Creep Testing and Modeling

Although, for computational wear analyses purposes, the UHMWPE viscoelastic/viscoplastic deformations may play a very important role in the wear phenomenon, this type of behavior has been scarcely considered in the literature [139]. Moreover the crack initiation from a notch in UHMWPE is a complex phenomenon that is governed by the viscoelastic fracture theory [140]. In brief, the importance of considering the viscoelatic/viscoplastic behavior of UHMWPE for medical application has been further emphasized by recent research. 

The discussion that follows will be restricted to the field of infinitesimal strain theory for viscoelastic and viscoplastic solids. For the finite strain 

The research work developed by Green and Rivlin [143-144], Green, Rivlinand Spencer [145], Noll [146] and Pipkin [147], based on functional analysis, resulted into the representation of constitutive equations for creep, or relaxation, in integral series. For the uniaxial case we have 

The complete determination of the kernels p. (...) is very involving and time consuming [148]. 

A simpler and effective approach was given by Schapery [149]. The theory has proven to describe reasonably well the non-linear viscoelastic behaviour for several polymers [150]. The constitutive equation in terms of strain is restricted to small strains by the rmderlying thermodynamic theory. For the unidirectional case is given as 

---