EMAT-generated shear wave transmission for NDE of composite laminates

Abstract

Errors in ply orientation and layup stacking sequence adversely affect many properties of a composite laminate such as stiffness, strength, and hygrothermal properties. Normal incident polarized shear waves interact strongly with the fibers. Shear wave transmission by electromagnetic acoustic transducer (EMAT) has been used for the detection of the layup errors in composite laminates. First, a complete analytical model has been developed for the shear wave transmission through a composite laminate. The model indicates that the transmission output is a function of both angle and frequency. Extensive model calculation for various types of laminates with various types of layup errors has been made. The results show that the angular-frequency patterns, either “aligned” (the transmitter and the receiver are parallel to each other) or “crossed” (the transmitter and the receiver are perpendicular to each other), give a sensitive and reliable indication of the layup errors. Second, a motorized EMAT azimuthal scan system has been developed for layup error detection for composite laminates. Experimental results on several types of laminates show reasonable agreement with the model prediction. Changes in angle-frequency patterns as well as angle-time patterns due to the layup errors are observed. The EMAT-generated shear wave transmission can therefore be a very valuable nondestructive testing technique for detecting errors in ply orientation and stacking sequence for both cured and green composite laminates.—This work was supported by the Center for NDE at Iowa State University.