Finally, Figure 5 shows the experimental measurements and finite element predictions of crack growth rate for the case of a specimen with a cold expanded hole subject to creep relaxation. The crack growth rates for small holes are essentially the same as for cold expanded holes without creep. For cracks of intermediate length, from about 1 to 2 mm, the growth rates are higher. Again, the finite element model under-predicts the growth rate for small crack lengths and over-predicts the rate for large crack lengths. Cycles N, x 104 0 5 10 15 20 25 30 Average surface crack length (mm) 0 1 2 3 4 Non cold expanded Cold expanded Cold expanded and creep Figure 3: Average fatigue crack lengths versus number of cycles. Surface crack length, c (mm) 0 1 2 3 4 5 6 Surface crack growth rate, dc/dN (mm/cycle) 10-6 10-5 10-4 10-3 10-2 Experimental (left) Experimental (right) Finite element prediction Figure 4: Measured fatigue crack growth rates for cold expanded specimens compared with averaged finite element results.
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