Two parameter engineering fracture mechanics

TJ Mechanical engineering and machinery

MacLennan, Iain James ; Hancock, John W.

University:University of Glasgow

Department:School of Engineering

关键词: TJ Mechanical engineering and machinery;   

Others  :  http://theses.gla.ac.uk/6756/1/1996MacLennanPhd.pdf

来源: University of Glasgow

【 摘 要 】

The object of this work was to investigate and expand on previously carried outresearch into elastic-plastic crack tip fields using the first two terms of the Williamsexpansion to characterise the degree of crack tip constraint. As a precursor to this research ahistory of fracture mechanics is also presented.In the present work crack tip fields in small scale yielding have been detennined usingmodified boundary layer formulations in an attempt to model the influence of the secondorder term of the Williams expansion, the T -stress. The prime object of this thesis was toinvestigate and expand on previously carried out research into a two parametercharacterisation of elastic-plastic crack tip fields using the second parameter of the Williamsexpansion(T), which attempts to characterise the degree of crack tip constraint. ModifiedBoundary Layer formulations in conditions of plane strain were implemented to derive asuitable reference solutions, against which the effects of out of plane strains can be comparedand the validity of presently established reference fields can be gauged. The effect of out ofplane non-singular stress, S, on the crack tip stress field were also considered, whereconstraint was largely determined by T. A wide range of analyses have been carried out, from the microstructural scale tocomplete engineering components in an attempt to characterise crack tip stress fields. Theability to apply two parameter fracture concepts to real engineering structures requiresmethods for calculating T for complex components with realistic semi-elliptical defects. Asimple engineering method for achieving this was developed making use of linespringelements in the finite element package ABAQUS. This approach was validated by thecalculation of T for semi-elliptical cracks at the chord-brace intersection of a tubular weldedjoint, modelled using the mesh generation program PATRAN.The micromechanics of cleavage, using the Ritchie-Knott-Rice model have also beenconstructed. This work relates the ratio of J for unconstrained and constrained geometries tocritical microstructural distance, critical cleavage stress and the toughness ratio on the strainhardening effect. The elastic-plastic behaviour of short and deeply cracked bend bars haspreviously been described by Betegon and Hancock based on the first two terms of theWilliams expansion. A local cleavage criterion has been applied to these fields to indicate theeffect of loss of constraint on lower shelf toughness of shallow cracked bend bars. Thework models the maximum temperature at which cleavage can occur in these geometries toshow the effect of constraint and aJW ratio of cracked bend bars on the ductile-brittletransition temperature. This has also been backed by a significant experimental researchprogram.Finally constraint dependent toughness has been considered in relation to failureassessment methodologies. A simple engineering method for modifying these FailureAssesssment Diagrams has been presented, this consists of considering the constraintmatched toughness of the strucutre. This procedure recovers the original Failure AssessmentLine and unifies the constraint dependent fracture toughness within defect assessmentschemes which utilise Failure Assessment Diagrams.