Pulse Phase Thermography and its Application to Defects in Adhesively Bonded Joints R. C. Waugh, J. M. Dulieu-Barton and S. Quinn Faculty of Engineering and the Environment, University of Southampton, UK. Fluid Structure Interactions Research Group FSI Away Day 2012 Introduction Adhesive joints are often preferred to their mechanical counterparts due to: Reduction in weight. Improved load transfer through the joint. Adhesive joints are currently being used in the membrane style liquid natural gas (LNG) containment system used in carriers such as the Mk III, Fig. 1. Figure 1: LNG MKIII carrier insulation construction. Kissing Defects Kissing defects are: The improper adhesion at the adhesive/adherend interface without any identifiable void or volume associated with it. The cause of kissing defects is unknown. Work here is based on a liquid layer approach to recreating kissing defects [1,2], which are made by introducing contamination to one of the adhesive/adherend interfaces in a joint, Fig. 2. Figure 2: Liquid layer method of recreating kissing defects. No current NDE methods are able to detect kissing defects. Aim To develop a non destructive method to assess the integrity of adhesive bonds using pulse phase thermography (PPT). References: 1. Yan, D., Drinkwater, B.W. and Neild, S.A. NDT&E International, : p Brotherhood, C.J., Drinkwater, B.W. and Guild, F.J. Journal of Nondestructive Evaluation, (3): p Maldague, X. and Marinetti, S. Journal of Applied Physics, (5): p Conclusions and Further Work Pulse Phase Thermography (PPT) A heat pulse is applied to the surface of a sample and the thermal response of that surface is monitored using an infrared camera, Fig. 3. Variation in conductivity through the material where there is a defect will lead to an area of different temperature on the surface over the defect. Figure 3: PPT set-up and equipment. Thermography data is recorded in a series of k thermal images over an observation period following the pulse. The data for is transformed from the time domain to the frequency domain using a 1D fast Fourier transform (FFT), (1). (1) Phase values for each pixel may then be calculated from these real and imaginary components, (2). (2) Phase images can then be obtained [3]. : Artificial insert defects were successfully identified in a range of materials including CFRP and GFRP. PTFE, silicon grease and petroleum jelly were all detectable in the CFRP lap joint sample. Defects in a film adhesive lap joint were found to be more detectable than in the spreadable adhesive. Artificial defects in the spreadable adhesively joined sample were found to be indistinguishable from other adhesive variations. Further investigation and development of the technique is required. Primary membrane Primary insulation Inner hull System support Secondary insulation Secondary membrane - adhesively joined Insulation plugs Support pins Adhesive Contamination Adherend Results and Discussion Materials with inserts Figure 4: Phase image and phase profile plot across the upper two defects. A carbon fibre reinforced plastic (CFRP) panel with polytetrafluoroethylene (PTFE) inserts between plies at various depths was tested. The defects were clearly visible to the maximum tested depth of mm for the minimum tested defect diameter of 5 mm. Tests were also carried out on glass fibre reinforced plastic (GFRP) with PTFE defects. A reduced phase contrast was found due to a reduced contrast in diffusivity between the sample and defect. Bonded samples Liquid layer defects were added to a CFRP lap joint along with PTFE inserts which were bonded using the normal curing cycle for the material. The PTFE inserts are clearly visible in the PPT data whereas the petroleum jelly and silicon grease contaminations are only just visible as a slight variation in the phase contrast image, Fig. 5. The chalk dust and Frekote defects were not visible. Figure 5: Phase images for PTFE and liquid layer defects in CFRP. PTFE inserts were easily detected in a CFRP bonded joint constructed using epoxy film adhesive, which allows the adhesive thickness to be accurately controlled, Fig. 6. When this control is reduced by using spreadable epoxy adhesive the contaminations and inserts become much harder to distinguish. The silicon grease contamination used is visible in the phase values however other variations in the adhesive thickness are also causing a similar magnitude variation in the phase values, Fig. 7. Figure 6: Phase image and profile plot for PTFE inserts in a CFRP bonded joint using film adhesive. Figure 7: Phase image and profile plot for spreadable adhesive bond with silicon grease contamination. Silicon grease Phase (°)