Crack and slip band formation in iron during ultrasonic fatigue at various temperatures

Abstract

Armco iron specimens with notches were cyclic loaded at stress amplitudes that produced fatigue lives greater than 4 × 10 7 cycles at temperatures of 23, 60 and 100°C and a frequency of 23 kHz. Simultaneously crack initiation and propagation as well as slip bands formation were observed by optical microscopy. The main fatigue cracks initiates crystallographically from a surface layer at the grain boundaries and in regions with a low density of slip bands or non-crystallographically in regions without slip bands. The crack path is surrounded by a thin plastic region formed by slip bands which tend to be in a shear direction or may be in other directions. The thickness of the plastic region and orientation of cracks nuclei are a function of both the grain orientation and temperature during loading. The ultrasonic plastic deformation at higher temperature is found in greater number of grains in contrast to observations at room temperature. At higher temperatures the concentration of external stress in the notch root is less significant than that room temperature also at high frequency loading.