| Two advanced nondestructive evaluation systems have been developed for imaging surface-breaking cracks in aerospace materials. The systems use scanning heterodyne interferometry and frequency-translated holography principles to image ultrasonic displacement fields on material surfaces with high resolution and sensitivity. Surface-breaking cracks are detected and characterized through near-field ultrasonic scattering processes, which result in a local intensification of the ultrasonic displacement field in the immediate vicinity of the crack. The local intensification permits the cracks to be easily distinguished from background levels, and creates unique displacement field images of the cracks which follow the contours and morphology of the cracks with microscopic precision. The interferometric and holographic imaging approaches provide the non-contact and optical-diffraction-limited measurement capabilities that are essential for probing the ultrasonic displacement fields in the immediate vicinity of the cracks. Several representative crack-imaging results are provided, along with detailed descriptions of both experimental techniques, and the capabilities and limitations of each method. The resulting systems provide a simple, yet very powerful means for evaluating surface-breaking cracks in detail. |
