Comparison of the defect detection capabilities of flash thermography and vibration excitation shearography

Abstract

The defect detection capabilities of transient thermography and shearography have been compared using optimum excitation methods for each technique: short pulse heating for thermography and vibration excitation using a piezoelectric transducer for shearography. A signal-to-noise ratio and limit of detection analysis has been performed on defect images obtained by the two techniques using the different excitation methods. Test samples considered in this paper are flat-plate samples made from aluminium, mild steel, stainless steel, CFRP and thermoplastic, containing flat-bottomed hole artificial defects of 20mm diameter at depths ranging from 0.5mm to 3.0mm. The technique of flash thermography involves using a short duration (~1ms) high intensity flash of light to heat the surface of a test piece. The test piece surface temperature is recorded by an infrared camera and computer system as it decays due to heat being conducted into the part after its deposition on the surface. Sub- surface defects reduce the conduction of heat away from the surface and therefore reduce the surface cooling rate compared to that occurring over non-defective regions. Consequently, a surface temperature contrast develops over a defect that can be used to detect a defective region. Shearography, otherwise known as shearing speckle interferometry, is a technique that uses interferometry to measure the out of plane displacement gradient of a sample's surface. The presence of defects will alter the way in which a sample reacts to an applied stress (in this case created by vibrating the test sample) and this change can be observed using shearography thereby inferring the presence of the defect. In shearography a speckle pattern is applied to the surface of the test sample and the sample is illuminated using expanded collimated (i.e. laser) light. The light is reflected diffusely from the surface of the sample and passes through a lens and shearing device to be recorded by a video camera and processed. The shearing device acts to slightly change the path of half of the rays of light reflected by the speckles, causing the reflected light from neighbouring speckles to overlap and produce an image sheared in the shearing direction. A reference sheared image is stored with the sample in an unstressed condition and the sample is then stressed. The stressing can take many forms, mechanical strain (e.g. with a vacuum hood or by applying a bending load with a clamp), thermal stress (generated by heating or cooling the sample), or vibration excitation (using a piezoelectric transducer as is the case in this testing). A second sheared image is recorded with the sample in the stressed state, and the interferometric superposition of these two images creates an interferogram that represents the phase difference of neighbouring speckle sources. From this phase difference the gradient of the surface displacement can be calculated.