A calculation-experimental study of crack-tip opening displacement and residual stresses upon warm prestressing

Abstract

V. N. EzhovFor a reactor pressure vessel steel 15Kh2MFA(III) experiments and calculations have been carried out to study the factors that have an influence on the increase of the lower-shelf fracture toughness in the temperature dependence diagrams upon warm prestressing. Stereoscopic fractography and numerical investigation have demonstrated that after the warm prestressing the crack tip remains blunt. This reduces the stress singularity during subsequent loading and raises the material fracture toughness. The paper gives the calculated data on residual stresses and crack-tip opening displacement during warm prestressing and upon relief. The calculated results are shown to agree well with the known analytical relations and experimental data.Keywords: warm prestressing, fracture toughness, heat-resistant reactor pressure vessel steels. Introduction.A review of fracture-mechanics publications [1][2][3][4][5][6][7][8][9][10][11] has revealed that for crack-containing bodies made of bcc-structured materials a promising method for improving their brittle fracture resistance is the plastic prestraining at an upper shelf of the fracture toughness vs. temperature curves.The available published data demonstrate that depending on the level of warm prestressing (WPS) there are three main factors that have an effect on the increase of brittle fracture toughness: (i) strain hardening, (ii) crack tip blunting, and (iii) residual compression stresses [1] (Fig. 1). However, the contribution of each of these factors to the increase of fracture toughness depending on the WPS level has not been adequately studied. Furthermore, the influence of factor such as the body geometry and dimensions (the scale effect) on the magnitude of fracture toughness upon WPS is still poorly understood. Ignoring these factors and disregarding the significant role of crack blunting that may take place upon WPS are the drawbacks of the Chell model [2] which has been widely used recently.It was shown in [5] that the applicability of the Curry model [3] was limited due to disregarded crack tip blunting and insignificant role of strain hardening. Thus, that approach is suitable for the cases where the relation of loads and displacements during WPS is close to linear.Reed and Knott [6] studied the contribution of crack tip blunting to the WPS effect. The warm-prestressed specimens were tempered to relieve residual stresses. The researchers observed a considerable decrease of fracture toughness and thus inferred that the contribution of the crack tip blunting was small. It should be mentioned that tempering results in a lower fracture toughness of non-WPS specimens too, and that the WPS level studied was such that the crack blunting was but insignificant and it was the residual stresses which made the major contribution to the increase of fracture toughness upon WPS.Despite the development of analytical models for predicting the WPS effect, insufficient consideration is given to the numerical analysis of this phenomenon. Nevertheless, there have been public...