Effect of direction of approach of test temperature on fracture toughness of Zr-2.5Nb pressure tube material

Previous work by one of the authors reported the stress-field for a fully-constrained hydride embedded in matrix computed using finite element method to develop an understanding of the temperature dependence of susceptibility of Zr-alloys to hydride embrittlement. It was observed that the nature and magnitude of stress field in the matrix and hydride depended on whether the hydride is expanding (experienced while cooling from a peak temperature) or contracting (experienced while heating to the test temperature). On the basis of the observed dependence of the nature and magnitude of stress components in the matrix and hydride, it was suggested that the fracture toughness of hydrided Zr-alloys may depend on the direction of approach of the test temperature. In order to verify this, fracture tests were carried out using samples machined from hydrogen charged Zr-2.5Nb alloy pressure tube material as per ASTM standard E1820-11 in which the test temperature was attained by either heating to the test temperature or cooling from a peak temperature. The direction of approach of the test temperature did not affect the fracture toughness in the lower and upper shelf temperature regime, it indeed had an effect in the transition regime. The computed stress field could provide explanation for the observed axial splits on the fracture surfaces for both the heating and cooling cases.