Studies on short fatigue crack growth behavior of a plain carbon steel using a new specimen configuration

A simple experimental technique has been developed to examine short crack growth behavior vis-à-vis microstructural features of a material. The procedure involves the use of a new specimen configuration, fatigue testing with the help of a rotating bending machine and optical microscopic examination of crack profiles and their associated microstructures using an image analyzer. The growth characteristics of several small fatigue cracks in a 0.25% carbon steel have been studied using this technique. The critical crack lengths above which linear elastic fracture mechanics may be applicable, the fatigue threshold values at the transition of short to long cracks and the influence of microstructure on the crack path have been examined. The estimated critical crack lengths are considered to be dependent on the nature of the short cracks; but the fatigue threshold values obtained from short crack growth experiments are found to lie in a narrow range. The cracks were found to have affinity to grow through interfaces, but the length of a crack passing through a specific phase was found to be approximately proportional to the amount of the latter.