Determination of damage mechanisms and damage evolution in Fiber Metal Laminates containing friction stir welded thin foils

Kurzfassung

The need for weight reduction in aircrafts has promoted the employment of hybrid materials such as Fiber Metal Laminates (FML) in the fuselage. Since thin aluminum foils can only be produced in limited width by the available rolling process, several layers of metal must be overlapped, forming a so-called splicing, to build large fuselage structures increasing weight and manufacturing costs due to its manual assembly. Friction Stir Welding (FSW) represents a plausible alternative for avoiding splicing by joining the aluminum foils to obtain a wider metal sheet configuration. In this study, tomographic investigations of post-mortem fatigue and in-situ tensile tests were carried out for fiber metal laminates (FML) containing 3 layers of friction stir welded AlMgSc AA5024 alloy foil and 2 layer of glass fiber reinforced polymer (GFRP).The aim of these investigations is to identify the damage mechanisms and sequence of damage during quasi static and fatigue loading conditions of the FML as well as to determine microstructural features responsible for tensile and fatigue damage. The tensile tests were carried out using the stop/go technique. The specimen were loaded until a pre-determined stress level and held. Examination of the radiographies taken during the loading period show that delamination is first observed at the interface between the welding flash of the outset aluminum thin foil and the GFRP-layer normal to the loading direction As the load increases cracking initiates and propagates within the polymer matrix. The aluminum sheets, then, abruptly fail without much evidence of bulk deformation. The FML containing friction stir welds only reach 65% of the strength of the conventional FML. In addition, fatigue crack initiates at the welding flash of the external aluminum foil and propagates through the polymer matrix of GFRP reaching the second and third aluminum layer.