Porosity determination of carbon fiber reinforced plastics (CFRP) in aviation applications using ultrasound without a back wall echo

Abstract

The use of CFRP increased not only in the field of aviation structures. Also regarding lightweight construction in the automotive industry CFRP is moving forward. Porosity is one of the main quality parameters of CRFP. X-ray computer tomography (CT) is nowadays one of the standard reference techniques for quantitative imaging of porosity. However, CT sample size is limited if µm local resolution is required. For process control ultrasonic attenuation measurements are therefore prevalent, either in pulse-echo mode by evaluating the back wall echo amplitude, or in transmission mode. However other approaches are required, when only single sided access is given or when no back wall echo is detectable due to thickness of the component or due to damping effects in case of increased porosity. Every individual time-of-flight signal carries phase and frequency information in addition to the amplitude information. The phase and the frequency variation of the transmitted and backscattered ultrasonic signals can be correlated to the inner fibre and matrix structure of the material to be assessed and to additional material inhomogeneity like pores, cracks and delaminations. Within this presentation results based on phase and frequency analysis of ultrasonic A-scans when no back wall information is present will be discussed. X-ray computer tomography data were used for calibration of the ultrasound results. These results are based on the analysis and processing of ultrasonic A-scans acquired with commercial single element transducers and using conventional state-of-the-art ultrasonic hardware