Non-destructive Thermographic Inspection of Multilayer Dry Fiber Reinforcement Stacks for Polymer Composites

Abstract

This paper discusses thermographic techniques for the inspection of dry multilayer fibre reinforcement stacks used in the manufacturing of polymer-matrix composites (PMCs) using liquid moulding (LM) processes. The aim is to identify defects in thick multilayer stacks of dry textile reinforcements before resin injection and the manufacturing of the composites. Inspecting dry preforms is not aimed at replacing the final inspection of PMC parts, but it can avoid wasteful manufacturing of PMC parts from preforms that are defective prior to injection. Defects sought in this paper cannot be detected using visual inspection. Tests were conducted on textile stacks only, and on stacks laid on a metal mould and vacuumed. Different devices, techniques and heat sources were used. The first technique used a hand-help camera. Stacks featured 2 to 4 reinforcement layers with defects of different widths, types and orientations. Defects were identified. Detection was influenced by the orthotropic fibre thermal conductivity, textile architecture, relative orientations of defects and yarns, and number of layers. In the second technique, optical excitation thermography and advanced image processing were used for inspecting stacks featuring different textile structures, thicknesses and defects. The performance of different techniques was rated through signal-to-noise ratio analysis. Monitoring modalities most suitable to industrial manufacturing were identified. In the last part of the work, stacks were positioned on a mould and inspected either on a heated mould or by heating from the front of the stack, under vacuum but without moulding ancillaries beyond the vacuum, using a number of different heat sources. The potential of the technique in an industrial setting is demonstrated; additional work remains in terms of image analysis in order to automate the technique and generalize its use.