Objectives: Long-term implants are subjected to both mechanical stresses and destructive fluids within the living body. While aging, they may change their mechanical and biological properties. These implants may work under both static and dynamic loading conditions.
Methods: The investigations presented in this study show the effects of simulated human body conditions on the mechanical properties of
biostable (polysulfone) and bioresorbable [poly(lactid-co-glycolid)] polymers and their composites reinforced with medium-modulus carbon fibers. The stability of investigated materials was estimated based on creep tests (static loading) as well as on fatigue tests (cyclic loading).
Result: The data obtained during the creep tests enabled us to estimate the long-term behavior of the examined samples under mechanical stress, as well as the effects of biological environment on this behavior. On the basis of obtained experimental data the critical stress was determined under which the material can work for long period of time without failure. Both the lifetime for extended loading and the acceptable deformation of the investigated samples were estimated. Formation and development of cracks leading to failure were observed as a result of cyclic stress.
Conclusion: In the analysis of fatigue behavior of composite elements, it has be taken into account that their fatigue behavior is quite different from that of their components. Hence, it is important to carry out the experiments on the medical device itself in conditions approximating the real working environment. In this study, an analysis of fatigue behavior of polymer composite screws was performed. On the basis of the micromechanical model selected, a lifetime prediction trial of a joining screw was undertaken, in conditions simulating the presence of body fluid and under cyclic load.

Keywords: Polymer Composites, implants, Creep, Cyclic Load, Life Time