Showing papers on "Hydrostatic stress published in 1989"

Numerical analysis of hydrogen transport near a blunting crack tip

Abstract: T he hydrogen transport problem is studied in conjunction with large deformation elastic—plastic behavior of a material. Oriani's equilibrium theory is used to relate the hydrogen in traps (micro-structural defects) to concentration in normal interstitial lattice sites (NILS). The resulting non-linear transient hydrogen diffusion equations are integrated using a modified backward Euler method. Coupled diffusion and plastic straining is analysed with this numerical procedure in the area around a blunting crack tip. A uniform NILS concentration as dictated by Sievert's law at the pressure and temperature of interest is used as initial condition throughout the body. The crack is initially blunted by plane strain mode I (tensile) loading. The finite element results show that hydrogen residing at NILS is generally very small in comparison with the population that develops in trapping sites near the crack surface. That is, lattice diffusion delivers the hydrogen but it is predominantly the trapping that determines its distribution at temperatures of interest. The predominance of trapped hydrogen over lattice concentration prevails even in the case when hydrogen migrates under steady state conditions. Hence, the hydrostatic stress effect is less important than traps created by plastic straining as far as the creation of high total hydrogen concentration is concerned. The trapping site locations and the temperature determine the amounts and locations of high hydrogen concentrations. Consequently, ahead of a blunting crack tip, the total hydrogen concentration and plastic strain diminish with distance from the crack tip whereas the hydrostatic stress rises. This would seem to have significant consequences for fractures induced by the presence of hydrogen.

Magma transport and reservoir formation by a system of propagating cracks

Abstract: A system of propagating cracks may explain magma transport and the evolution of a volcano. This paper considers only a basaltic magma. The system is controlled by two boundary conditions: the stress field, and the production rate of the magma-filled cracks in the mantle. Numerical solutions of crack propagation for various stress conditions, with a constant production rate high enough to coalesce isolated cracks, were performed, and the results applied to different tectonic conditions. For the hydrostatic stress conditions, most magma-filled cracks beneath a polygenetic volcano become trapped either in the lower crust, because there the density difference between magma and the host rocks (Δρ) becomes suddenly small, compared with that in the mantle, or trapped in the upper crust, because there Δρ is near to zero. Magma traps composed of such cracks may grow into magma reservoirs if the production rate of cracks in the mantle is large. If horizontal stress with a vertical gradient is superimposed on the hydrostatic condition in the crust, that is, tensile stress which increases upward or compressional stress which increases downward, magmafilled cracks, even if the density of magma is higher than that of the crust, may ascend directly without trapping. When the crust undergoes relative tension, magma-filled cracks may become trapped. Then, the lower part of the trap may grow into a magma reservoir, while the upper part may become filled with dikes. When the production rate of cracks is small, an initial magma-filled crack can rise directly to the surface only when the stress with a gradient is superimposed as mentioned above, or when the average density in a crack decreases, owing to, for example, vesiculation of volatile components.

The effect of void shape on void growth and ductility in axisymmetric tension tests

Abstract: The effect of void growth and ductility in axisymmetric tension tests was examined theoretically. The inelastic response of periodic arrays of initially oblate spheroidal voids in a homogeneous isotropically hardening matrix was analyzed for loading histories representative of axisymmetric tension tests. The failure strains were taken as the point where the deformation mode became that of uniaxial strain. All subsequent deformation was localized to ligaments between the voids. The results showed that, for stress states with a low hydrostatic stress component, the void growth rates were governed primarily by the void dimension perpendicular to the tensile axis. They also indicated that the failure strains were proportional to the projected area of the voids onto a plane perpendicular to the tensile direction, in agreement with published experimental results. For stress states with higher triaxiality, the influence of the initial aspect ratio was diminished.

Influence of Stress Dependent Elastic Moduli On Plane Strain Solutions For Boreholes

Abstract: The elastic moduli of porous or clastic rocks are not always constant but may increase with increasing minor principal stress. Previous studies of circular excavations in hydrostatic stress fields have shown that, for this class of rock, classical constant modulus linear elastic theory can lead to erroneous predictions of the initiation and extent of failure around excavations. A plane strain finite element Program has been adapted to extend these solutions to the case in which the in situ stresses in the plane of the excavation cross-section are not hydrostatic. Previous conclusions are confirmed for non-axisymmetric loading. The influences of the ratio of the major and minor principal stresses in the plane of the cross-section and of the internal borehole pressure are evaluated. The boundary conditions used are shown to influence the numerical results obtained for cases involving stress dependent elastic Moduli.

Analytical estimation for hydrogen induced stress corrosion cracking in duplex stainless steel and its weld.

Abstract: Hydrogen embrittlement type stress corrosion cracking of duplex stainless steel and its welded joint in acid environment has been studied. It was found that the electrode potential and ferrite content affected the susceptibility of duplex stainless steel to HESCC. At a less noble potential the time to failure decreased, while it increased at the noble side. Also, it was found that the crack growth rate decreased significantly with decreasing ferrite content. When the ferrite content was less than 40%, cracks did not propagate. The observation of the fracture surface showed the cleavage type fracture in α phase and the ductile fracture in γ phase. This means that γ phase would block the cracks propagating through α phase. According to the analysis of hydrogen diffusion, it was shown that the concentrations of hydrostatic stress and hydrogen appeared in α phase near the tip of γ phase, and this position became the most probable crack initiation site for HESCC.

Methods of Determining Stress-Strain Curves at Elevated Temperatures

Abstract: Determination of flow curves by tension, torsion and compression testing is discussed. The difficulties, advantages and disadvantages associated with each method are described.

Numerical Schemes in FEM for Plane Strain Deformation of Incompressible Solids with Applications to EPFM

Abstract: A finite element program for incompressible deformation was developed to obtain scalable fully plastic solutions of various flawed geometries. An automatic mesh generation scheme applicable to fracture mechanics is presented. In many respects, the method used is efficient and economic. For. instance, displacements and crack parameters can be determined without the computation of the hydrostatic stress. Some examples are presented.

A Micromechanics Based Model for the Creep Of Ice Including the Effects of General Microcracking

Abstract: A constitutive law for the creep response of ice during general microcracking is developed in this paper. A simple three-bar structure is introduced for uniaxial loading and used to identify the general features of the material response. The processes of crack nucleation and creep deformation of heavily microcracked material are examined in detail and a self consistent model is developed to describe the material response under multiaxial states of stress. The observed effect of hydrostatic pressure on the material response is predicted by the model.