MIXED-MODE CRACK MOUTH REFLECTION IN TIME-RESOLVED ACOUSTIC MICROSCOPY OF SHORT FATIGUE CRACKS IN SINGLE-CRYSTAL ALUMINUM

An Al single crystal with the axial direction of [452BAR] was fatigued in air, at a constant resolved shear stress amplitude (4 MPa), a freq uency of 20 Hz and at room temperature. Time-resolved measurements wer e then carried out on short cracks in persistent slip bands (PSBS) on the top surface, which has the largest slip steps, using the acoustic microscope. A new acoustical signal from the crack, when the acoustic lens was scanned over the crack sending convergent acoustic beams down to the specimen, was detected and interpreted quantitatively. The sig nal results from the reflections of a mixed mode surface wave (Rayleig h longitudinal lateral wave) from the crack mouth. At 1.2 x 10(6) cycl es, two short cracks in the PSB were measured to be 17 and 27 mum in d epth. 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 me thod.