On Experimental Procedure Development for Evaluating the Effect of Biaxial Loading on the Static Crack Resistance Characteristics of Reactor Vessel Steels

According to modern normative documents, the loading of postulated cracks in reactor vessels and standard specimens during crack resistance tests has a number of differences, namely: biaxiality of certain load configurations on the crack faces can change during non-isothermal, nonmonotonic processes under emergency modes of thermal shock, while crack front have a semi-elliptical shape of with relative depth ratio of 0.1–0.15. These factors prompt the development of an experimental technique to study crack resistance on non-standard type specimens. Cross-shaped (cruciform) bending specimens with a semi-elliptic surface crack and a through linear short crack, as well as model strip specimens with similar cracks in previous tests, were designed and studied for their static fracture strength. A method for monitoring fatigue crack initiation on a cross-shaped specimen by changes in its suppleness has been proposed. Evolution of semi-elliptic crack front shape at its initiation from a surface stress raiser under cyclic bending load has been investigated and the possibility of obtaining fatigue semi-elliptic crack of a certain configuration close to the postulated, according to the normative documents of cracks was shown. On the basis of test results, it was established that at temperature 20°C for 15Kh2NMFA-A steel, the standard conditions of a plane deformation in the used specimens were not satisfied, leading to the overestimated values of crack resistance since the steel ductile-brittle transition temperature T₀ (according to the Master-curve) was much lower. It was shown that the obtained increase in crack resistance under equal-axial bending is comparable with literature sources for the conditions of large-scale yielding with uniaxial loading.