A Brief Review and Advances of Thermographic Image - Processing Methods for IRT Inspection: a Case of Study on GFRP Plate

Abstract

The present work introduces a different data processing strategy, proposed in order to improve sub-surface defect detection on industrial composites; in addition, a resume of thermal data processing with most common algorithms in literature is presented and applied with new data. A deep comparison between the common absolute contrast, DAC, PCT, TSR and derivative methods and a new proposed contrast mapping procedure is implemented. Thermographic inspection was done in reflection mode on a Glass Fiber Reinforced Plastic plate, with flat bottom hole defects. Thermal data computation method is found to be critical for simultaneous defect detection and automatic mapping, optimized to identify defect boundaries at specific depth, with help of accurate image processing, implemented in a Matlab GUI for a reliable and rapid characterization of internal damage. The new processing approach, the Local Boundary Contrast method, elaborates different contrast maps and facilitates recognition of damage extension. Tanimoto criterion and the signal-to-noise ratio method were applied as a criterion to assess defect detectability of various processing methods.

Application of Pulsed Thermography and Post-processing Techniques for CFRP Industrial Components

“…As seen in Eq. 6, the PCT processing method rearranges the input three-dimensional matrix A to produce EOF and PC images sequences; the first few EOFs here described show large variability in the thermal sequence, as shown in 14a [30]. The analysis of EOF profiles shows an evident data variability of all k n profiles in the case of 5 selected ROI couples, showing alternatively both positive and negative contrast values, maximized generally in the first frames, but identifying defects in optimal way, as the 2nd and 3rd EOFs maps highlight respectively in Fig.…”

“…In this paper, MATLAB codes are implemented in a Graphical user interface (GUI) for image-processing evaluation of thermal data, comparing relative thermal profiles during time in selected Regions of Interest (ROIs) in most interesting defective and defect-free zones, as showed in other works [30]. The origin of different processing techniques lies in the thermal contrast evaluation [19] and several contrasts definitions have been proposed by different authors, i. e. absolute, normalized, running or standard [19,20], finally differential absolute contrast (DAC) [21] and interpolated differential absolute contrast approach (IDAC) [31].…”

“…In previous works, the authors proposed new thermal Image-mapping technique [denoted as Local Boundary Contrast (LBC)] [26] to increase a defect contrast parameter with damage zone surrounding area and to reduce negative effects induced by non-uniform heating, providing an improvement of defect shape and extension visibility [27,30]. This thermal contrast evaluation combines the pros of DAC approach to remove the manual selection of reference non-defective zone for relative contrast computation for each given defect, together with characteristics of Digital Image Correlation (DIC) approach [44], to automatically elaborate thermal image and correlating a selected ROI to a reference ROI through automatic digital processing of the whole thermal data [30]. Therefore, at a given time t , the computation of proposed absolute local contrast, denoted as C std LBC−I(i,j) [°C] is evaluated for each pixel of general location [i, j] as where I ROI(i,j) (t) and N ROI(i,j) (t) represents the average temperatures of a subset matrix (Dim.…”

Active Thermography for the Detection of Sub-Surface Defects on a Curved and Coated GFRP-Structure

“…Wei et al have achieved similar results by the combination of long-pulse thermography and PCA on a GFRP plate modified with flat bottom holes [16]. Panella et al performed a case study on a GFRP plate modified with various flat bottom holes by using long-pulse thermography with halogen lamps as excitation [15]. Different heating times between 5 s and 40 s were applied to the test specimen.…”

“…Due to the lateral heat transfer that takes place in the material between the surface and the defect, a defect with a depth-to-diameter ratio of ≥1 is considered to be barely detectable. In most experimental investigations, flat and uncoated reference objects (GFRP and CFRP) modified with flat bottom holes were studied [15][16][17][18] to reduce unfavourable influences on the results due to a curved geometry or a coated surface. Kaminska et al, for example, carried out investigations on a GFRP plate modified with flat bottom holes [19].…”