Mixed mode crack mouth reflection in time-resolved acoustic microscopy of short fatigue cracks in single crystal aluminium

An Al single crystal with the axial direction of (452) was fatigued in air, at a constant resolved shear stress amplitude (4 MPa), a frequency of 20 Hz and at room temperature. Time-resolved measurements were then carried out on short cracks in persistent slip bands (PSBS) on the top surface, which has the largest slip steps, using the acoustic signal from the crack, when the acoustic lens was scanned over the crack sending convergent acoustic beams down the specimen, was detected and interpreted quantitatively. The signal results from the reflections of a mixed mode surface wave (Rayleigh longitudinal lateral wave) from the crack mouth. At 1.2×10⁶ cycles, two short cracks in the PSB were measured to be 17 and 27 mu m in depth. The angle with the specimen surface was observed to be about 54 degrees, compared with a value of 51 degrees determined by an X-ray method.