Measuring mechanical fields in tribology: physical quantities, methods, and insights

Abstract

Contact interfaces are inherently heterogeneous across various length scales. Capturing the interface behaviour therefore requires not only macroscopic force and displacement measurements, but also local measurements along the interface. Such a refined view of the interface offers a unique opportunity to identify the elementary mechanisms that take place along the contact. It also provides highly constraining data to validate or falsify models. In this Chapter, we give an overview of the interface-related mechanical quantities whose spatial distribution can be experimentally measured. For each quantity, we describe the principal measurement techniques, illustrate their applications through examples and highlight some of the tribological insights that were obtained thanks to those local measurements. The accessible quantities include contact area, slip, gap, forces/stresses, stiffness and strains. We distinguish direct interfacial measurements and bulk measurements, based on methods spanning direct imaging, molecular probes, particle tracking velocimetry, digital image correlation, fluorescent liquids, interferences, Raman spectroscopy, ultrasound, X-ray computed tomography, photoelasticimetry and various local sensors. This overview aims to serve as a guide for selecting the appropriate techniques and interpreting local measurements in tribological contacts.</div