The effect of prior plastic strain on fracture initiation

  • Molly A Probert

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

In engineering structural integrity assessment procedures such as BS7910, prior plastic deformation during crack introduction is not taken into account when predicting the performance of components containing existing cracks. This leads to potentially overly-conservative assumptions about the integrity of the cracked component. If this conservatism can be reduced by taking account of the increase in apparent fracture toughness from the prior plastic deformation then life extension of existing components may be possible where previously costly repair or scrappage would have taken place.
As a crack grows though a residually stressed component the stresses are redistributed. For example if the tensile stress at the crack tip is greater than yield at any point during the crack growth a plastic zone around the crack tip will form. If the plastic zone merges with the plastic zone of the next increment of crack growth the larger cumulative plastically deformed area is termed a plastic wake. The energy diffused by the formation of this plastic wake reduced the energy available for initation of fracture after the crack has grown compared to a crack without a plastic wake.
Cracks were introduced into aluminium alloy 7475-T7351 compact tension specimens by fatigue. This material was selected for its combination of very consistent fracture toughness, favourable toughness-to-yield-strength ratio and favourable neutron scattering qualities. By increasing the mean load of fatigue whilst keeping the load range consistent, different levels of plastic wake were introduced into specimens. Simulating the effect of increasing levels of tensile residual stress field around a growing crack. Three groups of specimens were studied, the first group had no plastic wake, the second had a relatively low level of plastic wake, and the third was fatigued at the highest mean load possible whilst still keeping the crack growth stable. After fatigue crack introduction the specimens were fractured and apparent fracture toughness was determined.
Neutron diffraction measurements were taken of the specimens before and during fracture to measure the extent of residual strain field introduced into the specimens during fatigue crack growth. During loading to fracture further measurements allow the development of this residual strain field to be recorded. Comparison of the measured strain field and finite element model of the cracked specimens allowed for detailed validation of crack introduction modeling by symmetry plane node release. The validated finite element models of the different crack introductions were used to determine the extent of the plastic wake that was formed by fatigue cracking with different mean loadings.
The strain energy release rate of the crack tip with different levels of plastic wake showed that the higher level of plastic wake increased the apparent fracture toughness of the specimen. There is no significant influence of triaxiality or plastic hardening on the ability of the crack tip to withstand fracture and the effect on apparent fracture toughness is based on the localised compressive residual stress introduced by the material yielding during crack growth. It was found that the compressive residual stress field introduced by prior plastic deformation during crack introduction causes an increase in the subsequent apparent fracture toughness, compared to cracks introduced without prior plastic deformation. But the effect of plastic hardening during crack introduction has a comparatively small effect on the apparent fracture toughness. This is applicable to a range of materials though further experimental work is required, especially on materials with low hardening modulus that are more prone to plastic hardening.
Date of Award27 Sept 2022
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorHarry Coules (Supervisor) & Christopher E Truman (Supervisor)

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