Effect of specimen thickness on near-threshold fatigue crack propagation of nuclear structural materials

Abstract

In this study, the effect of specimen thickness on near-threshold fatigue crack propagation (FCP) was investigated in consideration of microstructural parameters, such as the effective grain size and pearlite content. Fatigue tests were performed with compact tension (CT) specimens of SA106 Gr.C steel and its weld, SA508 Cl.3 steel and 12% Cr steel. Specimen thicknesses were 6, 12, and 24 mm The tests were carried out at room temperature in ambient air with a load ratio of 0.1 and a sinusoidal waveform of 10 Hz. Near-threshold fatigue crack propagation rates decrease and threshold stress intensity factor range, $ΔK_{th}$ values increase with increasing specimen thickness for all microstructures. As the effective grain size decreases, the extent of increase in $ΔK_{th}$ value with increasing specimen thickness decreases. It was observed that the effect of specimen thickness on $ΔK_{th}$ was attributed to roughness- and oxide-induced crack closures enhanced by plastic constraint at the crack tip. As specimen thickness increases, the strain gradient at the crack tip will be steeper and plane strain condition will be prevalent internally. At a given ΔK, crack tip opening displacement under plane strain would be smaller than that under plane stress due to the plastic constraint and the contribution of roughness-induced crack closure would be relatively higher across the thickness. By the prolonged fretting contact of the crack front, oxide-induced crack closure would be enhanced and thus the fatigue thresholds increased. In the case of SA106 Gr.C steel, the $ΔK_{th}$ of the spheroidized microstructure was lower than that of the as-received microstructure in 24mm thick specimens, while there was little difference in 6mm thick specimens. It was observed that the crack was propagated continuously in the ferrite region, while at the same time it was deflected through large angle in the pearlite region by branching. The mismatching asperities caused by pearlites would ...