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Vibratory stress relief (vsr) of metal is the process of neutralizing residual stress by means of mechanically induced cyclic force. To further understand the term one must understand the terms vibratory, cyclic force, residual stress, and stress relief.

Vibration (or vibratory) can be defined as the back and forth motion of an elastic body that is subjected to cyclic force. Cyclic force can be either a constant repetition of unidirectional applied force (like a jack-hammer) or the constant application of a sinusoidal force (like an off-balanced wheel on a car).

Residual Stress
Residual stress is a pressure that remains within a metal work piece after its source has been removed. Problems associated with residual stress in metal are distortion and fatigue. Residual stress results following thermal or mechanical straining.

Therefore there are two kinds of residual stress in metal – thermal and mechanical. Thermal residual stress results following manufacturing processes that subject the metal to a sharp temperature drop such as welding, casting, hot rolling, hardening, burning, cutting, hot forging, Electrical discharge machining (EDM), and at times machining, Grinding (abrasive cutting), extruding and heat treating. Mechanical residual stress results following manufacturing processes that forcibly change the shape of the grains such as cold rolling, bending, stamping (metalworking), cold forging, and some extruding.

Stress Relieving
Stress relieving is for the purpose of reducing the potential of having distortion and/or fatigue related problems. There are various ways to stress relieve metal – natural aging or seasoning, heat treating or thermal, peening, cryogenics, (stretch - deformation) (compression - Physical compression), and vibration.

There are two main practices of using vibration to attempt to stress relieve metal – resonant and non-resonant. Benefits of vibratory stress relief include low cost, speed of treatment, wide size and weight capacity, no discoloration, and portability of the equipment in comparison to other stress relief methods.

Resonant-vsr can be defined as using “at-peak-amplitude” or at-resonance vibrations to stress relieve metal. The mode of operation of resonant-vsr is to induce a high amplitude vibrational energy into the work piece.

Non-resonant-vsr (non-resonant is also known as sub-resonant and sub-harmonic) can be defined as using any level of vibrational energy except the peak of resonance to stress relieve metal. However, the promoter of non-resonance-vsr always promotes the sub-harmonic zone which can be identified as “at or near the leading lower portion of the harmonic curve” which produces low amplitudes in comparison to the at-resonant-vsr approach.

Metallurgically, sub-harmonic-vsr will maintain the mechanical properties of the treated workpiece including strength, ductility, hardness, and toughness. Residual stress levels are significantly reduced as verified by neutron diffraction and Barkhausen Effect testing methods.

Sub-harmonic-vsr can be used to neutralize the thermal residual stress, that is, the stress that was caused by a sharp temperature drop in the metal work piece.

Limitations. Vibratory stress relief cannot be used to harden, soften, or straighten work pieces, nor can it toughen material. Vibratory stress relief is not effective in reducing the mechanically induced residual stress. Nor can vibratory stress relief be effective on copper and high copper content alloys.