Damage state assessment of fiber reinforced metal laminate composites

Fiber Reinforced Metal Laminate (FRML) composites are widely used as a structural material, mainly in the aerospace industry. One of the main challenges using FRML composites is to detect and asses/quantify damage during in-service conditions. For these reasons, the research presented in this article targets the development of a nondestructive testing and evaluation (NDT&E) method capable to assess damage development in FMRL composites as a function of applied loading. Specifically, the current research presents an approach based on multiscale observations which aims to monitor major failure modes in Glare®1A FRML specimens subjected to quasi-static loading conditions. Specifically, a combination of NDT&E methods is used including the Acoustic Emission (AE) and Digital Image Correlation (DIC) which are coupled with in situ Scanning Electron Microscope (SEM) level tensile tests. The post-processing of the ensemble of recorded AE activity reveals characteristics that were associated with the composite's constituents (Al-alloy as well as glass fibers within an epoxy matrix) along with features that were associated with delaminations and fiber/matrix interface failure. Furthermore, pronounced AE activity was detected at the elastic to plastic transition region which was found to evolve in a way similar to micro-crack density trends suggested by micro-mechanical models. The similarity noticed, motivated the use of specific AE features to evaluate the damage state evolution in FRML composites.