Single Mode Lamb Wave Inspection of Composite Laminates

Abstract

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, ob1ique incidence inspection can be expected to give better sensitivity since Lamb modes propagat ing para11el to the plate are more attenuated. At the same time, the lateral reso1ution wi1l be inherent1y lower, therefore the u1trasonic contrast must be carefu1ly optimizerl on a case-by-case basis.