Frictional Boundary Conditions in Plastic Compression

Distributions of normal stress, shear stress, and coefficient of friction were determined at the interface between a steel tool and a plastically deforming aluminum disk, using two pressure-sensitive pins imbedded in the tool. Comparisons were made with the normal-stress predictions of plasticity analyses, assuming constant shear stress or friction coefficient across the interface. Agreement was improved by considering actual variations in shear stress from lubricant-film deterioration but interesting and unpredicted features were still encountered. The course of liquid-lubricant breakdown was modified significantly by compression with periodic cycles of loading and unloading which resulted in lower friction stress for reactive lubricants, such as fatty acids, and in higher friction stress for inert lubricants, such as mineral oil.