Void Nucleation Model Research Articles

ANALYZING DUCTILE FRACTURE USING DUAL DILATIONAL CONSTITUTIVE EQUATIONS

Abstract— By adopting a suggestion made by Thomason, a new failure criterion for the Gurson‐Tvergaard model has been recently introduced by the authors. In this study, a method based on the Gurson‐Tvergaard constitutive model and the new failure criterion is applied to the analysis of ductile fracture. The main features of the method are that the material failure is a natural process of the development of Thomason's dual dilational constitutive responses, and the void volume fraction corresponding to the failure by void coalescence is not necessarily a material constant and is not needed to be fitted beforehand. Furthermore, void nucleation parameter(s) can be numerically fitted from experimental tension results. This method has been implemented into the ABAQUS finite element program via a user material subroutine and is applied to the prediction of tension problems conducted by the authors. In the analyses, two strain‐controlled void nucleation models have been studied and compared. The void nucleation parameters corresponding to the two models have been calibrated. The crack initiation of both smooth and notched axisymmetric tensile specimens are well predicted by the method. Finally, several critical issues in the analysis of ductile fracture are discussed.

A model of void nucleation from ellipsoidal inclusions in ductile fracture

A model of void nucleation from ellipsoidal inclusions in ductile fracture

A Continuum Model for Void Nucleation by Inclusion Debonding

A cohesive zone model, taking full account of finite geometry changes, is used to provide a unified framework for describing the process of void nucleation from initial debonding through complete decohesion. A boundary value problem simulating a periodic array of rigid spherical inclusions in an isotropically hardening elastic-viscoplastic matrix is analyzed. Dimensional considerations introduce a characteristic length into the formulation and, depending on the ratio of this characteristic length to the inclusion radius, decohesion occurs either in a “ductile” or “brittle” manner. The effect of the triaxiality of the imposed stress state on nucleation is studied and the numerical results are related to the description of void nucleation within a phenomenological constitutive framework for progressively cavitating solids.

Nucleation and growth of voids by radiation: III. The role of sink strengths in the kinetics of swelling

Chemical rate equations are a common means of describing the variation of different defect species with time during irradiation. Various methods are in use for deriving the necessary rate constants in these rate equations. The rate constants obtained from three different methods are compared and it is concluded that only in the case of point defects diffusing to voids considerable differences between the various methods arise. The equations necessary to complete the model of void nucleation and growth described in a preceding paper are listed in an Appendix.

Digital model of void nucleation in irradiated metals : By D. H. Hall; Thesis, Massachusetts Institute of Technology (Supervisor: K. C. Russell)

Digital model of void nucleation in irradiated metals : By D. H. Hall; Thesis, Massachusetts Institute of Technology (Supervisor: K. C. Russell)

Void nucleation during thermal cycling of adjusted uranium

The nucleation of voids in adjusted uranium during thermal cycling between 400 and 600°C and during rapid quenching from the β-phase has been studied, using an accurate density measuring technique and scanning electron microscopy. Voids are observed to nucleate at carbide inclusion/matrix interfaces before subsequent void growth occurs intergranularly. The length of the transient period between intragranular nucleation and intergranular growth is 300 cycles for adjusted uranium specimens cycled between 400 and 600° C. A model for void nucleation based upon parting at the inclusion/matrix interface as a result of interfacial strains set up by the differential expansion of the carbide inclusion and surrounding matrix is proposed.

High temperature creep cavitation mechanisms in a continuously cast high purity copper

Continuously cast high purity copper was used to study intergranular high temperature creep fracture mechanisms. With the help of an internal marker system due to impurity segregation, grain boundary sliding, GBS, was found to have occurred to a similar extent on cavitated and uncavitated boundaries. To explain this phenomenon a void nucleation model involving small nonwetting shearable particles is suggested. Metallographic observations and the apparent activation energy derived from fracture time data indicate the operation of the vacancy condensation mechanism at the lower temperatures and higher stresses. At the higher temperatures and lower stresses void growth is enhanced by GBS. This cavitation mechanism obtains strong support from measurements of the distribution of voids on grain boundaries as a function of the boundary angle with respect to the tensile direction. Computer analysis of these distributions, in terms of a model which properly accounts for the distribution of potential nuclei, yields bimodal curves exhibiting peaks at grain boundaries oriented for high shear stress (peak I), and for high normal stress (peak II). A phenomenological equation is proposed for the dependence of peak I on test conditions. Peak II is thought to be caused by nucleation by local GBS and growth by vacancy condensation under locally enhanced normal stress.