Monahan, Craig C.; (1994) Early fatigue crack growth in offshore structures. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis is concerned with the problem of early fatigue crack growth in welded tubular joints of the type used to construct fixed offshore structures for oil and gas production. Offshore structures are inspected periodically to ensure safety in operation, but cost effective inspection planning demands that accurate nondestructive techniques be used to size fatigue cracks and reliable fracture mechanics models be used to predict their growth. ACPD, ACFM and MPI were employed in this study to collect detailed crack growth data for small surface fatigue cracks in welded T-plates, machined T-plates and welded tubular joints. In all cases, crack closure was monitored using a highly sensitive clip gauge mounted across the mouth of the crack. High cycle fatigue tests were conducted to study the influences of mean stress and notch plasticity on small crack growth behaviour. Crack propagation rates were correlated using the effective range of the crack tip stress intensity factor. Low cycle fatigue tests were carried out to investigate the influence of general plasticity on surface fatigue crack growth. The change in crack tip opening displacement was used to characterise fatigue crack growth behaviour under conditions of large-scale yielding. The small crack sizing abilities of ACPD and ACFM were assessed by comparing predicted depths with optical measurements taken from several fracture surfaces. Calibration curves were derived from the results. The experimental data was used to develop a fracture mechanics model for predicting the early stages of fatigue crack growth in welded tubular joints. Empirical equations were found to characterise material behaviour and early crack shape development trends. The Newman-Raju equations were used with corrections for non-uniform stress and structural restraint to estimate stress intensity factors. Model predictions are compared with actual test results for the cases of IPB and OPB in tubular T- and Y-joints.

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