SHORT CRACK EFFECTS IN FATIGUE : A CONSEQUENCE OF CRACK TIP SHIELDING

The so-called anomalous behavior of small fatigue cracks is examined with reference to the phenomenon of crack tip shielding, i. e. , where the 'effective crack driving force' actually experienced in the near-tip region is reduced from the applied (far-field) value. Sources of shielding are described in terms of mechanisms which act primarily on the wake of the crack, and rely on the production of elastically constrained zones which envelop the crack (zone shielding), on the generation of wedging, bridging or sliding forces between the crack surfaces (contact shielding), and on crack path deflection and meandering. Moreover, the development of shielding with initial crack extension can result in progressively decreasing growth rate behavior as the threshold is approached.