Composites joined to avoid fatigue

In order to repair aluminium or composite structures in areas subject to high fatigue, it is often desirable to use fasteners that expand uniformly within the hole, putting it into compression.

Problem: The practice of pounding in fasteners slightly oversized for the holes to achieve the same effect is not satisfactory. Composites in particular are liable to become delaminated. Solution: Los Angeles based Monogram Aerospace Fasteners offers a solution called the 'Radial-Lok' blind fastener, which achieves uniform radial expansion throughout the length of a clearance fit hole. The installation tool applies torque to the screw, while the drive nut is held stationary. The screw advances through the centre of the fastener, forcing the nut body into the expansion sleeve, causing it to expand uniformly. Once the nut body is fully seated in the expansion sleeve, initial blind head formation commences. Continued tightening removes the sheet gap, clamps the sheets tightly together, and forms a large blind head, appropriate for joining composite materials. When the blind side sleeve forms tight against the structure, the screw fractures at the break groove. The tool is pulled away and the drive nut assembly discarded. Applications: Repair and construction of aerospace and motor sport automotive constructions, especially those involving use of composite sheets and subject to high fatigue loadings. Current applications include the F15, F18 and E-2C. Hole tolerance is 0.003in (0.076mm), avoiding the need to make the close tolerance or tapered holes required by some fasteners. Grip accommodation is 0.1 in (2.54mm). Sizes are 3/16 in to 3/8 in. Keith Brunell, director of sales and marketing, says the company has been making precision fasteners for more than a century. The fasteners are completely compatible with graphite and not subject to galvanic corrosion. TS Monogram Aerospace Fasteners