Recurrence quantification analysis as an approach for ultrasonic testing of porous carbon fibre reinforced polymers

Abstract

A first investigation of Recurrence Quantification Analysis (RQA) for the assessment of porosity in ultrasonic testing of Carbon Fibre Reinforced Polymers (CFRP) is presented. The standard method for detecting porosity with Non-Destructive Testing (NDT) using ultrasonic pulse-echo inspection is the evaluation of the back-wall echo (BWE) from the side opposite to the ultrasonic probe. The work presented aims at determining a BWE-equivalent out of the ultrasonic intermediate echoes from the inner of the part if a BWE cannot be evaluated, as for e.g. CFRP sandwich structures. Ultrasonic measurements on three CFRP samples with artificial porosity were performed. A delay embedding to reconstruct the state space for the intermediate echo time series and a subsequent creation of Recurrence Plots are carried out. The features Recurrence Rate RR and determinism DET are calculated with Euclidean and angular distance as metric. RQA parameters are largely varied and the results are evaluated on best correlations of RR and DET, respectively, with the BWE. The feature DET presents an appropriate BWE-equivalent. When using Euclidean distance, higher values of determinism are obtained for higher porosity, based on reduced amplitude due to additional reflections not going back to the ultrasonic transducer. This effect can be obtained with simpler evaluation (Quartile Coefficient of Dispersion), too, with a correlation similar to RQA. For angular distance, determinism decreases with higher porosity based on the randomness introduced by the pores. This first step of ongoing research will be followed by investigations on CFRP samples of other material and with “natural” porosity.