Pressure vessel fracture safety investigations using as criterion the onset of crack propagation

For the nuclear pressure vessel frangible behavior can be normally excluded. The critical material characteristic at fracture, Ksub(IC), is with linear elastic fracture behavior of steels near to the onset of stable crack growth. With elastoplastic material behavior the onset point of the stable crack growth and the actual fracture may be well apart. The 5% secant method is here of little use. The onset point can be down to 60% of the Psub(max). As a lower bound criterion the onset of stable crack growth is proposed. This point is detected on RPV steel by acoustic emission method and confirmed with heat tinting tests. From load at initiation of propagation Psub(i) the value of Ksub(Ii) can be derived. Similarly deltasub(i) can be measured. For the purpose of measuring deltasub(i) clip-in gauges were used according to the 'Swiss guidelines for the measurement of COD'. From the load/clip gauge displacement record, the load and clip gauge displacement at the onset of stable crack growth can be determined. These values are then used to calculate Ksub(Q), Ksub(Ii), deltasub(i) and Ksub(ICD). It is the purpose of this paper to show experimentally that in the elasto-plastic range a similarity exists between the critical crack sizes as determined by Ksub(Ii), COD, J and equivalent energy methods