The issue of residual strength tests of thin fibre metal laminates
Patryk Jakubczak, Jarosław Bieniaś, Konrad Dadej, Wojciech Zawiejski
Quarterly No. 3, 2014 pages 134-138
DOI:
keywords: fibre metal laminates, low-velocity impact, residual strength, Compression After Impact
abstract Modern aircraft structures contain sheathing elements which are supposed to not only carry loads, e.g static ones, but also at the same time possess resistance to corrosion or dynamic impact. As a consequence, new kinds of hybrid materials, e.g fibre metal laminates, were created. They combine the mechanical and physical proper-ties of various materials. Until now, the most common and widespread structures are GLARE® laminates (alu-minium/glass-epoxy composites), characterised by high fatigue and static properties, as well as by impact resi-stance. The concurrent influence of many negative factors during exploitation causes a gradual decrease in the functional properties of these materials. One of the factors affecting e.g. static strength is low-velocity impact. Low-velocity impact often leads to macroscopically invisible damage of the composite structure, with delamina-tions and ply cracking occurring during impact energy absorption. Fibre metal laminates possess a much better dynamic load-carrying capacity, limiting negative ply cracking in the composite and absorbing some impact energy through elastic-plastic deformation. In order to assess the influence of low-velocity impact on the residu-al strength of composite materials, Compression After Impact (CAI) tests are carried out. Normalised CAI te-sting is used for classic 5 mm thick composite structures. However, as the literature suggests, it is not effective in the case of fibre metal laminates, particularly those with a thickness more then 1.1 mm. The work presents an analysis of the possibility of conducting an effective (ensuring valid assessment of strength reduction) CAI test for 1.5 mm thick FML panels after dynamic impact. An alternative workstation construction was proposed, and simulations and experimental verifications were conducted. It was observed that a solution based on the ASTM standard does not apply to thin FML laminated panels. Deformation of the specimen occurs in areas located far from the impact site. As a consequence, the strength values differ neither for plates with impact-induced damage nor ones without it. The proposed alternative holder construction for compression after impact of thin fibre me-tal laminates plates testing eliminates premature material damage. On the basis of the conducted numerical si-mulations, it was stated that using the ASTM holder for CAI test leads to the occurrence of the first buckling mode in the damage area, with stress concentration in its vicinity. Such a form of deformation may allow one to correctly assess the influence of impact damage on FML composites.