Discussion: “Interpretation of Irradiation Effects on the Fracture Toughness of a Pressure Vessel Steel in Terms of Crack Tip Stress Analysis” (Parks, D. M., 1976, ASME J. Eng. Mater. Technol., 98, pp. 30–35)

Abstract

where t is the total time required to load the specimen to failure. But the intense strains associated with blunting extend only over a distance of size roughly 25 [11], at which point the strains have decayed to the order of an initial yield strain. Further, this decay has occured over a distance which is of order yield strain times plastic zone dimension. Thus, in by far the bulk of the plastic zone, a typical strain rate would be of order 10-3 times the crack tip strain rate. The ASTM specifications for Kic testing [23] prescribe loading rates such that the total fracture test time for this material is of order one minute. Thus a typical strain rate in the plastic zone is of order 10-4 /sec, and, in the temperature range studied, this strain rate occasions a yield stress which is essentially the same as quasi-static. An estimate of an irradiated fracture toughness transition temperature can be made in the same manner as for the unirradiated material. Using the maximum achievable stress intensification for non-hardening materials, 2.97, gives a transition yield stress of 570 MNm~2 (82 ksi). Depending on the particular irradiating temperature and total neutron fluence, the temperature corresponding to this yield stress could range from 94°C (200°F), to nearly 261°C (500°F), again complicated by the flatness of the Oo versus temperature curve. Although valid Kic measurements on irradiated material are scarce, there are indications that an irradiated fracture transition temperature is near 94°C(200°F), [27] where the yield stress is roughly 590 MNm~2.