G. Hendorfer

Quantitative Determination of Porosity by Active Thermography

We present a new approach based on Active Thermography by which it is possible to produce images of porosity in Carbon Fibre Reinforced Plastics that correspond striking well with results obtained by ultrasonic testing. We applied the method of Pulsed Thermography by using flashes of light for the generation of heat. The evolution of surface temperatures which depends on the diffusivity of the sample and on the sample’s geometry could be well fitted by means of a heat conduction model. Images of porosity are generated by means of an evaluation by which the influence of the light intensity distribution on the data is eliminated. Thus corrections with respect to the lateral distribution of the heat generation are not necessary, nor are emissivity corrections. Corrections due to the influence of geometry, however, had to be taken into account. The quantitative evaluation of the porosity is based on its linear relation to the diffusivity. Images of porosity obtained thermographically are compared with corresp...

Wavelet-based subsurface defect characterization in pulsed phase thermography for non-destructive evaluation

Active infrared thermography is a method for non-destructive testing (NDT) of materials and components. In pulsed thermography (PT), a brief and high intensity flash is used to heat the sample. The decay of the sample surface temperature is detected and recorded by an infrared camera. Any subsurface anomaly (e.g. inclusion, delamination, etc.) gives rise to a local temperature increase (thermal contrast) on the sample surface. Conventionally, in Pulsed Phase Thermography (PPT) the analysis of PT time series is done by means of Discrete Fourier Transform producing phase images which can suppress unwanted physical effects (due to surface emissivity variations or non-uniform heating). The drawback of the Fourier-based approach is the loss of temporal information, making quantitative inversion procedures tricky (e.g. defect depth measurements). In this paper the complex Morlet-Wavelet transform is used to preserve the time information of the signal and thus provides information about the depth of a subsurface defect. Additionally, we propose to use the according phase contrast value to derive supplementary information about the thermal reflection properties at the defect interface. This provides additional information (e.g. about the thermal mismatch factor between the specimen and the defect) making interpretation of PPT results easier and perhaps unequivocal.

Analytical and numerical computations of heat transfer in pulsed thermography applied to porous CFRP

In this paper we show a detailed verification of an analytical thermal diffusivity model using finite element simulations. The real pore morphology for the simulation models are obtained by computed tomography measurements. The heat transfer by conduction is simulated in transient analyses. The thermal diffusivity values are calculated from the temperature field on the front (reflection mode) and on the back sides (transmission mode). Investigations show that the analytical model correlates well with the thermal diffusivity values calculated in transmission mode. Furthermore, the simulations show a discrepancy in the reflection mode measurements due to the stronger influence of the pore morphology. These findings are similar to those seen in pulsed thermography experiments. In conclusion, the analytical thermal diffusivity model allows a precise quantification of porosity in carbon fiber reinforced polymer structures using transmission mode measurements.

Application of wavelet analysis in active thermography for non-destructive testing of CFRP composites

Active infrared thermography is a non-destructive testing (NDT) technique used for non-contact inspection of components and materials by temporal mapping of thermal patterns by means of infrared imaging. Through the application of a short heat pulse, thermal waves of various amplitudes and frequencies are launched into the specimen allowing a signal analysis based on amplitude and phase information (pulsed phase thermography PPT). The wavelet transform (with complex wavelets) can be used with PPT data in a similar way as the classical Fourier transform however with the advantage of preserving time information of the signal which can then be correlated to defect depth, and in this way allowing a quantitative evaluation. In this paper we review the methodology of PPT and the associated signal analysis (Fourier analysis, wavelet analysis) to obtain quantitative defect depth information. We compare and discuss the results of thermal FEM simulations with experimental data and show the advantages of wavelet based signal analysis for defect depth measurements and material characterization.

Characterization of defects in curved carbon fiber reinforced plastics using pulsed thermography

Results obtained from pulsed thermography on curved specimens made out of carbon fiber reinforced plastics are presented. In the specimens, inclusions of Teflon stripes with different sizes and orientation are positioned at different depths. Especially in the curved regions, it is important to separate defect-related effects on the surface temperature from effects due to the complex sample shape like curvatures or edges. The usage of 3D finite element simulation, taking into account anisotropic heat conduction and inhomogeneous heat excitation as well as orientation dependent heat absorption, makes it possible to interpret the thermographic result in order to reduce geometry effects.

Comparative defect evaluation of aircraft components by active thermography

Active Thermography has become a powerful tool in the field of non-destructive testing (NDT) in recent years. This infrared thermal imaging technique is used for non-contact inspection of materials and components by visualizing thermal surface contrasts after a thermal excitation. The imaging modality combined with the possibility of detecting and characterizing flaws as well as determining material properties makes Active Thermography a fast and robust testing method even in industrial-/production environments. Nevertheless, depending on the kind of defect (thermal properties, size, depth) and sample material (CFRP carbon fiber reinforced plastics, metal, glass fiber) or sample structure (honeycomb, composite layers, foam), active thermography can sometimes produce equivocal results or completely fails in certain test situations. The aim of this paper is to present examples of results of Active Thermography methods conducted on aircraft components compared to various other (imaging) NDT techniques, namely digital shearography, industrial x-ray imaging and 3D-computed tomography. In particular we focus on detection limits of thermal methods compared to the above-mentioned NDT methods with regard to: porosity characterization in CFRP, detection of delamination, detection of inclusions and characterization of glass fiber distributions.

Temperature mapping in heat treatment process with a standard color-video camera by means of image processing

Thermal imaging systems are usually based on IR-camera systems analyzing the infrared part of the electromagnetic spectrum to carry out temperature measurements. These systems are very accurate but also very expensive. A “low-cost” thermal-vision-system based on a standard CCD color video camera in combination with an image processing system is presented. The system was developed to visualize the temperature distribution inside a plasma-reactor. The temperature of the heat-treated work pieces lies in the range of 450°C to 650°C. By analyzing the thermal light emission of these objects a temperature map can be made to visualize and measure temperature differences of the reactor interior.

Estimation of material parameters from pulse phase thermography data

The use of new materials and new production techniques enables the industrial production of the ever more complex parts. This increases the requirements to the non-destructive testing methods. Previous studies have shown that pulse-phase thermography (PPT) is a fast and effective method to detect flaws in Carbon Fiber Reinforced Plastic (CFRP) parts. Currently PPT is mostly used for qualitative analysis. This study shows a possibility to estimate material parameters from PPT data. To avoid distortions which may occur by the use of DFT, a new method to transform recorded data from the time domain into the frequency domain will be elucidated in this study. Material parameter can be estimated by fitting the phase curve of the analytical solution of the heat conduction equation to the phase curve of recorded data. The capability of this material parameter estimation procedure was tested on finite element simulations and on experimental data.

Prospects and limitations of digital Shearography and Active Thermography in finding and rating flaws in CFRP sandwich parts with honeycomb core

This work shows the prospects and limitations of the non-destructive testing methods Digital Shearography and Active Thermography when applied to CFRP sandwich parts with honeycomb cores. Two specimens with different core materials (aluminum, NOMEX) and artificial flaws such as delaminations, disbonds and inclusions of foreign material, are tested with Digital Shearography and Pulse Thermography including Pulse Phase Thermography. Both methods provide a good ability for finding and rating the flaws.

SIZE AND DEPTH DETERMINATION OF DEFECTS IN PLASTIC MATERIALS, ESPECIALLY IN CFRP, BY MEANS OF SHEAROGRAPHY

Interferometric methods are known to be very sensitive, allowing metrology with resolutions better than the wavelength of the light used for illumination. On the other hand, those methods are susceptible to environmental and mechanical noise, usually. We use Shearography, a version of Speckle interferometry, which, in contrast, is a robust method, resistant to noise and vibrations and compatible to industrial applications. We survey thermally‐induced Shearography in order to detect defects in plastic materials, especially in carbon fiber reinforced plastics. We show that by analyzing out‐of‐plane‐deformations, it is possible to evaluate those data quantitatively, enabling the determination of the size as well as the depth of defects. The method of depth determination is based on a gray‐scale evaluation with respect to the deformations induced. It has been applied for defects localized in depths up to 10 mm, so far. The method of size determination is based on modeling the dependence of the apparent defect...