6. Delamination Detection in Fiber Metal Laminates Using Ultrasonic Wavefield Imaging

This work presents a novel, non-destructive evaluation (NDE) method for detecting delaminations in fiber metal laminate (FML) plate-like structures. FMLs are rapidly replacing other materials in many aerospace applications because of their superior mechanical properties, including improved tolerance to fatigue, corrosion, and impact damage. However, delaminations can occur deep in the plate, and since access is limited to the composite face during most operations, the ability of traditional NDE techniques to discern these defects is limited. Many researchers have proposed using ultrasonic guided waves to image defects, but the anisotropic nature of wave propagation in FMLs and the subtlety of defects between metal and fiber-reinforced composite layers necessitate a new approach. In contrast to repeated transient excitations proposed in other literature, the method proposed here utilizes the full-field, steady-state response of an FML plate to ultrasonic excitation. Thus, the inspection time is shortened as the delay between measurements is removed, and a higher-energy input improves the signal-to-noise ratio. A 2D scanning laser Doppler vibrometer (LDV) is used to record the measurements at discrete points, while a piezoelectric transducer supplies the ultrasonic excitation. The steady-state response is processed to visualize defects on a pixel-by-pixel basis and locate potential regions of delamination in the FML plate. In this study, the one-dimensional response of a plate-like T800 graphite composite Ti-6Al-4V FML specimen with known areas of delamination is simulated. Two defect-detection features, based on simulated physical phenomena—detrended Hilbert envelope magnitude (DHEM) and low-pass local phase derivative (LLPD)—are subsequently evaluated over a wide range of excitation frequencies, to determine an optimal input for increased precision. Results from these simulations suggest potential guidelines to achieve a rapid and reliable NDE method for delamination detection in FML structures.