Strain measurements on carbon-epoxy composites by lamb waves piezopolymer interdigital transducers

In this work we studied Lamb waves in composite materials for pure bending stress measurements. According to a theoretical analysis of the relationship between the strain due to pure bending and the variations of phase velocity, it turns out that this effect can be measured with a combined guided mode, obtained by the superposition of A1 and S1 modes propagating at the same group velocity. This operating condition has been obtained by proper design of interdigital transducers (IDT). In our experiment we have fabricated piezopolymer IDTs operating at 450 kHz and mounted on a cross-ply (0°/90°) 4 mm thick composite laminate, subjected to a pure bending stress on 4 points. The results obtained with this ultrasonic method are compared with standard strain gages measurements in the range (0-1500) µe.