Leibniz-Institut für Verbundwerkstoffe GmbH
Combining fibers reinforced laminates with thin metal sheets (t<0.5 mm) leads to a hybrid material with high specific strength and damage tolerance, by leveraging the metal's plasticity and high strength of e.g. carbon fiber reinforced plastics (CFRP). Despite this beneficial combination, residual stresses are typically present from a thermal curing cycle. Low thermal expansion of the carbon fibers subjects the laminate to a compressive residual stress, whereas the metal sheets are pre-tensioned. This state can be highly detrimental, as it promotes crack initiation and propagation in the metal sheets and compressive failure within the CFRP. The primary controlling factor is the polymer’s state during curing, as any elongation of the metal sheets at elevated temperatures persists once the material reaches operational temperatures. Modified curing cycles were already described as potential remedy. The presented work focuses on different approaches to alter the residual stresses. The effect of varied curing cycles on the polymer properties and the metal-laminate adhesion will be discussed. For this, different curing cycles of epoxy resin (Sicomin SR1660/SD7820) were investigated. This epoxy system features a particularly wide range of curing temperatures. In addition, two alternative modification methods are presented: pre-yielding and irradiation treatment. Pre-yielding is a process similar to autofrettage of metallic pressure vessels. The aim is to offset stresses into the less critical compressive range. Irradiation induced relaxation is also a post-curing method acting mainly on the molecular structure and was investigated with the frequently used RIM135/RIMH137 from Hexion. The presented results will discuss the benefits and potential drawbacks of each investigated modification method when targeting a selectively altered stress state within FMLs. This work forms the foundation for the examination of the influence of residual stresses on fatigue performance.
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