Analysis of the continuous mode conversion of Lamb waves in fiber composites by a stochastic material model and laser vibrometer experiments

The complex internal structure of modern laminate composite materials, for example, carbon fiber reinforced plastics (CFRP), has a pronounced influence on the propagation of elastic guided waves (GWs). Along with the well‐known anisotropy‐conditioned amplitude and dispersion directivity of source‐induced wavefields and their remarkable attenuation due to the polymer matrix viscosity, continuous mode conversion (CMC) of GWs in multilayered composites fabricated from unidirectional prepregs has been recently observed experimentally. The latter means that the symmetric fundamental mode continuously converts into the antisymmetric one without passing a damage, that is, even in an intact structure. Being strongly related to the inhomogeneous CFRP microstructure and possessing remarkable intensity for specific propagation directions, this phenomenon should be accounted for in ultrasonic nondestructive testing and structural health monitoring systems. In this work, we investigate the CMC phenomenon in an important class of laminates, namely cross‐ply ones, experimentally as well as numerically. In the computational model it is addressed through the concept of spatially varying material properties, and the finite element method (FEM) is employed for the GW propagation simulation. Experimental measurements are performed for piezoelectrically excited GWs with scanning laser Doppler vibrometry technique, allowing clear visualization of the CMC phenomenon and confirming theoretically predicted dependence of its intensity from propagation direction.