CINCINNATI - In 1989, the San Francisco-Oakland Bay Bridge's upper deck suffered a catastrophic collapse, in part because of a corroded bolt. The Fastener Quality Act of 1990 was enacted by Congress after several such high-profile fastener failures.

While highly engineered bolts aren’t required for assembling products like furniture, areas where performance is critical, such as airplane body assembly, bolting a turbine engine disc to a rotor, or racecar suspension, demand top performance.

Lambda Technologies Group www.lambdatechs.com has released a study showing how threaded fastener fatigue life can be dramatically improved by using a technique called “compressive residual stress.” The approach Lambda uses, called “Low Plasticity Burnishing (LPB),” is proving superior to another more widely used technique for surface enhancement – shot peening – which tends to dimple and distort the thread face, while not adequately treating the thread root. 

“LPB works because we can design custom-shaped tooling that fits into the root of the thread," says Doug Hornbach, CEO of Lambda Research, Inc., the research and materials testing laboratory that is part of Lambda Technologies Group. In an experiment conducted on a threaded rod manufactured from B7 steel (AISI 4140, ASTM) AI93), Lambda Technologies used Low Plasticity Burnishing (LPB®) to introduce high compressive residual stress into a threaded fillet. Fatigue testing showed the LPB samples had a fatigue life 15X greater than untreated specimens.

LPB is a patented mechanical process that applies pressure to the surface of metal components using a custom-designed hydrostatic burnishing tool. The pressure is delivered with enough force to generate plastic deformation, compressing a layer of the metal so that it resists damage from foreign objects, corrosion fatigue, fretting, stress corrosion cracking, and other common damage mechanisms.

"The LPB process provides a means of introducing an optimal compressive residual stress in the thread root where failures occur,” says Hornbach. Lambda conducted this experiment on steel, though LPB can be performed on almost any alloy.

"The speed at which we can treat the screws and bolts is another major benefit of the process," says Paul Prevéy, CEO of Surface Enhancement Technologies (SET), the division of Lambda Technologies Group that develops surface treatment solutions for commercial engineering applications. "LPB is performed on conventional CNC machines. With customized tooling installed on a high-speed lathe application, LPB is applied in a second or less."

Lambda Technologies Group, headquartered in Cincinnati, Ohio, is an innovative company incorporating a premier materials research laboratory with a world-class engineering and production enterprise dedicated to the development and optimization of surface treatments to improve component performance. The complete case study on improving threaded fastener fatigue performance is available at www.lambdatechs.com/diffraction-notes.