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 degrees]( 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 (Delta K-ini.), the test temperature and the e nvironment. Initial fatigue crack growth rates in air were found to in crease with increasing test temperature. Delta K-ini. transition value s leading to fatigue crack arrest compared with eventual specimen cata strophic failure are, at ambient temperature, approximately equal to 1 4 MPa root m in load control but are approximately 40% higher in posit ion control. In addition, at a temperature of 450 degrees C in air, th e limiting value of Delta K-ini. promoting crack arrest decreases to 8 MPa root m in load control. In contrast, the Delta K-ini. transition in vacuum at a temperature of 450 degrees C is identical to that at am bient temperature in air under load control. Under position control at the elevated test temperature, a period of constant nominal stress in tensity factor range (Delta K-nom.) occurs. The crack growth rate at a test temperature of 450 degrees C, in vacuum, is approximately 4 x 10 (-7) mm/cycle in the constant Delta K-nom. region, and this is somewha t less than that observed in air at the same test temperature (5 x 10( -6) mm/cycle), while crack arrest at ambient temperature in air is obs erved. Finally, it is suggested that position control experiments are of some advantage for the studies of the underlying crack growth mecha nisms. (C) 1998 Elsevier Science Ltd.