Lockin thermography parameter investigation

Bwembya, Emmanuel Mutale (2017-03)

Thesis (MEng)--Stellenbosch University, 2017.

Thesis

ENGLISH SUMMARY: Infrared thermography has emerged as one of the leading nondestructive testing techniques used for composite materials testing. It is fast, safe, effective and data acquisition is done remotely. As such it has attracted significant research to improve its performance in detection of material flaws in carbon and glass fibre composites. The aims of this thesis were to investigate parameters that affect defect detectability with optical lockin thermography. These were the frequency, intensity, number of wave cycles of the excitation and material type for which both simulations and experiments were conducted. The materials contrasted were carbon fibre composite and mild steel as they display a large disparity in thermal properties and density. A major note to make was that the material with lower thermal conductivity and heat capacity produced better results in experimental data while results in simulated data were similar. Low values of thermal effusivity enhanced defect detection and reduced the effect of experimental noise. Furthermore, other results showed that low frequencies probe deeper into the material and as such increase the probability of detecting deep defects. In the case of intensity of thermal excitation, low excitation intensities resulted in a low temperature rise which if too low equalled the camera noise and deterred defect detectability. This was especially so for mild steel which has a high heat capacity. As for the number of wave cycles, increasing this parameter had little influence on increasing the probability of defect detection in simulations as the phase response of the curve was barely altered by using more cycles as there was no noise in simulated data. Experimental results showed improved results for both materials.

AFRIKAANS OPSOMMING: Infrarooi termografie het navore gekom as een van die uitstaande nie-destruktiewe toets tegnieke vir saamgestelde materiale. Dit is vinnig, veilig, effektief en behels geen direkte kontak nie. Daarom is daar ‘n beduidende hoeveelheid navorsing na die vermoë van die tegniek om materiaal defekte in koolstof- of glasvesel materiale te identifiseer. Die doelwitte van hierdie tesis was om die parameters te ondersoek wat die vermoë van optiese vaspen termografie beïnvloed om defekte suksesvol te identifiseer. Dit het ingesluit frekwensie, intensiteit, aantal warmtebron golf siklusse en die materiaal tipe. Beide simulasies en eksperimente is uitgevoer. Koolstofvesel saamgestelde materiale is met staal gekontrasteer omdat hulle uiteenlopende termiese eienskappe en digthede het. ‘n Belangrike bevinding is dat die materiaal met laer termiese geleiding en warmtekapasiteit beter resultate opgelewer het in die eksperimente terwyl die simulasie resultate soortgelyk is. Lae termiese effusiwiteit verbeter die defek herkenning en verminder die effek van eksperimentele geraas. Verdere resultate toon dat lae frekwensies dieper defekte in die materiaal kan herken en verhoog dus die waarskynlikheid dat defekte herken sal word. Wat die intensiteit van die hittebron betref, veroorsaak lae intensiteite lae temperatuur stygings wat in sekere gevalle gelei het tot stygings wat kleiner was as die kamera resolusie en dus defek herkenning belemmer het. ‘n Toename in die aantal hittebron siklusse het nie ‘n groot invloed gehad op die waarskynlikheid om defekte in simulasies te herken nie, want die fase terugvoer van die kurwe was kwalik verander omdat daar geen geraas in die simulasie data was nie. Eksperimentele data toon verbeterde resultate vir beide materiale.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/101284
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