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Typical fractography of multi-point crack initiation with three origin sites of A, B and C (notched specimen, R = 0.1, σa = 150 MPa, Nf = 7.460 × 106 cycles); (a) the macro profile with three crack initiation regions and crack propagation region; (b) enlargement of crack initiation region B; (c) enlargement of crack initiation region C.
