Non-destructive detection of glue deficiency in laminated wood using thermography
Abstract: The purpose of the work described in this doctorate thesis has been to study the potential of the non-contact and non-destructive test (NDT) method, thermography, to detect glue deficiency beneath a thin laminated layer of wood. The technological applications can for example be found in the parquet flooring industry. The thesis is presented as a monography mainly based on five appended papers. The aim of the theoretical part, based on a literature survey, was to understand how thermal phenomena coupled to thermography theoretically can be described, how different factors and thermography methods affect the results of a thermography measurement and to know the status of delamination detection with thermography. The principal aim of the experimental measurements was to determine penetration depth, resolution, inspection time and repeatability for pulse, heating-up, and lock-in thermography. Using image thresholding as defect classification method, a method comparison has also been performed. Furthermore, the performances of another NDT method, air coupled ultrasound, was briefly investigated. Finally, the contrast detected by the two NDT methods has been validated with the crack test as a destructive reference test method. The results have showed/confirmed that it is possible to detect glue deficiency in laminated wood products with pulse, heating-up and lock-in thermography. Whenever time is not limiting, lock-in thermography should be used for each defect depth and is capable of detecting defects which are 4 times wider than the defect depth with at least a 2.0 mm thick surface layer. Provided the signal to noise ratio (SNR) can be increased, pulse thermography is suitable for defect depths of 0.5-1.0 mm in an on-line application where inspection time is a critical parameter and lies around a few seconds. If the SNR cannot be sufficiently increased for pulse thermography, heating-up thermography is an alternative and should be used for defect depths between 0.5-1.4 mm. For air coupled ultrasound in transmission mode, the contrast and resolution did not decrease with defect depth down to 2 mm, which qualitatively makes it a competetive alternative to thermography. However, its inspection time is proportional to the scanned surface and might be very long if a good resolution is needed. The glue deficiency pattern of the fracture surfaces corresponded very well to the contrast pattern detected by thermography and air coupled ultrasound. Thereby, contrast detected by thermography and air coupled ultrasound can be regarded as being able to indicate reduction of the mechanical resistance of the glue line.
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