Fatigue crack growth behavior of Ti-6AI-4V alloy forging

Results of two test programs (TP-1 and TP-2) are presented here to investigate: (1) the transition from Regime I to Regime II fatigue-crack growth behavior in a room-temperature high-humidity environment (TP-1); and (2) elevated-temperature fatigue-crack growth behavior of Ti-6Al-4V alloy (TP-2). Constant-amplitude tests were conducted for both at several stress ratios (R = 0.05, 0.4 and 0.7) to study the transition of structure-sensitive fatigue crack growth in TP-1, and several temperatures were studied at R = 0.1 in TP-2. Middle-tension (MT) specimens were used for the former case, whereas compact-tension specimens were used for the latter. Conventionally forged Ti-6Al-4V alloy processed to a solution-treated and over-aged condition was used for both programs. Temperatures for the TP-2 were: room temperature, 175, 230, 290 and 345 degreesC. An increase in stress ratio and temperature lowered the transitional stress intensity factor range where Regime I shifts to Regime II (steady-state region). A higher stress ratio accelerated the fatigue crack growth rates. However, higher temperature up to 345 degreesC influenced the crack growth rates only marginally. An empirical correlation equation was developed and validated to predict the transitional stress intensity factor for a number of materials. Distinct transition and crack growth behaviors were recorded for each stress ratio and temperature for TP-1 and TP-2, respectively. (C) 2003 Elsevier Science Ltd. All rights reserved.