NONDESTRUCTIVE EVALUATION OF COMPOSITE LAMINATES BY LEAKY LAMB WAVES

A method utilizing leaky Lamb wave (LLW) spectra for the nondestructive evaluation of fibre reinforced composite laminates is discussed. Lamb wave modes in composites depend on elastic properties of the laminate, plate thickness, fibre orientation, lay-up, and on the presence of internal discontinuities. Coupling to the Lamb waves is accomplished by mode conversion of an incident compressional wave in a fluid surrounding the plate, where a second transducer in the fluid detects the LLW. NDE applications of these wave modes are reported in multi-ply graphite-epoxy laminates using swept frequency r.f. tone bursts in the range of 0.5 to 10.0 MHz. Experiments have demonstrated useful sensitivity to various types of defects such as delaminations, porosity, ply gaps, foreign matter, and changes in fibre volume fraction. Results of two-transducer LLW C-scans of composite plates with seeded defects and with induced damage are presented, showing the nature and degree of the LLW spectrum perturbation caused by the various types of flaws. To prevent minor spatial variations in either fibre volume fraction or plate thickness from complicating the detection of important defects, a signal coding scheme, incorporating a frequency modulated tone burst, has been devised to analyse the ultrasonic reflection signals from the plate. Improved discrimination and defect sensitivity in the presence of thickness and fibre-fraction variations are demonstrated in several specimens. Digital techniques to manipulate the frequency modulated signals have been developed and are discussed in the context of the experimental results.