Papers by Keyword: Lock-In Thermography

Abstract: Nowadays there are many diagnostic methods for the detection of different defects in building structures. Destructive methods, which do not enable purposeful redevelopment action or a potential cause prediction of the defect without damage to the structure, are mostly used. Lock-in thermography is based on the modulation of the controlled heat flow, which impact on the diagnosed object. Heat (usually sinusoidal) wave penetrates inside the element and in the place of environmental change, ie. anomalies / deviations in the structure of the material of the tested object are reflected back to the surface. The paper focuses on the possibility of using lock-in thermography in the detection of various defects in building structures.

Abstract: This paper presents a two-stage scheme of damage identification for plate-type structure. In the first stage, probable damaged regions and their relative severities can be detected based on lock-in thermography technique. In the second stage, the relation between the damage level and its corresponding natural frequencies of the plate is constructed by means of Kriging surrogate model based on dynamic analysis. The inverse problem of damage quantification over the surrogate model is then solved by using a robust stochastic particle swarm optimization algorithm. Experimental study on a double-damaged plate demonstrates the feasibility and effectiveness of the proposed scheme.

Abstract: In this contribution the basic principles of DLIT (dark lock-in thermography) are introduced and typical application examples are shown. These results are compared with that of other solar cell characterization techniques like electroluminescence (EL) and photoluminescence (PL) imaging, which are also very popular in Germany. It will be shown that these techniques are largely complementary to each other. Luminescence techniques are most sensitive for the detection of local recombination centers in the bulk and of series resistance problems of the cells, whereas DLIT is most effective for investigating all problems being connected with the dark current of the cells. A new DLIT technique is introduced which allows a separate imaging of the so-called diffusion current and the recombination current. These two contributions of the dark current are based on different physical mechanisms (recombination in the bulk and in the depletion region, respectively), and their spatial distributions differ significantly. Such investigations are impossible by applying luminescence-based imaging techniques.

Abstract: This paper aims, to investigate the shunts in multi-crystalline (m-c) Si solar cells by lock-in infrared thermography (LIT) technique and to study their effect on the cell performance by PSpice simulations. LIT provided useful information about the location and nature of shunts which was used in the simulation. Based on the shunt location and shunt resistance of the cell obtained experimentally from the I-V characteristic of the cell, shunt resistance at the shunted region have been estimated by simulation using the distributed diode model approach of solar cell by fitting. Based on these values of shunts, simulation has been performed to obtain the information about the deterioration in cells performance caused by the shunts. This type of simulation is useful to study different types and severity of shunts at different locations of cells. Solar cells which have been used in this study show a power reduction in the range of 3% to 15% due the shunts. This reduction was more severe for the shunt which was on the bus-bar compared to the edges.

Abstract: This paper describes a set of theoretical and experimental results based on thermoelastic effect measurement from Glass-Fiber-Reinforced-Polymer (GFRP) composite structure for full-filed stress analysis. The sum of the principle stress (The first stress invariant) can be measured by means of thermoelastic stress analysis (TSA), and this method is used to determine the stress concentration of the composite structure. A finite element analysis is proposed to predict the stress distribution for GFRP gluing structure. The lock-in thermography has been applied to measure the structure stress distribution by its high thermal resolution. The experiment was carried out with different GFRP composite structures by lock in thermography. The experimental results show the stress distribution can be measured and evaluated with good accuracies by using of lock-in thermography

Abstract: The application of thermoelastic stress analysis in compound structure is particularly complicated because of the different material components, which determines the different thermoelastic effect to be depended on the different material property and mechanical performance. This paper describes a theoretical and experimental analysis on full-filed stress distribution from thermoelastic measurements and its application to determination of stress concentration for compound Steel-Foam-GFRP structure. A finite element modeling is proposed to calculate the sum of the principal stress under the condition of dynamic cyclic load. The sum of the principal stress can be measured by means of thermal stress analysis (TSA). Lock-in thermography has been applied to measure the sum of principal stress distribution of component by its high thermal resolution. In this study, Experiments were carried out with Steel-Foam-GFRP compound structure under dynamic periodic load. The thermoelastic constant is calibrated for different component of compound structure, respectively. An artificial neural network (ANN) is proposed to identify the different component stress distribution on whole compound structure. The experimental result shows that the stress distribution of compound structure can be measured and analyzed using lock-in thermography. It is found that the stress distribution of compound structure can be evaluated with good accuracies by lock-in thermography.

Abstract: This paper describes a theoretical and experimental analysis on full-filed stress distribution from thermoelastic measurements and its application to determination of stress concentration. The sum of the principle stress can be measured by Thermal Stress Analysis (TSA). Lock-in Thermography is very effective tool to measure the structure stress distribution by its high thermal resolving. In this study, the thermoelastic effect theory is described and the relationship between the temperature and the applied stress is developed in an elastic material. Experiments were carried out with 2A12 aluminium alloys plate and ones with hole structure under cyclic load. The thermoelastic effect coefficient is obtained for 2A12 aluminium alloys materials, and the effect law is analyzed that the stress value measured was affected by load frequencies. The optional load frequency is obtained, and that is, the load frequency is selected greater than 3.5Hz for 2Al12 materilas, and it was found that the structure stress can be evaluated with good accuracies by the lock in thermography. The experiment was carried out for aircraft components stress distribution measurement and structure stress analysis. The experimental results show the stress concentration position is easy found from stress distribution by lock-in thermography.

Abstract: The application of thermoelastic stress analysis in composite materials is particularly complicated because of the anisotropy of the material, which determines the thermoelastic effect to be depended on the material property and mechanical performance. This paper describes a theoretical and experimental analysis on full-filed stress distribution from thermoelastic measurements and its application to determination of stress concentration. The sum of the principal stress can be measured by Thermal Stress Analysis (TSA). Lock-in Thermography has been applied to measure the sum of principal stress distribution of component structure by its high thermal resolving. In this study, Experiments were carried out with GFRP composite ply and foam materials under cyclic load. The thermoelastic constant is obtained for GFRP and foam composite materials. The stress concentration is analyzed for a specimen with a hole. The experimental results show the stress distribution can be measured and analyzed using Lock-in thermography. It is found that the composite material structure stress can be evaluated with good accuracies by lock in thermography.

Abstract: This paper describes a theoretical and experimental analysis on full-filed stress distribution from thermoelastic measurements and its application to determination of stress concentration. The sum of the principal stress can be measured by Thermal Stress Analysis (TSA). Lock-in Thermography has been applied to measure the sum of principal stress distribution of component structure by its high thermal resolving. In this study, Finite element method is used to calculate the sum of principal stress distribution, and the thermoelastic effect model is developed to study the relationship between the temperature deviation and the applied stress in an elastic material. Experiments were carried out with ANSI 7071 high strength aluminum alloys ply and ones with a crack under cyclic load. The thermoelastic constant is obtained for ANSI 7071 high strength aluminum alloys materials. The stress concentration factor is calculated for a ply with modeling crack under the condition of different loads. The experiment was carried out with high strength aluminum alloys component structure with rivet joints. The experimental results show the stress distribution can be measured and analyzed the contact stress distribution between ply and rivet by using Lock-in thermography. It was found that the structure stress can be evaluated with good accuracies by the lock in thermography.

Abstract: Lock-in thermography (LT), that is active infrared testing technology, mainly includes optical lock-in thermography (OLT) and ultrasound lock-in thermography (ULT). LT can be used to detect unbonds between honeycomb core and face sheet of sandwich structures. However, modulation frequency is an important influencing factor. In this paper, the principles of LT are represented, in experimental detections of simulated unbonds in honeycomb sandwich structures with Al-face sheet and CFRP-face sheet using OLT and ULT, detectability of OLT and ULT is compared and analyzed, effect of modulation frequency is researched and the optimal frequencies are obtained.