Fringing Electric Field Sensors for the Detection of Incipient Thermal Damage in Composite Materials

Abstract

Commercial and military aviation has rapidly adopted the use of carbon composites due to their structural strength and light weight in comparison to metal alloys. As with metal aircraft, composite aircraft require regular inspection to maintain flight worthiness and safety for passengers and crew, however, thermal damage in composites is not detectable using conventional non-destructive evaluation (NDE) methods until after the material loses as much as 50% of its mechanical strength. Additionally, many NDE methods are expensive or severely limited by depth, resolution, or access to both sides of the material under Test (MUT). Dielectric spectroscopy combined with electric field sensors presents the possibility of a more effective means of identifying thermal damage in composite materials and enhance existing composite NDE methods. The goal of this thesis is to evaluate the effectiveness of detecting non-visible incipient thermal damage in composites using Fringing Electric Field (FEF) sensors and dielectric spectroscopy. We designed and implemented two sensors developed using established parameters for electric field sensors and analyzed sensor response using custom algorithms developed in MATLAB. Six aerospace composite samples, one undamaged and five exposed to temperatures ranging from 450°F to 626°F, were used to characterize the sensor response. An additional five composite samples with unknown thermal damage were measured for blind identification and validation. Spectroscopic FEF sensor measurements demonstrated thermal correlation above 70 kHz in calibrated gain sensor response with a low resolution of 0.03 dBV across all samples. Intra-sample overlap in sensor gain response resulted in the successful identification of only three of the five blind composite samples. Variation in sample measurements is attributed to the limited thermal resolution of the sensor and a standard deviation of individual sample measurements exceeding 8 percent due to sample geometry and physical damage. This thesis demonstrates a partial correlation between thermal damage in aerospace composites and their dielectric response via electric field sensor measurements above 70 kHz. With additional development, dielectroscopy with FEF sensors presents the potential to supplement and improve upon existing aerospace composite NDE technologies.