VISCOELASTIC CONSTITUTIVE LAW IN LARGE DEFORMATIONS - APPLICATION TO HUMAN KNEE LIGAMENTS AND TENDONS

Traction tests on soft tissues show that the shape of the stress-strai n curves depends on the strain rate at which the tests are performed. Many of the constitutive models that have been proposed fail to proper ly consider the effect of the strain rate when large deformations are encountered. In the present study, a framework based on elastic and vi scous potentials is developed. The resulting constitutive law is valid for large deformations and satisfies the principles of thermodynamics . Three parameters - two for the elasticity and one for the viscosity - were enough to precisely fit the non-linear stress-strain curves obt ained at different strain rates with human cruciate ligaments and pate llar tendons. The identification results then in a realistic, three-di mensional viscoelastic constitutive law. The developed constitutive la w can be used regardless of the strain or rotation values. It can be i ncorporated into a finite element program to model the viscoelastic be havior of ligaments and tendons under dynamic situations. (C) 1998 Els evier Science Ltd. All rights reserved.