Detection of embedment of plastic protective film in a composite sandwich structure - a case study

Abstract

High performance Carbon Fibre Reinforced Plastic (CFRP) composites are widely used for launch vehicle and spacecraft structural applications. They offer the advantages such as high specific strength, specific stiffness and tailorability. Sandwich construction with CFRP skins and aluminium honeycomb core is extensively used for light weight construction. Embedment of plastic protective films in the CFRP skins is a potential failure mode that needs to be addressed in the composite manufacturing process as part of online quality control. In one of the launch vehicle components, a control coupon extracted from the parent CFRP sandwich structure failed at less than half the load compared to that of the remaining coupons. The failure was observed to be due to the presence of a plastic protective film in one of the skins. Ultrasonic through transmission testing, the primary Non-Destructive Testing (NDT) technique could not detect the embedment of the protective film. Due to this observation, the acceptance of an entire set of panels realised was in question. An alternate NDT method was required to confirm absence of such films in them. In this paper, the feasibility studies carried out and use of laser shearography technique in the clearance of the sandwich panels are presented. Comparison of test results on reference specimens using pulsed thermography is also presented.