Variable-amplitude fatigue testing of a high hardness armour steel

This study investigated the effect of variable amplitude load sequences on the fatigue behaviour of a high hardness steel similar to that used in the manufacture of light armoured millitary vehicles. Fatigue testing was undertaken on specimens, notched in the L-T and T-L orientations, using service load spectra obtained from an army vehicle subjected to different road conditions. A three stage computer program was used to convert the raw stress data and generate variable amplitude analog signals for testing on a servo-hydraulic test machine. The study showed that the crack growth rate was greater in T-L notch orientation when compared with the L-T notch orientation. Of the two variable load spectra used, crack growth rate was greater under cross country spectrum compared with growth rate under rough spectrum. This paper describes the above results in terms of the inter-relationship between crack orientation, load spectra and the role of microstructure in influencing the crack growth process.