Showing papers on "Thermography published in 2008"

Matched excitation energy comparison of the pulse and lock-in thermography NDE techniques

Abstract: The defect detection capabilities of pulse transient thermography and lock-in thermography were compared using equal excitation energies. A signal-to-noise ratio analysis was performed on defect images obtained by the two techniques. The test piece imaged by both techniques was a carbon fibre composite plate containing back-drilled flat-bottomed hole artificial defects of 4, 6 and 12 mm diameters at depths ranging from 0.25 to 3.5 mm. Similar limits of defect detection were found for the two techniques. Lock-in thermography phase images were found to exhibit anomalous switches between positive and negative phase values for different modulation frequencies and for different combinations of defect diameter and depth. This effect resulted in values of defect phase response that differed substantially from the one-dimensional thermal wave interpretation of lock-in thermography imaging. The consequences of this anomalous effect and its physical origins are discussed.

Pulse compression approach to infrared nondestructive characterization

Abstract: Infrared thermography is a whole field, noncontact, and nondestructive characterization technique widely used for the investigation of subsurface features in various solid materials (conductors, semiconductors, and composites). Increased demand for greater subsurface probing in thermal nondestructive testing is often thwarted by the probing high peak power into the sample, for which narrow pulse operation is usually used. The technique of pulse compression offers a means of increasing the average power available to illuminate test specimen without any loss of the depth resolution needed for the tactical requirements. This is accomplished by transmitting a wide pulse in which the incident heat flux is frequency modulated and then, by proper signal processing methods, causing a time compression of the received signal to a much narrower pulse of high effective peak power. For the demonstration, a mild steel sample having flat bottom holes at various depths is introduced and detection capability of the proposed approach has been studied.

Quantitative determination of a subsurface defect of reference specimen by lock-in infrared thermography

Abstract: This paper describes the quantitative determination of the sizes and locations of subsurface defects using lock-in infrared thermography. A phase (or temperature) difference between the defect area and the healthy area indicates the qualitative location and size of the defect. To accurately estimate these parameters, the shearing-phase technique has been employed, where the inspected image is shifted by a certain number of pixels to obtain a shifted image while subtraction of one image from the other gives the shearing-phase distribution. The shearing-phase distribution has maximum, minimum, and zero points that help determine quantitatively the size and location of the subsurface defect. Experimental results for a steel plate with artificial subsurface defects show good agreement with actual values.

Advanced integrated technique in breast cancer thermography.

Abstract: Thermography is a passive and non-contact imaging technique used extensively in the medical arena, but in relation to breast care, it has not been accepted as being on a par with mammography. This paper proposes the analysis of thermograms with the use of artificial neural networks (ANN) and bio-statistical methods, including regression and receiver operating characteristics (ROC). It is desired that through these approaches, highly accurate diagnosis using thermography techniques can be achieved. The suggested method is a multi-pronged approach comprising of linear regression, radial basis function network (RBFN) and ROC analysis. It is a novel, integrative and powerful technique that can be used to analyse large amounts of complicated measured data such as temperature values extracted from abnormal and healthy breast thermograms. The use of regression allows the correlation between the variables and the actual health status of the subject, which is decided by other traditional means such as the gold standard of mammography for breast cancer detection. This is important as it helps to select the appropriate variables to be used as inputs for building the neural network. RBFN is next trained to produce the desired outcome that is either positive or negative. When this is done, the RBFN possess the ability to predict the outcome when there are new input variables. The advantages of using RBFN include fast training of superior classification and decision-making abilities as compared to other networks such as backpropagation. Lastly, ROC is applied to evaluate the sensitivity, specificity and accuracy of the outcome for the RBFN test files. The proposed technique has an accuracy rate of 80.95%, with 100% sensitivity and 70.6% specificity in identifying breast cancer. The results are promising as compared to clinical examination by experienced radiologists, which has an accuracy rate of approximately 60-70%. To sum up, technological advances in the field of infrared thermography over the last 20 years warrant a re-evaluation of the use of high-resolution digital thermographic camera systems in the diagnosis and management of breast cancer. Thermography seeks to identify the presence of a tumour by the elevated temperature associated with increase blood flow and cellular activity. Of particular interest would be investigation in younger women and men, for whom mammography is either unsuitable or of limited effectiveness. The paper evaluated the high-definition digital infrared thermographic technology and knowledge base; and supports the development of future diagnostic and therapeutic services in breast cancer imaging. Through the use of integrative ANN and bio-statistical methods, advances are made in thermography application with regard to achieving a higher level of consistency. For breast cancer care, it has become possible to use thermography as a powerful adjunct and biomarker tool, together with mammography for diagnosis purposes.

Local energy analysis of high-cycle fatigue using digital image correlation and infrared thermography

Abstract: This paper presents the first results provided by an experimental set-up developed to estimate locally the terms of the energy balance associated with the high-cycle fatigue (HCF) of DP 600 steel. The experimental approach involves two quantitative imaging techniques: digital image correlation and infrared thermography. First, a variational method is used to derive stress fields from the displacement fields. Patterns of deformation energy per cycle can then be determined on the basis of stress and strain data. Second, a local form of the heat equation is used to derive separately the thermoelastic and dissipative sources accompanying HCF. Energy balances show that around 50 per cent of the deformation energy associated with the mechanical hysteresis loop is dissipated while the rest corresponds to stored energy variations.

The infrared thermography control of the laser welding of amorphous polymers

Abstract: In laser welding technique, a real-time control of temperature distribution inside the irradiated materials is essential when attempting to optimize the process. For all laser welding methods that operate by the transmission principle, the difficulty of recording the developed temperature at the interface derives from the fact that materials to be welded are in contact throughout the entire process. In the present study, in order to overcome this issue, a contact-free method such the infrared thermography is used for surface temperature measurement. Corroborating this data with a numerical simulation of the temperature field evolution inside the components, an assessment of optimal process parameters is possible. The experimental investigations are made on amorphous polymers, in a typical configuration for through-transmission laser welding. The fine agreement obtained between the experimentally and calculated data, validate the infrared thermography as a non-destructive method for real-time monitoring of the welding process.

Non-destructive Inspection of Composites Using Step Heating Thermography

Abstract: The article explores the use of step heating thermography for defect identification in glass fiber reinforced composites. Step heating methods have potential for increasing the maximum detectable defect depth as compared with pulse heating methods. Inspection of thick fiberglass composites containing flat-bottom holes and resistive embedded defects using step heating thermography is investigated using 1-D analytical models, axisymmetric finite element modes, and experimental methods. 1-D heat transfer models of embedded defects under step heating are developed based on results from previous research of pulse heating thermography. Finite element models are used to explore the range of validity of defect identification techniques based on correlation with 1-D analytical methods, and it is found that accurate results are obtained for a defect radius to thickness ratio greater than two. Experimental results demonstrate the validity of the 1-D models for high defect resistance cases. For low resistance defects...

Imaging physical parameters of pre‐breakdown Sites by lock‐in thermography techniques

Abstract: Local pre-breakdown sites in solar cells can be studied by lock-in thermography (LIT). Three new LIT techniques are proposed and demonstrated here, which are TC-DLIT for studying the local temperature coefficient of pre-breakdown sites, Slope-DLIT for measuring the normalized local slope of the I–V characteristics, and MF-ILIT for imaging the local carrier multiplication factor. First results on multicrystalline silicon cells show that the pre-breakdown mechanism cannot completely be described by the conventional impact ionization and internal field emission models. Copyright # 2008 John Wiley & Sons, Ltd.

Surface crack detection by flash thermography on concrete surface

Abstract: . The follow-up actions and the corresponding severity index on different building defects are listed in Table 1. Therefore, the range of crack width of interest in this study is from 0.15 mm to 1 mm. A cement panel with major cracks with widths of 0.5-1 mm and micro-cracks having widths of 0.1-0.5 mm are inspected and the results are presented. Moreover, this paper provides a comparison of the effectiveness of two traditional crack detection techniques, Sobel and Canny, and one proposed method, the sheared image subtraction method, and the results are also presented. Cracking may impair the durability of concrete by allowing immigration of external aggressive agents; therefore, crack monitoring is always a vital part in building pathology. This study proposes to apply short-duration pulsed thermography - flash thermography (FT) - for surface crack detection. This method allows full-field and non-contact qualitative observation of thermal radiation from an object surface and is highly accepted in the aerospace industry. It is superior to the common practice of surface crack detection - visual inspection. The overall inspection time is reduced and hence maintenance costs lowered. During inspection, the inspected surface is excited with a heat-pulse of short duration (~3 ms). Surface cracking is detected based on the difference in heat emission between cracks and intact region. The results show that FT can detect surface cracks with 0.5 mm to 1 mm crack width successfully but micro-cracks (0.1 mm-0.5 mm) can only be detected by adding water with FT. In addition, this study also compared the performances of traditional Sobel and Canny edge detectors and a proposed shear image subtraction method, for crack detection. The results show that the sheared image subtraction method is significantly more effective than the other two edge detection techniques in identifying cracks.

Induction Thermography as a Tool for Reliable Detection of Surface Defects in Forged Components

Abstract: Dynamic thermography with inductive excitation is analysed as an alternative to magnetic particle inspection or to eddy current testing. Given by the relation of the electromagnetic skin depth, the thermal penetration depth and the crack dimensions to be detected, different regimes for defect detection are identified. The effect of the crack parameters length, depth and inclination angle are discussed. In ferritic steel, at induction frequencies of 100-200 kHz, perpendicular open cracks with a length of 7.5 mm were detectable when their depth was minimum 0.15 mm. For inclined cracks, the sensitivity is even higher. Experiments were performed on cold and warm forged steel components. The signal-to noise ratio obtained from defects was usually high, the critical limitation on technical surfaces is the background due to surface roughness and due to surface contamination. A series investigation on forged components showed a good probability of detection and a low false alarm rate compared to magnetic particle testing. The short testing times of a few 100 ms per object view will allow short cycle times for mass products.

Thermal conductivity measurements of laser crystals by infrared thermography. Application to Nd:doped crystals.

Abstract: We present a thermal conductivity measurement method for laser crystals based on thermal mapping of the crystal face by an infrared camera. Those measurements are performed under end-pumping of the laser crystal and during laser operation. The calculation of the fraction of pump power converted into heat is therefore simplified, and it is possible to link easily the temperature in the crystal to the thermal conductivity. We demonstrate the efficiency of this measurement method with a Nd:YAG crystal, before using it to compare Nd:YVO(4) and Nd:GdVO(4) crystals.

Infrared Thermography for Buried Landmine Detection: Inverse Problem Setting

Abstract: This paper deals with an inverse problem arising in infrared (IR) thermography for buried landmine detection. It is aimed at using a thermal model and measured IR images to detect the presence of buried objects and characterize them in terms of thermal and geometrical properties. The inverse problem is mathematically stated as an optimization one using the well-known least-square approach. The main difficulty in solving this problem comes from the fact that it is severely ill posed due to lack of information in measured data. A two-step algorithm is proposed for solving it. The performance of the algorithm is illustrated using some simulated and real experimental data. The sensitivity of the proposed algorithm to various factors is analyzed. A data processing chain including anomaly detection and characterization is also introduced and discussed.

Temperature Measurement of Carbon Nanotubes Using Infrared Thermography

Abstract: Multiwalled carbon nanotubes (MWNTs) have a very high absorbance in the near-infrared (NIR) region of the electromagnetic spectrum. It is demonstrated that the temperature of MWNTs, measured using infrared thermography during NIR laser irradiation, is much higher than that of other carbonaceous materials such as graphite controls. Since most biological materials have a low absorbance in the same region, carbon nanotubes could be used for selective photothermal hyperthermia of biological or nonbiological systems. This study was performed to prove the concept of direct measurement of the temperature distribution of carbon nanotubes when stimulated with laser irradiation. The maximum temperature increase of MWNTs measured with a high-sensitivity infrared camera, continuously irradiated with four 2.5-mW/650-nm lasers, was 7 °C vs a graphite control, while irradiation with a 390-mW/1064-nm laser yielded maximum temperature increases of more than 100 °C above the graphite control. The use of infrared thermograp...

Subsurface defect characterization in artworks by quantitative pulsed phase thermography and holographic interferometry

Abstract: In this study, experimental data from two artwork specimens was acquired and processed by pulsed phase thermography (PPT) and holographic interferometry. The first specimen was a wood painting with a variety of damages typical of this kind of pieces. A comparative study between thermography and interferometry results showed the potential complementarities of both techniques. The second inspected specimen was a fresco with fabricated inserts inspected by PPT to detect and characterize the subsurface defects. The well-known concept of Signal-to-Noise Ratio (SNR) is proposed for the selection of the proper phasegram frequency at which defect sizing is performed. A de-noising step was required prior to the application of the Canny edge detection algorithm. It is demonstrated with this investigation that PPT and holographic interferometry are valuable tools for the qualitative and quantitative assessment of artworks.

Improving reliability and decreasing losses of electrical system with infrared thermography

Abstract: Temperature and the resulting thermal behavior of electric power generation and distribution equipment and industrial electrical systems and processes are the most critical factors in the reliability of any operation or facility. Temperature is by far the most measured quantity in any industrial environment. For these reasons, monitoring the thermal operating condition of electrical and electromechanical equipment is considered to be key to increasing operational reliability and decreasing electrical losses.

Pulse thermography applied on a complex structure sample: comparison and analysis of numerical and experimental results

Abstract: The purpose of this work was to determine the possibility of modeling complex structure samples inspected by infrared thermography, as well as the possibility of identifying the defect characteristics, such as the defect type, using the results obtained. The paper presents the analysis of results obtained by pulse thermography experiments on a complex structure sample containing defects. The sample consists of two different honeycomb panels as a core between two graphite-epoxy layers joined with appropriate specified adhesive. Each of the two sample parts contains the same number of defects of different type, size and depth. A pulse thermography experiment was conducted and the results were recorded by high resolution infrared camera as presented in the article. The images obtained were processed so as to enhance the defect visibility. Once the location of the defects was determined, surface temperature evolution curves were extracted above each of the defects as well as in non-defective regions. These curves were used afterwards for numerical model evaluation. As mentioned, the numerical model of the tested sample was made in order to simulate the behavior of the sample under inspection. COMSOL software based on Finite Element Method (FEM) was used to make a 3D model of transient heat transfer. In a similar way as for the results obtained experimentally, the surface temperature evolution curves were extracted from the results of the simulation. Since many of the model parameters were not precisely known (power density of the heat source used in experiment, convective heat transfer coefficients, sample surface emissivity...), they were adjusted to obtain the results of numerical simulation as close as possible to those obtained experimentally. Nevertheless, all of the parameter values determined in such a way were within the range of the expected values. In general, the behavior of the temperature evolution curves obtained numerically fit the experimental results. Some difficulties in comparison aroused as the nonuniform heating was present in the experiment while the model assumed a uniform heating lamp power density distribution over the whole sample surface. This required a careful choice of the reference defect-free regions that was made for each of the defects separately in the recorded thermograms. A difference in behavior obtained experimentally and numerically was observed for only one of the defects. The comparisons performed in this paper enable us to have confidence in the developed model and thus to evaluate, through modelling, the potential performance of infrared thermography on similar specimens.

Infrared imaging of the anterior deltoid during overhead static exertions

Abstract: Infrared imaging has been used to detect the presence of neuromuscular disorders of the cervical spine and upper extremities. Despite diagnostic uses, evaluative or prognostic uses of thermography are limited. The objective of this study was to quantify surface temperature changes over the anterior deltoid and evaluate efficacy of thermography as an assessment tool. Surface temperature, discomfort ratings and endurance time were quantified during overhead static exertions until exhaustion at two work loads (15 and 30% maximum voluntary contraction) and shoulder angles (90 degrees and 115 degrees ). Ten participants free of confounding conditions participated in the study. The 90 degrees shoulder angle and 30% exertion level resulted in significantly faster thermal image rates of change, shorter endurance times and faster perceived discomfort increases. Thermography readings were more sensitive to changes in shoulder posture than load changes. This study provides preliminary evidence that thermography may be a useful exposure assessment tool. There is a need for new evaluation tools to quantify risk factor exposure for injury. Thermography was sensitive to changes in task loadings, illustrating its potential use for risk assessment. Specifically, changes in observed blood flow patterns during task performance are likely to conform to known physiological responses to injury.

Focal plane array based infrared thermography in fine physical experiment

Abstract: By two examples of dissimilar physical phenomena causing thermophysical effects, the unique capabilities of one of the up-to-date methods of experimental physics—focal plane array (FPA) based infrared (IR) thermography (IRT), are demonstrated distinctly. Experimenters inexperienced in IRT can grasp how this method provides a means for combining real-time visualization with quantitative analysis. A narrow-band short-wavelength IR camera was used in the experiments. It is discussed and stated that IRT is best matched and suited to the next two test conditions—when a heated specimen is thin and when heat is generated in the immediate region of a surface of a solid. The first prerequisite is realized in the search for directional patterns of combined low-power radiation sources with the use of the IRT-aided method. The second one is realized in studies of water vapour adsorption on uneven (irregular) surfaces of solid materials. With multiple swatches taken from a set of different fabrics and used as experimental samples, a sharp distinction between adsorptivities of their surfaces is strikingly illustrated by IRT time-domain measurements exhibiting the associated thermal effect ranging within an order of magnitude. It is justified that the described IRT-aided test can find practical implementation at least in the light industry. Emissivities of different fabrics are evaluated experimentally with the described reflection method based on the narrow spectral range of IRT. On the basis of direct IR observations, attention is paid to the need for close control over the surface temperature increase while the adsorption isotherms are being measured. Sensitivity of the FPA-based IRT method, as applied to examine the kinetics of initial stages of adsorption of gaseous molecules on the solid surface, is evaluated analytically and quantitatively. The relationship between the amount of adsorbate and the measurable excess of adsorbent temperature is found. It is discovered that the method makes it possible to control nano-quantities of the adsorbed matter, namely, it is sensitive to an incipient molecular film of 1/300-monolayer effective thickness.

Condition monitoring of exhaust system blowers using infrared thermography

Abstract: Electrical and mechanical machines are integral parts of engineering plants. Machine failures can occur due to many reasons. Thermal energy produced during the operation of all machines can be in the form of friction losses within machines, energy losses within machines, as a characteristic of the process media, or any combination thereof. Infrared thermography (IRT) is an ideal Non-Destructive Testing (NDT) technique to investigate abnormal temperature distribution on machine surfaces, because it provides thermal images of a machine or component remotely. In this paper, we discuss an important application of thermal imaging for condition monitoring of blower bearings, shaft and the motors in ventilation systems used in nuclear plant areas. Sources of abnormal temperatures are detected at the bearings and shaft at the impeller end of the blower of an exhaust system. Also, abnormal temperature distribution is detected in one of the belts of the pulley drive system due to over-tightening. IRT was effectively utilised to pinpoint the sources of excessive heat in the blower and to carry out necessary corrections. We demonstrate the usefulness of IRT to detect the sources of abnormal temperatures in various components of the blower at an early stage of impending malfunction failure that could prevent major breakdowns.

Infrared thermography and its applications in civil engineering

Abstract: Infrared thermography is a powerful tool to investigate structural condition and equally useful for damage assessment. It is a non-contact and non-destructive method that enables rapid investigations. Highly efficient infrared cameras and versatile software have simplified thermography considerably over the years. While infrared thermography has wide applications in process industries, it is not yet extensively adopted in the investigation of buildings. The paper presents a brief historical account of infrared thermography, the phenomenon of electromagnetic radiation, thermal imaging and applications in civil engineering. Numerous other applications of thermal imaging are also discussed briefly along with the advantages and limitations.

Thermographic signal processing through correlation operators in pulsed thermography

Abstract: In non-destructive testing by Infrared Thermography it is usually needed to locate defects and region of interests suspected to contain defects. The defects cannot always be observed directly from one single IR image taken at a single given time t. Thus, in the case of pulsed thermography, direct course techniques as the Fourier transform process the information of many images recorded for a given duration into one resulting image. Another way to compile the temporal information of a sequence of images into a single one is to compute a correlation image. This paper details an approach to use a statistical correlation operator to help improving defect detection in pulsed infrared thermography.