Sub-Harmonic Energy for Stress Relief

The expression “beauty is only skin deep” applies to metal parts as well. The quality of metal parts lies mostly beneath the surface. Quality can vary greatly, even between two supposedly identically prepared parts and will affect machine stability, long-term shape stabilization and the part’s resistance to cracking in service.

Quality of parts can be affected by several factors including chemical composition, heat treatment, speeds and feeds of cuts, and sharpness of cutters. Another significant factor that affects the consistency of performance is stress.

Thermal stress is caused by sharp temperature drops in a metal part from operations such as welding, casting, hot rolling, machining, grinding, EDM machining and hardening. The sharper the temperature change, the more intense the pressure will be.

Thermal stress can lead to distortion immediately following machining or grinding, delayed distortion or premature cracking. It is extremely difficult to calculate and is not understood by most engineers.

Thermal stress can vary in strength from a negligible amount to an amount that is equal to the material’s yield strength. Therefore, reducing this stress is important to achieving and maintaining consistently high quality in metal parts.

Ignoring thermal stress can lead to increased stock removal, more cutting to achieve flatness tolerances, higher scrap rates, more frequent and extensive repairs, and early replacement of parts. One strategy for coping with thermal stress is to minimize thermal shock on parts by examining every manufacturing step along the way. Whenever a sharp temperature drop occurs, a slower cooling rate can be incorporated.

Stress relief of the materials is a more practical solution for controlling distortion after machining and reducing premature cracking to improve fatigue life. Certain stress relief methods, such as heat treating, natural aging, cryogenics, and stretch and compression, bring along with them drawbacks that include extended time, expense, treatment distortion, surface oxidation, and changed mechanical properties of the material, as well as size, weight and shape limitations. Vibrational stress relief can offset many of these drawbacks.

One vibration stress relief process that has experienced consistently successful results in the metalworking industry uses what is called sub-harmonic technology or Meta-Lax. Bonal Technologies developed sub-harmonic vibration technology in its own planermill machine shop after first experiencing inconsistent results when using the resonant vibration approach.

Companies can realize a number of benefits by using sub-harmonic vibration stress relief. The process requires only about 5 to 15 percent of the cost associated with heat treat stress relieving and less than 5 percent of the time, with no treatment distortion or changing of the mechanical properties. It can eliminate transportation and cleaning costs while providing improved workpiece quality.

The sub-harmonic vibration stress relief process has two fundamental principles. Sub-harmonic energy must be used for the stress relief dwell frequency; and the harmonic curve of a thermally stressed metal part will shift and stabilize to a new frequency location as the workpiece becomes relieved of thermal stress.

Metal components exhibit a harmonic reaction to induced energy. The harmonic curve occurs when the vibrated component cannot dissipate any more energy from the force inducer and it responds with an out-of-portion amplitude movement. At and near the leading edge of the harmonic curve is the optimum frequency for using vibrational energy for stress relief. The significance of plotting the harmonic curve is to establish where the sub-harmonic zone is.

Following a scan of the part, the stress relief dwell frequency of vibration is set at the appropriate sub-harmonic level and maintained at this level for 15 to 60 minutes depending on the metal and weight of the workpiece.

All metal components have a natural harmonic peak. If the part was subjected to a thermal shock (which causes thermal stress) during manufacturing, the harmonic peak would be in an unnatural frequency location. As the workpiece becomes stress relieved, by applying sub-harmonic vibrations, the harmonic peak will shift and eventually stabilize in a new frequency location. The stabilized frequency would be the natural harmonic frequency.

Beyond the initial treatment of the part following the first scan, the workpiece should be monitored periodically and the sub-harmonic frequency readjusted every time the harmonic curve shifts. Once the harmonic curve stabilizes in a new frequency location, stress relief is complete.

Stress relief is important to ensure consistent quality of performance as evidenced by machine distortion control, long-term shape stabilization and reducing premature cracking in service. The sub-harmonic vibration stress relief process has been verified academically as well as proven in field applications to be an effective stress relief alternative to other methods, and is well-suited in advance of any manufacturing step where distortion and/or cracking are likely to cause rework or scrap.