THROUGH SKIN IMAGING for AIRCRAFT ASSEMBLY using PULSED - TRANSIENT THERMOGRAPHY

An investigation has been made of the potential of pulsed transient thermography for identifying the location of fixtures beneath aircraft skins to facilitate accurate automated assembly operations. Experimental studies have been made of 1.6mm thick aluminium skins and both 2mm and 4mm thick carbon fibre reinforced plastic (CFRP) composite skins over both aluminium and composite struts, representing fixtures. For the thin CFRP case (i.e. 2mm skin), the strut behind the skin can be easily imaged and centre line location determined to an accuracy of 0.5mm. Greater difficulty is experienced in the thick CFRP (i.e. 4mm skin) and aluminium cases caused by, in the first case, the thickness of the skin material, and in the second instance, the sensitivity to the thermal contact resistance between the surfaces. For the aluminium case, modelling results show a thermal contact resistance between two high thermal conductivity surfaces to have far greater effect on thermal image degradation than between two lower thermal conductivity CFRP surfaces. A modest loading of such parts, however, reduces the effect of the thermal contact resistance and enables a sub-skin strut to be imaged and located to a similar accuracy to that achieved with the 2mm CFPR skin positioned over the CFRP strut. Introduction: In the 1980’s, Vavilov and Taylor [1] discussed the principles of active infrared thermographic non-destructive testing as a means of providing quantitative information about hidden defects or features in a material. Since then, numerous groups worldwide have used infrared investigation techniques in the inspection of subsurface defects and features, thermophysical properties, coating thickness and hidden structures [2 4]. The capabilities of the technique for detecting and imaging subsurface defects have been greatly enhanced and the defect imaging process is now well understood [5, 6]. In this work, the objective was to study the ability of pulsed – transient thermography to locate anchoring points beneath the outer skin of aircraft structures, to facilitate automated drilling and fixing. Typical test structures, comprising of 1.6mm thick aluminium skins and both 2mm and 4mm thick carbon fibre reinforced plastic (CFRP) composite skins over both aluminium and composite struts respectively, were investigated experimentally and analysed using finite difference thermal modelling software, taking into consideration the size and depth of the features, as well as their thermal properties [7]. The ability of the technique to detect a subsurface fixing and to offer information about its location was investigated. Experimental: The representative test structures (figure 1), comprising an Al aircraft skin (1.6mm) positioned over a thick Al strut and of CFRP skin (2mm or 4mm) over a thick CFRP strut, were analysed using the ThermoCalc-3D software. The dimensions of all investigated panels were 500 x 500 mm. The width of the strut was 100 mm. The size and depth of the features, as well as the thermal properties of the investigated materials were taken into account. Furthermore, the effect of thermal contact resistance (the air gap between skin and strut) was also considered. For this reason, the models were run using different values of air gap [7] such as 1, 10, 50 and 100 μm, as well as with having the perfect contact between the two surfaces (zero air gap). The thermal and physical properties of the materials that were used during modelling are shown in table 1, whilst the heating time parameters shown in table 2 were used for the models. Figure 1: Rear view of modelled structures Material k (WmK) Cp (JKgK) ρ (Kgm) Al 2024-T3 126 961 2768 CFRP (⊥ fibre) 0.8 120