Evaluating the industrial application of non-destructive inspection of composites using transient thermography

Kretzmann, Jared Eric (2016-03)

Thesis (MEng)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: Transient thermography is a non-destructive testing method used in the detection and visualization of sub-surface flaws. Transient thermography could use one of two heating methods: step and square-pulse heating. Both these methods rely on observing the temperature rise of a surface that is subjected to a constant heat flux, while square pulse thermography also observes the subsequent thermal decay after the heat has been removed. The transient methods have not been thoroughly explored in literature with respect to the more popular methods, such as pulsed and lock-in thermography. Particular interest has been placed on investigating transient thermography on fiber-reinforced polymer (FRP) materials and its application in industry. Composites are prone to flaws such as delaminations, voids and inclusions that do not accurately represent flat-bottom holes, which are commonly evaluated in experimental work. Therefore, the inspection of thin artificial air-gaps and Teflon® delaminations were investigated. These artificial flaws can be considered to represent either a fully-separated or contacting delamination. A significant reduction in defect contrast and definition was observed for the thin delaminations, which is ascribed to the lower thermal resistance than that for flat-bottom holes. Further studies investigated the qualitative and quantitative performance of thermographic inspection on defective samples provided by an industrial partner. Experimental results demonstrated that variability in core geometry, ply arrangement, surface and sub-surface anomalies could be identiffied. The smallest detectable anomaly was found to be 1 mm wide, which was a spatial resolution limitation of the infrared camera. The investigated samples exhibited small radius and low resistance defects. It was found that current techniques to quantify defect depth are inadequate, especially if an accurate reference depth cannot be found. Thermography data is typically associated with subtle defect signatures that are strongly affected by non-uniform heating and surface variability. Advanced processing methods have been shown to help mitigate these effects. Various processing methods are reviewed from literature. Several methods were tested here for the first time, such as: multiscale retinex, matched filters, Markov error contrast and modified differential absolute contrast (IDAC) for step thermography. Transient thermography has shown to be a strong competitor amongst other thermographic methods for its simple application, relatively fast inspection times, and high thermal contrast for low defect resistance cases. It further enables the use of an entry-level infrared camera. The ndings of the artificial samples reported a maximum defect depth up to 7 mm was observed for clear Plexiglas®. The clear Plexiglas® can be considered to be the least optimal case of heating with optical excitation and has a low thermal emissivity. For the carbon and glass fibre reinforced polymers, a maximum detectable defect depth of 5 mm was observed, which is considered to be comparative or even better than pulsed thermography. The method was particularly better for low diffusivity materials, such as glass fibre composites.

AFRIKAANSE OPSOMMING: Oorgangstermografie is 'n nie-destruktiewe tegniek om defekte onder die oppervlak waar te neem en te visualiseer. Oorgangstermografie kan een van twee verhittingsmetodes gebruik: stap en vierkant puls verhitting. Beide tegnieke is gebaseer op die waarneming van die temperatuur styging van 'n oppervlak onderwerp aan 'n konstante warmtelas, terwyl vierkant puls verhitting ook die temperatuur daling waarneem nadat die warmtelas verwyder is. In vergelyking met meer populêre metodes, soos gepulseerde en geslote termografie, is die oorgangsmetodes nog nie ewe deeglik beskryf in die literatuur nie. Daar is veral belangstelling in ondersoeke na oorgangstermografie vir veselversterkte polimere en die toepassing daarvan in industrie. Saamgesteldemateriale is geneig om defekte soos delaminasie, leemtes en inklusies te hê wat nie goed voorgestel word deur plat bodem gate nie, soos algemeen gebruik in eksperimentele werk. Hier is die gebruik van dun, kunsmatige, luggapings en Te on® delaminasies ondersoek. 'n Beduidende verlaging in kontras en definisie is waargeneem vir dun delaminasies wat toegeskryf kan word aan die feit dat dit 'n laer termiese weerstand het as plat bodem gate. Verdere ondersoeke na die kwalitatiewe en kwantitatiewe vermoë van die termografiese inspeksie van defektiewe onderdele voorsien deur 'n industriële vennoot is gedoen. Eksperimentele resultate het getoon dat variasies in die kern geometrie, laag oriëntasie, oppervlak en sub-oppervlak afwykings geïdenti fiseer kan word. Die kleinste, waarneembare afwyking was 1 mm wyd, wat toegeskryf word aan die beperkte ruimtelike resolusie van die infrarooikamera. Die ondersoekte voorbeelde het klein radius en lae weerstand defekte getoon. Dit is gevind dat bestaande tegnieke om defek diepte te vind deur die gebruik van inversie metodes ontoereikend is, veral wanneer 'n verwysingsdiepte nie akkuraat bepaal kan word nie. Termografiese data word dikwels geassosieer met fyn defek kenmerke wat sterk beïnvloed word deur oneweredige verhitting en oppervlakte variasies. Dit is al gevind dat gevorderde verwerkingsmetodes die effek hiervan kan verminder. Verskeie van hierdie tegnieke, soos gevind in die literatuur, is oorweeg. Nuwe metodes, soos multiskaal retinex, bypassende lters, Markov fout kontras en aangepaste differensiële absolute kontras, word ook beskryf en ge-evalueer. Die prosesseringsmetodes is geïmplimenteer in 'n oopbron sagteware pakket en is getoets met voorbeelde uit die industrie. Dit is getoon dat oorgangstermografie 'n sterk mededinger is in die versameling termografiese tegnieke vernaamlik as gevolg van die eenvoudige toepassing daarvan, relatief vinnige inspeksie tye en hoë termiese kontras vir gevalle waar die termiese weerstand van die defek laag is. Verder is dit moontlik om intreevlak infrarooikameras te gebruik met hierdie tegnieke. Gebaseer op toetse met kunsmatige defekte kon foute so diep as 7 mm onder die oppervlak gevind word in helder Plexiglas®. Helder Plexiglas® is nie 'n ideale materiaal vir hierdie tegnieke nie as gevolg van die materiaal se lae termiese emmisiwiteit. Defekte so diep as 5 mm kon gevind word in koolstof- en glasvesel versterkte polimere. Dit is vergelykbaar met en selfs beter as gepulseerde termografie. Die tegniek het veral beter resultate gelewer met materiale met lae diffusiwiteit, soos saamgeselde veselglas materiale.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/98636
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