“…Early implementations are reported in Hughes et al [1,2], Hallquist [3], and have been extended to more general cases in Bathe and Chaudhary 380 G. ZAVARISE AND L. DE LORENZIS [4], Hallquist et al [5], Wriggers and Simo [6], Wriggers et al [7], Papadopoulos et al [8], Stupkiewicz [9], Areias et al [10], see also Wriggers and Zavarise [11]. The non-penetration conditions are enforced by preventing the nodes on one contact surface (the 'slave' one) from penetrating the contact segments on the counterpart contact surface (the 'master' one).…”