Can simple estimates from neat polymers provide safe fatigue fracture design limits for fiber-reinforced polymer matrix composites?

Abstract

There have been extensive efforts aiming at standardization of a test method for characterizing fatigue fracture propagation in FRP composites under tensile opening Mode I loads. These efforts, however, have not yielded a validated standard procedure yet. The effects of experimental scatter, fiber bridging, and the two-dimensional delamination propagation in planar or shell-like structural components versus that in beam-like test specimens are still debated. Selected literature data seem to indicate that quasi-static or fatigue fracture tests on neat polymers might yield rough estimates for fatigue fracture design limits for the respective FRP composites. The feasibility and the limitations of this approach will be discussed in detail. The development of a fatigue fracture test method for neat, particle or short fiber-reinforced polymers seems to be fairly straight-forward. Such a procedure may also be useful for generating fatigue pre-cracks for quasi-static fracture mechanics testing of polymers. Whether data from such tests yield safe design limits for FRP composites may depend on the specific toughening mechanisms of the matrix polymer, since these are not always fully transferred from the polymer to the FRP composite. Fatigue fracture data for toughened polymers may hence overestimate the resulting toughness or delamination resistance of the FRP composites. Therefore, further investigations are recommended to explore the limits of applicability of such data.