Crack growth behavior of small surface fatigue cracks is investigated for constant amplitude loading using in-plane bending specimens of 2024-T351 aluminum alloy. As small as 50㎛-radius semi-spherical starter notch is prepared by one pulse laser striking and subsequent mechanical polishing. Fatigue crack growth tests are performed under a stress ratio of zero at a frequency of 20Hz. Crack growth is monitored continuously during the test by an unloading elastic compliance method using back face strain gauge. Crack path morphology is observed.
The initial growth rate of small surface cracks decreases and reaches to a minimum at a crack depth of about 0.4mm regardless of applied loads. Crack growth rates of small surface cracks in terms of the stress intensity factor are dependent on the magnitude of applied load. Crack growth rates of small surface cracks can not be expressed by the effective stress intensity factor range based on crack closure measurements.
For small surface cracks deeper than 0.4mm, crack growth predictions are performed using the result of through-thickness short cracks and long surface cracks.