Radiation-induced changes in the fracture extension resistance (R-curve) of structural steels

The effects of irradiation on the fracture extension resistance of low- and medium-strength steel plates were explored using R-curve assessment procedures newly developed at the Naval Research Laboratory. The study employed four thick plates of A212-B, A302-B, A533-B, and A543-1 steel. The plates were specifically selected to depict a broad range of pre-irradiation dynamic tear (DT) upper-shelf toughness. Irradiations were conducted at low temperatures, < 450 F (232 C), and at an elevated temperature, ∼ 550 F (288 C). A subsize R-specimen (0.4 in. thick), patterned after the 5/8-in. dynamic tear specimen, was employed for pre- and postirradiation R-curve determinations. R-curves were constructed by plotting specimen energy absorption per unit fracture surface area against relative crack extension distance. Radiation exposure was revealed to have a highly detrimental effect on R-curve performance. The effect was observed for all test plates regardless of their pre-irradiation shelf toughness level. The effect encompassed R-curve performance at temperatures corresponding to the dynamic tear upper shelf and at temperatures within the dynamic tear transition regime. The primary detrimental change for the upper-shelf condition was a reduction in R-curve level. A tendency for irradiation to reduce R-curve slope was also evident. The analysis suggests that, with sufficiently high fluence, neutron irradiation can cause a transition from rising R-curve to flat R-curve behavior at upper-shelf temperatures.