Comparative Study of Basic Test and Resistance Curve Methods for Fracture Toughness Evaluation of Heat-Treated Zr-2.5Nb Alloy

ASTM E1820-11, the most widely adopted standard for the determination of fracture toughness parameters, recommends two ductile crack growth correction methods for the evaluation of the J-integral parameter, viz., the basic test (BT) method and the resistance curve test (RC) method. In the present work, a comparison between the fracture toughness parameters obtained using these two methods for heat-treated Zr-2.5Nb alloy at 25°C and 300°C is presented. In order to examine the influence of a material's microstructural condition on the fracture toughness results obtained via these two methods, Zr-2.5Nb alloy was investigated under six solution heat-treated conditions after getting soaked at 850°C, 870°C, or 890°C for either 15 min or 30 min followed by water quenching. The BT method predicted a higher J parameter than the RC method for a given crack length. This deviation in the magnitude of J increased with increasing crack length and was found to be almost twice as much at room temperature as the deviation observed at 300°C. For smaller crack lengths (i.e., up to a/W < 0.54), the J parameters determined using the two methods showed insignificant deviation (<5 %). However, such deviation reached ∼20 %, corresponding to ∼0.63 a/W. Values of fracture toughness parameters such as J_Q, J_max, and dJ/da evaluated using the BT method were found to be higher than those obtained using the RC method. However, deviations in the J_Q and J_max parameters determined by the two methods were insignificant (less than ∼6 %), whereas the deviation in the dJ/da parameter was ∼15 %. The RC method, in contrast to the BT method, provides a marginally conservative estimation of the fracture toughness. The parameter α used in the BT method was observed to control the extent of deviation in the J–R curves derived using these methods.