STRESS RELIEF BY VIBRATION (VSR)
Based on the weight of the piece, the VSR method introduces into it high amplitude and low frequency vibrations for a given period of time. This relieves residual stress without distortion or alteration of tensile strength, yield point or resistance to fatigue, and the static equilibrium is restored.
The most efficient vibrations are the resonant ones, because in the resonance frequency vibrations stress is better distributed, if compared with sub-resonant frequency.
Low frequency vibrations carry high amplitude energy and are very efficient in the significant decrease of peak residual stress in metallic parts and welds. The equipment usually employed consists of a sturdy vibrator of variable speed which is attached to the piece and an electronic control panel. Both are mounted into a portable cabinet.
Also attached to the piece is an accelerometer that detects vibrations and transmits a signal to the control panel. The resonance point is then determined and displayed on a dial. If the vibrator is equipped with a recorder, a chart can also be obtained.
The point of resonance is attained by varying the frequency of the vibrator until the proper one is reached. Two minutes is the average time required to reach the resonance frequency. At this point, vibration is maintained for a given time, depending on the weight of the piece and its intended application. The time may range from ten minutes to an hour or more, but if it is exceeded, the piece will not suffer any damage due to fatigue or loss of tensile strength.
If structures are very big, long or have open spaces, it may be necessary to apply the procedure in several points.
Some equipment carries out the vibration process automatically. Vibration is maintained for 15 minutes, in a sequence of three different selected frequencies, each lasting five minutes. This setting is efficient to treat pieces weighing up to ten tons. For pieces weighing more than ten tons two consecutive 15 minute periods can be used, without the piece suffering any harm.
Two simple rules should be followed for all applications:
a) Support the piece in the best possible manner, isolating it from the floor or rigid structures, thus leaving it free to vibrate.
b) The vibrator should be directly connected to the piece, in order to transfer the entire vibratory energy generated.
The method can be used on a wide range of ferrous and nonferrous metals, including carbon and stainless steel, cast iron, aluminum, titanium etc., in a large variety of shapes. Sizes can vary from small welded parts, shafts and gears, to large welded and machined steel structures. However, it presents some limitations: it is not efficient for extruded, cold worked and precipitation hardened materials.
One of the most important benefits of the use of the VSR method is its capacity to relieve stress at any point of the manufacturing process, such as after machining, snagging, drilling or grinding. In welded parts, stress relief can be performed during welding, which is very useful to prevent concentration of residual stress that may cause warping of the piece. The method is especially compatible with SMAW, GMAW and GTAW welding processes, popularly known as stick electrode, MIG and TIG. With other welding processes some logistical problems may arise.