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Detection of thinning of homogeneous material using active thermography and classification trees

Dudzik Sebastian, Dudek Grzegorz

Metrology and Measurement Systems

2021

Vol. 28, nr 1

89--105

Active thermography is an efficient tool for defect detection and characterization as it does not change the properties of tested materials. The detection and characterization process involves heating a sample and then analysing the thermal response. In this paper, a long heating pulse was used on samples with a low thermal diffusivity and artificially created holes of various depths. As a result of the experiments, heating and cooling curves were obtained. These curves, which describe local characteristics of the material, are recognized using a classification tree and divided into categories depending on the material thickness (hole depths). Two advantages of the proposed use of classification trees are: an in-built mechanism for feature selection and a strong reduction in the dimensions of the pattern. Based on the experimental study, it can be concluded that classification trees are a useful tool for the thinning detection of homogeneous material.

Kontrast filtrowany w charakteryzacji wad materiałowych metodą aktywnej termografii

Gryś S., Minkina W.

Pomiary Automatyka Kontrola

2010

R. 56, nr 8

893-896

W artykule przedstawiono możliwości zastosowania "kontrastu filtrowanego" do wykrywania i charakteryzacji defektów w materiale metodą aktywnej termografii. Do wyznaczenia "kontrastu filtrowanego" nie jest wymagana znajomość obszaru referencyjnego bez defektu oraz jest mniej wrażliwy na niejednorodność napromienienia powierzchni materiału w porównaniu do klasycznych rodzajów kontrastów cieplnych. Eksperyment przeprowadzono na stanowisku wyposażonym w kamerę ThermaCAM PM 595 oraz kartę rejestracji danych. Oszacowano wpływ parametrów cieplnych badanego materiału i defektu oraz parametru filtru wygładzającego, niezbędnego do implementacji idei kontrastu filtrowanego, na niepewność estymacji głębokości defektów. W analizie zastosowano zasadę propagacji rozkładów prawdo-podobieństwa i symulację Monte Carlo.

The paper presents the possibility of the use of new kind of thermal contrast in subsurface defects detection [1,3-6]. It allows detecting defects taking advantages [2,6] of an active thermography - Table 1. In opposition to known definitions of the thermal contrast [1], a defect-free area is no necessary and this contrast is less sensitive to nonuniformity of heat disposal to the material surface. The measurements were per-formed on a setup presented in Fig. 1 [7]. A special sample of Plexiglas was made with bottom-holes simulating defects - Figs 2 and 3. The material parameters - Table 2, were taken from [1]. The step heating was chosen as heat excitation. Exemplary, raw and processed thermograms for symmetrical and asymmetrical heat disposal are shown in Figs 4 and 5. The influence of the parameter B of the smoothing filter, thermal parameters of the tested material and defect on expanded uncertainty of determination of defect depth were analyzed. Due to significant complexity of the model the numerical method, i.e. Monte Carlo simulation was applied. According to this procedure [9,10] an expectation and 95% coverage intervals are presented in Tab 4 and 5. In the Table 6 there is fixed if the 95% coverage interval contains the true value of depth. For the inspected sample, B=10, and assumed accuracy of evaluation of diffusivity [8] of Plexiglas, the accuracy of the method does not exceed 20%. The optimal value of B corresponds to the diameter of defects. This aspect will be examined in further work.