BEHAVIOR OF SHORT CRACKS IN ALLOY 800HT UNDER BIAXIAL FATIGUE

Biaxial in phase fatigue tests were carried out on thin walled tube sp ecimens of alloy 800HT at ambient temperature. The loading modes inclu ded tension, torsion, and combined tension-torsion with a tensile/shea r plastic strain range ratio Delta epsilon(p)/Delta gamma(p) = 3(1/2). The influence of effective strain amplitudes and biaxiality on the in itial growth of fatigue cracks was investigated using the replica tech nique. The results indicated that the loading conditions strongly affe cted the growth rates of short cracks. In torsion the cracks grew sign ificantly more slowly than under axial or biaxial loading. A mean tens ile stress perpendicular to the shear crack promoted its growth and re duced the fatigue life. The growth of the cracks could be described by the Delta J integral for axial and biaxial loading; the integration p redicted the fatigue life under axial and biaxial loading correctly. H owever, significantly conservative lifetime predictions were obtained for pure torsional loading since Delta J does not include crack closur e and crack surface rubbing. (C) 1996 The Institute of Materials.