Peening Proves Promising Process

You'll never look at metal the same after attending shot peening training classes at an Electronics Incorporated seminar.

Shot peening is one of those techniques that we've all heard about but about which few of us know much beyond the fact that it involves beating up a piece of metal with little pieces of metal or shot. But shot peening is an important process that plays a key role in aircraft maintenance.

Jack Champaigne, president of Electronics Incorporated (www.electronics-inc.com), is a long-time expert in the art of shot peening. His company sells many of the materials needed for shot peening, devices with exotic names like Almen gages and Magna valves, plus runs annual shot peening workshops and trade shows and publishes a quarterly magazine called The Shot Peener. Who knew there was so much information about what appears to be a fairly ordinary process?

Shot peening works by adding compressive stress to metal, which helps the metal resist cracking and prevents small cracks from growing. Part-manufacturing operations like machining tend to promote tensile stress, which tends to pull metal apart, and shot peening adds compressive stress to counteract the tensile stresses.

Nowadays, shot peening is used for both manufacturing and maintenance. Helicopter parts are peened to remove excess adhesive, turbine engine blades receive smoothing surface peening, gears get peened to remove scaling, and all sorts of parts are peened during overhaul processes.

Peening was invented, according to Champaigne, by John Almen, who worked for the Buick Division of General Motors. Almen discovered that a cleaning process like shot peening, used to clean engine valve springs, imparted extra strength to the springs. The springs that were cleaned lasted a lot longer, and thus shot peening was born.

Shot peening has over the years developed into a rigorous process, with plenty of research to support the various techniques used today. A new development is that non-destructive testing techniques are now being employed to inspect shot-peened parts to verify that the treatment achieved the desired results. Another new development, according to Champaigne, is the use of shot peening to strengthen welds. During recent testing by the SAE Fatigue Design and Evaluation Committee, unpeened weld specimens failed after 250,000 cycles, while the peened specimens reached more than 2.5 million cycles before failing in an area outside the weld.

There is a lot more to shot peening and the best way to learn about it is to attend one of the Electronics Incorporated workshops. Like the NDT field, there is no FAA certification of shot peening, but within the shot peening industry, there are standards for the process and for tool calibration that must be followed in order to achieve the desired result. -- By Matt Thurber