CRACK INITIATION CRITERION AT NOTCHES IN ZR-2.5NB ALLOYS

This work investigates the experimental conditions necessary for initi ation of delayed hydride cracking in Zr-2.5Nb alloys. The experiments were performed on 240 notched cantiliver beam specimens loaded in pure bending to a wide range of stresses. The test temperature was 250 deg rees C with weekly cycles between 60 and 295 degrees C. The results ar e interpreted in the light of a recently proposed fracture criterion f or hydrides at cracks and notch tips. This criterion is based on the p remise that crack initiation occurs if the local tensile stress at the hydride exceeds that for hydride fracture. The local stress at the hy dride is expressed as the sum of the peak stress at the notch and the stress in the hydride arising from the hydride formation process. The fracture stress of the hydride is obtained experimentally. An approxim ate analytical method, based on a modified Neuber's rule, was used to calculate the plastic zone size needed to estimate the peak stress at the notch. These approximate calculations show that the threshold notc h tip stress for cracking is between 675 and 750 MPa.