Date of Award
Doctor of Philosophy (PhD)
Metallurgy and Materials Science
In considering the fabrication of engineering components from metals in the form of thin sheets (<6 mm in thickness), attention must be given to two important problems: a) the mechanisms by which plastic strain is distributed in the component, and b) the competition between continued deformation and fracture. The description of these processes from a theoretical viewpoint and the completion of an experimental program are complicated by the fact that both problems involve intrinsic properties of the sheet as well as external parameters such as temperature, strain rate and stress state. The objective of this thesis is to characterize the sequence of events which occur during the forming process by giving attention to both types of variables.
The first portion of this work involves a macroscopic approach. An experimental program has been conducted to measure (by using grid techniques) the distribution of strain in a variety of aluminum alloys subject to loading trajectories which range from pure shear to biaxial tension. Equations have been derived to express the occurrence of localized necking and fracture in terms of strain and stress coordinates. These curves, generated by plotting the two events for a wide range of stress systems, may be termed failure maps. If one considers their position relative to the initial yield event, the maps are capable of representing the entire strain history of the sample. In addition, their shape and level can be used a) to establish the relevance of empirical failure criteria, b) to compare the behavior of materials (e.g to illustrate the competition between alloys to form a given component), and c) to examine the influence of stress state on the occurence of a well defined fracture mode.
By varying the heat treatment of some of these alloys, it was possible to promote different fracture modes such as fibrous, shear or intergranular fracture and to examine these modes in terms of the resultant fracture map. The results indicate that the failure criteria applicable to sheet metal forming operations are in good accord with the fractographic evidence, and that one should not only consider the competition between continued deformation and fracture, but also between available fracture modes depending on heat treatment and stress state.
Although the macroscopic approach outlined above gives a broad summary of the straining history of the sample, it is not sufficiently specific to enable a correlation with the microstructure. The second approach of this thesis, at a microscopic level, is concerned with local events occuring inside the material and attempts to elucidate the metallurgical factors which influence the conditions under which a well characterized alloy undergoes the transition from uniform deformation to some form of localized deformation and finally fracture.
A model material (in the form of spheroidized carbon steels) was thus studied to obtain a quantitative description of the microvoiding process leading to fibrous fracture. Careful examinations of the fracture surfaces and the zones adjacent to them enabled a measurement of structural damage resulting from plastic deformation to be established. A detailed theoretical model was developed to relate the damage to continuum quantities such as strain and stress. The model was then extended to describe the process of void nucleation and growth at inclusions under more general stress systems.
Finally, a new description of work hardening characteristics of materials at large strains was presented and related to the foregoing concepts of strain localization and fracture.
Le Roy, Ghislain Henry, "Large Scale Plastic Deformation and Fracture for Multiaxial Stress States" (1978). Open Access Dissertations and Theses. Paper 3178.