Experimental evaluation of block loading effects on fatigue crack growth in offshore structural steels

Abstract

Offshore structures are subjected to operational and environmental conditions which by nature are variable in magnitude. For design or assessment purposes a real load spectrum is translated into a sequence of blocks with different but constant amplitude stress reversals. The block load variations can generate non-linear transients in the crack growth rate, i.e. crack growth acceleration or retardation effects. However these are generally ignored in design and a linear damage accumulation method is used to estimate fatigue lifetime. To investigate the effect of block loading transients on fatigue crack growth, different ΔK block loading schemes have been designed. So-called ESE(T) specimens manufactured from two offshore steel grades have been subjected to variable amplitude tests in air according to these loading schemes. To avoid additional effects of weld process parameters, possible weld flaws, weld microstructure heterogeneity, residual stress on crack path and crack growth, plain material has been chosen for this study. Retardation of crack growth has been evidenced for transitions to blocks with lower ΔK amplitude, further called high-low sequences. This retardation effect increased for larger jumps in amplitude and lower ΔK values. Crack arrest was evidenced at transition block ratios (ΔKi-1/ΔKi) higher than 2.2 to 2.4; the exact values depending on how close the stress intensity range is to the threshold value.