Fatigue crack growth retardation and acceleration in coupon specimens and tubular joint specimens

This experimental study considers load sequence effects in fatigue crack growth in coupon specimens and in element specimens. The coupon specimens were standard edge crack configurations cyclically loaded under four point bending on base metal and welds, whereas the element specimens were full-scale axially loaded tubular joints. Whereas load sequence effects such as crack growth retardation following high stress peaks (overloads) in base metal are already known, this paper makes a qualitative comparison to more realistic conditions of random variable amplitude loading and welded joints. The results of the coupon specimens show that the crack growth retards following an overload or a block of ranges with high mean stress in a further constant amplitude load regime for the steel grades investigated, whereas an underload applied after an overload reduces or cancels out the retarding effect. Test results on full scale tubular joint elements show that in case of realistic load sequences on realistic structural details, the net effect of overloads and underloads on the crack growth rate measured over the entire life is insignificant. As the majority of the fatigue life consists of growth of small cracks, the limited significance of load sequences is attributed to the limited crack growth experienced between events such as storms, so that retardation effects do not have the possibility to fully develop and are cancelled out by underloads. On the other hand, load sequence effects appear more significant for large cracks - and hence for inspections - in realistic joints as compared to coupon specimens