Photothermal porosity estimation in carbon fiber reinforced plastics based on the virtual wave concept

High strength and light weight, justify the frequent use of carbon fibre reinforced plastics for aeronautical applications. The manufacturing process of such material systems is a multi-stage process and susceptible to the formation of air-filled voids. This porosity weakens the epoxy matrix and causes noticeable degradation of mechanical properties. Active thermography with optical-excitation is an advantageous photothermal method because due to the infrared camera it is a non-contacting, fast testing method for the estimation of material properties or for defect detection. We use the Virtual Wave Concept, which allows ultrasonic testing methods for photothermal measurement data. Based on this ability, we apply the through-transmission method to determine the Time-of-Flight of virtual waves, which is directly related to the porosity dependent diffusion time. A signalto-noise ratio dependent approach is used for the temporal truncation of measurement data to get the optimum evaluation time. This ensures to evaluate only time-ranges which contain information of the heat diffusion inside the sample. In addition, undesired effects of heat losses due to convection and radiation are reduced. After the evaluation procedure is shown for simulated data, we demonstrate the experimental pixel-wise estimation of the porosity affected thermal diffusion times on a real aerospace part in transmission configuration. The results are validated by X-ray computed tomography reference measurements, where a good match can be achieved with active thermography results.