The influence of strain hardening of polymers on the piling-up phenomenon in scratch tests: Experiments and numerical modelling

The aim of this study was to relate the scratching behaviour of polymers to their mechanical properties. A thermosetting resin (CR39) and a thermoplastic polymer (PMMA) were studied using a microscratch tester allowing in situ observation of the contact area. These two polymers exhibit different elastic and viscoplastic properties, the main difference being the large ability of CR39 to strain harden, whereas PMMA softens. A spherical indenter was used to vary the level of deformation imposed on the samples. The response was initially elastic, then viscoelastic and finally mainly viscoplastic with increasing penetration of the indenter into the material. The two polymers displayed the same response for small levels of deformation, while at larger strains PMMA showed more pronounced plastic behaviour. The origin of this difference in behaviour was investigated by means of a three dimensional finite element analysis. The rheology of PMMA and CR39 was simplified and modelled by assuming linear elastic behaviour and a viscoplastic law taking into account their strain hardening capacity at high strains. Strain hardening was found to be a key factor to correctly model the material flow around the indenter. The response of the polymers was governed by the ratio between the plastic and elastic strains involved in the deformation in the contact region. In first approximation, the representative strain was imposed mainly by the geometry of the indenter, while the elastic deformation was controlled by the mechanical properties of the material, a larger strain hardening leading to a greater elastic deformation and a lower plastic strain thus a better scratch resistance of the specimen.