Single Mode Lamb Wave Inspection of Composite Laminates

Flaw detection in composite plates presents a very difficult problem. Conventional echoscopic techniques are not feasible because the back-scattered signal from a possible defect is usually overshadowed by much stronger reflections from the walls of the thin plate and by additional scattering from inherent inhomogeneities such as imbedded fibers and alternating plies. Single or double transmission attenuation measurements offer a convenient, but rather insensitive alternative. Fig. 1 shows the schematic diagrams of the conventional normal incidence and the oblique incidence so-called Lamb wave inspection techniques. At normal incidence, we can use a focused transducer of very good lateral resolution, but the sensitivity might be rather low when the defect exhibits a very small scattering cross-section from this particular direction of interrogation. This occurs, for instance, in the case of weak porosity when the defects tend to be concentrated in a thin layer parallel with the plies, or as is shown in Fig. 1, in the case of transverse cracks. In such cases, oblique incidence inspection can be expected to give better sensitivity since Lamb modes propagating parallel to the plate are more attenuated. At the same time, the lateral resolution will be inherently lower, therefore the ultrasonic contrast must be carefully optimized on a case-by-case basis.