DISLOCATION EMISSION AND FATIGUE CRACK-GROWTH THRESHOLD

The stress intensity range DELTA-K below which no cyclic plastic deformation at the crack tip, and hence, no fatigue crack propagation occurs is investigated. The emission of dislocations from the crack tip is assumed as mechanism for the dislocation generation. For a mode III crack, a computer simulation is carried out to study the influence of the number of dislocations, the friction stress and the critical stress intensity, k(e), to emit a dislocation. If during loading only one dislocation is emitted, the return of this dislocation to the crack tip and the emission of a dislocation with an opposite sign and the recombination with the first dislocation are possible during unloading. The DELTA-K necessary for both mechanisms is about 2k(e). If during loading more than one dislocation is emitted, during unloading at first a certain number of dislocations return to the crack tip before a dislocation of opposite sign is emitted. The necessary DELTA-K to move one dislocation back to the crack tip during unloading decreases with increasing number of dislocations and reaches a constant values of about 1.1 k(e). This value of DELTA-K then is roughly independent of the friction stress and k(e).