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

Abstract

The fatigue crack growth resistance of a 0/90°]_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 (?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° C in air. However, tests carried out at a temperature of up to 450° C in vacuum have shown that crack arrest conditions are similar to those observed from specimens studied at room temperature and at a temperature of 300° C in air. In these cases, initial ?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°C in air, even for tests performed at a frequency of 10 Hz, the limiting value of initial ?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°C in air. Finally, for such composites, sudden increases in fatigue crack growth rates have been attributed unequivocally to the failure of bridging fi bres, which were detected using acoustic emission.