CRACK INITIATION AT LONG RADIAL HYDRIDES IN ZR-2.5NB PRESSURE TUBE MATERIAL AT ELEVATED-TEMPERATURES

Crack initiation at hydrides in smooth tensile specimens of Zr-2.5Nb p ressure tube material was investigated at elevated temperatures up to 300-degrees-C using an acoustic emission (AE) technique. The test spec imens contained long, radial hydride platelets. These hydrides have th eir plate normals oriented in the applied stress direction. Below appr oximately 100-degrees-C, widespread hydride cracking was initiated at stresses close to the yield stress. An estimate of the hydride's fract ure strength from this data yielded a value of approximately 520 MPa a t 100-degrees-C. Metallography showed that up to this temperature, cra cking occurred along the length of the hydrides. However, at higher te mperatures, there was no clear evidence of lengthwise cracking of hydr ides, and fewer of the total hydride population fractured during defor mation, as indicated by the AE record and the metallography. Moreover, the hydrides showed significant plasticity by being able to flow alon g with the matrix material and align themselves parallel to the applie d stress direction without fracturing. Near the fracture surface of th e specimen, transverse cracking of the flow-reoriented hydrides had oc curred at various points along the lengths of the hydrides. These frac tures appear to be the result of stresses produced by large plastic st rains imposed by the surrounding matrix on the less ductile hydrides.