Effect Of Residual Stress On Creep Crack Initiation And Growth For Specimens With Different Geometric And Material Constraints

In this paper, different residual stress fields were introduced into compact tensile (C(T)) specimens by using the local compression method, and then the finite element method (FEM) based on ductility exhaustion damage model was used to simulate the effect of residual stress on creep damage, creep crack initiation (CCI) and growth (CCG) for homogeneous Cr-Mo-V steel and P92 steel welded joint with different material constraints. The mechanism of residual stress effect on creep crack initiation and growth was analyzed by the distributions of equivalent stress and stress triaxiality ahead of the crack tips. The main work and conclusions are as follows:(1) The tensile residual stress was introduced near the notch tips in the 2-D notched C(T) specimens by the in-plane compression method, then the effect of residual stress on creep damage and rupture was investigated. The results show, with the increase of notch radius, the peak tensile residual stress decreases, the CCI time increases and the CCG time decreases. With increasing the pre-compression displacement, the residual stress increases and the CCI time decreases, but the pre-compression displacement almost has no effect on the CCG rate. The mechanism of above results was analyzed by the distributions of stress fields and creep damage ahead of the crack tips.(2) For the pre-cracked C(T) specimens of Cr-Mo-V steel with different geometric constraints (in-plane and out-of-plane constraints), the residual stress was introduced by using the local out-of-plane compression (LOPC) method, then the effect of residual stress and creep ductility on the creep damage and creep crack initiation was investigated. The results show that by increasing the pre-compression depth, the residual stress ahead of the crack tips increases, the CCI time deceases; with the increase of creep ductility, the CCI time increases. The residual stress distributions for specimens with different in-plane and out-of-plane constraints and the effect of residual stress on CCI time and location are obtained by the calculation.(3) By moving the punching tools location agaist the crack tip in the LOPC method, the different residual stress distributions from tensile residual stress to compressive residual stress were introduced into the crack tips, then the effect of residual stress on the CCI and CCG behavior of Cr-Mo-V steel was investigated. The results show that the higher compressive residual stress at crack tips delays creep crack initiation and increases the initiation life, while the higher tensile residual stress promotes creep crack initiation and decreases the initiation life. The residual stress essentially has no effect on the creep crack growth rate, thus its effect on creep fracture life mainly arises from its effect on creep initiation life.(4) The residual stress was introduced into the C(T) specimens of P92 welded joint with different initial crack positions by the LOPC method, then the effect of the interaction between residual stress and material constraint with different initial crack position on the CCI and CCG was researched. The results show that the interaction between residual stress and material constraint determines the creep damage accumulation around the crack, the crack growth path, CCI and CCG life. The tensile residual stress ahead of the crack tip promotes creep crack initiation and growth, thus decreases CCI time and rupture life, and increases CCG rate. The interface cracks have shorter CCI time and higher CCG rates due to the existence of significant material constraint.(5) The residual stress was introduced into the C(T) specimens of P92 welded joint with different material mismatches in creep properties by the LOPC method, then the effect of residual stress and material constraint caused by the mismatch in creep properties on the CCI and CCG was researched. The results show that the interaction between residual stress and crack-tip material constraint determines the crack-tip creep damage, creep crack growth path, CCI and CCG behavior. The tensile residual stress at crack tips and the high local stress triaxiality caused by the material constraint effect promote the CCI and CCG, decrease the creep rupture life. When the crack-tip material is creep soft material, the CCI time decreases and the CCG rate increases.