Defect visualization of cylindrical and conical CFRP lattice structures using rotational ultrasonic propagation imager

Abstract

A composite lattice structure has been used in an aircraft and a space launcher to reduce the structural weight. Defects can occur during fabrication because of structural complexity and they are mostly invisible as occurring inside a structure. Also, they lead to the deterioration of structural performance and expensive failures. Therefore, non-destructive evaluation (NDE) to detect defects should be conducted not to damage an initially intact part. So far, various NDEs methods have been attempted, however, a standard inspection method has not been proposed for the large and curved lattice structures. In this study, two modified rotational through-transmission (TT) ultrasonic propagation imagers (UPIs) are proposed to inspect a skinless cylindrical lattice structure and a conical lattice structure with skin. In the first case of the skinless cylindrical specimen, there is a problem with the breakdown of the laser Doppler vibrometer (LDV) when an ultrasonic generation beam directly entered the LDV through an empty space of the specimen. For this issue, a wavelength domain beam splitting mechanism was included in the system. A proof-of-concept for the proposed inspection setup was performed using unit cells of the lattice structure, and defects could be visualized. The results were reliable compared with the radiation tomographic results. We also inspected the full-scale skinless lattice structure and visualized its internal defects. Conventional radiographic and visual inspection verified the location to be the same as that obtained using the TT UPI. In the second case of the conic specimen with skin, there is a problem with a change in a stand-off distance due to its shape. The change of the stand-off distance causes a steady decrease in signal-to-noise ratio (SNR), a change in the incident angle of the LDV beam and a change in a time-of-flight (ToF) at a received signal. These make defect visibility worse. For this issue, an additional manual rotational stage was incorporated into the rotational TT UPI to ensure that the sensing beam from the LDV would be perpendicularly incident on the skin surface. First, an appropriate scanning width was determined by measuring the variation in the SNR based on the stand-off distance. Furthermore, a ToF compensation algorithm was used to match the arrival times of signals for conic geometry. Then, the full-scale conical lattice structure with skin was inspected, and de-bonding defect was visualized via ultrasonic wave propagation imaging. As a result, it was possible to quickly inspect a large area of composite lattice structures regardless of the shape condition.