TWI Industrial Member Report Summary 7/1976
By K Tanaka and J D Harrison
The report describes investigations of the fracture toughness of a Type 316 stainless steel by means of delta R and JR curves. Both COD (delta) and J values rise dramatically after crack initiation and reach plateau values after some crack extension. These plateaus are much higher than the value of delta and J at maximum load and are considered to represent the maximum fracture toughness of the material.
A load/crack mouth opening record is shown to be sufficient to obtain a JR as well as a delta R curve. The maximum plateau value can be obtained only in specimens with sufficient ligament depth. It is shown that this is achieved for a specimen whose depth, W, is four times the thickness, B, with a ligament length of 2.8B.
The relationship between the delta R concept and the model of crack tip behaviour proposed by Green and Knott for an extending ductile tear is also investigated.