Effect of material anisotropy on guided wave propagation and scattering in CFRP laminates

Abstract

Carbon fiber laminates, consisting of highly anisotropic fiber-matrix ply-layers, are widely used in aerospace applications due to their good strength to weight ratio. However, poor interlaminar strength makes them prone to barely visible impact damage (BVID), significantly reducing the load bearing capacity of aircraft components. Guided ultrasonic waves have been widely used for structural health monitoring (SHM) of composite structures. Guided wave propagation and scattering at circular delaminations in a quasi-isotropic laminate was modelled using full three-dimensional (3D) Finite Element (FE) simulations in ABAQUS. Non-contact laser measurements were performed to obtain the scattered wavefield at a film insert delamination. The influence of ply layer anisotropy and incident wave direction were investigated both numerically and experimentally. Scattering directivity patterns were calculated using a baseline subtraction method and 2D scattering matrices were obtained for all incident wave directions. Circular magnets were used as a scattering target and numerical and measured scattering patterns were compared with those of the insert delamination. Strong directional dependency was observed for incident and scattered waves around both delamination and magnets, indicating energy focusing along the outer ply layers of the laminate. For the delamination a strong forward wave was observed, with low amplitude in other directions, whereas the magnet blocked forward transmission of the wave, demonstrating distinct scattering behavior. The anisotropic effects and different scattering patterns should be considered for guided wave sparse array SHM to ensure the robustness of imaging algorithms.

Anisotropy influence on guided wave scattering for composite structure monitoring

“…Regions of low scattered amplitude are observed in most directions, which could result in delaminations being missed from distributed sensors depending on their location. Strong forward scattering and low backscattered amplitude has been observed for delaminations at different depths, [33][34][35][39][40][41][42][43] thus qualitatively the scattering matrices would be expected to resemble Figure 10(a), although the relative amplitudes, for example, of the diagonal band, will vary with delamination depth. 43 The scattering matrix for a delamination located at the midplane of the laminate is shown in Appendix A.…”

Guided wave energy focusing and steering in composite laminates

“…Ultrasonic guided waves are a promising structural health monitoring (SHM) technique for composites as they propagate along structures, enabling rapid, long-range inspection of large areas [3]. However, guided wave propagation is influenced by material anisotropy resulting in a number of effects including energy focusing [4], directional dependency of wave velocities [5], wave skewing, and beam spreading [6,7]. If unaccounted for, these anisotropic effects could lead to inaccurate damage localization; therefore, understanding guided wave propagation behavior in anisotropic structures is required for accurate and reliable SHM of composites.…”