plasticity model is developed for an isotropic porous material, accounting for both diffuse and localized modes of plasticity at the micro-scale. A multi-surface approach is adopted, and two existing plasticity models that separately account for the two modes of yielding, above, are synthesized to propose an effective isotropic yield criterion and associated state evolution equations. The yield criterion is validated by comparison with quasi-exact numerical yield loci computed using a finite elements based limit analysis procedure. It is shown that the new criterion is in better agreement with the numerical loci than the Gurson model, particularly for large values of the porosity for which the loading path dependence of the yield stress is well predicted by the new model. Even at small porosities, it is shown that the new model predicts marginally lower yield stresses under low triaxiality shear dominated loadings compared to the Gurson model, in agreement with the numerical limit analysis data. Predictions for the strains to the onset of coalescence under proportional loading, obtained by numerically integrating the model, indicate that void coalescence tends to occur at relatively small plastic strain and porosity levels under shear dominated loadings. Implications on the prediction of ductility using the new model in fracture simulations are discussed.

Mechanical properties and microstructure of ultrafine grained commercial purity aluminium prepared by cryo-hydrostatic extrusion

Abstract: CP 99.5% aluminium was processed by cryo-hydrostatic extrusion with true strains up to 3.4 in one pass. The aim was to refine its microstructure and improve its mechanical properties. The properties of the thus processed material were compared with those obtained after the same process but run at room temperature. Cooling the billet with liquid nitrogen combined with water cooling of the extruded wire enabled suppressing partially the dynamic and static structural processes. The grain size was reduced to ~400 nm in the cryo-extrusion, and to ~450 nm in the room-extrusion. In the cryo-extrusion the increase of the yield strength to 168 MPa and the hardness to 56HV0.2 with the respective reduction of the elongation to fracture to 13.6% were obtained. The cryo-cooling effectiveness and the influence of the adiabatic heat generated during the plastic processing on the structure, mechanical properties, hardness, and tensile impact toughness just after hydrostatic extrusion, an also after the post deformation annealing are discussed. In view of the intensive adiabatic heating amounting to 0.57Tm no special improvement of the mechanical properties after the post-deformation annealing was observed. The cryo-cooling became effective at the true strain e>2, where the extrusion pressures clearly differed from the room-extrusion pressures and the defect density substantially increased. After the cryo-hydrostatic extrusion the mechanical properties were comparable to the highest values reported in the literature for cryo-rolling but, since they were obtained in a single deformation step and with twice as large subgrains, the ductility of the extruded aluminium was higher. During the cryo-hydrostatic extrusion conducted at high strains the reduction in ductility of the aluminium is hindered and thanks to the beneficial role of the hydrostatic stress active in the material the structural and mechanical effects which occur during severe plastic deformation are enhanced.

A fiber-reinforced thermoelastic medium with an internal heat source due to hydrostatic initial stress and gravity for the three-phase-lag model

Abstract: Purpose The purpose of this paper is to investigate the effect of a hydrostatic initial stress and the gravity field on a fiber-reinforced thermoelastic medium with an internal heat source that is moving with a constant speed. Design/methodology/approach A general model of the equations of the formulation in the context of the three-phase-lag model and Green-Naghdi theory without energy dissipation. Findings The exact expressions for the displacement components, force stresses, and the thermal temperature for the thermal shock problem obtained by using normal mode analysis. Originality/value A comparison made between the results of the two models for different values of a hydrostatic initial stress as well as an internal heat source. Comparisons also made with the results of the two models in the absence and presence of the gravity field as well as the reinforcement.

Normal load moving on magneto-elastic transversely isotropic half-space with irregular and hydrostatic initial stress

Abstract: The present paper investigates the effect of initial stress, irregularity depth, irregularity factor and magneto-elastic coupling parameter on the dynamic response due to a normal moving load with constant velocity on the free surface of an irregular magneto-elastic transversely isotropic half-space under the state of hydrostatic initial stress. The expressions for normal stress and shear stress are obtained in closed form analytically. The considerable effect of initial stress, irregularity depth, irregularity factor and magneto-elastic coupling parameter on normal stress and shear stress are computed numerically and depicted by means of graphs. Moreover, comparative study highlighting the effect of various types of irregularity viz. rectangular irregularity, parabolic irregularity and no irregularity on the normal stress and shear stress is a key feature of the study.

Elastic waves in particulate glass-rubber mixture : Experimental and numerical investigations/studies

Abstract: In this paper we study by wave propagation the elastic response of granular mixtures made of soft and stiff particles subjected under hydrostatic pressure/stress. This allows inferring fundamental properties of granular materials such as elastic moduli and dissipation mechanisms. We compare physical experiments in a triaxial cell equipped with piezoelectric wave transducers and Discrete Element Method simulations (DEM). In the experimental part, dense, static packings made of monodisperse glass and rubber beads are prepared at various levels of hydrostatic stress and species fractions. Small perturbations are generated on one side and the time of flight through the glass-rubber mixtures are measured to quantify the effect of the mixture composition on the elastic moduli. Interestingly, the experiments show that the behavior is non-linear and nonmonotonic with increasing percentage of rubber particles. Wave velocity and modulus remain fairly constant when increasing the fraction of rubber to 30%, while they experience a sudden drop between 30% and 60%, to become again constant between 60% to 100%. DEM simulations offer deeper insights into the micromechanics in and at the transition between the glass- and rubber-dominated regimes. The simplest analysis with Hertzian spherical particles of different stiffness is performed as a preliminary step. The behavior of mixtures with high glass content is very well captured by the simulations, without need of any additional calibration, whereas the complex interaction between rubber and glass leave open questions for further study.

Determination of stress intensity factor with direct stress approach using finite element analysis

Abstract: In this article, a direct stress approach based on finite element analysis to determine the stress intensity factor is improved. Firstly, by comparing the rigorous solution against the asymptotic solution for a problem of an infinite plate embedded a central crack, we found that the stresses in a restrictive interval near the crack tip given by the rigorous solution can be used to determine the stress intensity factor, which is nearly equal to the stress intensity factor given by the asymptotic solution. Secondly, the crack problem is solved numerically by the finite element method. Depending on the modeling capability of the software, we designed an adaptive mesh model to simulate the stress singularity. Thus, the stress result in an appropriate interval near the crack tip is fairly approximated to the rigorous solution of the corresponding crack problem. Therefore, the stress intensity factor may be calculated from the stress distribution in the appropriate interval, with a high accuracy.

Stresses due to moving load on the surface of an irregular magneto-elastic monoclinic half-space under hydrostatic initial stress

Abstract: The present article investigates the characteristic of moving load and effect of irregularity, hydrostatic stress, and magneto-elastic coupling parameter on the stresses developed due to moving load on the surface of an irregular magneto-elastic monoclinic half-space subjected to the friction associated with a rough surface. Boundary conditions are listed for which corresponding analyses can be performed analogously. The expressions for stresses are obtained analytically. Numerical computation of the obtained results are carried out for three different materials with monoclinic symmetry (lithium niobate, lithium tantalate, and quartz); the significant effects of affecting parameters on these relations are distinctly marked by means of graphs.

Experimental research on strain-hardening effect of 40CrMnMo in turning process based on the Oxley-Welsh theory

Abstract: Based on the Oxley-Welsh theory, this paper focuses on the influence of strain-hardening behavior of oil country tubular goods (OCTG) 40CrMnMo (workpiece material) on a cutting model and the influence of cutting parameters on a stress distribution in the shear zone and the chip formation in turning processes by establishing a reasonable cutting experimental platform. Then a relationship model between the shear flow stress and the hydrostatic stress in the shear zone is built, and the size effect and strain-hardening effect of the shear zone in the turning process are explained reasonably, especially for the influence of them on the chip morphology. The research results show that the slope K of the relationship model is not caused by the Bridgman effect but results from a synergistic action of the size effect and the strain-hardening effect in the shear zone of the workpiece material. The workpiece 40CrMnMo in the shear zone can obtain a better resistance to the inhomogeneous plastic deformation under a certain cutting condition.

Simulation experiment device for hydrostatic stress corrosion electrochemical action of magnesium alloy and experiment method of simulation experiment device

Abstract: The invention discloses a simulation experiment device for hydrostatic stress corrosion electrochemical action of a magnesium alloy and an experiment method of the simulation experiment device The simulation experiment device comprises an electrolyte tank, a test device, a computer terminal and an electrochemical workstation, wherein the test device is placed in the electrolyte tank; electrolyte in the electrolyte tank is forced by a high-precision peristaltic pump to cyclically crawl The simulation experiment device is simple in overall structure, and static pressure can be continuously adjusted The experiment method is to adjust a screw rod of the test device to apply different sizes of hydrostatic stress on a magnesium alloy test sample, and then adjust the flow rate of the electrolyte through the peristaltic pump to really simulate a living body microenvironment in vitro, so as to carry out a series of electrochemical test operations on open circuit potential, a polarization curve, an alternating current impedance spectrum and electrochemical noise; test results are real and stable, and the reappearance and the reproducibility are high The simulation experiment device and the simulation method can fully meet the requirement of the magnesium alloy for relevant research use on bearing of the hydrostatic stress in a human bone microenvironment

Synthesis, characterization and forming behavior of hybrid copper matrix composites produced using powder metallurgy

Abstract: Hybrid copper matrix composites containing 5 wt.% of titanium dioxide and varying graphite content (0 wt.%, 2 wt.% and 4 wt.%) were synthesized using powder metallurgy. Metallurgical studies were carried out to examine the presence and distribution of reinforcements in the copper matrix. To investigate the forming behavior of the sintered composite preforms, cold upset tests were conducted from which the true axial stress, the true hoop stress, the true hydrostatic stress and the true effective stress were evaluated and their relationship with the true axial strain was analyzed and presented. It is observed that the increase in addition of weight percentage of graphite into the copper matrix increases the true axial, the true hoop, the true hydrostatic and the true effective stresses. The variation of hardness, strength coefficient and strain hardening with respect to the addition of graphite content is also evaluated and reported.

A numerical model to assess the role of crack-tip hydrostatic stress and plastic deformation in Environmental Assisted Fatigue Cracking

Abstract: To better understand the mechanics of environmentally assisted cracking, and particularly hydrogen embrittlement, a correct description of the hydrostatic stress field is indispensable. The concentration of hydrogen in the proximity of the crack tip is indeed dependent of the hydrostatic stress effect on the microstructural lattice of the material. The overall parameters of the hydrostatic stress, including peak value, its location, gradient, and distribution size are fundamental to assess the effect on hydrogen distribution near the crack tip, specifically considering hydrogen-enhanced decohesion mechanism, or the HEDE mechanism. Hydrostatic stress is hence widely analysed in studies related to hydrogen embrittlement contribution in stress corrosion cracking or corrosion fatigue of metallic alloys. However, recent studies highlighted that the hydrogen-enhanced local plasticity (HELP) mechanism can be more relevant than HEDE in hydrogen-assisted fatigue failure of metallic alloys. In order to investigate the contribution of the HELP mechanism, detailed finite element modelling is reported for notched Ti-6Al-4V specimens, based on experimental fatigue data. The material is modelled with elastic-perfectly plastic behaviour, reproducing actual geometry of the notches and the fatigue crack, from measurements and replicas conducted during testing. Strain data are obtained in initial and final crack configuration, to discuss the HELP contribution on environmentally assisted cracking, and compare it with HEDE contribution linked to hydrostatic stress.

Far-Field Boundary Conditions for Calculation of Hole-Drilling Residual Stress Calibration Coefficients

Abstract: The Hole-Drilling method for residual stress measurement, both in its standard version based on strain gauge rosettes (ASTM E837-08e1 2008) and its derivative using optical methods for estimating the displacement field around the hole (Baldi (2005) J Eng Mater Technol 127(2):165–169; Schajer and Steinzig (2005) Exp Mech 45(6):526–532; Schajer (2010) Exp Mech 50(2):159–168), relies on numerical calibrated coefficients (A and B) to correlate the experimentally acquired strains (displacements) with residual stress components. To estimate the A and B coefficients, two FEM (Finite Element Method) computations are required, the former related to a hydrostatic stress state, the latter to a pure shear case. Both can be implemented using either a semi-analytical approach (i.e. an axis-symmetric mesh expanded in the tangential direction using a Fourier series) or a tri-dimensional mesh, usually exploiting the double symmetry of the problem. Whatever the approach selected, the definition of constraints to be applied to the outer boundary is critical because the hole-drilling method assumes an infinite plate, thus both the usual solutions—fully constrained or free boundaries—are unable to correctly describe the theoretical situation. In the following, the problem of correct simulation of the infinite domain will be discussed and two simple and effective solutions will be proposed.