An investigation into fatigue crack closure of materials under constant amplitude loading

A thorough examination of the correlations of fatigue crack closure with crack growth behaviour was made. Both experimental and analytical studies, under various conditions, were conducted on two aluminium and four steel alloys. The study systematically investigated a number of factors influencing crack closure. In addition, a computer simulation for crack closure was developed by an interface model using finite elements. To check on the application of crack closure developed in this program, a numerical approach was employed to predict the fatigue life with crack closure correction.;It was found that fatigue crack closure was affected strongly by various factors. The crack closure effect denoted by the crack opening load was a function of fracture toughness of materials, which was applicable to both nonferrous and ferrous metals. Examination of the fracture appearances indicated that a coarse fracture surface due to high fracture toughness of a material was associated closely with a significant crack closure effect and a slow crack growth rate. Crack opening stress intensity factor was found to be independent of crack length and specimen thickness. Crack opening load is insensitive to the applied loading levels.;Crack opening level was found to be an important parameter in describing crack growth behaviour for peened and unpeened specimens, as well as for humid environments. Various peened locations on the specimens and the technique of peening under load resulted in a marked change in crack closure effect. The study addressed the correlations among the crack reinitiation life, shot peening intensity and crack opening level. A relationship between crack closure effect and relative humidity was established.;Crack closure phenomenon is due to a crack wake effect. Therefore, an interface model using finite elements was performed to physically simulate the crack closure phenomenon. The analytical prediction of crack opening load was greatly consistent with experimental results. The analysis of stress distribution in the ligament indicated a shift of the neutral axis.;Based on the improved fatigue data processing and the crack closure correction, a numerical method to predict fatigue life was conduced. It shows an improvement of prediction accuracy over that ignoring the closure effect.