Carbon fibre reinforced polymer (CFRP) laminates are widely used for
aerospace applications as they reduce the weight of structures whilst
maintaining mechanical strength. Composites have highly anisotropic material
properties and high in-plane strength but poor interlaminar strength, making
them vulnerable to barely visible impact damage (BVID) caused by low velocity
impacts. Composite damage is multi-modal, consisting of fibre breakage, matrix
cracking, and delaminations, with delaminations causing the most significant
strength reduction. Guided ultrasonic waves, often generated using a sparse
network of sensors bonded to a structure, provide a promising structural health
monitoring (SHM) technique for composites. Guided waves propagate along a
structure, with energy throughout the entire thickness, making them ideal for
rapid, long-range inspection of large areas. In anisotropic materials wave
energy is focused along the high stiffness (fibre) directions, resulting in higher
amplitude and wave speed in these directions. Waves launched away from the
fibre direction are steered towards the fibres. These anisotropic effects could
lead to inaccuracies in damage localization if not accounted for.
Propagation of the fundamental, flexural (A0) guided wave mode was
investigated in an undamaged unidirectional CFRP panel. Anisotropic effects
including the directionality of wave velocities, skew angles, and beam spreading
were quantified through both finite element simulations and experiments,
achieving good agreement with predictions obtained from dispersion curves.
Scattering of the A0 mode at an artificial delamination was studied for a quasiisotropic CFRP plate layup. Wave-trapping on top of the delamination, and
strong forward scattering at the delamination exit was found. Significantly
different scattering behaviour was observed to that of a magnet target, often
used to develop SHM systems. Scattering around both damage targets was
found to be directionally dependent, with higher amplitudes in the fibre
directions of the outermost laminae. Implications for the SHM of composites
were discussed.