Relaxation and recovery measurements by the microhardness indentation technique for transparent polymeric materials

A simple microhardness testing technique is described that enables the identation process to be continuously observed for transparent materials Photographic recording gave measurements of the indentation area and its changes with time, both during the on‐load period and during the subsequent period following the removal of the load. To illustrate the techniques, the time‐dependent mechanical properties of poly(methyl methacrylate) (Perspex) and diallyl diglycol carbonate (CR39) have been studied. Two parameters are defined that could be useful in identifying time‐dependent deformation properties of polymeric materials. The relaxation hardness, analogous to the relaxation modulus, describes the stress–relaxation that occurs for an identation loaded material. The surfaces resistance to damage, defined as the ratio of the applied indentation load to the remaining deformation area after load removal, is useful as an abrasion or wear indicator, particularly for assessing optical deterioration. The changes in this parameter with time describe the mechanical recovery that occurs at an indentation site in the material.