Elastic plastic fracture toughness values for a hastelloy alloy sample submitted to a static load

In the High Temperature Reactor research programm of the french CEA, Hastelloy alloy plates contain mineral fibers used as insulating material and as such prevent the concrete core vessel from receiving excess heat. Due to the high pressure exerted on this insulating material, the Hastelloy plates are heavily stressed and, therefore, it is important to know their fracture strength. In the case of an elastic, brittle material, the fracture toughness is commonly defined through the well knownKsub(1c) parameter. But, for plastic materials, measuring Ksub(1c) requires very large samples and it was found more convenient to measure the fracture parameter Jsub(1c) instead of Ksub(1c). This parameter was determined according to the RICE or to the more sophisticated HERKLE and CORTEN formula. Two independent ways were followed to measure Jsub(1c): In the first one the shape of the crack after loading was made apparent by some fatigue cycles. After complete fracture of the sample, the amount of crack extension appears clearly and it becomes possible to generate a J resistance curve. As an alternative, we partially unloaded the specimen to get its compliance: the change in compliance was then supposed to coincide with the start of the crack extension. Finally a finite element code was used to compute the specimen state at the beginning of the crack extension. So we are able to obtain the shape of the Load-Displacement curve and to compare it with the experimental one. (orig.)