Small depth indentation of plastic materials

Abstract

A new apparatus was developed to perform creep, recovery, relaxation or indentation tests based on the local deformation of polymeric solids by harder indenters. This paper outlines the design concepts of this device that fits in a typical universal tensile testing machine. A feature was added to allow the precise detection of the first contact between the indenter and the sample. The rig was used to perform creep and recovery tests on UHMW-polyethylene to measure the yield strength and Young's modulus. The values so generated of these parameters were used in this study to model indentation behaviour. Using flat samples, indentation tests were also conducted and the resistance to loading was recorded as a function of the depth of indentation. Small depths not higher than 1 mm were considered and five types of indenter including a cone, spheres of different diameters, a cylinder and pyramids (square and equilateral based) were assessed. The analysis of indentation test data shows a very good fit of data by a power law. Values of the constants for each type of indenter are reported. For spherical indenters, an empirical model relating the indentation force to the depth was developed. Finally, a modified form of Johnson's model is proposed to predict the indentation pressure in plastic materials.