Carbon fibre-reinforced polymer (CFRP) composites have been increasingly used in aircraft industry for their superior stiffness and strength properties. However, they are vulnerable to impact damage. Some of this damage could not be easily identified by visual inspection. The non-destructive testing (NDT) research community has been looking into better solutions for damage evaluation. An alternative method is radio frequency (RF)-based. In this work, three RF sensors (i.e., coupled spiral inductors (CSI), time domain reflectometry (TDR) and passive spiral electromagnetic sensor with wireless interrogation) have been further developed and evaluated. The complementary split-ring resonator (CSRR) sensing method and ultra-wideband (UWB) 3D imaging are applied for the first time for impact damage detection. A comprehensive experimental study of impact damage detection of carbon fibre composites using an optimised electromagnetic CSI sensor is carried out. A quantitative relationship between the signal response and the location, depth and width of the damage is established. Multi-frequency inspection is also performed, where the resultant images indicate great potential in the characterisation of the extent of the subsurface impact damage. The passive spiral electromagnetic sensor is used with a wireless setup, which improves its non-contact ability for easy implementation. A detailed equivalent lumped circuit model is proposed for providing an estimation of the resonance frequency. Both the resolution and the sensitivity provided by the designed sensors are significantly improved compared with those in the literature. The CSRR sensor is capable of impact damage detection by measuring the shift of the resonance frequency. A UWB 3D imaging system is introduced to rapidly scan a relatively large area and identify surface dents. The experimental results demonstrate that all the sensors developed can offer alternative inspection methods that can be fully automated for structural monitoring and maintenance of aircraft and other composite structures currently used in marine, automotive and wind energy industries.
Radio frequency non-destructive evaluation of impact damage in carbon fibre composites
Li, Z. (Author). 31 Dec 2017
Student thesis: Phd