Comparison of Non-Destructive Evaluation Techniques for Damage Detection in Fibre-Reinforced Composites

Context:
Fibre-reinforced composite materials are used across a wide range of industries, including aerospace and military, as their high strength and low weight offer an advantage over other materials. Defects in composite materials (e.g. cracks, debonding, delaminations) are caused by stress, fatigue or impact events. These defects affect the structural integrity of the composite plates, potentially leading to the complete failure of the material. The detection and characterisation of such defects are thus of primary importance in predicting the composite's health and performance capabilities.

Non-Destructive Evaluation (NDE) or Testing (NDT) aims to identify and characterise damage in a given structure, both internally and externally, while conserving its structural integrity. Such evaluation is conducted using different imaging techniques (e.g. radiography, ultrasonic testing, thermography, infrared testing, etc.). Ultrasonic testing enables the detection and localisation of defects, as well as the characterisation and sizing of these defects, and is widely used for NDE of composite structures for damage detection. Ultrasonic testing presents several advantages, including relatively high resolution and the ability to detect internal defects and inconsistencies in materials, both homogeneous and inhomogeneous. However, in the case of immersion ultrasonic imaging, it is harder to set up, requires the full immersion the sample, and results in longer acquisition times in order to achieve high accuracy.

Conventional radiography was first applied on metals and alloys to study their structural integrity and look for manufacturing defects. Later on this technique was applied to the study of composite materials and impact damage, and is widely used today for the detection of defects. X-ray computed tomography (CT) results in the production of a 3D cross-section image of the sample, and enables the detection of defects such as delaminations, porosity, or cracks, which are not visible in 2D radiography. X-ray CT images are usually compared with more established NDE techniques, mainly ultrasonic C-scan imaging. However, as the nature of the components used for the manufacturing of composite plates tend to have similar absorption coefficients, or, alternatively, if defects are smaller in size and cannot lead to a clear variation in intensity, damage cannot always be resolved. This is usually the case for carbon fibre/epoxy resin composite plates, which are widely used in the aerospace industry.


Aims of project and methodology:
Edge-Illumination Phase Contrast X-ray imaging (EI XPCi) is an X-ray imaging technique which uses phase shift effects induced in an X-ray beam when going through an object to create contrast. This resolves the problem of the poor contrast resulting from similar absorption coefficients or smaller defects, and thus results in better contrast.

This project aims to benchmark EI XPCi as a viable non-destructive evaluation technique for damage detection and characterisation in fibre-reinforced composite plates. This will be achieved by comparing EI XPCi with established ultrasonic testing techniques, foremostly immersion C-scan ultrasonic imaging, both qualitatively and quantitatively.

The two imaging techniques' capability of detecting, localising and characterising different types of defects induced in composite plates during service, as well as common manufacturing defects will be studied and quantified. Several types of samples will be used, ranging from artificial defects induced in simple materials to simulate the damage in composite plates, to varying degrees of damage induced in composite plates with different structural complexity. By doing this, a new interpretation of damage detection using ultrasonic C-scan imaging can be obtained, complemented by EI XPCi, as well as a better understanding of the detection capabilities and limitations of each imaging technique.