Peter T. Kantzos

Fatigue crack growth study of SCS6/Ti-15-3 composite

A study was performed to determine the fatigue crack growth (FCG) behavior and the associated fatigue damage processes in a (0)(8) and (90)(8) oriented SCS(6)/Ti-15-3 composite. Companion testing (CT) was also done on identically processed Ti-15-3 unreinforced material. The active fatigue crack growth failure processes were very similar for both composite orientations tested. For both orientations, fatigue crack growth was along the fiber direction. It was found that the composite constituent most susceptible to fatigue damage was the interface region and in particular the carbon coating surrounding the fiber. The failure of the interface region lead to crack initiation and also strongly influenced the FCG behavior in this composite. The failure of the interface region was apparently driven by normal stresses perpendicular to the fiber direction. The FCG rates were considerably higher for the (90)(8) oriented CT specimens in comparison to the unreinforced material. This is consistent with the scenario in which the interface has lower fatigue resistance than the matrix, causing lower composite fatigue resistance. The FCG rates of the (0)(8) composite could not be directly compared to the (90)(8) composite but were shown to increase with an increase in the crack length.

Influence of Alloy Behavior on Multiaxial Fatigue Lifing Criteria for Turbine Disk Bores

In a previous paper, a criterion for multiaxial lifing of turbine disk bores made from a nickel base super alloy, DP-718, was substantiated using results from spin pit testing of mini disks. In this paper, another turbine and compressor disk alloy, Alloy 10, exhibiting a different material behavior than DP-718 is examined from multiaxial point of view. This new alloy manufactured by powder metallurgy processing, has a coarser grain size, lower tensile yield strength, contains a much finer distribution of brittle carbides in its microstructure and shows a significant difference in failure initiation behavior under low cycle fatigue when compared to DP-718. Under the multiaxiality conditions experienced by disk bores, Alloy-10 lives seem to correlate well using an effective stress based criterion, whereas DP-718 was found to follow principal stress based criterion. Interesting differences between DP-718 and Alloy-10 in alloy and fractographic behavior under uniaxial and multiaxial conditions are discussed.Copyright