Virtual wave concept for thermographic porosity estimation in carbon fiber reinforced plastics

In this work the quantitative evaluation of porosity with pulsed thermography in carbon fiber reinforced plastics is shown by applying the virtual wave concept. A virtual temperature signal is calculated by applying a local transformation to the measured temperature data. This virtual temperature is a solution of the wave equation, whereby for the parameter estimation ultrasonic test methods can be used, e.g. pulse-echo method for time-of-flight measurements. The time-of-flight determined from the virtual temperature is directly related to the thermal diffusion time. Therefore, if the specimen thickness is known the effective thermal diffusivity can be carried out. The main advantage of the virtual wave concept is the possibility to use the same data algorithm for pulsed thermography measurements in reflection as well as in transmission mode for parameter estimation. The estimation of the porosity based on the virtual wave concept matches the porosity based on the well-known Parker method very well. Effective medium theories are used to derive the porosity from the estimated thermal diffusivity. In the first step, this model-based porosity evaluation is validated on a range of different calibrated porous CFRP specimens with different number of plies and varying porosity contents. Furthermore, porosity evaluation on real aerospace parts will be demonstrated. All thermography results are validated with X-ray computed tomography.