Shear Wave Ultrasonic Technique as an NDE Tool for Composite Laminates Before and After Curing

The highly anisotropic elastic properties of the plies in a composite laminate, especially those manufactured from unidirectional prepregs, interact strongly with the inplane vibration of shear ultrasonic waves propagating through its thickness1–3. The transmitted signals in a “crossed polarizer” configuration (with the transmitting and receiving transducers perpendicular to each other) were found to be particularly sensitive to ply orientation and layup sequence in a laminate4,5. This technique therefore holds good potential to be an NDE tool for detecting layup errors during the manufacturing of composite components. In such measurements, the transmitting transducer and the receiving transducer were rotated simultaneously, referred to as an azimuthal scan, while maintaining perpendicularity between them. The overall peak-to-peak amplitude of the RF waveform of the transmitted shear wave was recorded and plotted as a function of the transmitting transducer orientation. It was demonstrated experimentally that a single misoriented ply at the center of a 24-ply quasi-isotropic laminate can be detected with ease. Sensitivity to other errors in ply orientation and layup sequence have also been demonstrated.6 To continue the assessment of detection sensitivity, 48-ply graphite-epoxy laminates with and without intentional ply errors were fabricated. Azimuthal scans were performed to detect the errors and the results were quite successful, as described below.