In-situ observation of strain and cracking in coated laminates by digital image correlation

Abstract

Overlay coatings are widely used in engineering components to impart a range of surface functionalities including thermal, wear, and corrosion protection, as well as material reclamation. In most surface engineering applications, the coating's role is restricted to the surface, with limited integration with the underlying substrate. However, the situation is changing: there is an emerging need for so-called structurally integrated coatings, where the coating and substrate are intimately bonded, resulting in a coupled system. Of interest are the emerging applications of thermal spray and cold spray overlay coatings applied on loaded engineering components such as landing gear, heavy machinery hydraulics, and steel infrastructure. These coatings, even metals or cermets, respond in a brittle manner associated with their layered processing and ultra-fine grain structures resulting from rapid quenching. As such, it is of importance to understand their coupled mechanical response, especially stress-strain behavior and strain to fracture.In this study, an approach involving strain monitoring of coated steel via digital image correlation has been developed. Both elastic response and strain beyond the yield point of the system are assessed to examine load transfer between the coating and substrate and onset of cracking. Three different coating materials (Ni, WC-CoCr and Al2O3) were deposited to near full density via high velocity thermal spray. The results point to a powerful new approach for understanding mechanical behavior of heterogeneous composites using advanced imaging techniques.