Showing papers on "Hydrostatic stress published in 1986"

The biaxial deformation and yield behavior of bisphenol‐a polycarbonate: Effect of anisotropy

Abstract: The biaxial tension stress-strain-yield behavior of glassy bisphenol A-polycarbonate has been investigated at constant octahedral shear stress rate and at 25°C and −40°C. The specimens possess a small amount of anisotropy. Hill′s criterion for yielding of anisotropic materials is modified to take into account the sensitivity of the yield locus to the hydrostatic stress component. This modified yield criterion fits the 25°C data quite well. However, the data at −40°C cannot be fitted using reasonable parameters.

Hydrostatic stress and crack opening displacement in three-point bend specimens with shallow cracks

Abstract: A theoretical analysis is given for the slip-line field in a three-point bend specimen containing a sharp short crack. It is shown that the hydrostatic stresses are larger for deeper cracks. These results support the observation that the crack opening displacement at the initiation of a ductile tear is bigger for shallow cracks than deep ones. An analytical relationship for the ratio of plastic crack tip opening to crack mouth opening displacement is also established which is in agreement with experimental replica measurements. Application of the analysis to practical crack opening displacement measurements for shallow cracks is discussed.

Model calculation of nitrogen properties in III-IV compounds

Abstract: We present a semiphenomenological model calculation of the nitrogen-related traps in GaP and related compounds. Working in the one-band--one-site approximation, the complex character of the lowest conduction band is taken into account by using the model density of states previously introduced by Kleiman. In the case of NN pairs in GaP, this results in different series of levels whose energy position depends on structure factors which couple the nitrogen-nitrogen relative positions in real space and the extrema of the conduction band in k space. A tentative identification of these levels is done, in the light of the present calculation, by taking account of the experimentally determined symmetry. A comparison is next performed with recent hydrostatic stress experiments and with the pair dependence of the local-field parameter which splits the ${\ensuremath{\Gamma}}_{8}$ bound hole. Concerning isolated nitrogen, the model predicts composition and pressure dependences in very good agreement with experimental results in ${\mathrm{Ga}}_{\mathrm{x}}$${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$P, ${\mathrm{GaAs}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{P}}_{\mathrm{x}}$ or GaAs.

A multiaxial theory of viscoplasticity for isotropic materials

Abstract: Many viscoplastic constitutive models for high temperature structural alloys are based exclusively on uniaxial test data. Generalization to multiaxial states of stress is made by assuming the stress dependence to be on the second principal invariant (J sub 2) of the deviatoric stress, frequently called the effective stress. If such a J sub 2 theory, based on uniaxial testing, is called upon to predict behavior under conditions other than uniaxial, e.g., pure shear, and it does so poorly, nothing is left to adjust in the theory. For a fully isotropic material whose inelastic deformation behavior is relatively independent of hydrostatic stress, the most general stress dependence is on the two (non-zero) principal invariants of the deviatoric stress, J sub 2 and J sub 3. These invariants constitute what is known as an integrity basis for the material. A time dependent constitutive theory with stress dependence on J sub 2 and J sub 3 is presented, that reduces to a known J sub 2 theory as a special case.

Hydrogen-induced delayed fracture under mode II loading

Abstract: Hydrogen induced cracking (HIC) and stress corrosion cracking (SCC) of a high-strength steel 34CrNi3Mo (T.S = 1700 MPa) under Mode II loading were investigated using notched specimens. The stress field around the notch tip was analyzed by means of finite element method. The result shows HIC and SCC under Mode II loading initiated at the back of the notch tip,i.e., θ = -110 deg, where hydrostatic stress has maximum value. However, cracking is oriented along the shear stress direction at the site, not normal to the direction of maximum principal stress component. On the contrary, if the specimens are loaded to fracture in air under Mode II loading, cracking at the maximum shear stress site around the notch tip and the cracking direction coincide with the direction of the maximum shear stress. The above facts indicate that hydrogen induced delayed plastic deformation is a necessary condition for HIC, and the nature of SCC for high-strength steel in 3.5 pct NaCl solution is HIC. The results show that HIC and SCC under Mode II loading can occur during dynamic charging with hydrogen and in 3.5 pct NaCl solution, respectively. The normalized threshold stress intensity factors under Mode II loading during dynamic charging in 1 N H2SO4 + 0.25 g As2O3/L solution and in 3.5 pct NaCl solution are KIIH/KIIX = 0.1 and KIISCC/KIIX = 0.45, respectively. The corresponding values under Mode I loading are KIH/KIX = 0.02 and KISCC/KIX = 0.37, where KIIX and K,IX are critical values loaded to failure in air under Mode II and Mode I loading, respectively. Thus, (KIIH/KIIX)/ KIH/KIX) = 5 and (KIISCC/KIIX)/K,(ISCC/KIX) = 1.2. A typical intergranular fracture was observed during HIC and SCC under Modes II and I loading. But the fracture surfaces of specimens failed in air are composed of dimples for both kinds of loading.

Elastic-Plastic Three-Dimensional Finite-Element Analysis of Bulk Metalforming Processes

Abstract: An elastic-plastic three-dimensional finite-element formulation is presented for the study of bulk metalforming problems. The incremental technique is based upon the Prandtl-Reuss flow rule and von Mises’ yield criterion, and incorporates a finite-deformation formulation using correct definitions of stress and strain increment for accurate and efficient solution of large-strain analyses.