Crack growth and closure behavior of short fatigue cracks are investigated for both constant amplitude and random loading. Also the effects of the frequency characteristics and history length of the random block loading are discussed. Tests are performed for various stress ratios using in-plane bending specimens of 2024-T351 aluminium alloy. Artificially prepared short through-thickness cracks are used. Crack length and closure of short cracks are measured by an unloading elastic compliance method.
Under constant amplitude loading, short cracks grow faster than long cracks in the low stress intensity factor range region and growth rates of short cracks are influenced by the applied stress ranges. The growth rates of short cracks are well described by the effective stress intensity factor range. But the growth rates of short cracks lower than $3x10^{-9}m/cycle$ are much lower than that of long cracks for the same effective stress intensity factor range.
It is observed that crack opening point of short fatigue crack under random loading is nearly constant within a block of random loading and is much lower than that under constant amplitude loading.
The importance of the crack closure phenomenon is examined by predicting the growth lives of short cracks using obtained crack opening behavior. As in the constant amplitude loading, the fatigue crack growth and closure behavior of short cracks under random loading can be well described in terms of the effective stress intensity factor range.
