Quality assessment of CFRP bonded structures with active thermography techniques

Adhesive bonding for lightweight application, especially with carbon fiber reinforced polymers (CFRP) materials regarding aerospace application has a large development potential. Requirements involve a strict control of the bonding process to ensure the quality of the assembly. Several factors may however influence the adherent surface prior to bonding and so induce weak adhesive bond performances. If NDT inspections are involved in the defect detection for the bondline (delaminations, disbonds, porosity, voids, etc.), their potential is limited. They are not capable of assessing the adhesion quality of bonded structures yet. This paper presents work performed with active thermography for the extensive characterization of CFRP structures before and after bonding operations. Multispectral thermography with optical excitation has been used to characterize CFRP adherent surfaces prior to bonding and determine the presence of common contaminations (water, release agent). At the same time, another set-up involving a standard active thermography with pulsed excitation has been investigated for the characterization of contaminated bonded structures. This second approach presents an additional challenge since the contamination itself is not detected directly anymore but instead, the induced loss of adhesion performance via the influence of thermal resistances. To help appraise this challenge, simulations based on material behavior due to the contamination and their effects on the thermal conductivity of the material are presented.