Void/pore distributions and ductile fracture

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I.

INTRODUCTION

A common problem in alloys which have been cast, processed by powder metallurgy techniques, or welded is the presence of porosity. In addition, wrought alloys often contain large inclusions which form voids at small strains when fracture of the matrix is still remote. The linking of the large voids, usually by the formation of microvoids between the large voids, causes failure. In either instance, the deleterious influence of pores and/or voids on strength and especially tensile ductility is well known. It is also well established that the severity of the effect depends on the volume fraction of pores or void-nucleating inclusions (for example, see Reference 1); however, factors such as the inclusion size 2'3 and the degree of inclusion clustering 4 are also known to influence fracture resistance. A difficulty in analyzing the influence of porosity or voiding on ductile fracture is that changes in one feature of the pore/void "microstructure" (for example, the volume fraction of porosity) are usually accompanied by changes in other parameters, such as the distributions of pore sizes, shapes, and spacings. Thus, an experimental separation of the individual effects of a pore/void microstructure on strength and fracture behavior does not exist. Existing theories of ductile fracture also fail to establish the effects of the random nature of void/pore distributions. Nearly all theoretical analyses of fracture based on void growth and linking assume regular arrays of equi-sized holes or cavities. 5-1~ Only Melander has attempted to analyze a random distribution of voids, but no experimental verification was attempted. ~2'~3'14 As will be discussed later, the present experimental data are not consistent with Melander's analysis. Many previous studies, both experimental and theoretical, have used through-thickness holes as a two-dimenE . M . DUBENSKY is with Boride Products, 2879 Aero Park Drive, Traverse City, MI 49684. D. A. KOSS is Chairman, Metals Science and Engineering Program, The Pennsylvania State University, University Park, PA 16802. This paper is based on a presentation made at the symposium "Stochastic Aspects of Fracture" held at the 1986 annual AIME meeting in New Orleans, LA, on March 2-6, 1986, under the auspices of the ASM/MSD Flow and Fracture Committee.

METALLURGICAL TRANSACTIONS A

sional analog of three-dimensional voids or pores. 5'8'1~,~3'15In the present study, void distributions are modeled in two dimensions as arrays of holes whose positions are predicted using a random number generator. Experiments are conducted on arrays of equi-sized cylindrical holes in which (a) the area fraction of holes, (b) the diameter of the holes, and (c) the minimum spacing between the holes (or degree of clustering) are controlled. The associated effects of these parameters on the deformation and fracture behavior are determined by experimental testing of two materials (1100-0 A1 and 7075-T6 A1) of differing work-hardening rates under conditions of either predominantly plane stress or plane strain:

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