Matrix Fatigue

Continuous fiber reinforced materials are a class of materials with exceptionally good fatigue properties, due to the durability of the fibers under cyclic loading. However, the polymer and the fiber-matrix interface are basic prerequisites to utilize these properties in a composite. The initiation can be for example a small crack or an increased fiber misalignment under compressive loading, due to lacking support from the matrix. Even though, these damage mechanisms are typically on a microscale, it is established that this initial damage grows and threatens the laminate’s integrity on a macroscale. A systematic investigation of the relationship between matrix polymer and the laminate’s fatigue damage can be accomplished by high-energy irradiation. This treatment mainly affects the polymer. Fatigue damage initiation and degradation for transverse tension-tension loading and on-axis compression-compression loading were studied for different irradiation configuration of both the neat polymer and the laminate. An extensive characterization campaign of the neat polymer specimens linked the properties directly to the composite’s fatigue damage. The results indicate that the actual polymer response as part of a fatigue-loaded laminate will only be evident under identical conditions, for example quasi-brittle failure of polycarbonate because of long-term loading or fatigue loading. Numerical models were used to supplement experimental observations by in-situ microscopy. These models were built to replicate the actual specimen, including inhomogeneity. The necessary elastic and visco-elastic properties are determined from homogenization of the constituent properties and can account for local variations in fiber volume content. Overall, it could be shown that inhomogeneity dictates the failure location and residual stresses along with the fatigue-crack-growth resistance of the polymer are related to the number of cycles or load level to initiate damage.

The project’s goal is a basic and systematic understanding of dependencies between the mechanical matrix properties and the fatigue behavior of fiber-reinforced composites on a micromechanical level.