A large critical part is made of steel for which KIc=60 MPa·m1/2 determined in air at room temperature. Nondestructive inspection has found no cracks in the part.
The minimum detectable crack length for the nondestructive inspection equipment is 2a=0.20×10-3 m. The part is to be subject to a stress range of ??=300 MPa about a mean stress of 150 MPa. Laboratory tests in air at room temperature show that the crack growth rate in this material follows the relationship da/dN=1×10-13 ?K6 in units of MPa and meters.
(a) Predict the crack length at fracture, assuming an internal crack with Y=1.
(b) Calculate the lifetime of this part based upon fatigue-crack propagation in air at room temperature.
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