Date of Award
Doctor of Philosophy (PhD)
Prof. R. Sowerby
The topic of this thesis is the phenomenon of ductile fracture, and stems from a government research contract aimed at assessing the utility of different steels designated for cold forging applications.
Recourse has been made to experimental data, available in the published literature, of upsetting tests on certain steels in different heat treated conditions. Some upsetting tests have also been performed as part of the present study, and following earlier work conducted at McMaster University, specimens of different geometry were also investigated. The materials were examined metallographically, before and after deformation.
It was found that in addition to material characteristics the specimen geometry can have a marked effect on the extent of the deformation before surface cracking. The so-called collar specimen resulted in smaller strains to fracture vis à vis the other specimen geometries investigated.
The principal surface strains were determined throughout the course of the upsetting tests by measuring a grid of lines marked on the equatorial free surface of the specimens.
A knowledge of the strain path enables the stress history of a point on the equatorial free surface to be determined using simple plasticity theory. The details are described herein. The upsetting tests were also modelled using a finite element technique based on a rigid, work-hardening material. The predicted and measured strains at the free surface of the specimens showed good agreement, and consequently, the predicted stresses agreed with those determined using simple plasticity theory. However, the finite element technique enables the stress history to be determined at any interior point within the specimens.
A knowledge of the stress history permits an evaluation of certain damage integrals. The ones proposed by Cockcroft and Latham and Oyane were investigated in the present work, but they failed to provide an adequate account of the observed behaviour in the upsetting tests. Failure based on the attainment of a critical shear seemed more appropriate. The experimental data also demonstrated that the fracture strains, i.e. the.hoop, ε₀₀, and axial strains, ε₀₀/ε₂₂ = -1/2, as proposed by Kuhn.
Reliance was also placed on experimental data (gathered by other researchers at McMaster University) from tensile tests performed on spheroidized steels subjected to superimposed hydrostatic pressure. Quantitative metallography had revealed information on certain aspects of void initiation, growth and coalescence as a function of volume fraction of inclusions and hydrostatic pressure.
In the present work the void growth stage was predicted using a void growth model due to McClintock, but modified mutatis mutandis to allow for the continuous nucleation of voids. The model is shown to be capable of describing certain experimental observations regarding void growth.
McClintock's model was also applied to the data gathered from the upsetting tests. Again it appeared capable of predicting the damage rate and damage accumulation in a manner consistent with experimental observations. Furthermore, it was possible to predict a fracture locus, plotted in ε₀₀-ε₂₂ space and based on the attainment of a critical accumulated damage, which correlated very well with the experimental data. McClintock's model is outlined in the thesis, and discussed in the context of other void growth models given in the published literature.
Chandrasekaran, N., "The Phenomenon of Ductile Fracture" (1985). Open Access Dissertations and Theses. Paper 1181.