  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.
