The effect of [formula omitted] on near threshold fatigue crack growth in a metallic glass and a stainless steel

Abstract

Fatigue crack growth tests have been performed on very thin amorphous Ni 78Si 10B 12 and Type 316 stainless steel samples at low ΔK levels and different R- ratios . Crack tip displacements of the metallic glass alloy were determined using a direct replication technique. In this alloy near threshold growth rates deviated from the Paris-Erdogan Law and cracks continued to grow at remarkably low ΔK values, i.e. down to a threshold value of ΔK TH = 0.5 MPa√m for R = 0.1. In this regime an anomalous R- ratio effect occurred with ΔK TH increasing to 1.0 MPa√m when R was raised to 0.5. The same increase in R- ratio caused a decrease in crack growth rate at all ΔK values below 5 MPa √m. This decrease in growth rate was accompanied by a decrease in the size of shear facets that characterize the fracture surfaces. Contrary to the metallic glass, in thin stainless steel specimens ΔK TH decreased and crack growth rates increased with increasing R- ratio . The effects was similar to that reported from work on conventional thick specimens for which crack closure was held to be responsible. Crack closure was detected at low R- values in the metallic glass but since crack growth rates were found to be slower at high R- ratios , closure arguments could be used to account for this effect. The crack tip measurements showed that the decrease in crack growth rates with increasing R- ratio was due to an decrease in the local crack tip compliance as measured by ΔCTOD. It is postulated that this decrease in compliance is due to buckling of the specimen.