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Inspection of Fastener Holes Using Ultrasonic Phased Arrays


 An Air Force Aging Aircraft Project was initiated to identify a suitable replacement for the Autoscan. The Autoscan is a unique piece of equipment that was originally identified for specific inspections for detection of first-layer, faying surface fatigue cracks (.030″ and larger) around fastener holes beneath the fastener heads, without removal of the fastener. The currently inspected parent material ranges from .125 to .3 inches thick, 2024T3 or 7075-T73 aluminum. Potential also exists for replacing other aging inspection equipment such as the Rotoscan. A secondary objective was to evaluate the feasibility of expanding the inspection capabilities to detect corrosion as well as cracks.
 A Trade Study was conducted initially to consider existing technologies available, trade-offs, and technology insertion in order to meet the required performance parameters. The trade study showed Phased Array Ultrasonics to have the greatest potential, so it was chosen as the inspection method to pursue. Using phased arrays, a novel inspection technique for rapidly and reliably inspecting the area around fastener holes for cracks and corrosion has been developed with no moving parts. Specially designed probes are used for the aircraft inspections.
 This design consists of a three-dimensional matrix of 504 ultrasonic elements on a cone that encircles the fastener head. The two-dimensional arrangement of elements permits deflection of the ultrasonic beam in three dimensions. Full circumferential scans are performed by programming the phased array focal laws to scan 360° of the fastener holes, using a combination of the following scan patterns: pulse-echo at 45° incident on the crack, pulse-echo at 90°, pitch-catch, plus local scanning. This capability allows flexible coverage of the fastener hole and surrounding area, again with no moving parts. Additionally, the beam deflection capability means that one probe is adaptable to a wide range of fastener diameters and skin thickness.
 Several conical sub-arrays were built to evaluate the feasibility of the concept experimentally. The experimental results along with numerical modeling were used to determine optimal values for inner and outer radii of the cone, angle of the cone, number of elements and arrangement of the elements. A complete prototype conical array was subsequently built.
 The final portion of this project includes developing the specific inspection procedures, and performing a Probability of Detection study (POD) developed by the FAA’s Airworthiness Assurance Nondestructive Tested Validation Center at Sandia National Laboratory.