Environmental interactions in high-temperature fatigue crack growth of Ti-1100

The crack growth behavior of Ti-1100 is investigated for loading frequencies ranging from 30 to 0.0031 Hz at temperature levels extending from 23 °C to 650 °C in both air and vacuum environments. Two types of time-dependent damage mechanisms have been identified: oxidation and creep effects. It is concluded that the effect of oxidation on the crack growth acceleration is rapidly developed and only weakly dependent on total cycle time. Creep effects, on the other hand, are dominant at low frequencies in both air and vacuum and are loading rate dependent. The degree of contribution of each of these two damage modes during the steady state growth region has been phenomenologically determined by examining the frequency dependence on the exponent and coefficient parameters of the Paris-type crack growth equation. It is found that these parameters are largely determined by the extent of the viscoplastic response of the crack tip region below a specific, environment-sensitive transition loading frequency. Furthermore, the physical mechanisms involved in the environment-affected damage are identified with the nature of crack tip plastic work input as a function of loading frequency. The influence of frequency and environment on the anomalous appearance of pronounced stage I/stage II knee regions is also discussed with respect to closure levels and creep transient response. © 1993 The Minerals, Metals and Materials Society, and ASM International