ON CORROSION FATIGUE EMANATING FROM NOTCHES: STRESS FIELD AND ELECTROCHEMISTRY

摘要

We analyze the processes of initiation and growth of corrosion fatigue cracks on the surface of a semicircular notch depending on the stress field and local electrochemical processes running inside the notch. For this purpose, we consider a typical corrosion-active system formed by low-strength carbon steel placed in a 3 percent NaCl solution. In this system, the electrochemical dissolution of the metal is predominant. A semicircular notch is regarded both as a stress concentrator and as a local corrosion electrolyzer. The field of elastoplastic stresses near the notch is computed by the finite-element method with the help of the CASTEM-2000 software package. The early stages of corrosion fatigue are investigated and the existence of a critical density of cracks q_*. On the surface of the notch is demonstrated (above this density, the cracks undergo intense coalescence leading to the formation of the macrocrack). We propose a model of this process and an analytic relation for the prediction of the number of cycles N_* corresponding to the critical density q_*. This relation takes into account the constants of electrochemical dissolution of the metal, the density of corrosion current I_cor, and the effective stress sigma_eff, which can be regarded as the mean level of stresses in a segment 0 < = x < = x_eff from the notch tip.