Stress Triaxiality Values Research Articles

An analysis of inclusion morphology effects on void nucleation

Deformations of a planar doubly periodic array of square elasticinclusions in an isotropically hardening elastic-viscoplastic matrixare analysed. The arrays considered have multiple inclusions per unitcell, but in each array all inclusions have the same size. Overallplane strain tension with a superposed tensile biaxial stress isimposed. A finite deformation formulation is used with a cohesivesurface constitutive relation describing the bonding between theinclusion and the matrix. A characteristic length is introduced fromdimensional considerations since the cohesive properties include thework of separation and the cohesive strength. The system analysed isused to study inclusion distribution effects on void nucleation, withthe aim of providing background for incorporating the effect ofclustering on void nucleation into phenomenological constitutiverelations for progressively cavitating plastic solids. For low valuesof the triaxiality of the imposed stress state, void nucleation occursafter extensive overall plastic straining and regular distributionshave a higher value of the void nucleation strain than randomdistributions. For larger values of stress triaxiality, where void nucleation occurs at relatively small overall plastic strains, theeffect of inclusion size dominates the effect of inclusiondistribution and smaller inclusions give rise to higher void nucleation strains. The ability of various scalar measures ofclustering to characterize the computed dependence of void nucleationon inclusion distribution is explored. Within the context of aphenomenological description of void nucleation, it is found that theeffective void nucleation stress is approximately a linear function ofthe overall hydrostatic tension with a coefficient 0.40-0.44 forregular distributions and 0.25-0.35 for random distributions.The results also suggest a possible dependence of the effective voidnucleation stress on a simple scalar measure of clustering.

Influence of temperature, strain rate and specimen geometry on the microscopic cleavage fracture stress

The aim of this work is to determine the influence of temperature, strain rate and specimen geometry on the microscopic cleavage fracture stress σ f*. Besides, the dependence of the initiation temperature for shear fracture T i and the temperature for general yield T gy on strain rate is investigated. Finally, the local values of stress triaxiality and equivalent plastic strain at the occurrence of cleavage fracture for several steels and specimen types are compared with the failure curve for ductile fracture to check the validity of the theory of stress controlled cleavage fracture and the strain/triaxiality controlled shear fracture in the transition region. Based on experimental tests, the results are obtained by finite element analyses. The investigation shows, that σ f* is dependent on temperature and strain rate and increases with decreasing test temperature and increasing strain rate. The transition temperatures T i and T gy increase with increasing strain rate. The theories of stress controlled cleavage fracture and of shear fracture controlled by equivalent plastic strain and stress triaxiality seem to be valid. That fracture mechanism occurs for which the critical condition is reached first.