Fatigue crack extension behaviour in porous plasma spray 80%Ni20%Cr material: The influence of [formula omitted

Abstract

This paper attempts to describe the fatigue crack growth response of a plasma spray 80%20%Cr material, utilised in the corrosion protection of engineering components, whose microstructure consisted of (i) an austenitic matrix, (ii) a secondary dispersion of a chromite non-metallic inclusion phase and (iii) regions of closed and connected porosity. It was demonstrated that little or no effect of R- ratio was observed on the threshold stress intensity range ΔK th , which was attributed to both the materials fine to intermediate grain size and probable plain stress testing conditions which significantly decrease crack closure effects. At intermediate fatigue crack growth rates high ratio results were an order of magnitude faster than the low R- ratio data. This was the result of the high R- ratio case seeking out more regions of porosity which then increased the local ΔK on the remaining ligaments leading to accelerated crack growth rates. Porosity was shown to significantly decrease the value of ΔK th and the extent of porosity observed on fatigue fracture surfaces increased with ΔK level and was well in excess of that of 5% recorded by metallography. Hence the growing fatigue crack preferentially sought out regions of porosity as they represented locations of low fracture energy.