Showing papers on "Hydrostatic stress published in 1994"

Analysis of elastoplastic sharp notches

Abstract: In this paper the elastoplastic solutions with higher-order terms for apex V-notches in power-law hardening materials have been discussed. Two-term expansions of the plane strain and the plane stress solutions have been obtained. It has been shown that the leading-order singularity approaches the value for a crack when the notch angle is not too large. In plane strain cases the elasticity does not enter the second-order solutions when the notch opening angle is too small. For a large notch angle, the two-term expansions of the plane strain near-tip fields are described by a single amplitude parameter. The plane stress solutions generally contain the elasticity terms. The boundary layer formulations based on the small-strain plasticity theory confirm that a dominance zone exists ahead of the notch tip. Finite element results give good agreement to the asymptotic solutions under both plane strain and plane stress conditions. The second-order terms cannot improve the predictions significantly. The near-tip fields are dominated by a single parameter. Finite element calculations under the finite strain J2-flow plasticity theory revealed that the finite strains can only affect local characterization of the asymptotic solution. The asymptotic solution has a large dominance zone around the notch tip. For an apex notch bounded to a rigid substrate the leading-order singularity falls with the notch angle significantly more slowly than in the homogeneous material. It vanishes at the notch angle about 135° for all power-hardening exponents. The elasticity effects enter the second-order solutions when the notch angle becomes large enough. The tip fields are characterized by the hydrostatic stress and the shear stress ahead of the notch.

Nylon-6/rubber blends, Part III: Stresses in and around rubber particles and cavities in a nylon matrix

Abstract: The stress field around a rubber particle and a cavitated particle in a nylon/rubber blend has been studied using an analytical and a finite element approach. Attention was paid to the influence of the mechanical properties of the dispersed phase and the applied stress state. The results show that the choice of the bulk modulus of the elastomer is crucial. It appeared that especially with a triaxial stress, the Von Mises stress increased strongly upon cavitation (a more than five-fold increase close to the particle) while the hydrostatic stress only increased slightly. Also, the stresses in particles in the neighbourhood of a cavity have been calculated. Stresses in particles lying in or close to the equatorial plane of the cavity were higher than stresses in the other particles

Effect of hydride precipitation on the elastoplastic stress field near a crack tip

Abstract: Dissolved hydrogen in zirconium diffuses up the hydrostatic stress gradient and forms hydride platelets in the region of high stress. The hydride formation is accompanied by an expansion. The effect of hydride expansion on the elastoplastic stress, strain and displacement fields, is investigated in this paper by a finite-element discretization technique. It is shown that hydride formation causes an elastic unloading in the crack tip (reduction of the peak stress) and appearance of a peak stress at the front end of the hydride. Further unloading occurs along the hydride platelet, and depending on the hydride expansion, reversed plastic deformation may take place. Effects of the hydride length, its location with respect to the crack tip, and geometry of the front end extremity of the hydride, are also investigated.

Thermal Stresses in Passivated Copper Interconnects Determined by X-Ray Analysis and Finite Element Modeling

Abstract: Stresses in passivated copper lines were determined from strains measured using x-ray methods and compared to stresses calculated using finite element methods. Copper lines, 2 μm wide by 0.8 p,m thick, were fabricated by DC magnetron sputtering and lift off patterning. The cross section of these lines was trapezoidal, with approximately 50° sidewalls. One set of samples was left bare, the others passivated with Si3N4 in several different thicknesses. Strain was determined using x-rays. Stress was calculated using elastic moduli adjusted for predominantly (111) fiber texture. Elastic-plastic finite element models provided comparisons of the effects of passivation thickness, rectangular and trapezoidal cross section, and mechanical anisotropy due to preferred texture on the hydrostatic stress in a line. A similar model for aluminum also provided a comparison case of stress as a function of passivation thickness.

Micromechanics Characterization of Unidirectional Composites during Multiaxial Plastic Deformation

Abstract: This study examined the micromechanics at the scale of reinforcement in a short fiber/whisker reinforced unidirectional composite undergoing elastic/plastic deformation. The equivalent inclusion method was extended to consider composite systems comprising fiber/whisker of variable aspect ratio. The flow rule of matrix material is incorporated such that the composite deformation involved multiaxial loading can be handled. Theoretical predictions are reported on the effects of variable fiber aspect ratio, loading direction, and hydrostatic stress on the macroscopic behavior of the composite and the stress state at the reinforcement scale.

A hydrostatic stress-dependent anisotropic model of viscoplasticity

Abstract: A hydrostatic stress-dependent, anisotropic model of viscoplasticity is formulated as an extension of Bodner's model. This represents a further extension of the isotropic Bodner model over that made to anisotropy by Robinson and MitiKavuma. Account is made of the inelastic deformation that can occur in metallic composites under hydrostatic stress. A procedure for determining the material parameters is identified that is virtually identical to the established characterization procedure for the original Bodner model. Characterization can be achieved using longitudinal/transverse tensile and shear tests and hydrostatic stress tests; alternatively, four off-axis tensile tests can be used. Conditions for a yield stress minimum under off-axis tension are discussed. The model is applied to a W/Cu composite; characterization is made using off-axis tensile data generated at NASA Lewis Research Center (LeRC).

Numerical Simulations of Tensile Deformation Behavior in Two Phase α2 + γ Lamellar Tial Alloys

Abstract: Deformation microstructures in γ-TiAl + α 2 Ti 3 Al based fully lamellar (FL) and nearly lamellar (NL) microstructures have been simulated using micro-mechanical methods. The deformation is extremely inhomogenous resulting in a large accumulation of hydrostatic stresses at the grain boundaries, thereby promoting intergranular fracture initiation. In particular, the increase in ductility with increasing equiaxed γ-grain volume fraction (with compliant deformation characteristics) in nearly lamellar alloys is explained by the reduction of the hydrostatic stress buildup at the boundaries, consequently mitigating fracture.

A Numerical Analysis of Fracture and High Temperature Creep Characteristics of Composites with Discontinuous Ductile Reinforcements

Abstract: The role of the material parameters on the fracture and creep behavior of discontinuous ductile fiber reinforced brittle matrix composite system was numerically investigated. For simulation of fracture behavior, the ductile fibers were modeled using a constitutive relationship that accounts for strength degradation resulting from nucleation and growth of the voids. The matrix is assumed to be elastic and fails according to requirements of a stress criterion. Results indicate that the contribution of ductile reinforcement to the work of fracture value (toughness) of the composite increases with less exhaustion of its work hardening capacity before the onset of matrix failure. At creep regime, for rigidly bonded interfaces, the creep rate of the composite is not significantly influenced by the material properties of the ductile reinforcing phase due to development of large hydrostatic stress and constrained deformation in the reinforcement. Significant increases in room temperature fracture toughness can be achieved without extensively sacrificing the creep strength by ductile discontinuous reinforcements.

Birefringence measurement under hydrostatic pressure utilizing twist-induced effects in highly birefringent fibers

Abstract: A new method of birefringence measurement in highly birefringent bow-tie fibers under hydrostatic pressure up to 100 MPa is presented. The birefringence measurement method is based on twist-induced effects and has never been applied before in a high-pressure environment. The experiments were conducted using a specially designed pressure facility, which made it possible to simultaneously generate several mechanical. Perturbations, including twist and hydrostatic stress, and to investigate their effects on mode transmission in optical fibers. The results indicate that in the case of ECB single-mode bow-tie fibers, hydrostatic pressure up to 100 MPa decreased beat length with a mean coefficient of 1/L/sub Bo/|dL/sub Bdp|=0.170%/MPa what is in a very close agreement with our previous measurement based on Rayleigh scattering. >

Mixed dynamic problem for a half plane and its application to rock mechanics

Abstract: The mixed dynamic problem of the theory of elasticity is solved for an isotropic half plane. The dynamic equations are reduced to integration of fourth-degree equations in partial derivatives with constant coefficients, after whose solution, the components of the stress tensor and displacement vector are written in a form similar to that introduced by Lekhnitskii for an anisotropic body. The stress state of a rock mass subjected to rapid face advance in a seam is investigated using the solution obtained. The stress distribution is analyzed numerically. The existence of a critical rate at which the stress increases without restriction is demonstrated.