Material point simulations as a basis for determining Johnson-Cook hardening parameters via instrumented scratch tests

Scratch tests are a powerful and inexpensive tool for studying the mechanical properties of materials. The tests are typically applied for determining the deformation behavior of materials and serves as quality assessment method for measuring the adhesion and delamination properties of coatings. However, the extraction of quantitative material parameters using scratch tests remains elusive and, contrary to instrumented indentation or tensile testing, hardly any procedures are available so far for determining the hardening behavior of materials. Such a procedure is of enormous relevance, since it allows a non-destructive determination of the material parameters of constitutive models commonly used in computer simulations for thin films, coatings, or surface changes due to loading. In this work we rely on extensive computational simulations of scratch tests using a meshless Material Point Method for finding relationships between the scratch forces, the scratch topography, and the material parameters. The simulations are performed for two large groups of metals with Young’s moduli corresponding to steel and copper. Within each group, the yield stresses and hardening parameters are varied in order to cover the widest possible range of hardening behaviors. The results show that the scratch topography serves to narrow down the value of the yield stress, which can be alternatively determined using indentation. Once the yield stress is known, the hardening parameter can be unequivocally determined for a fixed hardening exponent via the scratch topography using a single scratch, or via the scratch forces using two scratches, provided that they are done at different normal loads.