Design of test specimens for the determination of elastic through-thickness shear properties of thick composites from measured modal vibration frequencies

Recommendations are developed and presented regarding the most desirable test plate aspect ratio (length-to-thickness ratio) to use during the non-destructive measurement of elastic constants of thick composite plates by modal vibration testing. The effects of transverse shear were studied over the range of practical plate aspect ratios (length to thickness ratios) and mode numbers for three specific types of composite materials ranging from planar isotropic (E-glass/polyester sheet molding compound) to moderately orthotropic (unidirectional graphite/epoxy) to highly orthotropic (unidirectional pitch/epoxy). Frequency predictions from previously developed Rayleigh-type two-dimensional (2-D) analytical models with and without through-thickness (transverse) shear effects were compared with those from 2-D and 3-D finite-element analyses. Reasonable agreement between the models was observed, and results were used to determine the aspect ratios at which transverse shear effects become important enough to be taken into consideration during vibration tests. It was also found that the avoidance of undesirable in-plane modes must be factored into the process of selecting the proper plate aspect ratio.