Transient thermography as a through-skin imaging technique for aircraft assembly

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 1.6 mm-thick aluminium skins over aluminium struts and 2 mm-thick carbon fibre reinforced plastic (CFRP) composite skins over composite struts. For the CFRP case, the strut behind the skin can be easily imaged and its centre line location determined to an accuracy of 0.5 mm. Greater difficulty is experienced in the aluminium case caused by sensitivity to the thermal contact resistance between the surfaces. Modelling results show a thermal contact resistance between two high thermal conductivity aluminium surfaces to have far greater effect on thermal image degradation than between two lower thermal conductivity CFRP surfaces. A modest loading of aluminium parts, however, reduces the effect of the thermal contact resistance and enables a subskin strut to be imaged and located to a similar accuracy as achieved with CF'PR parts.