Nowadays total joint replacements are widely used in the world, so in average 800,000 joint surgeries are done yearly only in Europe and North America. However implant loosening is and remains as the major issue of all implant failures and therefore causes revision surgery procedures. Studies and experiments have identified poor fixation of implants most likely is the main cause of long term implant failure and in this case the cement-implant interface cavities are very effective due to resultant stress concentration. In this study the theory of this problem, continuum and mathematical equations for an inhomogeneous material by using Eshelby’s equivalent inclusion method with a spherical void as a special type of inhomogenities is addressed and a new yield criterion with respect to the void’s volume fraction is derived and changes in material elasticity tensor concerning Mori-Tanaka’s theorem also determined, then by using finite element method and remeshing technique a macro scale cement-implant interface cavity is modeled and concerning the loss of strength due to void existence and the interface stress concentration, the crack initiation and propagation  phenomenon is numerically investigated with respect to different orthopedic cement material properties. The results show that crack propagates at the interface at constant stress and strain by elastoplastic material and it propagates in cement bulk by considering elastic material properties for cement that both could cause implant loosening even in very small void’s volume fractions in which there are no significant changes in cement yield stress and elasticity tensor according to analytical solution. But numerical simulation shows that when a homogenous cement structure is achieved via high vacuum mixing method, there is a uniform stress distribution in the cement structure and no stress concentration zone forms even at high stress levels and also there is no appropriate local site for crack initiation.

The bearing capacity and deformation behavior of geostructures are highly dependent on the mechanical response of a thin layer of soil in contact with a structure that is technically called an interface. This paper presents a constitutive model for the mechanical behavior of sand-structure interfaces. To this goal, the critical state compatible bounding surface framework of Manzari & Dafalias is modified to capture the fundamental state dependent aspects of sand-structure interface behavior. The model has relatively few parameters which have clear physical meanings. The model predictions are compared with experimental results for various interface types, normal stress levels, and stiffness boundary conditions. Using a unique set of model parameters, it is shown that the model is capable of providing reasonable predictions for stress-displacements and normal displacement-tangential displacement behaviors of sand-structure interfaces.