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The Effects of Deformation Mode and Microstructure Fracture-Related Properties of Ti Alloys.
Final rept. 1 Nov 79-30 Jun 83,
CARNEGIE-MELLON UNIV PITTSBURGH PA
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Fatigue crack growth has been studied in two binary Ti-Al alloys, an 8 wt. Al and a 4 wt. Al alloy. It has been found that the crack growth rate is faster than in the 4 Al alloy. The principal difference between the alloys is that the higher Al alloy exhibits planar slip. The effect of this planar slip character is to cause a more irregular crack path. This increase in crack path irregularity causes roughness-induced closure. Back face strain measurements have been used to measure the variation in closure and to calculate an effective delta K. It lso has been shown that the crack growth rates for the two alloys coincide if the variations in closure are taken into account. Elevated temperature crack growth experiments have also been conducted and oxide-induced closure effects have been found in this case. This type of closure also affects the crack growth rate. Finally, crack growth experiments at high mean stress have been conducted and it has been found that the closure effects disappear. The implications of this work on the selection of materials for fatigue crack growth limited applications is discussed. Author
APPROVED FOR PUBLIC RELEASE