Ultrasonic metrics for large-area rapid wrinkle detection, classification and quantification in composites

Due to their high strength-to-weight ratio, composite materials are now in use in many high-stress applications, particularly where light weight is also a requirement. In these situations, the detrimental knock-down in mechanical strength due to an out-of-plane wrinkle defect can have serious consequences and is the reason for a requirement to rapidly detect and quantify any such wrinkles at manufacture. Unfortunately, these wrinkles do not perturb the ultrasound currently used for quality control at manufacture in a way that can be readily detected above coherent structural noise variations. This paper exploits the ply resonance that is a characteristic of multi-layer structures to generate two new metrics for both detection and quantification of out-of-plane wrinkles, due to their effect on the ply spacing. These can be measured at every three-dimensional location in a structure using the instantaneous frequency, which is the rate of change of phase in the pulse-echo ultrasonic response. The proposed two new metrics for detection and quantification of wrinkles are: Mean Spacing and Spacing Difference. Use of an analytical model to predict the ultrasonic response of the structure has allowed an understanding of how these metrics will be affected by various wrinkle types and how they can not only detect but also classify and quantify the wrinkle extent and severity. Three main types of wrinkles are considered: classic wrinkles near the mid-plane of a structure, back-surface wrinkles forming from a resin bulge near the back of a structure, and folded wrinkles where several plies can be folded over completely in the bulk of the structure. Both simulations and experimental results demonstrate the effectiveness of these metrics on various types of structure including carbon-fibre and glass-fibre composites with a range of ply thicknesses and wrinkle types.