Engineers have designed a new device, called the Smart Washer, that can electromagnetically detect bolthole cracks in metal structures. The device is a metal washer that contains a wire coil embedded onto the bottom surface of the washer. An alternating current drives this coil and induces eddy currents around the bolthole. These eddy currents develop electromagnetic fields that interact with the driving field of the coil.
A sensor is built into the Smart Washer, enabling direct contact with the bolthole lip, which is a natural crack-formation site. The sensor is therefore ideally positioned for crack monitoring, and since the Smart Washer is a permanent part of the structure, technicians can detect bolthole cracks without removing the bolt.
Another advantage is that unlike bolts, washers are not critical structures. Since washers distribute bolt loads onto surfaces, Smart Washers can accommodate a sensor without degrading structural strength.
The Smart Washer technology has several novel features. First, unlike many sensing systems, the design allows a long cable between the local electronics of the Smart Washer and the measurement circuitry. If this cable is routed to an access port, then technicians can inspect boltholes by connecting a hand-held instrument.
Secondly, it is easy to fabricate Smart Washer sensing coils, and the measurements involve well-understood techniques that require relatively few components. The simplicity of the Smart Washer provides a robust, rugged system that can withstand field conditions.
Finally, because washers are not critical structures, designers are more willing to replace standard washers with Smart Washers, thereby adding embedded bolthole monitors to new and existing designs.
The unique capabilities of the Smart Washer have been successfully demonstrated during destructive tests. Smart Washers were mounted on 1/8-in. (3.2-mm) thick aluminum 2219-T87 coupons, where each coupon contained a 0.5-in. (13-mm) diameter drilled hole. The coupon was installed in a fatigue test machine, and the Smart Washer was fastened to the coupon with a stainless steel nut and bolt. Cyclical tensile loading induced a bolthole crack, and the Smart Washer sensor signal was recorded as the crack grew.
The Smart Washer successfully detected crack initiation and growth in conditions of dynamic loading, static loading, and unloaded conditions. In all cases, the Smart Washer could reliably detect 0.050-in. (1.27-mm) long cracks, and could track crack growth out to lengths approaching 0.200 in. (5.08 mm).
This work was done by Bruce McKee, Yuri M. Shkarlet, Atteen Khatkate, Tom Banas, Richard Ingram, and David Perkins of Innovative Dynamics, Inc., Cornell Research Park, for Marshall Space Flight Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Mechanics category, or circle no. 176 on the TSP Order Card in this issue to receive a copy by mail ($5 charge). MFS-26479