Asynchronous Lock In Thermography of 3D Printed PLA and ABS samples

Lock-In thermography is a useful Non Destructive Technique (NDT) for enhanced detection of defects in components, as it amplifies the phase contrast where defects exist. This amplification was found to be around 2-3 times compared to constant heating. The current used a Fuse Deposition Modelling (FDM) 3D printer to print samples with known defects, in order to characterise the relative effects of different variables on the Lock-In phase data. Samples were printed using ABS (Acrylonitrile Butadiene Styrene) and PLA (Polylactic Acid) for comparisons, and variables such as print direction, cameras, heating power, Lock-In frequency, as well as thickness, width and depth of defects were explored. It was found that different materials resulted in different baselines, but had similar phase contrast. A novel asynchronous technique was derived to enable Lock-In measurements with 5 different infrared cameras, and similar results were found. Even cheap cameras like the Seek Thermal CompactXR were proven capable of detecting the same defects as other cameras such as the FLIR SC7500. Heating power did not affect phase contrast, except for shallower defects up to 1.0 mm deep, where higher power resulted in better contrast. As expected, deeper defects could only be detected using lower Lock-In frequencies, and there was better phase contrast with wider, thicker and shallower defects. It was shown that defects 4 mm in width could be detected automatically up to a depth of around 1.5 mm, based on the phase signal trends. Sub-sampling of frame data showed that at least 10 frames were required per Lock-In period for minimal deviations in Lock-In phase contrast. Also, it was shown that phase contrast was similar for shallower defects up to 1.5 mm deep, with data from 1 Lock-In period, as long as the first frame was synchronised with the heating cycle.