The paper presents a special shape plate specimen with high strain gradient fields which are similar to those at pressure vessel nozzles. On the specimens, experimental J parameter was obtained by means of the multiple specimen technique; the CTOD δt of three definitions was measured with the microphotography technique; the fracture criteria of the crack initiation were examined at the same time. Thus the estimation formula of the parameter J and the relation between J and δt are deduced for practical nozzle corner cracks. The suitability of the engineering methods of defect assessment being used popularly all over the world and the problem of J controlling crack growth are discussed at the end of the paper.

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