AN INVESTIGATION OF THE FATIGUE CRACK GROWTH RATE CHARACTERISTICS OF TITANIUM ALLOY IMI829

摘要

The primary objective of the investigation was to analyze fatigue crack growth (FCG) rate characteristics of IMI829(Ti-5.5A1-3.5Sn- 3Zr-1Nb-0.25Mo-0.3Si) between 20-600 degrees Celsius using quantitative and qualitative methods. The validity of the mode I stress intensity range (DELTA)K as a correlating parameter was also considered. FCG rate data was summarized as (DELTA)K vs. FCG rate and the main findings were as follows. At 20 degrees Celsius, the FCG rate at constant (DELTA)K increased with increase in R(minimum/maximum) load ratio. Visual estimate of threshold stress intensity range ((DELTA)K(,th)) was 9.0 MPa SQRT.(m) for R = +0.1 and 4.1 MPa SQRT.(m) for R = +0.6. Increasing temperature generally resulted in increased FCG rate for a constant (DELTA)K. The estimated (DELTA)K(,th) value decreased from 9.0 MPa SQRT.(m) at 20 degrees Celsius to 5.7 MPa SQRT.(m) at 475 degrees Celsius. At 600 degrees Celsius, the (DELTA)K(,th) increased to 6.4 MPa SQRT.(m). Very little change in (DELTA)K(,th) occurred with increase in temperature between 20 to 600 degrees Celsius for the R = +0.6 test conditions. Increase in temperature also resulted in greater sensitivity of FCG to (DELTA)K in the (DELTA)K(,th) region. The acquisition of low FCG rate data at elevated temper- ature was very difficult due to crack growth retardation effects. The elevated temperature fracture surfaces were coated with scale assumed to be TiO(,2). The most prominent fracture feature was cleavage-like facet formation through aligned (alpha) colonies. Other fracture features were furrow formation, ripple and regular striations, and mixed mode fracture. Fracture morphology remained unchanged between 20-600 degrees Celsius. The use of a mode I (DELTA)K parameter as a correlating parameter appears to be invalid because of cracking due to loading modes II and III. The relevance of an elevated temperature (DELTA)K(,th) was also placed in doubt because of the problems associated with the presence of scaling on the fracture surfaces and the test technique used to determine (DELTA)K(,th).