For a specimen under fatigue test with a maximum stress of 150 MPa (tensile stress) and a minimum stress of -20 MPa (compressive stress), if a fatigue life 1.5*10^6 cycles is expected, determine the largest initial crack length that can be allowed.
Assume the fracture toughness is 50 MPa sqrt(m), and the values for m and A in the fatigue life equation are 3.0 and 1.0*10^-12, respectivly, for stress in MPa and crack length in m. Also, assume Y has a value of 1.0 in the fracture toughness equaiton.