Vladimir P. Vavilov

Quantitative infrared thermography in buildings

A methodology, based on the solution of the inverse heat transfer problem, for the detection and evaluation of flaws in buildings is discussed. The temperature varying in space and time is recorded by a thermographic equipment and each point belonging to the inspected area is analysed quantitatively. Data are processed to give a map of defects of the wall, based on the most suitable local thermal parameter. The thermal-physical aspects of different defects are studied, along with the description of simplified models to interpret surface temperature data. The building envelope is examined mainly in transient thermal regime. Testing procedures using periodic or pulse heating of the surface are described. The theoretical analysis is used to predict temperature evolution and properly design the test. A detectability comparison among different procedures is performed. Experimental results are reported for insulation deficiencies and thermal bridges evaluation, air leakage detection and moisture content mapping.

Surface transient temperature inversion for hidden corrosion characterisation: theory and applications

Abstract Corrosion in metals is simulated with variations in plate thickness which are laterally infinite in a 1D model and are represented by milled flat-bottom holes in a 2D model. Temperature contrast over corroded areas is chosen as an informative parameter, quite independent of absorbed thermal energy in the infra-red thermographic test. It is shown that, due to lower sensitivity to rear-side effects at the beginning of the thermal process and increasing 3D heat diffusion effects at the end of the process, there is an optimum time to detect corrosion. A robust inversion function is proposed and its stability against variations in tested material, heat pulse duration and observation time is analysed using numerical modelling. Corrosion in a steel specimen of 1.3 mm thickness is experimentally studied, having proved the validity of the inversion algorithm with an average accuracy of 17% for material loss ranging from 74 to 14%.

Thermal Characterization of Defects in Building Envelopes Using Long Square Pulse and Slow Thermal Wave Techniques

Building infrared thermography is a well-established but still qualitative technique that is used mostly for detecting insulation deficiencies, air leaks, moist areas, and thermal bridges in envelopes. Characterization of buried defects requires replacing steady-state thermography with a transient technique. It is proposed to borrow for this purpose the inspection methodology developed in the thermal nondestructive evaluation of materials. This methodology is based on recording the image sequence in both the heating and cooling stages, with the thermal stimulus being delivered onto the surface with a Dirac, square, or harmonic heat pulse. The dedicated image treatment allows the substitution of a sequence of any length with a pair of images called the maxigram and timegram. These images contain information needed for the characterization of defects. Some explicit inversion formulas discussed in the paper enable the determination of defect depth and thermal resistance. The proposed approach is illustrated with experimental results. Evaluation of internal corrosion in concrete and detection of underplaster delaminations are reported.

Nondestructive Evaluation of Delaminations in Fresco Plaster Using Transient Infrared Thermography

A transient method of thermal nondestructive testing is applied to detect subsurface delaminations in fresco plaster as a part of restoration work that is being conducted in Italy. A two-dimensional cylindrical model is used to estimate temperature distribution in materials with defects of various sizes and depths. The possibility of testing plaster with thickness up to 30 mm is illustrated. The choice of heating technique, which is supposed to be really nondestructive to ancient frescoes and powerful enough to create reasonable temperatures signals, is considered. Test duration and optimum sampling interval are discussed also. The structural noise of plaster and the influence of surface painting are investigated.

Corrosion evaluation by thermal image processing and 3D modelling

Abstract Quantitative transient IR thermography has been applied to the characterization of hidden corrosion in metals. A dedicated 3D numerical model of heat transfer has been used to solve the direct thermal problem and to simulate the test. Theoretical modelling allows the verification of limits of the ID solution and the derivation of coefficients which take heat diffusion into account. An analysis of inversion accuracy was carried out. A simple algorithm based on a surface temperature time-derivative is proposed for detecting thickness variations. Then, material loss in an area of arbitrary shape is evaluated applying a modified algorithm, originally developed for a ID thermal model. The potential of dedicated image processing in enhancing a signal-to-noise ratio is explored. The feasibility of corrosion quantification by the proposed inversion algorithm is demonstrated with experimental results. Detection and evaluation of hidden material loss within a boiler section, typically used at a power plant station, has been performed. The external surface was heated with flash lamps and temperature response was analyzed both in time and space domains. The masking effect due to the noisy inspected surface (not painted and affected by a long time service) were substantially removed before evaluating corrosion. Obtained results have been compared with measurements produced by the ultrasonic method.

Thermal NDT: historical milestones, state-of-the-art and trends

The first practical attempts of sensing our world in the infrared spectrum were undertaken in the nineteenth century. One of the first implementations of the active thermal non-destructive testing (TNDT) was the inspection of Polaris rocket motor cases in 1965 but by the end of the 1970s applications of IR thermography inspired by successful developments of IR technology in military were still rather qualitative thus preventing the progressive competition of TNDT with other inspection techniques. A new level of TNDT was achieved after the wider use of elements of the heat conduction theory. Nowadays, IR thermographic diagnostics and TNDT represent mature high-technology fields which combine achievements in the understanding of heat conduction, material science, IR technology and computer data processing. The high interest to the IR/thermal inspection technique is related to its universal character, high testing productivity and in-service safety. The purpose of this review is to briefly review the basics of TNDT, which include elements of many disciplines of science and technology, such as heat conduction, IR engineering, optoelectronics, image processing and statistics.

Sensitivity Analysis of Classical Heat Conduction Solutions Applied to Materials Characterization

This paper presents the analysis of classical heat conduction solutions applied to materials characterization. The formulas for parametric derivatives are obtained and illustrated to demonstrate the evolution of the relative sensitivity functions in time. The potential of using both front-surface and rear-surface solutions for determining material thermal properties, sample thickness, and surface heat exchange parameters is discussed. The roots of the well-known transcendent equation for a non-adiabatic plate are approximated in a polynomial form. Some practical applications of the proposed formulas are reported.

Advanced modeling of thermal NDT problems: from buried landmines to defects in composites

Advanced thermal models that can be used in the detection of buried landmines and the TNDT (thermographic nondestructive testing) of composites are discussed. The interdependence between surface temperature signals and various complex parameters, such as surface and volumetric moisture, the shape of a heat pulse, material anisotropy, etc., is demonstrated.

Dynamic thermal tomography: new promise in the IR thermography of solids

The thermal tomography technique in the form of ordinary or cross-sectional timegrams completes the transition from the amplitude to the amplitude-time information criterion in thermal NDT. It improves some features of NDT based on the signal images: more efficient identification of defects, detection and separation of internal layers, and better SNR.

Thermal tomography, characterization and pulse phase thermography of impact damage in CFRP, or why end-users are still reluctant about practical use of transient IR thermography

Results of inspecting a CFRP specimen with the impact damage defect are reported. The accent is made on dedicated data processing including the so-called defect maps production which are of special interest for end-users. Comparison of thermal tomography and pulse phase thermography is made while applying these methods to both defect detecting and characterizing.