Year

1999

Degree Name

Doctor of Philosophy

Department

Department of Materials Engineering

Abstract

The effects of residual stress and fatigue crack closure have been investigated in an offshore application structural steel. Residual stresses were generated by deliberate overloading to generate localised plastic deformation and by welding.

The residual stresses in the specimens were measured by a variety of techniques, and fatigue cracks grown under controlled tensile load cycles. During the crack growth experiments, the fatigue loading was interrupted to perform fatigue crack closure measurements. The overall concepts, and experimental techniques, used were well established. However, the work represents a holistic and unique approach to correlate the combined effect of residual stresses and crack closure upon fatigue crack growth

The results indicated that pre-existing residual stresses were dissipated by the growing fatigue cracks. When a crack initiated in a compressive residual stress field, it continued grow in a stress field of the same sign. However, when a crack initiated in a tensile residual stress field, the tensile residual stress was dissipated more rapidly and the compressive residual stress and crack closure effects generated by the crack itself eventually dominated. The crack growth behaviour then tended toward that of a specimen relatively free of residual stresses.

Residual stresses acting normal to the crack plane affected the fatigue crack growth rates, and the shape (or profile) of the fatigue crack tip. However, those residual stresses parallel to the crack appeared to have little effect on crack growth rates or on the shape of the fatigue crack. In assessing the remaining fatigue life of a cracked structure or component, may often be sufficient to consider only the residual stress acting normal to the crack plane within the overall assessment

Residual stresses affect the crack opening closing response on loading and unloading. Compressive residual stresses lead to higher crack opening and closing stress intensities (KoP and Kcl). The effect of residual stress, due to welding or local plastic deformation due to overloading, are more influential on the fatigue crack closure effect (in the material investigated) than contributions due to fracture surface roughness or plasticity. In the absence of residual stresses, at near threshold crack growth rates roughness induced fatigue-crack closure, due to the mismatch of mating fracture surfaces and random contact of asperities, was found to be more important than plasticity induced crack-closure close the crack tip.

The crack opening/closing stress intensity (Kop/Kcl) provides the basis for reliable empirical estimates of the remaining high-cycle fatigue life of a specimen that was already cracked. a suitable means can be found to measure the crack opening/closing load response (Pop/Pcl) in situ is feasible, then this could be used in conjunction with application of suitable fracture mechanics to assess the remaining life

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