Modeling fatigue of pre-corroded body-centered cubic metals with unified mechanics theory

Corrosion is a common degradation problem in engineering structures. The unified mechanics theory (UMT) was used to develop a model to predict the fatigue life of pre-corroded steel samples with BCC structure. The model was also verified experimentally. For this purpose, A656-grade steel samples were immersed in a 5 wt% sodium chloride (NaCl) solution at a pH of 7. Electrochemical measurements were made with a potentiostat to monitor corrosion. Then, a series of ultrasonic vibration fatigue tests were conducted on these corroded samples at 20 kHz. UMT is ab-initio unification of the laws of Newton and the 2nd law of thermodynamics. Hence no empirical degradation/dissipation function is needed. However, the thermodynamic fundamental equation for metal corrosion and ultrasonic vibration must be analytically derived. The thermodynamic fundamental equation formulates all the entropy generation mechanisms during the corrosion and then during ultrasonic vibrations. The cumulative entropy generation was then used to calculate the Thermodynamic State Index (TSI), which starts at zero and asymptotically approaches one at failure. Hence, TSI predicts the life span. The proposed model results agree well with the experimental results, thus proving that the UMT-based model can predict the high cycle fatigue life of previously corroded samples accurately.