DescriptionActive Thermography is increasingly used as method for non-destructive evaluation in recent years. Its main application is in the field of defect detection, nevertheless the method is used also forthe visualization and determination of material properties – at least qualitatively. The measurements can be done on the rear side of the sample, opposite of the excitation (transmission mode) or on the front side (reflection mode). Transmission mode measurements work very well with respect to reproducibility and accuracy, in particular if Parker’s method oder the Linear Diffusivity Fitting (LDF)-method are used. In industrial applications, however, reflection mode measurements are preferred in most cases. Unfortunately, the results in reflection mode measurements scatter muchmore as compared to transmission mode measurements, at least, if material properties are under investigation. As preferred methods for the former Thermal Signal Reconstruction (TSR) in combination with fitting procedures are used frequently. The limitations and shortcomings of those methods suggest methodological enhancements. We present a new method, well suited for reflection mode measurements, for the determination of the effusivity of polymers. The method is based on Active Thermography, which is excited by laserlight (870 nm). The latter is modulated according to a square wave signal, which turns out to be crucial for the success of the method. The laser spot on the sample surface is about 3 cm and the thermal response is measured with help of an infrared camera with a spatial resolution of 50 µm. The thermal transients are transferred to the frequency domain by means of a numerical Laplace Transformation. After normalization of the Laplace-transformed signal the effusivity of the illuminated samples can be determined after a few evaluation steps unequivocally, without fitting and independent of the value of heat transfer. The method works for a sample thickness bigger than 1.5 mm. The validity of the method has been tested with help of simulations, which served as artificial measurement signals. Furthermore, the method has been tested also with help of thermography measurements applied on specimens of pure polymers and of Carbon Fibres Reinforced Polymers (CFRP). The CFRP samples have also been measured by X-ray Computed Tomography, which served as a reference method for a porosity determination. The examinations indicate that our proposed new approach may serve as a fast, robust and accurate method for the determination of thermal properties, which seems well suited for industrial applications. Subsequently, from the thermal properties also other material properties like porosity can be determined.
|Period||16 Jul 2017|
|Event title||44th Annual Review of Progress in Quantitative Nondestructive Evaluation (QNDE), Atlanta: null|
|Location||Provo, Utah, United States|