Determination of fracture toughness, J–R curve from experimental test results requires certain geometry factors. These expressions of geometry factors are available for standard geometries and loading configurations. However, these expressions are not available in open literature for complex geometries and loading configurations. R6 Failure Assessment Diagram (FAD) methodology is used for assessment of failure in the failure regime of cleavage fracture, ductile crack initiation and plastic collapse. In this paper, the use of this method is extended to estimate the entire J–R curve from the fracture test results. Under comprehensive test programme initiated at RSD (Reactor Safety Division), BARC (Bhabha Atomic Research Centre), large numbers of straight pipes with circumferential cracks have been tested. These experimental results have been used to calculate material J–R curves using R6 FAD, which are subsequently compared with conventionally calculated J–R curves using g factor approach. It can be observed that the J–R curves calculated using both approaches are in good agreements for three out of total six pipes studied. Further, the segregation of J–R curves based on the size of pipes has been studied in the light of prevalent crack tip constraints. Here, crack tip constraints are calculated in terms of stress triaxiality parameters ‘Q’ for all pipes. It can be noted that the J–R curves for 8 in. pipes are ‘toughened’ because they have lower constraints than 16 in. pipes. This establishes the theory that high/low crack tip constraint causes low/high material fracture toughness.