Experimental Investigation of the Effects of Pre-Preg Gaps for the Automated Production of Fiber Metal Laminates

Gaps in pre-preg plies, in a case when adjacent plies do not align perfectly, may represent a significant issue in an automated fibre placement manufacturing process and can be detrimental to the properties of the final laminates. In order to tackle the consequences of gaps in the composite material, a comprehensive research has to be performed. Starting from existing manufacturing processes, the effects of gaps were investigated in different ways, including the first indication of their impact on the mechanical properties. The focus of this paper is to extend the state of the art in gaps in composite materials to hybrid composite materials such as Glare, a member of Fibre Metal Laminates (FMLs). The investigation started with the manufacturing of Glare specimens having gaps with different widths, followed by a non-destructive ultrasonic inspection. Also, an optical evaluation of the gaps was performed by microscope image analysis of the cross sections of the specimens. Results from the ultrasonic inspections revealed the presence of areas corresponding to the gaps areas. The optical evaluations supported the ultrasonic results and showed the presence of fibre waviness and delaminations. Finally, the ultimate tensile strength of comparable specimens was determined, proving the detrimental effect of gaps on the strength of the final laminate.