Effects of control mode in a cross-ply Ti-MMC at ambient and high temperature in air and in vacuum

Fatigue crack growth rates from unbridged defects in a [0/90°]_2s Ti-6Al-4V/SCS-6 cross-ply laminate composite have been measured to assess the effects of the control conditions under a stress ratio of R = 0.1 and at a frequency of 10 Hz, whilst varying the initial stress intensity factor range (ΔK_ini.), the test temperature and the environment. Initial fatigue crack growth rates in air were found to increase with increasing test temperature. ΔK_ini. transition values leading to fatigue crack arrest compared with eventual specimen catastrophic failure are, at ambient temperature, approximately equal to 14 MPa√m in load control but are approximately 40% higher in position control. In addition, at a temperature of 450°C in air, the limiting value of ΔK_ini. promoting crack arrest decreases to 8 MPa√m in load control. In contrast, the ΔK_ini. transition in vacuum at a temperature of 450°C is identical to that at ambient temperature in air under load control. Under position control at the elevated test temperature, a period of constant nominal stress intensity factor range (ΔK_nom.) occurs. The crack growth rate at a test temperature of 450°C, in vacuum, is approximately 4 × 10^-7 mm/cycle in the constant ΔK_nom. region, and this is somewhat less than that observed in air at the same test temperature (5 × 10^-6 mm/cycle), while crack arrest at ambient temperature in air is observed. Finally, it is suggested that position control experiments are of some advantage for the studies of the underlying crack growth mechanisms.