Two parameter engineering fracture mechanics

MacLennan, Iain James (1996) Two parameter engineering fracture mechanics. PhD thesis, University of Glasgow.

Abstract

The object of this work was to investigate and expand on previously carried out
research into elastic-plastic crack tip fields using the first two terms of the Williams
expansion to characterise the degree of crack tip constraint. As a precursor to this research a
history of fracture mechanics is also presented.
In the present work crack tip fields in small scale yielding have been detennined using
modified boundary layer formulations in an attempt to model the influence of the second
order term of the Williams expansion, the T -stress. The prime object of this thesis was to
investigate and expand on previously carried out research into a two parameter
characterisation of elastic-plastic crack tip fields using the second parameter of the Williams
expansion(T), which attempts to characterise the degree of crack tip constraint. Modified
Boundary Layer formulations in conditions of plane strain were implemented to derive a
suitable reference solutions, against which the effects of out of plane strains can be compared
and the validity of presently established reference fields can be gauged. The effect of out of
plane non-singular stress, S, on the crack tip stress field were also considered, where
constraint was largely determined by T. A wide range of analyses have been carried out, from the microstructural scale to
complete engineering components in an attempt to characterise crack tip stress fields. The
ability to apply two parameter fracture concepts to real engineering structures requires
methods for calculating T for complex components with realistic semi-elliptical defects. A
simple engineering method for achieving this was developed making use of linespring
elements in the finite element package ABAQUS. This approach was validated by the
calculation of T for semi-elliptical cracks at the chord-brace intersection of a tubular welded
joint, modelled using the mesh generation program PATRAN.
The micromechanics of cleavage, using the Ritchie-Knott-Rice model have also been
constructed. This work relates the ratio of J for unconstrained and constrained geometries to
critical microstructural distance, critical cleavage stress and the toughness ratio on the strainhardening effect. The elastic-plastic behaviour of short and deeply cracked bend bars has
previously been described by Betegon and Hancock based on the first two terms of the
Williams expansion. A local cleavage criterion has been applied to these fields to indicate the
effect of loss of constraint on lower shelf toughness of shallow cracked bend bars. The
work models the maximum temperature at which cleavage can occur in these geometries to
show the effect of constraint and aJW ratio of cracked bend bars on the ductile-brittle
transition temperature. This has also been backed by a significant experimental research
program.
Finally constraint dependent toughness has been considered in relation to failure
assessment methodologies. A simple engineering method for modifying these Failure
Assesssment Diagrams has been presented, this consists of considering the constraint
matched toughness of the strucutre. This procedure recovers the original Failure Assessment
Line and unifies the constraint dependent fracture toughness within defect assessment
schemes which utilise Failure Assessment Diagrams.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Subjects: T Technology > TJ Mechanical engineering and machinery
• Colleges/Schools: College of Science and Engineering > School of Engineering
• Supervisor's Name: Hancock, Professor John W.
• Date of Award: 1996
• Depositing User: Ms Dawn Pike
• Unique ID: glathesis:1996-6756
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 30 Oct 2015 14:37
• Last Modified: 30 Oct 2015 14:37
• URI: https://theses.gla.ac.uk/id/eprint/6756