Experimental determination of mode I stress intensity factors for elevated temperature fretting fatigue of a nickel-based superalloy
Abstract
The elevated temperature environment and variable amplitude load history associated with the dovetail notch configuration of the turbine blade root and disk create a critical location for fretting induced crack nucleation. Fretting contact loads induces very large-gradient tensile subsurface stress, acting parallel to the surface. This stress decays rapidly into the depth of the material and possible arrest of initiated cracks. However, superposition of a tensile bulk load leads to crack growth. Throughout the course of this research, the mode I stress intensity factors have been determined for a typical elevated temperature R88 fretting fatigue crack using the well established marker banding back-calculation method. This was achieved by the application of a suitable load spectrum to enable reconstruction of crack front geometries using optical and scanning electron microscopy. The material da/dN versus ΔK properties were obtained from prior fatigue research conducted on R88 at elevated temperatures. A series of experiments were conducted at fixed nominal conditions in order to establish sufficient statistical evidence to demonstrate reasonable improvement in the fretting fatigue life of R88 at elevated temperatures due to shot-peening. Complementing the experimental data obtained during this research with prior experimental data, it was shown that some life improvement is obtainable with shot-peening, although appreciable scatter was observed in the life data. The experimental phase of this research was supported in part by fractographic observations using both optical and scanning electron microscopy. The observations provided insight into mechanisms associated with fretting fatigue crack initiation and dominant corner crack transition and propagation.
Degree
M.S.A.A.
Advisors
Farris, Purdue University.
Subject Area
Aerospace engineering|Materials science
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