Nitrile butadiene rubber (NBR) balls of radius R ¼ 10.7 mm were used in the present study. Young's modulus was estimated as E ¼ 3.5 MPa by performing instrumented indentation and by fitting the resulting force–displacement response using the Hertzian contact theory. However, the material exhibits hysteretic effects even at low loading rates, hence the estimated Young's modulus is regarded approximate. A polished low-carbon steel disc was used as a counter surface. Due to a high difference in elastic stiffness, the steel disc can be assumed rigid.