Infrared thermography is a well-known technique for the Nondestructive Testing (NDT) of industrial components. Typically, the raw results of a thermal inspection are processed with an algorithm to enhance the defect detectability and then analyzed by an expert. A challenging point of this workflow is the final step, as the assessment made by the operator could be biased or subjective. To tackle this issue, clustering algorithms could be used to define, in an unsupervised manner, whether a region under inspection is defective or sound. In this work, a steel sample with flat bottom-hole defect is investigated in a Flash Thermography setup. The recorded thermal sequence is then analyzed with a clustering algorithm (k-means). The algorithm is applied varying different parameters and assessing, for each scenario, the performance of the clustering in terms of defect detection, quantified through specificity and sensitivity.
Metal cutting by computer assisted laser is more and more utilized in advanced manufacturing. The increased accuracy, the non-contact processing, the higher productivity and the decrease of energy demand are some of the benefits that make competitive this technology in respect to the most traditional ones like the rotating tools or sewing. Typically, such systems are composed by a powerful diode sourcing light in continuous mode or periodically pulsed. The light is collected by an optical fiber and focused by the optical system on the manufact to be cutted. The optical system, on its turn, is mounted on a 2/3-axis head. Typical wavelength is around 1064 nm and average power ranges from 1 to several kW. Notwithstanding those appealing features, there is the concern on the effects of the high temperature developed during the melting of the metal on the cutting zone, and the surrounding as well, that can lead to the degradation of the mechanical characteristics of the final product. Several authors have proposed different kind of analysis on the Heat Affected Zone (HAZ): measure of the tensile strength, Vickers hardness, SEM and optical microscopy analysis. Correlation have been drawn with the speed of the laser head and the power of the source with the extension of the HAZ. The paper propose to utilise an IR camera to monitor the temperature in the surrounding of the cutting zone and a model to fit the data collected by the IR camera itself.
In the foundry industry the typical casting process is done by pouring the liquid (fused) metal on a mold shape and left until solidification. The inner part of the mold gets in contact with the liquid, high temperature metal. The mold is generally coated with a special refractory varnish that improves the flow of the liquid metal inside the mold and allows to obtain smoother surfaces of the solidified object. In the process of optimizing the performance of such refractory coatings, new recipes are experimented and the thermal properties of such new materials are paramount in the process of qualification of the product. In this work, three different kind of coatings varnishes are tested. These coatings are deposited on a metallic substrate and the thermal diffusivity is evaluated by one side thermal test. The laser shots the surface of the coating with a short pulse and the temperature variation is measured by an infrared camera. Knowing the density, and measuring the specific heat by Differential Scanning Calorimeter, it is then possible to derive the value of the thermal conductivity.
In ancient buildings, knowing the composition and the conservation status of walls is a crucial issue to ensure their durability. The presence of defects or discontinuities could be found using InfraRed Thermography (IRT) with different heating techniques. In order to properly calibrate the thermographic testing procedure, a mock-up wall is studied. The specimen is made of ancient bricks and has defects inside the plaster layer at different depths. Three kinds of heating patterns are delivered on the wall: step, sine wave, and chirp. All the three heating patterns are able to enhance the defect from the sound area after a proper image processing analysis. The differences between each technique are presented with a signal to noise ratio evaluation.
The thermal diffusivity of solid materials is usually measured with the well-known flash method. In the traditional setup, the tested specimens have the shape of a small disc. However, several industrial applications need to test different typologies of samples. This work is focused on ring specimens, that are widely used as joints or sealants in various applications. The goal is investigating the possibilities and limitations of the flash method, applying minimum adjustments to the traditional experimental setup.
A preliminary numerical study is conducted with the creation of a finite element model. Firstly, the model is checked with the standard case of a full disk. Then the simulation investigates the case of an aluminum oxide ring, that is taken as the reference case to determine the reliability of the proposed technique.
After the simulation, an experimental measurement is performed on the aluminum oxide ring reference case. Several samples are tested and useful information on the practical feasibility of the experimental setup are collected. The obtained thermal diffusivity values fall into the expected range for the material, confirming the validity of the suggested method.
Pulsed Thermography (PT) is one of the most common methods in Active Thermography procedures of the Thermography for NDT & E (Nondestructive Testing & Evaluation), due to the rapidity and convenience of this inspection technique. Flashes or lamps are often used to heat the samples in the traditional PT. This paper mainly explores exactly the opposite external stimulation in IR Thermography: cooling instead of heating. A steel sample with flat-bottom holes of different depths and sizes has been tested. Liquid nitrogen (LN2) is sprinkled on the surface of the specimen and the whole process is captured by a thermal camera. To obtain a good comparison, two other classic NDT techniques, Pulsed Thermography and Lock-In Thermography, are also employed. In particular, the Lock-in method is implemented with three different frequencies. In the image processing procedure, the Principal Component Thermography (PCT) method has been performed on all thermal images. For Lock-In results, both Phase and Amplitude images are generated by Fast Fourier Transform (FFT). Results show that all techniques presented part of the defects while the LN2 technique displays the flaws only at the beginning of the test. Moreover, a binary threshold post-processing is applied to the thermal images, and by comparing these images to a binary map of the location of the defects, the corresponding Receiver Operating Characteristic (ROC) curves are established and discussed. A comparison of the results indicates that the better ROC curve is obtained using the Flash technique with PCT processing method.
Infrared thermography is widely applied to the inspection of building, enabling the identification of thermal anomalies due to the presence of hidden structures, air leakages, and moisture. One of the main advantages of this technique is the possibility to acquire rapidly a temperature map of a surface. However, due to the actual low-resolution of thermal camera and the necessity of scanning surfaces with different orientation, during a building survey it is necessary to take multiple images.
In this work a device based on quantitative infrared thermography, called aIRview, has been applied during building surveys to automatically acquire thermograms with a camera mounted on a robotized pan tilt unit. The goal is to perform a first rapid survey of the building that could give useful information for the successive quantitative thermal investigations. For each data acquisition, the instrument covers a rotational field of view of 360° around the vertical axis and up to 180° around the horizontal one. The obtained images have been processed in order to create a full equirectangular projection of the ambient.
For this reason the images have been integrated into a web visualization tool, working with web panorama viewers such as Google Street View, creating a webpage where it is possible to have a three dimensional virtual visit of the building. The thermographic data are embedded with the visual imaging and with other sensor data, facilitating the understanding of the physical phenomena underlying the temperature distribution.
The accurate knowledge of the thermal performance could reduce significantly the impact of buildings on global energy consumption. Infrared thermography is widely recognized as one of the key technologies for building surveys, thanks to its ability to acquire at a glance thermal images of the building envelope. However, a spot measurement could be misleading when the building is under dynamic thermal conditions. In this case data should be acquired for hours or days, depending on the thermal properties of the walls. Long term thermographic monitoring are possible but imply strong challenges from a practical standpoint.
This work investigates the possibilities and limitations of spot thermographic surveys coupled with contact probes, that are able to acquire continuously the thermal signal for days, to investigate the thermal bridges of a building. The goal is the estimation of the reliability and accuracy of the measurement under realistic environmental conditions. Firstly, numerical simulations are performed to determine the reference value of an experimental case. Then a long term thermographic survey is performed and integrated with the contact probe measurement, assessing the feasibility of the method.
An innovative ceramic material has been developed as a possible substitute of the traditional rock-wool as thermal insulating material. It should be used in the future inside a machine working at a temperature greater than 200 °C. The effect of exposition to this temperature for several hours has been evaluated to check if a degradation of the insulating properties can be measured. Experiments did not show any evidence of degradation. Nonetheless the value of the thermal conductivity measured both at high and ambient temperature was not so good as expected. At the same time, the same measurements on rock-wool (the traditional choice for insulation in this machinery) revealed to be very difficult as it is not possible to prepare samples to be tested in a laser flash. To overcome this problem in the measurement of the performance at high temperature a new experiment was prepared by heating one side of the material by means of an electric heater and by looking and comparing (at least qualitatively) the temperature increase on the other side. On the purpose, two parallel-piped samples of the two rival materials, with the same thickness have been prepared and put in contact with the electric heater plate. The temperature evolution of the side facing the ambient has been measured by means of a thermographic camera for almost one hour. The experiment shows that the traditional material owns better insulation performance than the innovative one. Attention has been paid on the properties of the innovative material that, being highly hygroscopic, can maintain a low temperature during the drying process due to the very high value of the latent heat of water when changing from liquid to gas phase.
Infrared thermography is a valuable tool for non-destructive evaluation of antique artworks. Active thermographic
techniques can be applied on-site thanks to their contactless and non-invasive nature. On-site monitoring is a challenging
task. The observed objects are often hard to reach and of unknown thermal and physical properties. Moreover there are
usually hard constraints on the availability of the site, in terms of space and time. For these reasons the acquired data are
typically inhomogeneous and need to be reorganized and post-processed, with dedicated algorithms, to enhance the
The frescoes of the San Gottardo Church, located in Asolo, in the North-East of Italy, are showing multiple detachments
due to the ageing process. More than 60 frescoed surfaces have been selected for evaluation via an active thermography
procedure. Each area has been heated with handheld air heaters and a sequence of infrared images of the cooling process
has been recorded.
Several techniques are available for the post-processing of thermographic sequences. In this work standard algorithms,
such as correlated contrast and principal component thermography, are compared with new ones. We propose two new
algorithms, the first is based on sum and filtering, the second is an adaptation of the partial least squares method to
thermography. The obtained results allow to identify and locate the most important detachments on the surfaces.
The Infrared Images and Other Data Acquisition Station enables a user, who is located inside a laboratory, to acquire visible and infrared images and distances in an outdoor environment with the help of an Internet connection. This station can acquire data using an infrared camera, a visible camera, and a rangefinder. The system can be used through a web page or through Python functions.
Applications of Infrared Thermography in buildings surveys are not limited to the identification of the temperature distribution and heat losses on building envelopes. As it is well known from NDT testing in industrial applications, active IR thermographic methods such as heating-up/cooling-down or lock-in thermography improve the results in many investigations. In civil engineering these techniques have not been used widely. Mostly, thermography is used in a quasistatic manner. This paper illustrates a new approach to achieve, by the lock-in technique, an in depth view of the structure of the wall evidencing the presence of buried elements, interfaces and cracks. The idea is to take advantage of the periodic heating and cooling of earth surface due to the alternating of day and night. The corresponding thermal wave has a period equal to 24 hours that can probe the walls of buildings with a penetration depth of the order of some centimeters. The periodic temperature signal is analysed to extract amplitude and phase. It is expected that the phase image gives the indication of inhomogeneity buried in the wall structure. As a case study, the exterior surface of Palazzo Ducale in Venice is analysed and illustrated. In addition to IR images, visible electromagnetic band is considered to evaluate the strength of the solar radiation and the geometrical distortion. Indeed, the periodicity due to the Earth rotation is only approximately of 24 hours. The passing clouds or the possibility of rainy days can superimpose other heating or cooling frequencies to the main one. The Fourier analysis of the impinging radiation on the wall is performed. The façade of Palazzo Ducale is tiled with stone of two different colours and types. A final attempt to automatically classify the stone tiles in the visible and infrared images is conducted.
The frames of windows are typically made of wood in Italy, even though aluminum, PVC and other materials are more and more utilized in the building manufacture. On the other hand, the growing attention on the problem of energy saving makes more stringent the attention to the insulation properties of any component of the building envelope. Therefore, it is paramount to evaluate the thermal properties of wood that will be utilized in the windows frame manufacture.
Wood is a material characterized by a high anisotropy due to its characteristic growing. Mechanical properties, and thermal as well, are very different if considered along the direction of grain or perpendicular to it.
In manufacturing the frame for windows, the fiber or grain direction must be selected in such a way to maximize the thermal resistance along the inside to outside direction, that means the inside/outside direction of frame (i.e. inside/outside direction of window) must be perpendicular to the grain direction. Indeed the grain direction is the one with the maximum thermal conductivity while the perpendicular one (crossing the fiber direction) owns a lower conductivity value.
The anisotropic characteristics of wood made it a challenging material for the measurement of thermal conductivity. Three types of wood have been measured: oak, larch and spruce. Two instruments have been utilized: a) the hot disk apparatus; b) the IR thermography equipment in transmission (a variant of the Parker’s method) and reflection scheme complemented by density and specific heat measurements. In particular, IR thermography gives the possibility to evaluate by images the preferential direction of heat propagation by looking at the deformation of a localized heat source released on the surface (i.e. a circular shape can become an ellipse as heat diffuses on the surface). Results coming from different kind of measurements are compared and critically considered.