RESEARCH PAPER A Transparent Indenter Measurement Method for Mechanical Property Evaluation C. Feng & B.S.-J. Kang Received: 15 August 2005 / Accepted: 13 October 2005 # Society for Experimental Mechanics 2006 Abstract An optical Transparent Indenter Measure- ment (TIM) method was developed for measurement of material surface mechanical properties. During the spherical indentation test, in-situ measurements of indentation-induced surface out-of-plane displace- ments were obtained using an integrated phase-shifting Twyman-Green interferometer. Based on elastic re- covery theory and 2D finite element analyses, a procedure was developed to determine the material Young’s modulus using the measured surface out-of- plane displacements. During the spherical indentation test, contact radii were also measured and used to estimate the material post-yielding true stress-strain curve using Tabor’s empirical relation. An experimen- tal TIM apparatus was assembled to test on two engineering alloys and the results showed good agree- ment with known material properties. Keywords Transparent indenter measurement . Spherical indentation . Young’s modulus . Out-of-plane displacement . Stress-strain curve Introduction Since Tabor [1] showed the application of spherical indentation approach to obtain material post-yielding true stress-strain curve, spherical indentation technique has been applied to determine material mechanical properties such as Young’s modulus and post-yielding behavior [2–18, 20–23]. The indentation parameters such as load, indentation depth, contact radius and unloading characteristics have been studied extensively either experimentally or numerically [1–25]. In the early 1970s, with the work of Sneddon, Bulychev et al [2, 3], unloading stiffness method was successfully used to measure material Young’s modulus. Later, it was extended to brittle material with depth-sensing Vickers indentation by Loubet [4]. Doerner and Nix [5] studied the unloading characteristics using instrumented inden- tation. Oliver and Pharr [6] studied the unloading behavior of six different materials, and concluded that, instead of a straight line, the unloading load-depth curve actually has a similar power law form to that of the loading-depth curve. Therefore, only initial unload- ing stiffness is suitable for the Young’s modulus de- termination. With the development of various thin film materials, depth-sensing indentation is often applied to evaluate mechanical properties of thin film materials [6, 9, 11, 13–15]. However, indentation test is a complicate mechan- ical process. It involves complex contact mechanics and nonlinear plastic deformation. Analytical solutions are difficult to obtain. As a result, much of the un- derstanding of the indentation process has been acquired through experiments and finite element analyses. Thus accurate experimental measurement of indentation parameters is critical in determining ma- terial surface mechanical properties using indentation technique. With the advancement of computational mechanics, fully elastic-plastic finite element analyses have provided further understanding of indentation process. From finite element analyses, indentation parameters, such as loading-unloading characteristics, surface deformation, stress distribution and elastic Experimental Mechanics (2006) 46: 91–103 DOI 10.1007/s11340-006-5862-5 C. Feng (SEM member) I B.S. Kang (), SEM member) Mechanical and Aerospace Engineering Department, West Virginia University, Morgantown, WV 26506, USA e-mail: bruce.kang@mail.wvu.edu SEM