OVERLOAD RETARDATION OF FATIGUE CRACK GROWTH IN A 9%Cr 1%Mo STEEL AT ELEVATED TEMPERATURES

Abstract

This paper describes an investigation into the effects of a single-peak tensile overload on fatigue crack propagation in a 9%Cr 1 %Mo steel. Overloads were applied during cycling at a constant stress intensity range (AK), and any consequent transients in growth rate were recorded. The severity of retardation rises as the magnitude of the applied overload is increased. The effect of temperature is complex, but a 525°C retardation is significantly less marked than at 25 or 225°C. Signs of crack face contact are seen on post-overload fracture surfaces, but there is little crack branching. The dominant cause of overload retardation in this steel appears to be plasticity-induced closure. At 525"C, post-overload dwell periods significantly reduce the severity of retardations. This is not observed at lower temperatures, indicating that the residual clamping stresses that lead to closure are gradually relieved at 525°C. NOMENCLATURE aR = length of subsequent crack growth over which growth rates are affected by an overload do, = size of monotonic plastic zone resulting from an overload da/dN = crack growth increment per cycle AK = stress intensity range AKef = effective stress intensity range experienced at the crack tip when a closure mechanism is operating AK,,, = threshold stress intensity range, below which there is negligible crack growth K,,, = maximum stress intensity during the baseline fatigue cycle KO, = peak stress intensity applied during an overload Nd = number of cycles by which an overload delays crack growth N: = Nd normalised with respect to the number of cycles a crack would normally take to grow the distance aR during constant amplitude loading if no overload were imposed cry = uniaxial yield stress

Some Aspects of Fatigue Crack Growth Retardation Behaviour Following Tensile Overloads in a Structural Steel

“…This was supported by work on a 50B steel [4] where no differences in bulk and surface closure measurements on 3 mm-thick specimens were observed. The work reported here and that of others [4,9] suggests that crack closure is predominantly a near surface, plane stress phenomenon. At these lower R ratios, greater retardation might then be expected in thinner specimens since the proportion of the specimen in plane stress is larger, and therefore the near surface closure has a proportionally greater influence on behaviour than in thicker specimens.…”

Fatigue Crack Growth Due to Two Successive Single Overloads

“…A decrease in Δ δ with a decrease in the stress ratio of the overload cycle is observed in Fig. 1(b) as a result of the re‐sharpening effect on the crack tip profile [ 14, 15]. Increasing the level of σ 1 decreases the contribution of the overload cycle part which is higher than σ 1 , and hence a smaller crack tip blunting effect is expected, as confirmed in Fig.…”

Overload effect on the fatigue crack propagation in large‐scale tubular joints

“…Such approaches, however, do not include the effect of a single overloading event, which often occurs in the service life of a realistic structure, exemplified by an offshore platform subjected to the impact of a small or medium size ship or a bridge under an unexpected environmental action 5 . Past research evidences 6–11 have confirmed the retardation in the fatigue crack propagation caused by a single overloading event through experimental investigations on small‐scale fatigue specimens, e.g. the compact tension, C(T) specimen or the single edge‐notched bend, SE(B) specimen.…”