Fracture toughness evaluation of unidirectional fibre metal laminates using traditional CTOD (δ) and Schwalbe (δ5) methodologies

Fibre metal laminates (FMLs) were developed for the aeronautical industry, which requires thin sheets with high resistance to fatigue crack growth, high damage tolerance and high specific strength. Considering all these requirements, FMLs are an advantageous choice when compared to metal alloys currently used. In order to employ FMLs in aircraft structures, designers must have a deep knowledge of a wide set of properties including fracture toughness. The aim of this work was to evaluate the available methodologies for critical CTOD measurement of unidirectional FMLs. To achieve this, tests were performed to obtain traditional (BSI/ASTM) and Schwalbe’s CTODs by using experimental procedures especially adapted to these laminates. Results achieved point out that there are differences between both CTOD parameters, that Schwalbe method proved more appropriate, and also that the standard plastic-hinge model does not work properly in FMLs.