Application of microwave noiselets for nondestructive testing of unidirectional carbon fiber reinforced polymers

Fiber-reinforced polymer (FRP) composites are a class of materials consisting of polymer matrices infused with high-strength fibers, such as glass, aramid, or carbon. The uses of FRPs, such as carbon fiber reinforced polymers (CFRP), are widespread throughout many military and civil applications due to their high-strength properties and lightweight nature. During the lifespan of a structure reinforced with an FRP, many scenarios arise which threaten the integrity of the material, which may cause minor air gaps within the layers of the material. Microwave non-destructive testing serves as a potentially non-contact one-sided approach for detection of internal or superficial defects. An issue that arises is that the sample under test, in many cases, does not maintain a uniform shape, making assessment of the material's state difficult. This is a result of the microwave response being more heavily weighted on the shape of the material, rather than potential defects. To accommodate for this, elaborate systems must be designed to minimize the likelihood that non-visible defects may be overlooked. Ultrawideband (UWB) noiselet waveforms, along with the anisotropic nature of CFRP, allow for the reduction of this likelihood. In this research effort, we include the derivation of the noiselet waveform, give a description of the UWB system used for testing, and present results of tests conducted on a CFRP sample back with an aluminum sheet, which has an air gap fabricated within the layers.