Thermographic porosity estimation in composite structures using virtual A-scans

Description

In this work, the quantitative evaluation of porosity in carbon fiber reinforced plastics (CFRP) with pulsed thermography data is shown by applying the Virtual Wave Concept (VWC). Therefore, a virtual temperature signal Tvirt is calculated by applying a local transformation at the point r to the measured surface temperature data T. This transformation is a linear inverse problem and can be formulated as a Fredholm integral of the first kind. Since the problem is severely ill-posed, the alternating direction method of multipliers (ADMM) is used as regularization technique. Prior information, such as positivity and sparsity of the virtual wave field, is included in the ADMM procedure to improve the reconstruction quality. The A-Scan representation of the analytical virtual wave and the regularized solution with the classical truncated singular value decomposition (SVD) and ADMM is shown. The measurement of the thermal diffusion time with the VWC allows a thickness estimation of a CFRP step wedge sample as well as a porosity estimation of different CFRP test coupons. The measurement uncertainty of the VWC method was determined from the measurement data of the step wedge. Furthermore, effective medium theories are used to derive the porosity from the estimated thermal diffusivity on a range of different calibrated CFRP specimens. All thermography results are validated with X-ray computed tomography. The main advantage of the VWC is the possibility to use the same data algorithm for pulsed thermography measurements in reflection as well as in transmission configuration for parameter estimation.

Period	9 Jul 2019
Event title	20th International Conference on Photoacoustic and Photothermal Phenomena
Event type	Conference
Location	Moscow, Russian FederationShow on map