Development of Motorized Azimuthal Scanners for Ultrasonic NDE Of Composites

Composites are a material class for which nondestructive material property characterization is as important as flaw detection. Fiber reinforced composite laminates often possess strong in-plane elastic anisotropy attributable to the specific fiber orientation and layup sequence. Many of these elastic anisotropies may be investigated using ultrasound [1–6], among which angular measurements are often used. Hsu et al [3,4] used angular scan of acousto-ultrasonic signals to investigate fiber reinforced composite laminates. By placing and rotating two contact transducers on the same side of crossedplied composite laminates, the angular dependence of the acousto-ultrasonic signal was measured and found to have good correlation with the fiber orientation of the sample. Angular measurement of normal-incident shear wave has also been used to detect errors in layup sequence and ply orientation in both green (before cured) and cured composites [4–6]. The transmitted signals of normal incident shear wave in a “crossed polarizer” configuration were found to be particularly sensitive to ply misorientation and layup sequence in a laminate. For green composites, sandwiched between aluminum delay lines, EMATs (electro-meganetic transducer) were used so that the problem of changing coupling condition during the angular scan was avoided. This technique was believed to hold good potential as a practical NDE tool for detecting layup errors during the manufacturing process [5]

Application of Shear Waves for Composite Laminate Characterization

The layup sequence in a composite laminate greatly effects its properties. If one ply is misaligned in the layup sequence, it can result in the part being rejected and discarded. At the present time, most manufacturers cut a small coupon from the waste edge and use a microscope to optically verify the ply orientations on critical parts. This can add a substantial cost to the product since the test is both labor intensive and performed after the part is cured. A nondestructive technique which could be used to test the part after curing and require less time than the optical test would be very beneficial, and one that could be performed prior to curing would be extremely desirable. Preliminary tests demonstrate a high probability that the model and tests developed in this paper can be used for characterizing uncured layups as well.</p