Fatigue Behavior of Composites with Foamed Matrix

Abstract

Fiber reinforced epoxy foams laminates are a new group of fiber compos ite materials. The microporous structure of these fiber composites is achieved by adding siloxanes as an expanding agent during the hardening reaction with amino-hardeners. Hydrogen released during the cross-linking reaction expands the polymer matrix. Fiber reinforced epoxy foams laminates have already found widespread industrial use. A foam system with glass fiber reinforcement was used in the production of the first load- bearing primary structures of fiber reinforced thermosets. The underbody of the Z1 model of BMW exhibts a high degree of inherent stiffness and strength. Fiber reinforced epoxy foams systems have also become successfully established in sports and in the electrical and building industries. Epoxy foams reinforced with glass and carbon fibers were selected for mechanical and microscopic tests. Given a micropore content of 27.4 vol. %, densities of 1.4 g/cm 3 were achieved with glass fiber fabric reinforced epoxy foams with a glass fiber content of 42 vol. %. Carbon fiber fabric epoxy foams exhibit the following values: density of 1.25 g/cm3 with a carbon fiber content of 50 vol. % and micropore content of 15 vol. %. The dynamic increasing load tests showed that the damping progress of glass fiber rein forced epoxy foams laminates does not depend on the micropore content. The damping progress by carbon fiber reinforced epoxy foams laminates is largely independent of the micropore content, the stresses at break diminish with increasing micropore content. The Wöhler fatigue tests on glass fiber fabric reinforced epoxy foams laminates with different micropore content produced similar Wöhler curves, which are relatively close together. In view of the initial results it can be assumed that the structure of the fiber reinforced epoxy foams laminates can favourably influence the dynamic characteristics of these com posite materials. There is evidence that the porous structure which is developed during the expansion of epoxy resin can restrict the crack progression. The crack progression de pends on the size and distribution of the micropores in the resin matrix. This means that the dynamic behavior of completely pore-free laminates need not always be linked to the better dynamic behavior of such materials.