J resistance curve behaviour of S355NL structural steel using the unloading compliance technique

Room temperature toughness measurements performed on 32 mm thick flat surface and side grooved C(T) specimens of the S355NL structural steel are presented. As expected, stable ductile crack growth was observed during the tests and toughness characterization based on resistance curve (R curve) concepts was therefore used. The unloading compliance technique was chosen to estimate stable crack growth during the tests, and accurate load line displacement measurements were performed using a clip gage. Stress intensity factor and compliance calibrations as a function of crack length/width ratio, published by ASTM, were used to derive the J R-curve. The R-curve for the flat surface specimen is higher than for the grooved specimen. The estimated J at initiation of stable crack propagation is also higher for the flat surface specimen, although difference between the flat and grooved specimens is very small. Generally, a drop in fracture toughness was observed for the grooved specimens, as expected given its higher triaxiality. As an additional comparison of the two types of specimens, the maximum load J0,max value was obtained using the actuator displacement record. The maximum load measurements proved to be rather similar in both types of specimen.