Enhancement of Infused CFRP Primary Structure Mechanical Properties Using Interleaving Thermoplastic Veils

Abstract

Damage tolerance of aircraft primary structures is important to prolong the life of the structure and reduce maintenance costs. For composite structures, this is particularly crucial; unlike metallic structures which are largely made up of multiple components which can be easily visually inspected and replaced where necessary, advanced composite structures are frequently one piece, e.g. wingskins, fuselage sections, and therefore cannot be replaced. Inspection of these components require complex and time-consuming non-destructive inspection testing to check for delaminations within the laminates and any damage would require complex and costly repairs. Since this work focuses on infused CFRPs, it should also be noted that prepreg CFRPs in comparison are inherently more damage tolerant and hence one objective is to narrow the ‘performance gap’ between the two technologies. By including thermoplastic veils between the layers of a composite laminate stack, this reduces the brittleness of the matrix between reinforcement layers and also acts to reduce crack propagation, thus drastically reducing damage sizes as a result of an impact. Subsequently, residual CAI strengths are increased which for many aircraft structures is the primary design allowable. This paper outlines the mechanical performance evaluation of a toughened fabric by addition of interlaminar thermoplastic veils infused with epoxy resin. The objective of the toughened materials is to maximise the performance enhancements, particularly damage tolerance, whilst minimising any knock-downs in hot/wet performance, preform permeability for infusion and also electrical conductivity due to the additional insulating layers. This project work within AS09 evaluates the materials at coupon and sub-component level and feeds into IS12 for the materials of choice to manufacture the Bombardier lower wingskins.