Fatigue crack growth in fibre reinforced titanium MMC laminate at room and elevated temperatures

The fatigue crack growth resistance of a [0/90 degrees](2s) Ti-6Al-4V (wt-%) SCS-6 cross ply laminate has been assessed as a function of varying the initial nominal stress intensity factor range (Delta K), the test temperature, and the environment. In all cases, through thickness cracks have been grown from unbridged defects. Fatigue crack growth rates are higher at elevated temperatures of 300 and 450 degrees C in air. However, rests carried out at a temperature of up to 450 degrees C in vacuum have shown that clack arrest conditions ale similar to those observed from specimens studied at room temperature and at a temperature of 300 degrees C in air. in these cases, initial Delta K-ini transition values between fatigue crack arrest and eventual specimen catastrophic failure are close to 10 MPa m(1/2). In contrast, at a temperature of 450 degrees C in air, even for tests performed at a frequency of 10 Hz, the limiting value of initial Delta k(ini) to give crack arrest is less than 6 MPa m(1/2). This has been attributed to the action of an aggressive environment, and particularly to the attack of the carbon coating layers. In addition, correlations have been found between fibre pull out lengths and changes in both temperature and environment; these are negligible after tests at 450 degrees C in air. Finally, for such composites, sudden increases in fatigue clack growth rates have been attributed unequivocally to the failure of bridging fibres, which were detected using acoustic emission. MST/3719.