Showing papers on "Hydrostatic stress published in 2009"

Common Evolution of Mechanical and Transport Properties in Thermally Cracked Westerly Granite at Elevated Hydrostatic Pressure

Abstract: Increasing the damage and crack porosity in crustal rocks can result in significant changes to various key physical properties, including mechanical strength, elastic and mechanical anisotropy, and the enhancement of transport properties. Using a Non-Interactive Crack Effective Medium (NIC) theory as a fundamental tool, we show that elastic wave dispersion can be inverted to evaluate crack density as a function of temperature and is compared with optically determined crack density. Further, we show how the existence of embedded microcrack fabrics in rocks also significantly influences the fracture toughness (KIC) of rocks as measured via a suite of tensile failure experiments (chevron cracked notch Brazilian disk). Finally, we include fluid flow in our analysis via the Gueguen and Dienes crack porosity-permeability model. Using the crack density and aspect ratio recovered from the elastic-wave velocity inversion, we successfully compare permeability evolution with pressure with the laboratory measurements of permeability.

Brittle versus ductile deformation as the main control on the transport properties of low-porosity anhydrite rocks

Abstract: [1] An experimental approach has been taken to study the prefailure fluid flow properties of borehole samples of low-porosity anhydrites and the evolution of permeability (k) during deformation leading to brittle and ductile failure. The permeability measured under hydrostatic stress conditions prior to loading at room temperature is generally low (k = 10−21–10−19 m2) and displays an anisotropy and pressure sensitivity controlled by grain size and fabric orientation (foliation). Triaxial loading test results show that the brittle-ductile transition occurs for effective pressure Pe < 20 MPa and is almost independent of fabric orientation and grain size. All samples, whether deforming in a brittle (localized deformation) or ductile (distributed deformation) mode, show dilatancy after an initial phase of compaction. During loading, the k starts to increase prior to the phase of sample dilation and before the yield stress is attained. The k rise is characterized by an upward concave trend, prior to localized deformation (brittle failure), and by a downward concave trend, during distributed deformation (ductile failure). The k increase prior to brittle failure is about 1 order of magnitude higher than during ductile failure. We interpret the different shape of the k curve as due to the observed different degrees of fracture connectivity (widespread development of intragranular and intergranular fractures) reached during brittle (low) and ductile (high) deformation, respectively. Our experimental results imply that for low-porosity rocks the mode of failure, controlled by Pe, has an overwhelming effect on the evolution of permeability, compared to other factors such as grain size and fabric orientation.

On measuring the strength of metals at ultrahigh strain rates

Abstract: The strain rate sensitivity of materials is normally measured through a combination of quasistatic, Hopkinson bar, and pressure-shear experiments. Recent advances in uniaxial strain ramp loading provide a new means to reach strain rates significantly higher than achievable in pressure-shear experiments. One way to determine strength in ramp loading is by comparing the uniaxial stress-strain response to an appropriate pressure-density response obtained from an equation of state for the material. Using this approach, strengths for aluminum are obtained for strain rates of 105–108 s−1. Two issues arise in this calculation: heating due to plastic work and the effect of the superimposed hydrostatic stress on the strength. Heating due to plastic work is calculated and accounted for within the context of the equation of state for the material in a straightforward manner, but neglecting this heating can lead to significant errors in the calculated strength at higher compression levels. A simple scaling of strength with the pressure-dependent shear modulus is utilized to estimate the strength at zero pressure for ramp loading and pressure-shear experiments. When examined in this manner, the strain rate dependence of aluminum is found to be less than previously reported, with little increase in strength below strain rates of about 107s−1. The effects on ramp loading strength measurements of heating due to plastic work and of hydrostatic pressure are also examined for copper and tantalum using simple equation of state and strength models. The effect of plastic heating is similar for the three materials for a given strain level but quite different for a constant stress, with aluminum showing greater effects than the other materials. The effect of hydrostatic pressure in ramp loading experiments is similar for all three materials, but the effect is likely to be much greater in pressure-shear experiments for aluminum than the other materials.

Three energy-based multiaxial HCF criteria for fatigue life determination in components under random non-proportional stress fields

Abstract: The prominent high cycle fatigue (HCF) criteria have been generally proposed based on definition of an equivalent stress that is mainly a modified version of a static failure criterion or a static yield function. One or more effects including the mean normal and shear stresses, different phase shifts or random frequencies of the stress components, relative instantaneous time variations of the stress components, relative time locations of the extrema of the time histories of the stress components, etc. have not been considered by many of the previously proposed criteria. In the present paper, based on the proposed instantaneous stress amplitude and mean stress concepts, three new energy-based HCF criteria are proposed to overcome the mentioned shortcomings. A relevant fatigue life assessment algorithm is proposed and results of the prominent criteria are compared with results of the proposed criteria as well as the experimental results prepared by the author. To introduce a comprehensive study, the criteria are examined for components with complicated geometries under proportional, non-proportional and random loadings. Results confirm the efficiency and accuracy of the proposed criteria. Furthermore, it is deduced that the Liu-Zenner type criteria which include the hydrostatic stress implicitly are more accurate than the Papadopoulos-type criteria that consider the hydrostatic stress explicitly.

Basic characteristics and development of yield criteria for geomaterials

Abstract: The yield criteria of geomaterials play a crucial role in studying and designing the strength of materials and structures. The basic characteristics of yield criteria for geomaterials need to be studied under the framework of continuum mechanics. These characteristics include the effects of strength difference (SD) of materials in tension and compression, normal stress, intermediate principal stress, intermediate principal shear stress, hydrostatic stress, twin-shear stresses, and the convexity of yield surface. Most of the proposed yield criteria possess only one or some of these basic characteristics. For example, the Tresca yield criterion considers only single-shear stress effect, and ignores the effect of SD, normal stress, intermediate principal stress, intermediate principal shear stress, hydrostatic stress, and twin-shear stresses. The Mohr-Coulomb yield criterion accounts for the effect of SD, normal stress, single-shear stress and hydrostatic stress, but disregards the effect of intermediate principal stress, intermediate principal shear stress, and twin-shear stresses. The basic characteristics remain to be fully addressed in the development of yield criterion. In this paper, we propose a new yield criterion with three features, that is, newly developed, better than existing criteria and ready for application. It is shown that the proposed criterion performs better than the existing ones and is ready for application. The development of mechanical models for various yield criteria and the applications of the unified strength theory to engineering are also summarized. According to a new tetragonal mechanical model, a tension-cut condition is added to the unified strength theory. The unified strength theory is extended to the tension-tension region.

Fundamental and applied aspects of laser surface engineering

Abstract: The contribution to this jubilee issue of IJMR concentrates on the analysis of laser surface treatments. In particular laser cladding processes using coaxial and side set-ups are evaluated to determine the optimal processing window that prioritizes the clad quality and the efficiency of the coating method. The microstructural features in these so-called processing maps are illustrated with Inverse Pole Figure (IPF) mapping. texture pole figures and Electron Backscatter Pattern (EBSP) techniques. The distribution of stresses inside Co-based laser coating is measured with the so-called three-dimensional X-ray Diffraction microscopy (3DXRD) and it is found very dispersive, i.e. stresses may change from one grain to another considerably. The mean value of observed hydrostatic stress in all grains at specific laser track depth is gradually changed from tensile state in the upper part of the coating to the compressive state in its lower part close to the substrate. On the other hand the average value of shear stress component does not show any substantial chan-e with depth.

Finite element formulation for shear modulus reconstruction in transient elastography

Abstract: In order to image the shear modulus in soft tissue, for medical diagnosis, given one component of measured displacements as a function of time on an imaging plane, two related direct finite element-based inversion algorithms are presented. One algorithm is based on the governing equations expressed in the frequency domain, and the other is in the time domain. The algorithms consider the complete equations of isotropic, small deformation, elasto-dynamics, where the hydrostatic stress is also treated as an unknown. The algorithms reconstruct both the shear modulus and hydrostatic stress fields, and regularization is used to stabilize the hydrostatic stress recovery. An algorithm is also developed for reconstructing the second displacement component, while simultaneous finding a smooth approximation to the measured displacement component to reduce noise. Shear modulus reconstruction results from both algorithms, using experimental ultrasound measurements on a tissue-mimicking phantom, are presented, and the ...

The elastic stability, bifurcation and ideal strength of gold under hydrostatic stress: an ab initio calculation.

Abstract: In this paper, we employ an ab initio density functional theory calculation to investigate the elastic stability of face-centered cubic Au under hydrostatic deformation. We identify the elastic stiffness constant Bijkl as the coefficient in the stress–strain relation for an arbitrary deformed state, and use it to test the stability condition. We show that this criterion bears the same physics as that proposed earlier by Frenkel and Orowan and agrees with the Born–Hill criterion. The results from those two approaches agree well with each other. We show that the stability limit, or instability, of the perfect Au crystal under hydrostatic expansion is not associated with the bulk stiffness modulus as predicted in the previous work; rather it is caused by a shear instability associated with the vanishing rhombohedral shear stiffness modulus. The deviation of the deformation mode from the primary hydrostatic loading path signals a bifurcation or symmetry breaking in the ideal crystal. The corresponding ideal hydrostatic strength for Au is 19.2 GPa at the Lagrangian expansion strain of ~0.06. In the case of compression, Au remains stable over the entire pressure range in our calculation.

Probabilistic high cycle fatigue behaviour of nodular cast iron containing casting defects

Abstract: Theoretical and experimental investigations were combined to characterize the influence of surface casting defects (shrinkages) on the high cycle fatigue (HCF) reliability. On fracture surfaces of fatigue samples, the defect is located at the surface. The shape used for the calculation is a spherical void with variable radius. Finite-element simulations were then performed to determine stress distribution around defects for different sizes and different loadings. Correlated expressions of the maximum hydrostatic stress and the amplitude of the shear stress were obtained by using the response surface technique. The loading representative point in the HCF criterion was then transformed into a scattering surface, which has been obtained by a random sampling of the defect sizes. The HCF reliability has been computed by using the Monte Carlo simulation method. Tension and torsion fatigue tests were conducted on nodular cast iron with quantification of defect size on the fracture surface. The S-N curves show a large fatigue life scattering; shrinkages are at the origin of the fatal crack leading to the final failure. The comparison of the computed HCF reliability to the experimental results shows a good agreement. The capability of the proposed model to take into account the influence of the range of the defect sizes and the type of its statistical distribution has been demonstrated. It is shown that the stress distribution at the fatigue limit is log-normal, which can be explained by the log-normal defect distribution in the nodular cast iron tested.

Damage percolation modeling of void nucleation within heterogeneous particle distributions

Abstract: A damage percolation model has been developed to simulate damage development and ductile fracture within heterogeneous particle distributions. The percolation model has been extended to incorporate the stress state, material softening and a coalescence model linking the void geometry with the applied stress. A nucleation criterion is developed where void nucleation is related to the particle morphology, hydrostatic stress and shear loading. The nucleation criterion is calibrated by subjecting three particle fields from an aluminum–magnesium alloy to different loading conditions to achieve agreement with experimental forming limit data. The calibrated percolation model can successfully recreate the forming limit curve of the material and, more importantly, provide predictions for the average area fraction and size of nucleating particles which are in good agreement with the available experimental data.

Impact resistant polymeric glasses using compressive pre‐stress

Abstract: The application of an externally applied pre-stress on impact properties is studied on polymethyl methacrylate (PMMA) organic glass. Samples are tested under equi-biaxial compression, simple shear and a combination of biaxial compression and shear. Equi-biaxial compression is shown to increase the threshold stress level for projectile penetration whereas shear pre-stress has a large effect on the overall energy absorbed during an impact. There is also an apparent interaction observed between compression and shear to dramatically increase the threshold stress. Pre-stressed laminates show an increase in damage area because of the unique formation of a secondary cone. Higher levels of compressive equi-biaxial pre-stress significantly increase the stress relaxation time because of the corresponding increase in hydrostatic stress. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Finite element analysis of deformation characteristics in heavy slab rolling

Abstract: Plastic deformation that occurs in a heavy slab during plane-strain rolling was investigated by the finite element analysis. A cylindrical pore was assumed to be located along the transverse direction of a slab. The effective strain was found to be the largest at the sub-surface layer and the smallest at the middle layer, where the shear strain developed the least. Pore closure was most difficult at the middle layer. This is where hydrostatic stress in addition to effective strain developed the least. Rolling torques, rolling forces and pressure distributions at the roll/slab interface were investigated as well, under various conditions.

Micro‐statics and micro‐kinematics of capillary phenomena in dense granular materials

Abstract: A capillary stress tensor and an effective stress tensor are defined in DEM simulations of spheres packings with capillary effect. It is shown that induced fabric anisotropy results in an anisotropy of the capillary stress, so that any stress‐like variable used to represent the effect of capillarity in granular materials should be represented by a non‐spherical tensor. The response of a sample to small isotropic stress increments is also presented, the loading being imposed either by a small variation of the stress imposed at boundaries (method A) or by a variation of matric suction (method B). The comparison of the results, with emphasis on micromechanical aspects, shows some differences between the results obtained with methods A and B, even though the increment of effective stress is the same in both cases. The effective stress concept in unsaturated granular materials is questionned on the basis of these results.

Study of permeability evolutions in low permeability media under different stresses and temperatures

Abstract: Determination of permeabilities of low permeability rock(≤10-18 m2) under different stress conditons and temperatures is the basis of stability evaluation in geological disposal of radioactive waste,underground gas and oil storage tanks and construction of deep cavern groupsBased on the developed low permeability test equipment(T-M-PTS),gas transport properties of typical Jinping marbles are studied under different hydrostatic pressures and temperaturesBased on microcrack linkage model,evolution of permeability is also studied theoreticallySome results are achieved:(1) Klinkenberg effect is remarkable when gas transport through dense rocks,and should be considered;(2)based on statistic theory and percolation theory,a 3-D penny-shaped microcrack moded has been developed to predict and investigate the evolution of permeability due to microcrack closure in compressive regime;(3) permeability decreases as hydrostatic stress increases because of the decrease of porosity under compression,and permeability decreases faster at the first stage of compression;and(4) permeability decreases from 219×10-21 m2 to 39×10-21 m2 as temperature increases from 15℃ to 40℃ because of the decrease of elasticity,which lead to decrease of porosity under the same hydrostatic stress

Study of critical condition of borehole instability in granite under high temperature and high pressure

Abstract: Utilizing the 20 MN servo-controlled triaxial rock testing machine with high temperature and high pressure developed by the authors and using optical observation instrument of borehole deformation made by theory of optics,the intensive study of deformation laws and instability critical conditions of large-size granite with φ 40 mm borehole samples of φ 200 mm×400 mm within 600 ℃ and triaxial hydrostatic stress in depth of 6 000 m is carried out The research results are as follows:(1) The deformation of borehole in granite bored with the increase of temperature and stress performance is obvious at different stages with high temperature and high pressure Within hydrostatic stress of 4 000 m in depth and temperature at 400 ℃,drilling performance is obvious at the moment of viscoelastic stage Although borehole diameter reduces but still in stable condition, and damage does not occur The hydrostatic stress of 4 000–5 000 m in depth and the temperature at 400 ℃– 500 ℃,the deformation of borehole is at viscoelastic-plastic stage and the surrounding rock near drilling shows a destructive trend,and the aperture begins to increase Under 5 000 m hydrostatic stress in depth and temperature more than 500 ℃ when the cracked rock-like particles block in plastic zone gradually fall down from the hole wall,the borehole is in damage (2) When the surrounding rock of granite in more than creep threshold of stress and creep threshold of temperature,which is over hydrostatics stress of 5 000 m in depth and temperature than 500 ℃,the borehole is in destruction and the collapse of a borehole phenomenon occurs At the same time, the particles of surrounding rock in granite are falling down;and the hole diameter is increasing (3) At different temperatures and hydrostatic stresses in depths,the critical hydrostatic stress of rheodestruction of surrounding rock is 5 000–6 000 m (that is 125–150 MPa);and the critical temperature of rheodestruction is 500 ℃–600 ℃ The failure modes are of compression fracture,transpression or both of them (4) The critical condition of borehole instability in granite under high temperature and stress is hydrostatic stress of 4 000–5 000 m in depth and temperature at 400 ℃–500 ℃ These research can provide a scientific basis and theoretical guidance for hot dry rock(HDR) development to utilize the drilling in the maintenance of stability and Chinese continental scientific drilling(CCSD) in the aspect of ultra-deep hole and deep hole in the construction of drilling encountered in the process of stability

Theoretical Strength of Face-Centred-Cubic Single Crystal Copper Based on a Continuum Model

Abstract: The constitutive relation of single crystal copper based on atomistic potential is implemented to capture the nonlinear inter-atomic interactions. Uniaxial loading tests of single crystal copper with inter-atomic potential finite-element model are carried out to determine the corresponding ideal strength using the modified Born stability criteria. Dependence of the ideal strength on the crystallographic orientation is studied, and tension-compression asymmetry in ideal strength is also investigated. The results suggest that asymmetry for yielding strength of nano-materials may result from anisotropic character of crystal instability. Moreover, the results also reveal that the critical resolved shear stress in the direction of slip is not an accurate criterion for the ideal strength since it could not capture the dependence on the loading conditions and hydrostatic stress components for the ideal strength.

Simulation of Stress-Corrosion Cracking by the Cohesive Model

Abstract: The effect of hydrogen on the mechanical behaviour is twofold: It affects the local yield stress and it accelerates material damage. On the other hand, the diffusion behaviour is influenced by the hydrostatic stress, the plastic deformation and the strain rate. This requires a coupled model of deformation, damage and diffusion. The deformation behaviour is described by von Mises plasticity with pure isotropic hardening, and crack extension is simulated by a cohesive zone model. The local hydrogen concentration, which is obtained from the diffusion analysis, causes a reduction of the cohesive strength. Crack extension in a C(T) specimen of a ferritic steel under hydrogen charging is simulated by fully coupled diffusion and mechanical finite element analyses with ABAQUS and the results are compared with test results.

Coupled Analysis of Hydrogen Transport Within ABAQUS

Abstract: In this paper, the coupled model with hydrogen transport and elasto-plasticity behavior is introduced. This model is implemented to the general-purpose FE code, ABAQUS, via the user-defined subroutine UMAT and UMATHT. In UMAT, the spatial gradients of hydrostatic stress and hydrogen induced deformation are calculated, and then are passed into UMATHT. Heat transfer equation within UMATHT is substituted by hydrogen transport equation including the effects of stress states and strain hardening. To validate this model, the finite element analyses coupled with hydrogen transport and mechanical loading are performed for the boundary layer specimens with low and high strength steel properties. The FE results are compared with the previous studies by Taha and Sofronis (2001).

Inherent Shear-Dilatation Coexistence In Metallic Glass

Abstract: Shear deformation can induce normal stress or hydrostatic stress in metallic glasses [ Nature Mater. 2 ( 2003) 449, Intermetallics 14 ( 2006) 1033]. We perform the bulk deformation of three-dimensional Cu46Zr54 metallic glass (MG) and Cu single crystal model systems using molecular dynamics simulation. The results indicate that hydrostatic stress can incur shear stress in MG, but not in crystal. The resultant pronounced asymmetry between tension and compression originates from this inherent shear-dilatation coexistence in MG.