Low Thermal Conductivity Materials and Very Low Heat Power: A Demanding Challenge in the Detection of Flaws in Multi-Layer Wooden Cultural Heritage Objects Solved by Pulse-Compression Thermography Technique

Abstract

An inlay sample with artificial defects was inspected via the pulse-compression thermography (PuCT) technique. The sample belongs to the cultural heritage field, and it was realized by a professional restorer based on his long-time experience, imitating historical art crafting styles. The tesserae composing the inlay were not treated by any protective paints, so that external thermal stimuli may cause physical/mechanical alterations of the cell walls, with consequent colour changes, cracks, and eventually damage to its surface. To avoid any alteration of the sample, the PuCT technique was used for inspecting the inlay sample as it allows the heating power to be very low, while assuring enough thermal contrast for the defects to be detected after the exploitation of the pulse-compression algorithm. Even if a maximum ΔT slightly exceeding 1 °C was detected during the PuCT test of the inlay sample, it is shown that this is enough for detecting several defects. Further, image processing based on the Hilbert transform increases defect detection and characterization. In addition, a novel normalization technique, i.e., a pixel-by-pixel data normalization with respect to the absorbance estimated by considering a characteristic value of the compression peak, is introduced here for the first time. The proposed normalization enhances the defect detection capability with respect to the standard pixel-by-pixel amplitude visualization. This has been demonstrated for two experimental setups, both exploiting the same LED chips system as heating source but different thermal camera sensors, i.e., one in the mid-infrared spectrum, the other in the far infrared one. Thus, the present work is also the first small-scale test of a future portable system that will include low-power LED chip feed in DC by metal-oxide-semiconductor field-effect transistor (MOSFET) devices, and a handy far-infrared camera.

Correlation of Vulnerability and Conservation Between Artistic Assets and Structural Elements: Campaign of Thermographic Surveys on the Frescoes of the Templar Church of San Bevignate

“…The role of the latter approach in detection and qualification of deficiencies/discontinuities in the stonework is an important aspect [21,22,23,24]. Through the analysis of another part of the same wall, located near the apse and in the proximity of a window, it was possible to discover the presence of a blind opening in the original masonry, which now is totally bricked up (Figure 10).…”

Feasibility of a Thermography Nondestructive Technique for Determining the Quality of Historical Frescoed Masonries: Applications on the Templar Church of San Bevignate

“…In such a described approach to thermography, the role of the latter in detection and qualification of deficiencies/discontinuities in the stonework is a significant part [23][24][25][26]. Through the analysis of a portion of the same wall, located near the apse and in the vicinity of a window, it was possible to discover the presence of a blind opening in the original masonry, which now is totally bricked up (Figures 11 and 12).…”

Detection and Identification of Defects in 3D-Printed Dielectric Structures via Thermographic Inspection and Deep Neural Networks

“…Popular filaments used in AM can be classified as good thermal insulators. In the case of materials with a low value of thermal conductivity, such as the acrylonitrile butadiene styrene (ABS) plastic printout tested in the presented work, the heat conduction process is relatively slow, and the observed temperature differences between the healthy areas and the defected ones are hardly measurable, especially in the case of damages, such as very narrow defects or micro cracks [ 32 ].…”