Edge Illumination X-Ray Phase Contrast Imaging and Ultrasonic Attenuation for Porosity Quantification in Composite Structures

Carbon fiber reinforced composites are widely used in the aerospace industry, due to their low weight and high strength. Porosity often occurs during the manufacturing of composite structures, which can compromise the structural integrity of the part and affect its mechanical properties. In the aerospace industry a typical requirement for structural components is for the porosity content to be kept below 2%. Non-destructive evaluation (NDE) techniques are used to estimate the porosity content in composite components, the most common being ultrasonic attenuation and X-ray computed tomography (CT). Planar Edge Illumination X-ray Phase Contrast Imaging (EI XPCI) was used to quantify the porosity content in woven carbon fiber reinforced composite plates with porosity ranging between 0.7% and 10.7%. A new metric was introduced, the standard deviation of the differential phase (STDVDP) signal, which represents the variation of inhomogeneity in the plates for features of a scale equal to or above the system resolution (here 12µm). The SDTVDP was found to have a very high correlation with porosity content estimated from matrix digestion and ultrasonic attenuation, hence providing a promising new methodology to quantify porosity in composite plates

Composite impact damage detection and characterization using ultrasound and X-ray NDE techniques

Combining low weight and high strength, carbon fiber reinforced composites are widely used in the aerospace industry, including for primary aircraft structures. Barely visible impact damage can compromise the structural integrity and potentially lead to failures. Edge Illumination (EI) X-ray Phase Contrast imaging (XPCi) is a novel X-ray imaging technique that uses the phase effects induced by damage to create improved contrast. For a small cross-ply composite specimen with impact damage, damage detection was compared to ultrasonic immersion C-scans. Different defect types could be located and identified, verified from the conventional ultrasonic NDE measurement

Post-Acquisition Mask Misalignment Correction for Edge Illumination X-ray Phase Contrast Imaging

Edge illumination x-ray phase contrast imaging uses a set of apertured masks to translate phase effects into variation of detected intensity. While the system is relatively robust against misalignment, mask movement during acquisition can lead to gradient artifacts. A method has been developed to correct the images by quantifying the misalignment post-acquisition and implementing correction maps to remove the gradient artifact. Images of a woven carbon fiber composite plate containing porosity were used as examples to demonstrate the image correction process. The gradient formed during image acquisition was removed without affecting the image quality, and results were subsequently used for quantification of porosity, indicating that the gradient correction did not affect the quantitative content of the images

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

Increased material differentiation through multi-contrast x-ray imaging: a preliminary evaluation of potential applications to the detection of threat materials

Most material discrimination in security inspections is based on dual-energy x-ray imaging, which enables the determination of a material's effective atomic number (Zeff) as well as electron density and its consequent classification as organic or inorganic. Recently phase-based "dark-field" x-ray imaging approaches have emerged that are sensitive to complementary features of a material, namely its unresolved microstructure. It can therefore be speculated that their inclusion in the security-based imaging could enhance material discrimination, for example of materials with similar electron densities and Z eff but different microstructures. In this paper, we present a preliminary evaluation of the advantages that such a combination could bear. Utilising an energy-resolved detector for a phase-based dark-field technique provides dual-energy attenuation and dark-field images simultaneously. In addition, since we use a method based on attenuating x-ray masks to generate the dark-field images, a fifth (attenuation) image at a much higher photon energy is obtained by exploiting the x-rays transmitted through the highly absorbing mask septa. In a first test, a threat material is imaged against a non-threat one, and we show how their discrimination based on maximising their relative contrast through linear combinations of two and five imaging channels leads to an improvement in the latter case. We then present a second example to show how the method can be extended to discrimination against more than one non-threat material, obtaining similar results. Albeit admittedly preliminary, these results indicate that significant margins of improvement in material discrimination are available by including additional x-ray contrasts in the scanning process

Comparing signal intensity and refraction sensitivity of double and single mask edge illumination lab-based x-ray phase contrast imaging set-ups

Double mask edge illumination (DM-EI) set-ups can detect differential phase and attenuation information from a sample. However, analytical separation of the two signals often requires acquiring two frames with inverted differential phase contrast signals. Typically, between these two acquisitions, the first mask is moved to create a different illumination condition. This can lead to potential errors which adversely affect the data collected. In this paper, we implement a single mask EI laboratory set-up that allows for a single shot retrieval of the differential phase and attenuation images, without the need for a high resolution detector or high magnification. As well as simplifying mask alignment, the advantages of the proposed set-up can be exploited in one of two ways: either the total acquisition time can be halved with respect to the DM-EI set-up or, for the same acquisition time, twice the statistics can be collected. In this latter configuration, the signal-to-noise ratio and contrast in the mixed intensity images, and the angular sensitivity of the two set-ups were compared. We also show that the angular sensitivity of the single mask set-up can be well approximated from its illumination curve, which has been modelled as a convolution between the source spatial distribution at the detector plane, the pre-sample mask and the detector point spread function (PSF). A polychromatic wave optics simulation was developed on these bases and benchmarked against experimental data. It can also be used to predict the angular sensitivity and contrast of any set-up as a function of detector PSF

Enhanced composite plate impact damage detection and characterisation using X-Ray refraction and scattering contrast combined with ultrasonic imaging

Ultrasonic imaging and radiography are widely used in the aerospace industry for non-destructive evaluation of damage in fibre-reinforced composites. Novel phase-based X-ray imaging methods use phase effects occurring in inhomogeneous specimens to extract additional information and achieve improved contrast. Edge Illumination employs a coded aperture system to extract refraction and scattering driven signals in addition to conventional absorption. Comparison with ultrasonic immersion C-scan imaging and with a commercial X-ray CT system for impact damage analysis in a small cross-ply carbon fibre-reinforced plate sample was performed to evaluate the potential of this new technique. The retrieved refraction and scattering signals provide complementary information, revealing previously unavailable insight on the damage extent and scale, not observed in the conventional X-ray absorption and ultrasonic imaging, allowing improved damage characterisation

Quantification of porosity in composite plates using planar X-ray phase contrast imaging

The application of planar Edge-Illumination X-ray Phase-Contrast imaging (EI-XPCi) for the non-destructive quantification of porosity in carbon fiber reinforced polymer (CFRP) specimens, a significant concern in aerospace applications, was investigated. The method enables fast, planar (2D) scans providing access to large samples. A set of woven CFRP plates with porosity content ranging from 0.7% to 10.7% was examined. In addition to standard X-ray attenuation, EI-XPCi provides differential phase and dark-field signals, sensitive to inhomogeneities and interfaces at scales above and below the system spatial resolution, respectively. The correlation with the porosity content from matrix digestion obtained from the dark-field signal was comparable to that from ultrasonic attenuation. The novel analysis of the standard deviation of differential phase (STDP), sensitive to inhomogeneities above the system resolution (approximately 12 μm), resulted in a very high correlation (R2 = 0.995) with the matrix digestion porosity content, outperforming ultrasonic attenuation measurements

Laboratory implementation of edge illumination X-ray phase-contrast imaging with energy-resolved detectors

Edge illumination (EI) X-ray phase-contrast imaging (XPCI) has potential for applications in different fields of research, including materials science, non-destructive industrial testing, small-animal imaging, and medical imaging. One of its main advantages is the compatibility with laboratory equipment, in particular with conventional non-microfocal sources, which makes its exploitation in normal research laboratories possible. In this work, we demonstrate that the signal in laboratory implementations of EI can be correctly described with the use of the simplified geometrical optics. Besides enabling the derivation of simple expressions for the sensitivity and spatial resolution of a given EI setup, this model also highlights the EI’s achromaticity. With the aim of improving image quality, as well as to take advantage of the fact that all energies in the spectrum contribute to the image contrast, we carried out EI acquisitions using a photon-counting energy-resolved detector. The obtained results demonstrate that this approach has great potential for future laboratory implementations of EI. © (2015) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only