Improved Instrumented Indentation of Soft Materials through Surface Deformation Measurements

Abstract

Instrumented indentation is a commonly used technique to determine the mechanical properties of bulk materials and thin films by measuring the load and displacement during indentation into a specimen. However, in traditional indentation measurements, it can often be difficult to determine the true deformation of the specimen due to the machine compliance and drift in the system. The issue of drift is particularly problematic in tests that occur over extended time scales, such as creep tests on soft materials. In the present work, a new method, in which the full-field deformation of the sample material around the indenter is measured, is presented to overcome these challenges. Specifically, the deformation of specimens during cylindrical flat punch indentation tests is monitored by tracking discrete particles, such as microbeads, near the surface of the sample. This method allows for direct measurement of the specimen surface as it is deformed. In the current implementation, it is applicable to transparent materials, including many polymers, gels, and biological materials. An inverse method was developed to extract mechanical properties of the specimen from the measured displacement fields. A numerical parametric study was performed to quantify the effect of changes in Poisson’s ratio, magnification, particle density, and experimental noise on the elastic properties calculated using the inverse method.