Development of improved active thermographic techniques for nondestructive inspection of building and civil engineering structures

Abstract

Recently infrared thermography has received increasing acceptance for nondestructive testing applications. It has the advantages of being full-field, non-contact and yielding quasi real-time results. Basically, thermography measures surface radiation. When surface emissions are known, surface temperature distribution can be determined. A major industrial application of active thermography is nondestructive inspection. During inspection, a thermal excitation is applied to the test object and a series of thermal images (images of the object’s surface radiation) is recorded at equal timeintervals. The presence of a material defect will interfere with the heat conduction and cause a localized temperature distribution anomaly to appear on the surface of the object being tested. A subsurface flaw is revealed by the presence of an anomalous temperature rise on its surface. This phenomenon is the cause of a transient flow of heat which is partially interfered by the flaws. The flaw shape and size can be determined by the boundary of the localized anomalous temperature distribution. Through the consideration of the time parameter and from reference calculations, its depth-location can also be estimated. Although active thermography is applicable to any material, it can yield false results on objects having an anomalous surface emissivity distribution. This is commonly found in building materials. This problem can be alleviated by a novel algorithm developed by the research described in this dissertation. The reduction of the influence of this emission is based on the idea of self-referencing. Subsequent to the thermal excitation, a series of thermal images (typically 320x240 pixels image) is sequentially digitized with time-intervals and is stored in the computer memory. The sequential data of each pixel represents the thermal history at the pixel point. The data is first reconstructed according to thermal signal reconstruction (TSR) which is based on the one-dimensional diffusion theory, therefore the temporal hardware noise can be removed. Then, the emissivity reduction algorithm is applied to process the sequential data through self-referencing. The self-referencing reconstruction process successfully diminishes the influence of emissivity, since the parameters of emissivity (temperature, wavelength, and direction) are the same at each pixel within the inspection time. Therefore, the capability of flaw detection in this active thermographic technique can be greatly enhanced by this selfreferencing algorithm. The validity of the method is experimentally verified and demonstrated by applying the method to objects of non-uniform surface emissivity, such as concrete and composite materials. Another main contribution of this thesis is the development of an electro-thermographic technique. The technique of electro-thermography is a combination of electromagnetic induction excitation and thermographic inspection techniques. This method is superior to the traditional active thermography technique using the irradiative excitation method. The electro-thermographic technique diminishes the influence of emissivity during excitation since it can directly interact with the target material. During the inspection, the eddy current is induced directly on the surface of the conducting material only. Flaw detection is based on the anomalous “hot-spots” generated by the eddy current. The eddy current density is affected by the induction magnetic flux density correlated with the object’s geometry. The penetration depth of the magnetic field can be controlled by the driving frequency. Therefore, surface, micro-surface or subsurface flaws can be identified by this technique which was developed during the research described in this thesis. For instance, electro-thermographic technique has been successfully employed in several building and civil engineering applications due to its electromagnetic features, for example in the detection of reinforcement steel bars, fiber reinforcement bonding integrity inspection, and the flaw inspection for steel rail and pressure vessels. Unlike the traditional covermeter, ground penetration radar, ultrasonic or acoustic emission measurement which requires contact and point-by-point measurement, the proposed technique is full-field and non-contact. It has the advantages of both fast inspection speed and high reliability. It is believed that the self-referencing algorithm for reducing emissivity influence and the development of electro-thermographic technique reported in this thesis will significantly advance and facilitate the application of thermography in nondestructive testing.