Guided wave energy focusing and steering in composite laminates

Abstract

Lightweight, carbon fiber reinforced composites are often selected for aerospace components but are prone to barely visible impact damage, caused by low velocity impacts, during service. Guided-wave-based structural health monitoring (SHM) techniques can efficiently detect impact damage impact in composite structures. However, wave propagation is influenced by material anisotropy resulting in a number of effects. The phase and group velocity of propagating wave modes depend on the wave launching direction, with increased wave speeds in the high stiffness (fiber) directions. Wave energy tends to be focused along the fiber directions, resulting in beam steering or skewing away from the initial wave launching direction. These anisotropic effects, if unaccounted for, could lead to inaccurate localization of damage, and potential regions of the structure where guided waves cannot propagate with sufficient amplitude, reducing damage sensitivity. Wave propagation in an undamaged unidirectional carbon fiber reinforced polymer (CFRP) panel was investigated for the A0 mode for multiple wave launching directions. Finite Element (FE) modelling was carried out using homogenized anisotropic material properties to investigate the directional dependency of velocity. Point and line sources were modelled to investigate the influence of the excitation source on the guided wave evaluation and signal processing. Wave skewing behavior was visualized for the line source, and wave skew angles and beam spread angles were calculated for a range of propagation angles. Experimental non-contact guided wave measurements were obtained using a laser vibrometer. A PZT strip transducer was developed in order to measure wave skew angles. Experimental and numerical velocities and skew angles were compared with theoretical predictions and good agreement was observed.