Materials Science and Engineering A 408 (2005) 158–164 Materials characterization and classification on the basis of materials pile-up surrounding the indentation G. Das ∗ , Sabita Ghosh, Sukomal Ghosh, R.N. Ghosh Material Science and Technology Division, National Metallurgical Laboratory (CSIR), Jamshedpur 831 007, India Received in revised form 19 July 2005; accepted 19 July 2005 Abstract The ball indentation technique (BIT) has been used to classify various engineering materials on the basis of observed pile-up/sink-in in materials, surrounding the indentation. In respect of revealed pile-up, viz., prominent, moderate and negligible, the materials could be classified into three respective groups. The dependence of the pile-up on the yield ratio and the strain-hardening exponent of materials have also been established as a novel concept. The actual indentation diameters (both plastic and total) differ from the diameters calculated from the depth of indentation measured by linear variable transducers (LVDT). This difference is due to the accumulation of materials (pile-up) surrounding the indentation. The methodology to determine the correction factor, to be incorporated in the measurement of actual diameter, has been highlighted and is compared with the reported approach. © 2005 Elsevier B.V. All rights reserved. Keywords: Ball indentation technique; Pile-up; Mechanical properties; Yield ratio; Materials classification 1. Introduction The conventional methods to evaluate mechanical properties of materials are well established. Yet, evaluation of the same through tests conducted in situ or by using small amount of test materials, are in great demand for esti- mation of remaining lives of service-exposed components [1–13]. The ball indentation test requires a small amount of material. During indentation, underneath the indenter, the material undergoes plastic deformation. An elastic zone surrounds this plastic deformation. Therefore, the plastic zone is always in a state of confinement. The indentation impression grows with increase in load resulting in extension of the plastic zone. At the surroundings of the indentation, a material pile-up/sink-in are observed due to the forced flow of material caused by the multiaxial stresses acting at this region. The extension and the height of accumulation of materials was found to be independent of the size of impression [14], yet, dependent on the work hardening ∗ Corresponding author. Tel.: +91 657 271 710; fax: +91 657 270 527. E-mail address: gd@nmlindia.org (G. Das). characteristic of the material [15]. The formation of this pile-up/sink-in in materials has a significant influence on the accuracy of measurement of indentation diameter. The actual diameter of indentation differs from that calculated from the depth of indentation, measured by linear variable transducers (LVDT), often, the difference is too large to be ignored. Some of the present authors [16–20] did extensive work on the influence of pile-up in materials in measurement of indentation diameter using a hard spherical ball. They observed that the accumulation of materials surrounding the indentation was not only dependent on the work hardening characteristic of the material but also on the initial pre- straining of the test sample. It was shown [19] that the L ¨ uders band appearing during conventional test on pre-strained sam- ples had correlation with the formation of pile-up/sink-in. To determine the indentation diameter from the LVDT measured depth of indentation, they [16–21] did consider the pile-up phenomenon. Meyers and Chawla [22] worked on pile-up/sink-in behaviour of materials using a conical indenter. They divided the metallic materials into three categories according to the nature and extent of deformation surrounding the indentation. Those were (a) non-work hardening, (b) work hardening and 0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.07.026