Up till now,in the defect assessment standard of pressure vessel,a semi-experience method,equivalent K(stress intensity factor)conversion,usually used to deal with beried and surface three dimen-sional crack.In this paper,the method using elastic-plastic fracture parameter
δ(crack tip opening displace-ment)to replace K(stress intensity factor)is first proposed,and equivalent
K conversion in usual pressure vessel assessment is replaced by equivalent
δ conversion,and equivalent
K conversion and equivalent
δ conversion is demonstrated in penny shaped crack.It is observed that the equivalent
K conversion is conser-vative,and the equivalent
δ conversion is more reasonable than the equivalent
K conversion.