Porosity is an unavoidable defect in carbon fiber reinforced polymers and has noticeable effects on mechanical properties since gas filled voids weaken the epoxy matrix. Pulsed thermography is advantageous because it is a non-contacting, non-destructive and fast photothermal testing method that allows the estimation of material parameters. Using the Virtual Wave Concept for thermography data, ultrasonic evaluation methods are applicable. In this work, the pulse-echo method for Time-of-Flight measurements is used, whereby the determined Time-of-Flight is directly related to the thermal diffusion time of the examined material. We introduce a signal-to-noise dependent approach, the optimum evaluation time, for evaluating only relevant time ranges which contain information of heat diffusion. After the validation of the method for heterogeneous materials, effective medium theories can be used for quantitative porosity estimation from the estimated diffusion time. This model-based approach for porosity estimation delivers more accurate results for transmission and reflection configuration measurements compared to thermographic state-of-the-art methods. The results are validated by X-ray computed tomography reference measurements on a wide range of different porous carbon fiber reinforced plastic specimens with different number of plies and varying porosity contents.

High strength and low density make epoxy-based CFRP a highly interesting construction material for the aerospace manufacturing industry. Porosity represents an unavoidable defect and significantly weakens strength values dominated by the matrix. To evaluate the quality of safety-relevant components, non-destructive evaluation and thus the characterization of porous structures is indispensable. Pulsed thermography represents a fast, large-area and non-contact testing method that enables efficient estimation of material parameters. In this work, the authors demonstrate the quantitative estimation of porosity by pulsed thermography on a multi-axial laminate fabricated from unidirectional Prepregs for the first time. The characteristic, extensive expansion of the pores in fiber direction, is addressed by the 3D microstructure characterization of Cone beam X-ray computed tomography data. Hence, the application of effective medium theories and thus the model based porosity estimation is enabled. After the investigation of the effect of pore expansion on the effective thermal diffusivity in 3D finite element simulations, the quantitative photothermal porosity estimation on a sample with a global volume porosity of $$\Phi =1.51\%$$Φ = 1.51 % is demonstrated. The accuracy of this fast and non-contact method for porosity estimation with pulsed thermography ( $$\Delta \Phi =0.63\%$$Δ Φ = 0.63 % ) is comparable to the standard ultrasonic method. Consequently, an efficient estimation of porosity for large, complex shaped UD/Epoxy composite components is enabled.

In this work, the Virtual Wave Concept (VWC) is applied on measurement data from pulsed thermography. Therefore, a virtual temperature signal is calculated by applying a local transformation to the temperature data. This transformation is a linear inverse problem and can be formulated as a Fredholm integral of the first kind. The propagation of the virtual temperature can be described by the wave equation, whereby for the parameter estimation ultrasonic test methods can be used, e.g. pulse-echo method for time-of-flight measurements. If the specimen thickness is known, the effective thermal diffusivity can be derived from the virtual back wall echo. The VWC can be applied in reflection as well as in transmission configuration. Several experiments are carried out on carbon fiber reinforced plastics. The measurement of the thermal diffusion time with the VWC allows a thickness estimation of a step wedge sample as well as a porosity estimation of different test coupons.