TESTING SIMULATION
A virtual test starts with one of the above configured specimens. The specimen is subjected to a cyclic axial load corresponding to the parameters given for one of the experiments listed in Table 1. Consequently, some grains are plastically deformed. A grain is considered experiencing plastic deformation when the current applied stress is greater than its yield stress.
The extents of the monotonic and cyclic plastic zones ahead of the tip of a crack are assumed given by either (1) carefully calculated extents based on fracture mechanics concepts in terms of crack length, applied stress and yield stresses of the grains located ahead of the tip or (2) the extent of plasticity within the grains located ahead of the tip, whichever is greater. The applied load and the extents of the crack tip plastic zones calculate corresponding maximum and cyclic crack tip opening displacements which are, then, used for the calculation of the advance rate of the considered tip. The details of those calculations are given elsewhere 37.
All existing tips are subjected to the above calculations. An increment of load cycles is accommodated to capture the earliest event out of (1) a maximum tip advance of 0.1μm, (2) a maximum approach of adjacent tips of two neighboring cracks of 0.1 μm (3) the earliest merge of two approaching tips and (4) the instant of changing the applied load as given in Table 1. The resulting advance of the tips updates their coordinates within the surface grains. With the updated configuration of the existing cracks, a computational loop terminates. The flow chart of a computer software devised to follow and repeat the above algorithm for further computational loops is shown in Fig. 2.