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. |
