Numerical modeling of microhardness tests for polymer materials

A computational method in conjunction with indentation measurements is presented to determine stiffness properties of different polymeric materials. Experiments were performed using a Vickers-type indenter. Five different polymers were tested: polyethylene naphthalate (PEN), copolymer amorphous polyethylene terephthalate/polyethylene naphthalate (PET/PEN 70/30), copolymer crystalline polyethylene terephthalate/polyethylene naphthalate (PET/PEN 70/30), a random copolyester of polyethylene naphthalate/hydroxybenzoate (PEN/PHB 50/50), and a blend of isotactic polypropylene/ethylene-co-propylene rubber (i-PP/EPR 85/15). Elastic properties are determined from the unloading curve of indentation. Identification of plastic properties of polymers are performed from the loading curve of indentation. A novel approach based on experimental design and finite-element analysis is discussed. Yield stress and tangent modulus are obtained by minimizing the deviation between the calculated and the experimental data. Results are verified by the finite-element method. Good agreement between the experimental and numerical load-indentation depth curves is observed.