Enhanced photothermal parameter estimation of thick CFRP in reflection mode for rectangular pulse excitation by Halogen lamps

Abstract

In this paper, an improved estimation of the thermal diffusion time for thick (6.67 mm < L < 9.38 mm) monolithic CFRP is shown for a single-sided thermographic experiment. The Virtual Wave Concept, a local transformation of the measured temperature data, enables the application of ultrasonic evaluation methods also for arbitrary heating functions. Frequently, in optical excited pulsed thermography, the amount of energy absorbed by the specimen is limited by the flash lamp. To overcome this problem, we use cost-efficient Halogen lamps for optical excitation, where for constant power density the amount of energy introduced into the component is optimized by varying the pulse duration. The radiation emitted by Halogen lamps partly overlaps with the sensitivity range of quantum detector cameras. We propose a novel concept of a Spectral Fluid Filter based on the absorption properties of water to avoid undesired radiation and to enable the optical excitation of the sample simultaneously to the IR measurement. The estimation of the thermal diffusion time, based on a Time-of-Flight measurement of the virtual wave for excitation by Halogen lamps, requires prior knowledge of the applied heating function. For this purpose, we present an electronic circuit based on a monolithic photodiode for data acquisition as well as a model-based approach for the intensity of the emitted radiation of Halogen lamps. As excitation with Halogen lamps provides a much more robust signal-to-noise ratio, the standard deviation of the thermal diffusion time is significantly reduced and more accurate results are obtained than by photothermal state-of-the-art reflection mode methods.