Probing material properties with sharp indenters: a retrospective

A retrospective on the use of sharp, fixed-profile indenters as materials probes is presented. Indentation is proposed as a simple but powerful methodology for evaluating basic mechanical properties-elastic modulus, hardness, toughness-in all classes of materials. Indentation also provides unique insight into fundamental deformation and fracture processes. Of particular interest is the existence of intrinsic size effects as characteristic contact dimensions pass from macro- to micro- to nano-scale dimensions. The utility of indentations as 'controlled flaws' in the context of strength of materials is outlined. The roles of two other important material factors-rate effects and microstructure-are considered. Examples of technological and biological applications are presented as illustrations of the widespread power of the technique. Strengths and limitations of the methodology as a routine testing protocol are discussed.