Internal Necking Research Articles

A theoretical estimate of the surface energy in ductile fracture

An estimate of the surface energy for the coalescence of cavities by internal microscopic necking is derived from a rigid-plastic model [1] for ductile fracture. The. results suggest that the surface energy for coalescence will be a function of the mean-free-path between the cavities or second phase particles. The theoretical surface energy for coalescence of cavities in steels was found to be about an order of magnitude less than experimental estimates of the total surface energy for ductile crack propagation. Since the surface energy measured in experiments includes a contribution from pre-coalescence straining ahead of the advancing crack, it is concluded that the theoretical surface energy for coalescence is of the right order and that the theoretical model for ductile fracture by internal necking of cavities may be closely related to the true mechanism of ductile fracture in real materials.

Tensile plastic instability and ductile fracture criteria in uniaxial compression tests

Experimental measurements of the free surface strain increment components on high-carbon steel compression specimens were used to estimate the point at which tensile plastic instability occurred at the equatorial free surface. A number of compression tests, with various combinations of specimen geometry and platen friction conditions, were carried out to the point of macroscopic fracture on the barrelled surface, thus giving a wide variation in the history of the stress components and the strain increments at the equator. In these tests the equatorial strain to instability varied widely but the strain from instability to fracture varied only slightly. Metallographic examination of the equatorial free surface showed that there was no evidence of fracture damage in specimens prior to instability, but specimens compressed just beyond the point of instability showed abundant evidence of microscopic fracture damage. It was concluded that the start of tensile plastic instability marks the point where internal microscopic cavities begin the process of growth and coalescence by internal necking. A criterion for ductile fracture in metal-working operations is proposed which is based on an estimate of the point where tensile plastic instability begins. A number of suggested criteria for ductile fracture were examined and found to be unsatisfactory when applied to the present results. There was no evidence of local necking on the free surface of the compression specimens after instability had occurred.

A Theory for the Effects of Strain-Rate-Sensitivity on Ductile Fracture

AbstractA theory for ductile fracture in a material with a strain-rate-sensitive yield stress is proposed, based on the hypothesis that fracture is the result of internal necking between adjacent microscopic cavities. The theory is developed for a plane-strain element containing uniformly distributed cavities and this model is used to examine the effects of strain-rate-sensitivity on the strain to fracture in ductile materials. The results suggest possible explanations for both “blue-brittleness” behaviour and the “superplasticity” effect.

THE FRACTURE BEHAVIOUR OF COPPER COMPACTS CONTAINING RESIDUAL POROSITY

The surface and internal deformation taking place during tensile straining of sintered copper compacts has been studied, with particular reference to the way in which this contributes to the mode of fracture. The porosity was varied in the range 0-13% and the effects of size and shape of pores were investigated, together with changes in grain size. Comparison is made with the deformation and fracture behaviour of copper containing non-metallic inclusions instead of pores. The results emphasize the importance of internal necking between pores, in controlling the microscopic nature of the fractures and thus the overall ductility, and the geometrical and material conditions which affect internal necking are discussed.