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For assessing building façades using active thermography, in case of direct solar exposure the sun itself can be used as heat source. It is shown that active thermography can be applied to large areas successfully, if a shadow cast occurs. After performing a sequence reconstruction in order to correct the temporal behaviour of shadow movement, the data could be analysed by pulse phase thermography (PPT). Compared to raw thermograms, the obtained phase images display an improved image quality with a lot of details. The frequency of the phase images is related to the probing depth. The presented case study describes the thermographic investigation of an historical building façade, where an area of 17 m x 13 m has been investigated.
Up to now, the inspection of wind turbines with industrial climbers has been considered ”state of the art”. However, ever-larger wind turbines and advancing digitization make modern and automated inspection methods indispensable. Passive thermography can serve as such a digital and atomized method while it is well known for its applications in the inspection of buildings or electrical circuits. However, its application relies on thermal gradients in the inspected object such that a temperature contrast exists between damaged and sound areas. This also holds for unheated structures like rotor blades of wind turbines which show no intrinsic temperature gradient and can hardly be heated. Under certain weather conditions with sufficient solar loading and diurnal temperature variations, passive thermography is suitable for the in-service inspection of rotor blades. However, for a reliable use of passive thermography on ”thermal passive” components, the incorporation of these environmental conditions in the planning and evaluation of thermal inspections is crucial. Additionally, the complex inner structure of wind turbine blades in comparison to other objects and buildings require a specific method referencing the individual rotor blades to each other. This allows the distinction between the thermal response of design-specific structural features and damages or irregularities between the three blades. We show thermal signatures of damage in rotor blades and contrast them with structural characteristics by comparing the three blades. In addition to measurements in industrial environments, laboratory measurements are shown and compared to simulations. The long-term goal is to simulate the influence of different weather parameters and thus gain a better understanding of measurements in the field. The results shown here can be seen as one step towards industrial application.
Additive manufacturing (AM) technologies, generally called 3D printing, are widely used because their use provides a high added value in manufacturing complex-shaped components and objects. Defects may occur within the components at different time of manufacturing, and in this regard, non-destructive techniques (NDT) represent a key tool for the quality control of AM components in many industrial fields, such as aerospace, oil and gas, and power industries. In this work, the capability of active thermography and eddy current techniques to detect real imposed defects that are representative of the laser powder bed fusion process has been investigated. A 3D complex shape of defects was revealed by a μCT investigation used as reference results for the other NDT methods. The study was focused on two different types of defects: porosities generated in keyhole mode as well as in lack of fusion mode. Different thermographic and eddy current measurements were carried out on AM samples, providing the capability to detect volumetric irregularly shaped defects using non-destructive methods.
The knowledge of the thermal parameters of a particular concrete is essential for thermal design of a building, but also could help to identify and assess the state of a concrete structure. Active thermography has the potential to be applied onsite and to provide a fast investigation of thermal properties. In this work, three different concrete samples were investigated by active thermography in reflection and in transmission setup. It was found that this method yields the same results without direct contact as the Transient Plane Source (TPS) method as an established inspection tool.
The knowledge about energy release and transient behaviour of flash lamps, used as short-term energy source in flash thermography investigations, is essential for a quantitative analysis of the results as well as for simulations regarding such experiments. Usually, only a rectangular pulse shape of a typical duration of a few milliseconds is regarded when flash lamps pulses are described. This note considers the afterglow period of a flash lamp explicitly and shows that negligence of this afterglow effect leads to a systematic underestimation of the pulse length and the related total energy release. In the reported case, the intensity transient recorded by a Silicon photo diode was transformed into a temperature transient, allowing the study of the real energy release. It turns out that approximately only 50% of the entire pulse energy were released during the usually regarded period of 2.8 ms, while the other half was released within a period of 20 ms with an exponential decay.
As groundwork for thickness determination of polymeric surface protection systems for concrete, we present a method for measuring the thickness of isolated semitransparent solids using pulse thermography both in Transmission and reflection geometry. Since standard models do not capture semitransparency, an advanced analytical model by Salazar et al. is applied. Physical material parameters are deduced by fitting experimental data from samples of well-known
thickness. Using those, the thickness of samples of the material can be obtained by fitting, as demonstrated for different semitransparent polymer materials.
Recurrent non-destructive testing inspections are necessary to prevent damages in wind turbine rotor blades, but so far, there is no established method that detects defects in blades from greater distances – although this becomes increasingly important in the context of hardly accessible offshore wind parks.
Thermography is a promising method for detecting subsurface defects, but various challenges arise when this method is applied on-site to turbine blades in operation. Disturbing influences from the environment easily lead to a misinterpretation of thermograms (i.e. thermographic images), such as thermal signatures caused by reflections, dirt and other superficial inhomogeneities.
This study explores several problems and effects that arise, when (rotating) blades are monitored with thermography.
It will then be demonstrated that a meaningful defect inspection in this scenario is essentially restricted to a procedure following three steps: Firstly, calculating the so-called difference thermograms of all blade pairs for eliminating disturbing reflections. Secondly, identifying potentially relevant signals, which are associated neither with structural features nor with dynamical effects, and the identification of these signals’ allocations (through comparison of all difference thermograms with each other). And thirdly, comparing these signals with (processed) photos for excluding incorrect indications by surface effects. Unlike common thermographic analysis methods, which typically only include an aspect of this procedure, the composition presented in this contribution constitutes an advanced technique for minimizing disturbing influences in thermograms.
The proposed thermographic technique enables the detection of potential subsurface defects within rotating rotor blades from greater distances – such as from the ground, air crafts or vessels.
Flash lamps are widely used excitation sources in the field of non-destructive testing with active thermography. Though the realized energy density in front of the investigated object is a significant factor with regard to detection sensitivity, only few data concerning this issue have been published so far. It is shown here that local energy densities can be estimated by means of a simple metal plate, which exhibits a certain temperature increase after flash excitation. After discussing the underlying calorimetric principle the sensor concept is reviewed using constant blackbody radiation and short laser pulses, since both kinds of sources generate known energy densities. The relative uncertainty of measurements of the energy density is found to be in the range of 10%. The last part of the present paper describes an application for characterizing the radiation of a usual 6 kJ flash lamp. The energy conversion efficiency was found to be only about 11%.