Title : Relevance of indentation (Nano/Micro/Macro) in current materials research

The increasing application of advanced materials in aerospace systems demands reliable characterisation of mechanical properties across multiple length scales, particularly when conventional destructive testing is impractical for miniature components, thin sections, coatings, and limited-volume materials. Instrumented indentation provides a powerful alternative for evaluating mechanical properties from macro- to nano-scale loads with minimal material consumption and damage. However, the influence of indentation size, strain rate, and temperature on the extracted mechanical properties remains an important consideration in establishing reliable material-property databases.

The present investigation focuses on the mechanical characterisation of two technologically important aerospace alloys: Ti-6Al-4V (Ti64), widely employed in low-pressure compressor components, and the precipitation-hardenable Ni-base superalloy Haynes 242 (H242), used in high-temperature aerospace applications. A systematic study was undertaken to examine indentation size effects (ISE) and the influence of strain rate and temperature over a broad range of loading conditions.

Macro-indentation experiments were performed using spherical indenters of different radii, while instrumented micro- and nanoindentation tests were conducted using a Berkovich indenter. The experimentally obtained indentation responses were analysed using established Tabor and Johnson approaches for determining elastoplastic properties. Dao’s reverse-analysis methodology was employed to extract key plasticity parameters, including yield stress and strain-hardening exponent. In addition, finite-element (FE) simulations were carried out to investigate the temperature dependence of the mechanical response over the range of 300-673 K.

The results demonstrate that indentation size, strain rate, and temperature significantly influence key indentation-derived parameters, including Meyer hardness, constraint factor, pile-up/lip height, load–displacement response, yield stress, and strain-hardening exponent. The observed indentation size effects highlight the importance of accounting for length-scale dependence when interpreting nanoindentation data and correlating local mechanical properties with bulk material behaviour.

A qualitative correlation between the macro-, micro-, and nano-scale length regimes is established for Ti64 and H242, providing an integrated framework for understanding their mechanical response across multiple scales. The resulting characterisation methodology and material-property database can support improved interpretation of nanoindentation measurements and contribute to more reliable design and performance assessment of aerospace components operating under demanding mechanical and thermal environments.

Keywords: Nanoindentation; Instrumented indentation; Indentation size effect; Ti-6Al-4V; Haynes 242; Strain rate; Temperature; Aerospace materials; Finite-Element Analysis.

Dr. Kumaraswamy Adepu is Professor of Mechanical Engineering at DIAT (DRDO), with 31 years of experience in teaching, research and academic leadership. His research interests include mechanical characterisation through Nano/Micro/Macro indentation, High strain rate deformation behaviour, Tribology, and allied areas. He has served as Principal Investigator for sponsored R&D projects funded by DRDO and has contributed significantly to research and innovation through 4 patents, 75 peer-reviewed journal publications, 6 eBook chapters and 45 conference proceedings. He has supervised 10 PhD scholars and 75 plus M.Tech./MS(R) scholars, with 8 doctoral scholars currently pursuing research under his guidance. He has also adjudicated 30 plus PhD theses.