Nanoindentation of compliant materials using Berkovich tips and flat tips
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Donna M. Ebenstein Department of Biomedical Engineering, Bucknell University, Lewisburg, Pennsylvania 17837, USA (Received 23 July 2016; accepted 28 November 2016)
Nanoindentation testing of compliant materials has recently attracted substantial attention. However, nanoindentation is not readily applicable to softer materials, as numerous challenges remain to be overcome. One key concern is the significant effect of adhesion between the indenter tip and the sample, leading to larger contact areas and higher contact stiffness for a given applied force relative to the Hertz model. Although the nano-Johnson–Kendall–Roberts (JKR) force curve method has demonstrated its capabilities to correct for errors due to adhesion, it has not been widely adopted, mainly because it works only with perfectly spherical tips. In this paper, we successfully extend the nano-JKR force curve method to include Berkovich and flat indenter tips by conducting numerical simulations in which the adhesive interactions are represented by an interaction potential and the surface deformations are coupled by using half-space Green’s functions discretized on the surface.
I. INTRODUCTION
Nanoindentation, also known as instrumented or depth-sensing indentation, has become a widely accepted materials characterization technique, and international standards for instrumented indentation testing have been elaborated since 2007.1 During nanoindentation tests, a small indenter tip is pressed into the sample and both the applied load and the displacement of the material are measured, allowing its mechanical properties such as reduced modulus and hardness to be calculated. Although traditional nanoindentation techniques were developed for stiff materials, such as metals and ceramics, nanoindentation testing of compliant materials, such as soft tissues, cells, and hydrogels, has recently attracted substantial attention because the high spatial resolution of nanoindentation allows local testing of mechanical properties of soft matter that is not possible using macroscale techniques.2–25 For example, Ferguson et al. applied the nanoindentation technique to human articular calcified cartilage and subchondral bone from normal and osteoarthritic patients21; Leong and Morgan used the same technique to describe local changes in the mineralization across a rat fracture callus22; and the surface mechanical properties and deformation behavior of ultrahigh molecular weight polyethylene were examined by nanoindentation experiments performed with a surface force microscope.25 Contributing Editor: Jinju Chen a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.483
However, nanoindentation is not readily applicable to softer materials yet, as numerous challenges remain to be overcome. One of the key concerns associated with nanoindentation testing of soft materials is the significant effect of adhesion between the indenter tip and the sample. Adhesion leads to larger contact areas and higher contact stiffness for a given applied force r
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