POROSITY AND CRACK INITIATION DURING LOW-CYCLE FATIGUE

The influence of porosity on crack initiation during low cycle fatigue has been examined by both experimental observations and theoretical modeling. Experimental data based on powder-processed titanium indicate a porosity-induced enhancement of crack initiation which contributes to significant reductions in low cycle fatigue life. All the levels of porosity examined, which range from 0.4 to 6 vol.%, cause an order-of-magnitude or greater decrease in the number of cycles to initiate a 15 μm crack. Assuming porosity located at a surface can be modeled as a through-thickness hole under plane stress conditions, adapting a Coffin-Manson law as a failure criterion and applying cumulative damage theory, a theoretical analysis has been developed for predicting the number of cycles for microcrack initiation in the presence of porosity. The predictions, which rely on pore shapes and the low cycle fatigue response of the fully dense matrix, accurately predict the number of cycles necessary to initiate a 15 μm crack adjacent to both isolated and interconnected pores during low cycle fatigue. © 1990.