Indentation of elastic solids with rigid cones

Experimental investigations of the response of several elastic solids, carried out using an instrumented indentation machine, are described. These solids include a natural rubber compound, neoprene, and three different compositions of poly( dimethylsiloxane) (PDMS) when they were indented with tungsten carbide cones of included angles of 60degrees, 90degrees, 120degrees and 136degrees. It is shown that, except for the case of the 60degrees cone loading on to the blocks of PDMS (1:10) and PDMS (1:20), the indentation load versus indenter displacement behaviour of all the elastic solids corresponding to all the conical indenters is well fitted by the 1965 Sneddon theory for frictionless indentations. Video camera sequences of the process of indentation by cones of different included angles into the blocks of PDMS (1:10) and PDMS (1:20) have also been taken from which measurements of the elastic deformation of the indented surface have been made. It is shown that the magnitude of the elastic deformation of the indented surface closely agrees with the theory when the blocks are indented with cones of included angles of 90degrees, 120degrees and 136degrees. However, for the case of the 60degrees cone, there is again a discrepancy between experiment and theory; the magnitude of the deformation is significantly smaller than that predicted theoretically. It is suggested that the discrepancy between theory and experimental observations may be due to a non-zero coefficient of friction between the 60degrees conical indenter and the blocks of PDMS (1:10) and PDMS (1:20). It is also shown experimentally that, for indenter loading speeds in the range 0.2-200 mm min(-1), no time-dependent effects were observed for the loading curves, thus justifying the use of the theory given by Love in 1939 and of the Sneddon theory of conical indentations of elastic solids. Finally, using the findings described in this work, a brief discussion is included to explain the discrepancy between the results obtained by Sabey in 1958 and those obtained by Greenwood and Tabor in the same year when tyre-tread rubber specimens obtained from a single source were indented with steel cones of different included angles.