8.3 Thermografische Verfahren
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Thanks to its good strength/mass ratio, a glass fibre reinforced plastic (GFRP) composite is a common material widely used in aviation, power production, automotive and other industries. In its turn, active infrared (IR) nondestructive testing (NDT) is a common inspection technique for detecting and characterizing structural defects in GFRP. Materials to be tested are typically subjected to optical heating which is supposed to occur on the material surface. However, GFRP composite is semitransparent for optical radiation of both visual and IR spectral bands. Correspondingly, the inspection process represents a certain combination of both optical and thermal phenomena. Therefore, the known characterization algorithms based on pure heat diffusion cannot be applied to semi-transparent materials. In this study, the phenomenon of GFRP semi-transparency has been investigated numerically and experimentally in application to thermal NDT. Both Xenon flash tubes and a laser have been used for thermal stimulation of opaque and semi-transparent test objects. It has been shown that the Penetration of optical heating radiation into composite reduces detectability of shallower defects, and the signal-to-noise ratio can be enhanced by applying the technique of thermographic signal reconstruction (TSR). In the inspection of the semi-transparent GFRP composite, the most efficient has been the laser heating followed by the TSR data processing. The perspectives of defect characterization of semi-transparent materials by using laser heating are discussed. A neural network has been used as a candidate tool for evaluating defect depth in composite materials, but its training should be performed in identical with testing conditions.
Material defects in fiber reinforced polymers such as delaminations can rapidly degrade the material properties or can lead to the failure of a component. Pulse thermography (PT) has proven to be a valuable tool to identify and quantify such defects in opaque materials. However, quantification of delaminations within semitransparent materials is extremely challenging. We present an approach to quantify delaminations within materials being semitransparent within the wavelength ranges of the optical excitation sources as well as of the infrared (IR) camera. PT experimental data of a glass fiber reinforced polymer with a real delamination within the material were reconstructed by one dimensional (1D) mathematical models. These models describe the heat diffusion within the material and consider semitransparency to the excitation source as well to the IR camera, thermal losses at the samples surfaces and a thermal contact resistance between the two layers describing the delamination. By fitting the models to the PT data, we were able to determine the depth of the delamination very accurately. Additionally, we analyzed synthetic PT data from a 2D simulation with our 1D-models to show how the thermal contact resistance is influenced by lateral heat flow within the material.
Pulsed thermography is a well-known non-destructive testing technique and has proven to be a valuable tool for examination of material defects, to determine thermal material parameters, and the thickness of test specimens through calibration or mathematical models. However, the application to semitransparent materials is quite new and demanding, especially for semitransparent materials like epoxy, polyamide 12, or glass fiber reinforced polymers with epoxy or polyamide matrix.
In order to describe the temporal temperature evolution in such materials, which are recorded with an infrared camera during pulse thermography experiments, much more influences have to be considered, compared to opaque materials:
- The wavelength of the excitation source and the spectral range of the infrared camera
- The angles between the specimen, the excitation source and the infrared camera
- The area behind the specimen
- The roughness of the material surface
- The scattering mechanism within the material
Here, we will consider all these influences and describe how they can be treated mathematically in analytical or numerical models (using COMSOL Multiphysics software). These models describe the temperature development during the pulse thermography experiment in reflection and transmission configuration. By fitting the results of the mathematical models to experimental data it is possible to determine the thickness or the optical and thermal properties of the specimen.
Active thermography is a well suited non-destructive testing method for the challenging inspection of wind rotor blades. Since the GFRP structures are up to some centimetres thick, long pulse heating is required to provide an appropriate energy input into the structure. So far, no best practice exists to guarantee a reliable detection of deep-lying flaws. In this work, a step wedge specimen having a maximum thickness of 34mm is systematically investigated by experiment and well-matched simulations to assess the influence of the experimental parameters, like the absorbed energy, on thermal contrasts. Finally, a scheme to conduct full-scale test of a wind rotor blade in less than three hours is proposed.
Impact damages and delaminations in fibre-reinforced composites (FRC) might not be visible at the surface, but could have an influence on the resistance and on the long-term behaviour of the component. Therefore, and especially for safety relevant structures, non-destructive methods are required for the assessment of such damages.
Active thermography methods are suitable to characterize damages after loading using different kind of excitation techniques and various configurations of infrared (IR) camera and heating sources. Here, flash lamps, impulse excitation with infrared radiator and lock-in technique with halogen lamps or widened laser beams are suited. In addition, non-optical sources like sonotrodes (requiring direct contact to the structure) or induction generators (only suited for carbon fibre reinforced polymer (CFRP) structures) could be applied as well. For the investigation of the evolution of the damage during the impact, passive thermography can be applied in-situ. Elastic and plastic deformations alter the temperature of the structure and thus the temperature on the surface.
In this contribution, at first the general principles of quantitative defect characterisation in FRC using active thermography with flash, impulse and lock-in excitation are described. Optical and thermal properties of the FRC material and its anisotropy are considered. Results of phase differences obtained at flat bottom holes with flash and lock-in thermography are compared for qualifying both methods for quantitative defect characterization. Secondly, the damage evolution of CFRP and GFRP structures under impact load and static tensile loading is described. The spatial and temporal evolution of the surface temperature enables us to distinguish matrix cracks or fibre-matrix separation from delaminations between the layers. Afterwards, all results for loading defects, obtained by passive and active thermography, are compared with each other. Fig. 1 and 2 show the difference of passive and flash thermography obtained at impact and tensile loaded CFRP plates, respectively. As one purpose of these investigations is the development of standards within national (DIN) and European (CEN) standardisation bodies, new draft and final standards are presented and further needs are discussed at the end of the presentation.
Quantification of impact damages in CFRP and GFRP structures with thermography and ultrasonics
(2018)
The extent of damage caused by impacts in fibre reinforced composites depends on the energy of the impacts, on the velocity and the shape of the impacting body, on the material and structure of the composite and on the geometry of the structure. Here, mainly the thickness of the component is essential. The non-destructive evaluation of these damages can be carried out using both ultrasound and active thermography methods. A comparison of the detection sensitivity of these methods for the different damages is carried out in this paper depending on the fibre composite material used (CFRP and GFRP), the thickness of the material and the impact energy. The NDT methods used after the damage are supplemented by thermographic measurements with high temporal resolution, which were already recorded during the impact.
Quantification of impact damages in CFRP and GFRP structures with thermography and ultrasonics
(2018)
For the quantification of impact damage in CFRP and GFRP structures, first passive thermography measurements have been performed at the front and rear side of the test specimens during low velocity impact load. After loading, the damaged structures have been investigated non-destructively with active thermography with flash excitation and with step heating and with two different ultrasonic methods (phased array and air coupled ultrasonics). The damage sizes quantified with all methods are evaluated and compared. In addition, the depth of the greatest damage inside the different test specimens is analysed.
For assuring the safety and reliability of components and constructions in energy applications made of fiber-reinforced polymers (e.g., blades of wind turbines and tidal power plants, engine chassis, flexible oil and gas pipelines) innovative non-destructive testing methods are required. Within the European Project VITCEA complementary methods (shearography, microwave, ultrasonics and thermography) have been further developed and validated. Together with partners from the industry, test specimens have been constructed and selected on-site containing different artificial and natural defect artefacts. As base materials, carbon and glass fibers in different orientations and layering embedded in different matrix materials (epoxy, polyamide) have been considered. In this contribution, the validation of flash and lock-in thermography to these testing problems is presented. Data analysis is based on thermal contrasts and phase evaluation techniques. Experimental data are compared to analytical and numerical models. Among others, the influence of two different types of artificial defects (flat bottom holes and delaminations) with varying diameters and depths and of two different materials (CFRP and GFRP) with unidirectional and quasi-isotropic fiber alignment is discussed.
Pulse thermography (PT) has proven to be a valuable non-destructive testing method to identify and quantify defects in fiber-reinforced polymers. To perform a quantitative defect characterization, the heat diffusion within the material as well as the material parameters must be known. The heterogeneous material structure of glass fiber-reinforced polymers (GFRP) as well as the semitransparency of the material for optical excitation sources of PT is still challenging. For homogeneous semitransparent materials, 1D analytical models describing the temperature distribution are available.
Here, we present an analytical approach to model PT for laterally inhomogeneous semitransparent materials.We show the validity of the model by considering different configurations of the optical heating source, the IR camera, and the differently coated GFRP sample. The model considers the lateral inhomogeneity of the semitransparency by an additional absorption coefficient. It includes additional effects such as thermal losses at the samples surfaces, multilayer systems with thermal contact resistance, and a finite duration of the heating pulse. By using a sufficient complexity of the analytical model, similar values of the material parameters were found for all six investigated configurations by numerical fitting.