Hardware and Software for Thermal Nondestructive Testing of Metallic and Composite Materials

Modeling and processing software, as well as experimental units, developed at Tomsk Polytechnic University for the last decade in the field of thermal/infrared nondestructive testing, are shortly described in this paper along with some illustrations of using this technique in the detection of impact damage in composites and corrosion in metals

Comparing thermal stimulation techniques in infrared thermographic inspection of corrosion in steel

Remote detection of corrosion in metals is a developing application area of active thermal nondestructive testing. In this study, emphasis is made on the optimization of heating techniques that is of a major interest in practical surveys. Some popular data processing techniques, such as Fourier transform, correlation and principal component analysis, are also quantitatively compared in application to corrosion detection in 1-2 mm thick steel by applying a criterion of signal-to-noise ratio. The best inspection results have been obtained by using powerful halogen lamps and air blowers. Material loss of about 25 % with lateral dimensions greater than 10x10 mm can be reliably identified in practical tests. The use of Xenon flash tubes is inefficient because of significant steel thickness. LED panels have not provided expected results due to low absorption of LED quasi-monochromatic radiation

Evaluation of equivalent defect heat generation in carbon epoxy composite under powerful ultrasonic stimulation by using infrared thermography

Low velocity impact is a frequently observed event during the operation of an aircraft composite structure. This type of damage is aptly called as "blind-side impact damage" as it is barely visible as a dent on the impacted surface, but may produce extended delaminations closer to the rear surface. One-sided thermal nondestructive testing is considered as a promising technique for detecting impact damage but because of diffusive nature of optical thermal signals there is drop in detectability of deeper subsurface defects. Ultrasonic Infrared thermography is a potentially attractive nondestructive evaluation technique used to detect the defects through observation of vibration-induced heat generation. Evaluation of the energy released by such defects is a challenging task. In this study, the thin delaminations caused by impact damage in composites and which are subjected to ultrasonic excitation are considered as local heat sources. The actual impact damage in a carbon epoxy composite which was detected by applying a magnetostrictive ultrasonic device is then modeled as a pyramid-like defect with a set of delaminations acting as an air-filled heat sources. The temperature rise expected on the surface of the specimen was achieved by varying energy contribution from each delamination through trial and error. Finally, by comparing the experimental temperature elevations in defective area with the results of temperature simulations, we estimated the energy generated by each defect and defect power of impact damage as a whole. The results show good correlation between simulations and measurements, thus validating the simulation approach

Photothermocapillary Method for the Nondestructive Testing of Solid Materials and Thin Coatings

The photothermocapillary (PTC) effect is a deformation of the free surface of a thin liquid layer on a solid material that is caused by the dependence of the coefficient of surface tension on temperature. The PTC effect is highly sensitive to variations in the thermal conductivity of solids, and this is the basis for PTC techniques in the non-destructive testing of solid non-porous materials. These techniques analyze thermal conductivity and detect subsurface defects, evaluate the thickness of thin varnish-and-paint coatings (VPC), and detect air-filled voids between coatings and metal substrates. In this study, the PTC effect was excited by a “pumped” Helium-Neon laser, which provided the monochromatic light source that is required to produce optical interference patterns. The light of a small-diameter laser beam was reflected from a liquid surface, which was contoured by liquid capillary action and variations in the surface tension. A typical contour produces an interference pattern of concentric rings with a bright and wide outer ring. The minimal or maximal diameter of this pattern was designated as the PTC response. The PTC technique was evaluated to monitor the thickness of VPCs on thermally conductive solid materials. The same PTC technique has been used to measure the thickness of air-filled delaminations between a metal substrate and a coating

Inspecting aviation composites at the stage of airplane manufacturing by applying 'classical' active thermal NDT, ultrasonic thermography and laser vibrometry

The results of applying three nondestructive testing techniques to the inspection of parts of a new Russian TVS-2DTS airplane made of carbon fiber reinforced plastic are presented. A basic technique implemented in workshop conditions implements optical stimulation of inspected parts. The usefulness of ultrasonic infrared thermography combined with laser vibrometry in the evaluation of parts with complicated geometry is illustrated. Samples with artificial and real defects have been tested in workshop conditions

Defining the Thermal Features of Sub-Surface Reinforcing Fibres in Non-Polluting Thermo–Acoustic Insulating Panels: A Numerical–Thermographic–Segmentation Approach

Natural fibres present ozone-friendly solutions in the field of construction. The attenuation of the sound and heat losses is an important feature in such type of materials above all, when used in non-woven fabrics and fibre-reinforced composites. Hemp fibres show robust insulation performance; this research work should be considered beneficial to the development of a non-destructive thermographic methodology, which can address the thermal barrier (typically applied on multi-layer panel) effects. The intent is to assess the integrity of the sub-surface reinforcing glass fibres; such integrity state will help confer the rigidity and the resistance to mechanical stresses. The testing proposed in this study can be further developed in a laboratory right after the manufacturing process of similar type of components. The testing needs preliminary numerical simulations to help guide the selection of the appropriate pre- and post-processing algorithms combined with or without segmentation operators. A set of numerical and experimental tests were performed through controlled thermal stimulation while recording the thermal responses. The study also highlights the advantages, disadvantages, and future development of the presented technique and methodologies

Evaluating the Quality of Reinforced Concrete Electric Railway Poles by Thermal Nondestructive Testing

Thermal nondestructive testing can be used to inspect reinforced concrete supports that are widely used in various industries. Corrosion damage is a typical defect found in these supports. Corrosion usually starts as a separation between the concrete and the steel rebar. Damage is exacerbated by pressure that is caused by the formation of corrosion products. The most logical method for using IR to detect corrosion or a disbond would be to heat up the rebar by resistive or inductive heating. In both cases, monitoring the dynamic temperature distributions on the pole allows for the evaluation of reinforcement quality. The thermal properties of steel, concrete, air, and corrosion products differ greatly. The magnitude of temperature anomalies and their behavior over time depend on the presence of corrosion products, air gaps, and the quality of contact between rebar and concrete

Evaluating Thermal Properties of Sugarcane Bagasse-Based Composites by Using Active Infrared Thermography and Terahertz Imaging

This study is devoted to the analysis of impact damage in composite samples by applying techniques of IR and Terahertz imaging. The samples made of high-density homogeneous particleboards of sugarcane bagasse and castor oil polyurethane resin were subjected to impacts with the energy from 5 to 30 J. The impact damage defects were evaluated by applying one- and two-sided thermal non-destructive testing. Both the apparent thermal effusivity and diffusivity were determined to analyse their relationship to impact energy. In the above mentioned range of impact energies, the thermal effusivity varied from 5 to 18%, while the variations in thermal diffusivity were from 4 to 24%. The algorithm of dynamic thermal tomography was used to demonstrate that predominant subsurface cracking occurred at depths up to 1 mm. In addition, the defect areas were tested by applying the Terahertz technique, whose results qualitatively matched the IR thermographic ones to show that the damaged areas were larger than they appeared visually. However, both inspection techniques have revealed no significant dependence of analysed parameters on impact energy. This is believed to prove that non-uniform composite structure is the decisive factor in producing structural defects under impacting