A new approach for estimation of defects detection with infrared thermography

A new approach is proposed for a priori ascertaining the suitability of infrared thermography for the non-destructive evaluation purposes of materials. The novelty regards a cause/effect relationship between the thermal signature perceived by the infrared camera and a dimensionless group, which includes geometrical and thermal characteristics of the defect and host material. Such relationship, which was experimentally assessed through the evaluation of several specimens, is helpful to predict the thermal contrast associated with a type of defect embedded inside a certain material. This prediction of course is useful in decision making about the use of infrared thermography without preliminary feasibility tests and thus with economic benefits.     

Determination of thermal diffusivity of solids by use of periodic heat flow

This paper deals with the determination of thermal diffusivity in a cylindrical specimen by the use of a periodic heat flow in the axial direction. Heat transfer from the periphery is taken into account, and its influence upon the evaluation of thermal diffusivity from measurement of phase lag and amplitude decrement, respectively, is discussed. Experimental conditions are pointed out for which the evaluation can be done as for a semiinfinite specimen. Theoretical considerations are compared with experimental results.     

Quantitative analysis of the projectile impact on rock using infrared thermography

The transient process of rock, normally impacted by a gas gun, was monitored with a thermal infrared (TIR) imager. The geometric features and the radiant characteristics of the infrared thermography in target region were extracted and regressively analyzed. Both the projectile's impacting velocity and kinetic energy were back analyzed based on the extracted geometric features and the radiant characteristic parameters of target thermography. It was revealed that: (1) there is a critical velocity at which target thermography is regular and centrally symmetrical; (2) within the critical velocity, the impacting velocity is linearly correlated to the perimeter of target thermography and is quadratically correlated to the TIR temperature increment, while the projectile kinetic energy is linearly correlated to both the area and the increment of the TIR radiant flux of the target thermography; (3) for a quantitative back analysis of the impact-related parameters, both geometric features and radiant characteristics can be applied. It was suggested that the perimeter and the TIR radiant flux increment of the target thermography can applied as the principal indices (with an error of less than 5%), while the area and the TIR temperature increment of target thermography can act as other auxiliary indices (with an error of less than 15%). The possible mechanisms of variations in IR radiation for impact on rock are discussed. The results are of general significance for the impact on other types of rocks or solid materials, and might extend the application of infrared remote sensing from a static geosciences domain to a dynamic mechanical domain.     

Measurement of the thermal diffusivity by the thermal wave method using low-power heat sources

The errors in measuring the thermal diffusivity by the plane thermal wave method are considered as a function of the thermal flux power density. The minimum values of the thermal flux power density required for measurements with a specified error and the optimum parameters of the samples and of the heat source are determined. __________ Translated from Izmeritel’naya Tekhnika, No. 8, pp. 44–46, August, 2007.     

Effects of infrared detector nonlinearity on thermal diffusivity measurements using the flash method

When using an infrared detector to measure temperature changes as in the case of the flash technique, the effects of detector nonlinearity can have drastic effects on the experimental data. In the flash technique, the detector nonlinearity tends to shift the calculated half-time to larger values, resulting in underpredicted values of thermal diffusivity especially in experiments performed at room temperature. In order to predict the error in the diffusivity calculation, the nonlinear relationship between the detector signal and the temperature change was developed into a Taylor series expansion used in the flash technique's mathematical model. The nonlinear detector model proves to yield accurate correction factors for the presently calculated values of diffusivity. In order to utilize the model, it is necessary to estimate the maximum temperature rise of the back surface and the degree of detector nonlinearity.     

交流量热法测量SiO2薄膜的热扩散率

介绍了交流量热法测量薄膜热扩散率的原理和系统组建，用脉宽为纳秒级的超短激光脉冲作为热源，测量了Si衬底上厚度为100nm和500nm的SiO2薄膜水平方向上的热扩散率，实验结果表明该结构的热扩散率比SiO2体材料的要小，并且随着SiO2层厚度的减小，热扩散率也减少。     

Simultaneous measurement of thermal conductivity and thermal diffusivity of solids by the parallel-wire method

An improved parallel-wire technique for simultaneous measurement of thermal conductivity and thermal diffusivity is presented. The deviation between experimental results and recommended (or another author's) values is less than 5% for fused quartz and refractory brick.     

Application of infrared thermography and geophysical methods for defect detection in architectural structures

The scope of the present study was a multi-methodological approach to non-destructive evaluation of architectural structures. Three different techniques such as infrared thermography, ultrasonics and electric-type geophysical methods were analysed to acquire information for a synergic use of the different methods, which may be useful for the estimation of the buildings degradation sources. The investigation was carried out in laboratory by considering specimens, which were made of a plaster layer over a support of marble, brick, or tuff to simulate masonry structures. Air bubbles were intentionally created inside specimens to simulate detachments. Examples of in situ applications are also reported. In particular, infrared thermography was used for the inspection of the status of the tiles covering the walls of a building.     

Damage evolution and infrared thermography in woven composite laminates under fatigue loading

An analytical model based on cumulative damage has been used for predicting the damage evolution in composite materials. The model is verified with experimental data from a carbon/epoxy composite fatigued under tension–tension load. Fatigue tests of specimens have been monitored with an infra-red thermography system. By analysing the temperature of the external surface during the application of cyclic loading, it is possible to evaluate the damage evolution. The model agrees well with the experimental data, and it can be used to predict the evolution of damage in composites.     

Measurement of thermal diffusivity and anisotropy of plasma-sprayed coatings

The thermal diffusivity of free-standing tungsten and zirconia plasma-sprayed coatings was measured in the directions parallel and perpendicular to their surface. The parallel thermal diffusivity was evaluated by a double-sensing Laplace-transform technique and compared to the perpendicular values obtained by the (lash technique. Ratios between the parallel and the perpendicular thermal diffusivity values were in the range of 1.1 to 1.5 for zirconia and 4 to 6 for tungsten. The results are discussed in terms of the coating thickness and microstructure.On leave from Laboratoire d'Énergetique et de Méchanique Théorique et Appliquée, B.P. 160, 54504 Vandoeuvre les Nancy, France.     

Cyclic hardening/softening behaviour in AZ31B magnesium alloy based on infrared thermography

The work-hardening/softening behaviour of AZ31B magnesium alloy during high cycle fatigue was investigated. The superficial temperature evolution during fatigue tests was used as a criterion for the different levels of work-hardening/softening. The microstructures under different cycles were observed by transmission electron microscope. Tensile test (with post-fatigue) was conducted to quantify the work-hardening/softening behaviour which showed that high dislocation density after cyclic loading lead to high tensile strength. The temperature evolution of the specimens with different levels of work-hardening/softening during tensile tests is related to the microstructures; the results indicated that the temperature rise of the specimen with high density dislocation was lower. Microstructures after tensile tests showed that high dislocation density after cyclic loading would lead to high twinning density.     

A review of factors influencing defect detection in infrared thermography: Applications to coated materials

Infrared thermography is a technique that is used to nondestructively inspect parts for the presence of subsurface defects. The technique normally consists of applying heat to one surface of the part and observing the thermal response, using heat-sensing devices such as infrared cameras, as the part cools. Internal defects such as voids modify the thermal response and produce local hot or cold spots on the specimen surface. For the detection of subsurface defects, the sensitivity of the technique to different parameters such as defect depth, material properties, and heating methods has not been established due in part to the complex nature of the heat/flaw interaction. A finite element model is used here to examine the influence of these parameters on defect dectability. The model shows that the defect detectability decreases with increasing defect depth beneath the surface, and that the technique is most sensitive to the inspection of low thermal diffusivity coatings bonded to high thermal diffusivity substrates. The results also show that the heat pulse duration should be made as short as possible to maximize defect detectability.     

Application of laser flash method to penetrative materials for measurement of thermal diffusivity

The theoretical equation for the laser flash method is derived where the penetration and transmission of the input energy into the test piece is taken into account. The thermal diffusivity of penetrative materials and its measurement errors are calculated instantly by fitting the experimental data to this equation by using the method of least squares with an on-line personal computer connected to the laser flash device. The usefulness of this method is shown by comparing it with the conventional method.     

Measurement and evaluation of the thermal diffusivity of two-layered materials

Transient methods, such as those with pulse- or step-wise heating, have often been used to measure thermal diffusivity of various materials including layered composite materials. The aim of the present study is to investigate effects of various parameters on the measurement of thermal diffusivity when the transient methods are applied. Mainly a two-layered material in the pulsewise heating method is considered because of its simplicity and usefulness in identifying and determining the effects of the parameters. First, it has been shown that there exists a special condition for determining the thermal diffusivity of a component in the two-layered material whose other relevant thermophysical properties are known. Second, it has been shown that the thickness of the laserbeam absorption layer, which inevitably makes sample material into the twolayered material, may cause a relatively large error when the thermal diffusivity of the base material is high. Finally, it has been derived a definite relation between the apparent thermal diffusivity obtained from the temperature response and the mean thermal diffusivity, which has a physical meaning related to the thermal resistance.     

Defect detection in wire welded joints using thermography investigations

The formation of gas voids inside the wire joints during laser welding may cause internal defects (cracks and porosity), that are invisible from outside. Authors propose the application of active thermography for detection of such defects. Thermal camera was used to acquire sequences of thermograms showing the joints during transient heating. Fourier analysis enabled phase value calculation, which is different for defective and non-defective samples. Laboratory results were confirmed by simulations on prepared two-dimensional model.     

Remote nondestructive evaluation technique using infrared thermography for fatigue cracks in steel bridges

Long‐standing infrastructure is subject to structural deterioration. In this respect, steel bridges suffer fatigue cracks, which necessitate immediate inspection, structural integrity evaluation or repair. However, the inaccessibility of such structures makes inspection time consuming and labour intensive. Therefore, there is an urgent need for developing high‐performance nondestructive evaluation (NDE) methods to assist in effective maintenance of such structures. Recently, use of infrared cameras in nondestructive testing has been attracting increasing interest, as they provide highly efficient remote and wide area measurements. This paper first reviews the current situation of nondestructive inspection techniques used for fatigue crack detection in steel bridges, and then presents remote NDE techniques using infrared thermography developed by the author for fatigue crack detection and structural integrity assessments. Furthermore, results of applying fatigue crack evaluation to a steel bridge using the newly developed NDE techniques are presented.     

An experimental analysis of fatigue behavior of AZ31B magnesium alloy welded joint based on infrared thermography

The fatigue behavior of AZ31B magnesium alloy welded joint during high cycle fatigue test was investigated by infrared thermography. Five stages of superficial temperature evolution were observed: an initial temperature increase, a temperature decline, a temperature equilibrium, an abrupt temperature increase and a temperature drop after the failure. The theoretical models were formulated to explain the observed temperature evolution. The mean temperature decline caused by thermoelastic effect was observed and discussed when the maximum stresses were below 30 MPa. The influence of weld reinforcement on fatigue behavior was also investigated. A good precision was achieved in fatigue strength prediction by means of infrared thermography.     

Isothermal thermal diffusivity of iron oxide

Using the flash technique, the thermal diffusivity of iron oxide has been measured as a function of time at temperatures ranging from 623 to 753 K to study the isothermal decomposition of wustite to magnetite and iron. The results are briefly discussed in terms of transformation kinetics and it is shown that the data are consistent with the growth of a fixed number of nuclei, all of which are present at the start of transformation.     

Applying infrared thermography to analyse martensitic microstructures in a Cu-Al-Be shape-memory alloy subjected to a cyclic loading

This study aims at revealing martensitic microstructures that exist in a shape-memory alloy. Infrared thermography was used for this purpose. Experiments were performed on a Cu-Al-Be single crystal which features a pseudoelastic response at ambient temperature. The specimen was first partially transformed to martensite by mechanical loading. Then a small cyclic loading was applied while the temperature evolution on the specimen surface was captured by an infrared camera. Thermal images obtained were then processed to extract two types of quantities: the maps of heat sources produced by the material and the maps of temperature oscillation amplitudes. Two thermomechanical couplings are revealed: the thermoelastic coupling and the latent heat due to the small cyclic movement of austenite-martensite interfaces, thus highlighting the martensitic microstructure distribution in the specimen.