Crack shape development in plates and bonded joints.

Planar Mode I fatigue defects, in finite thickness steel plates and in bonded aluminium joints, have been modelled according to linear elastic fracture mechanics principles. An automatic 3D finite element modelling technique has been developed and used to establish the fatigue growth of arbitrary shaped defects. The variations of K and growth rate around a crack front are calculated. Software which automatically advances and remeshes the defect conveniently models the fatigue propagation of arbitrary shaped defects.

Reasonable estimates of defect stress intensity factors, growth rates, shape changes and fatigue lives can be made by elliptical and straight-fronted characterisations to actual crack profiles. More accurate estimates (errors within a few percent) require that the variation of K around a crack front and the deviations of defect shape from regular profiles be both considered. The extra exterior surface abutting the front of a corner crack increases K by some 10&37 from that of a similar surface crack. This corresponds to a reduction of 1.10^m ( m is the Paris growth rate exponent) in the fatigue lives of defects abutting extra external boundaries.

The tension fatigue shape development of a surface defect prior to leakage is related to its preceding configuration; crack dimensions immediately before through-thickness characterise defect growth subsequent to leakage. This permits the estimation of both leakage and straight-through defect dimensions and is important for safety evaluation procedures.

The same method has also been used to model defects in bonded T-Peel joints. Crack propagation within such joints is principally cohesive (within a bondline) rather than adhesive (interfacial between an adhesive bond and an adherend) and can be characterised using either K or J.K of cohesive defects varies with the ratio of adhesive to adherend elastic moduli ( K or K <IMG WIDTH=7 HEIGHT=7 ALIGN=BOTTOM SRC="/maths/alpha.gif">sqrt{E_a/E} ) and bondlinethickness ( K <IMG WIDTH=7 HEIGHT=7 ALIGN=BOTTOM SRC="/maths/alpha.gif">sqrt{t} ,for 1mm <IMG WIDTH=10 HEIGHT=24 ALIGN=MIDDLE SRC="/maths/leq.gif"> t <IMG WIDTH=10 HEIGHT=24 ALIGN=MIDDLE SRC="/maths/leq.gif"> 2mm). Crack growth within bonded and spot weld-bonded T-Peel joints initiates inside the adhesive fillet front edge as an embedded defect. Joint failure occurs almost immediately after breakout of such a defect, which rapidly forms a through-width defect. Joint strength is maximised by having maximum adhesive within the joint fillet region.