The Influence of Plastic Prestraining on Brittle Fracture Resistance of Metallic Materials With Cracks

Abstract

The purpose of the present work was to study the influence of different regimes of overloading of pressure vessel steels in different states which correspond to the steel properties at the beginning of a reactor operation and at different degrees of embrittlement (simulated by heat treatment). The experiments were performed on 25, 50 and 150 mm thick specimens with short and long cracks of various shape in the temperature range from 293 to 623 K corresponding to the service temperature range of those steels.The following factors were investigated contribution of different effects (residual stresses, strain hardening, crack tip blunting) into the enhancement of the brittle fracture resistance of steels after warm prestressing, stability of the positive warm prestressing effect during subsequent exposure of the steels to different service loading conditions; size effect on optimal regimes of thermo-mechanical prestressing and on the brittle fracture resistance characteristics of the steels studied after warm-prestressing. An approach is proposed to predict the increase in the brittle fracture resistance of steels with cracks after warm prestressing. NOMENCLATURE a = crack length or small semi-axis of a semi-elliptical surface crack Aa =crack increment B, W, S = 3PB specimen thickness, width and length, respectively c = large semi-axis of a semi-elliptical surface crack E = Young's modulus JI, =critical J-integral at the crack growth onset (ASTM standard E813) K = stress intensity factor K,c =critical stress intensity factor under plane strain conditions K , =critical stress intensity factor (non-plane strain condition) determined at crack growth onset by KCV =impact fracture energy determined by testing V-notched specimens with a 0.25 mm notch tip HB = hardness the 5% secant method (GOST 25.506-85, ASTM standard E616-82) radius (GOST 9454-60, type IV) K, = critical stress intensity factor after warm prestressing K h = stress intensity factor during holding under load K,, = critical stress intensity factor defined via JLc value K , = critical stress intensity factor determined by the 5% secant method n = strain hardening coefficient AN = number of cycles P = load rp = plastic zone size T = temperature & = temperature of holding under load V = crack faces displacement along the load Line 6 = crack tip opening displacement Kr = K,/Kc!T,) K,, =maximum stress intensity factor in a cycle 73 1 132 V. V. POKROVSKY et at. ai = critical crack tip opening displacement 6, =longitudinal critical strain v = Poisson's ratio u, = ultimate tensile strength uy = yield stress $ =transverse critical strain S , , = residual crack tip opening displacement at unloading after warm prestressing