Ultrasonic plate wave evaluation of natural fiber composite panels

Two key shortcomings of current ultrasonic nondestructive evaluation (NDE) techniques for plywood, medium density fiberboard (MDF), and oriented strandboard are the reliance on empirical correlations and the neglect of valuable waveform information. The research reported herein examined the feasibility of using fundamental mechanics, wave propagation, and laminated, shear deformable plate theories of nondestructively evaluate material properties in natural fiber-based composite panels. Dispersion curves were constructed exhibiting the variation of flexural plate wave phase velocity with frequency. Based on shear deformable laminated plate wave theory, flexural and transverse shear rigidity values for solid transversely isotropic, laminated transversely isotropic, and solid orthotropic natural fiber-based composite panels were obtained from the dispersion curves. Axial rigidity values were obtained directly from extensional plate wave phase velocity. Excellent agreement (within 3%) of flexural rigidity values was obtained between NDE and mechanical testing for most panels. Transverse shear modulus values obtained from plate wave tests were within 4% of values obtained from through-thickness ultrasonic shear wave speed. Tensile and compressive axial rigidity value obtained from NDE were 22% to 41% higher than mechanical tension and compression test results. These differences between NDE and axial mechanical testing results are likely due to load-rate effects; however, these large differences were not apparent in the flexural and transverse shear comparisons. This fundamental research advances the state-of-the-art of NDE of wood-based composites by replacing empirical approaches with a technique based on fundamental mechanics, shear deformation laminated plate theory, and plate wave propagation theory.