Cyclic R-curve measurements in nodular cast ironTuesday (26.05.2020) 17:20 - 17:40
The fatigue life of nodular cast iron is profoundly influenced by the growth of fatigue cracks from porosity generated by the casting process. According to the application of hydraulic presses, the crack growth might occur under negative load ratio. In this scenario, the initial stage of crack propagation is governed by the development of crack closure which determines a strong crack threshold dependence with the crack length. Consequently, it is of fundamental importance the proper measurement of the so-called cyclic R-curve which defines the change of the crack threshold as the crack advances from the porosity with limited initial crack closure to a steady-state condition where the combined effect of plasticity-induced, roughness, oxidation crack closure yield a higher crack threshold. In this study, the cyclic R-curve is measured from experiments employing micro-notched specimens for three stress ratios R: 0, -1 and -2.5. Following pre-cracking in compression to minimize the load history effects, the crack is subjected to block of cycles at constant DK in order to induce an initial crack advancement and a successive crack arrest due to the crack closure development. The Digital Image Correlation technique was employed to precisely pinpoint the crack opening and closing and to measure the crack advancements. The study points to discuss for the cast iron under investigation the possible application of the R-curve to estimate the Kitagawa-Takahashi diagram which defines the change of the fatigue strength with the crack size.