An investigation of bending fatigue crack propagation in structural steel by the measurement of indirect parameters

This work analyzes procedures to monitor fatigue crack propagation in smooth specimens made of high-strength structural steel based on the measurement of variations in the induced moment at the fixed support. Bending fatigue tests were performed with strain control for eight different levels of strain amplitude. The moment at the support was recorded during each test and the number of cycles to fracture determined. For each strain amplitude, the average moment at the support was plotted against the time and the point where a significant decrease occurred in the specimen flexibility was identified. From these results, a curve indicating the initial crack detection was built. To verify in each curve the relationship between the crack size and corresponding number of cycles, a set of interrupted fatigue tests was performed. Crack size and position were marked by specimen heating and oxidation followed by cycling to final fracture and fracture surface analyses. The initial results indicate that this methodology can identify the transition from the fatigue crack initiation to the propagation phase.