Composite porosity characterization using x-ray edge illumination phase contrast and ultrasonic techniques

Owing to their combination of low weight and high strength, carbon fiber reinforced composites are widely used in the aerospace industry, including for primary aircraft structures. Porosity introduced by the manufacturing process can compromise structural performance and integrity, with a maximum porosity content of 2% considered acceptable for many aerospace applications. The main nondestructive evaluation (NDE) techniques used in industry are ultrasonic imaging and X-ray computed tomography, however both techniques have limitations. Edge Illumination X-ray Phase Contrast Imaging (EI XPCi) is a novel technique that exploits the phase effects induced by damage and porosity on the X-ray beam to create improved contrast. EI XPCi is a differential (i.e., sensitive to the first derivative of the phase), multi-modal phase method that uses a set of coded aperture masks to acquire and retrieve the absorption, refraction, and ultra-small-angle scattering signals, the latter arising from sub-pixel sample features. For carbon fiber-reinforced woven composite specimens with varying levels of porosity, porosity quantification obtained through various signals produced by EI XPCi was compared to ultrasonic immersion absorption C-scans and matrix digestion. The standard deviation of the differential phase is introduced as a novel signal for the quantification of porosity in composite plates, with good correlation to ultrasonic attenuation.