Phased Array Ultrasonic Inspection of Fastener Holes in Thick Multiple-Layer Structures

Budgetary constraints on defense spending have highlighted the importance ofprograms aimed at extending the service life of aging aircraft. Nondestructiveinspection is a key component of such programs. One important capabilityis the detection of cracking and corrosion around fastener holes. Phased arrayultrasonics offer a novel and powerful solution. They can steer an ultrasonic beamat various angles around the hole, allowing a range of skin thicknesses and fastenerdiameters to be inspected without mechanical adjustments. Complete circumferentialscans can be performed, in some cases without any physical motion of the probe.The added flexibility of examining flaws at a whole range of angles simultaneouslyleads to a higher probability of detection. The elimination of some or all physicalmotion reduces weight, size and complexity.The feasibility of detecting first layer corner cracks on thefaying surface using phased arrays has been demonstrated in a prior USAFprogram. Similar technology could be applied to the detection of cracks inmultiple layer structures provided sealant exists between the layers to ensurecoupling. This proposal outlines our approach for overcoming the technical hurdlesof applying phased arrays to thick, multiple layer structures.It is anticipated that technology developed under this SBIR will have direct benefitsfor United States Air Force Aging Aircraft Programs by allowing safe, rapid andeffective nondestructive inspection of fastener holes in thick, multiple layerstructures and that such technology, if proven successul, would be acquired by theDoD. As the cost of the technology decreases, it is believed that commercialcarriers may also develop an interest.