The leak-before-break(LBB) behaviour of a structural component under high fatigue stress, low-cycle fatigue conditions is an important problem in nuclear power plants, liquid nitrogen gas tankers and chemical plants. This paper is an experimental study to clarify the crack shape at penetration, crack opening behaviour after penetration and the LBB condition for plate specimens and pipe specimens. The fatigue load which realizes the LBB condition, PEE,, is based on a net-stress criterion. It is found that Pf,, is dependent only on the initial crack length a,. Furthermore, it is shown that the crack opening displacement Vat the centre of a penetrated crack can be derived using the gross stress, uG, and the front surface crack length, us, together with the back surface crack length, a,. NOMENCLATURE a, = one half of initial surface crack length (mm) ub = one half of back surface crack length (mm) a, = one half of front surface crack length (mm) aL = one half of front surface crack length at penetration (mm) b, = initial depth of surface crack (mm) P,,, = maximum load of fatigue and static test (kN) P,,, = minimum load of fatigue and static test (kN) P[,,, = fatigue limit load for LBB realization (kN) P t B B = critical static load for LBB realization (kN) N L =number of cycles at penetration NF = number of cycles at fracture t = thickness of plate or pipe (mm) r = inside diameter of pipe (mm) V = crack opening displacement (mm) W = plate width (mm) da/dN = crack growth rate (mm/cycle) 0, = initial crack angle of pipe 8, = crack angle at penetration of pipe uf = flow stress [ = (0, + u,)/2] (MPa) uu =tensile strength (MPa) uy = yield stress (MPa)