A steel part must withstand 4000 cycles of a stress of 0 to 500 MPa. The steel has a KIc of 200 MPa·m1/2.
Laboratory tests have shown that the crack growth rate
for this stress cycle follows the relationship da/dN=A(?K)6, where A is equal to 2.5×10-14 in units of MPa and meters.
(a) With this stress cycle, what is the critical crack length for failure of the steel, assuming an internal crack with Y=1?
(b) What is the minimum detectable crack length required of the nondestructive inspection equipment to guarantee the 4000-cycle fatigue life?
(a) With this stress cycle the stress goes from zero to 500 MPa. The part should break when it is at the maximum stress of 500 MPa. The value of KIc is known to be 200 MPa·m1/2. From Equation 11.23
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