Vibratory stress relief
From Wikipedia, the free encyclopedia
Vibratory Stress Relief, often
abbreviated VSR, is a non-thermal stress relief method used by
the metal
working industry to enhance the dimensional stability and
mechanical integrity of castings, forgings,
and welded components,
chiefly for two categories of these metal workpieces:
-
-
Precision components, which are machined or
aligned to tight dimensional or geometric tolerances.
Examples include machine tool bases or columns,
components of paper
mill, mining
equipment, or other large-scale processing
machinery, and centrifuge rotors.
- Heavily loaded
metal workpieces, which are components designed and
built with the ability to withstand heavy loads.
Examples include lifting yokes, clamshell buckets, crane
bases, vibratory screening system frames, ingot
processing equipment, and rolling
mill equipment.
These stresses are called residual
stresses,[1] because
they reside within the metal workpiece. Residual stresses are
caused by rapid, unequal cooling as opposed to the stresses
caused by external loading. This unequal cooling occurs during
welding, casting, forging, rough machining or hot
rolling. These stresses often lead to distortion or warping of
the structure during machining, assembly, testing, transport,
field-use or over time. In extreme cases, residual stress can
cause structural
failure.
Almost all vibratory stress relief equipment
manufacturers and procedures use the workpiece’s own resonant
frequency to boost the loading experienced by induced
vibration, so to maximize the degree of stress relief achieved.
Some equipment and procedures are designed to operate near, but
not at, workpiece resonances (perhaps to extend equipment
life)(example WIAP research,[2] but
independent research[3] has
consistently shown resonant frequency vibration to be more
effective. See references 4, 6, and 9.
The effectiveness of vibratory stress relief
is highly questionable.[4] In
general, the strain amplitudes achieved during vibratory stress
relief are too low to exceed the critical stress required to
activate mechanical relaxation during the induced low amplitude
high cycle fatigue excitation of the transducer vibrations. If
the strain amplitudes were increased to a level sufficient to
cause instability in the residual stresses, fatigue damage would
occur.[5][6] For
most applications, conventional stress relief methodologies
should be applied to components that require the reduction of
residual stresses.[7]
Criteria for effective VSR treatment[edit]
Effective vibratory stress relief treatment
results from a combination of factors:
-
1. Material condition: The
material must be ductile.
Metal in the welded, cast, forged, or hot-rolled condition
can be treated. Material that has been severely cold-rolled
or through-hardened, which renders the metal non-ductile,
will resist effective treatment.
-
2. Component geometry: Large workpieces lend
themselves well to vibratory stress relief, likely due to
their being more able to be resonated, however a variety of
modest-sized workpieces (overall size less than 20" / 500
mm) have been effectively stress relieved, using vibration.
-
3. Setup for VSR Treatment involves several steps.
- Placing workpiece
upon load cushions. These cushions should be made of
soft, yet resilient material, typically urethane or
neoprene. The cushions should be placed away from the
corners of the workpiece, so that workpiece damping is
minimized, which promotes increased resonant response to
vibration.
- Positioning,
orienting, and securely clamping vibrator on workpiece. The
vibrator should be placed away from the corners of the
workpiece, and oriented so that the force-field output
of the vibrator, with rotary vibrators a plane
perpendicular to the vibrator’s axis of rotation, can
drive the workpiece into resonance. Dual-mount-flanged
vibrators are helpful in achieving effective
orientation. The vibrator must be securely clamped,
typically with machinist-grade clamps or high-tensile
bolts.
- Positioning and
orienting vibration sensor. The best location for
this sensor is on one of the corners of the workpiece,
and in-line with the force-plane of the vibrator (a
plane perpendicular to the vibrator’s axis of rotation [AOR]).
- Adjustment of the
vibrator unbalance. The unbalance of the vibrator
should be sufficient to drive the resonances of the
workpiece, minimally to a level of a few g’s of
acceleration. The unbalance might require further
increase, to cause peak growth (discussed later) during
stress relief treatment.
-
4. Finding Resonance(s). The vibrator speed range
must reach high enough to be greater than the resonance(s)
of the workpiece. A max speed capability of at least 6000 –
8000-RPM is recommended. Equally important is tight vibrator
motor speed regulation (±0.25%), which greatly improves the
ability to detect and drive the resonance(s) (abilities that
are required for stress relieving to occur). Driving a
resonance involves tuning the vibrator speed to the top of
the resonance peak. This is increasingly challenging as
workpiece rigidity increases, which causes resonances to
become very narrow. To record such resonances, a slow,
automated scan through the speed range and plotting of the
vibration response of the workpiece is made.
-
The scan rate must be slow, not only because the resonance
peaks are narrow, but also due to the high-inertia of the
workpiece. There is a significant time delay, caused by this
high workpiece inertia, in the response to vibration. This
can be best explained by first looking at the phenomenon
known as ring time.
-
Ring time is defined as the time period a resonating body
continues to vibrate after resonant excitation is stopped.
When the vibration is stopped, the waveform will decay, ie,
reduce in amplitude, due to frictional losses. See Figure
1
-

Figure 1: A waveform that demonstrates
ring time, which is the time period vibration
continues, after resonant excitation ceases.[8]
-
Most people have experienced ring time. A large bell, after
being struck, continues to emit sound, but at decreasing
(softer) sound levels. Over time, the sound level
dissipates, as the vibration amplitude decays to an
undetectable level.
-
When vibration is the excitation causing resonance (rather
than a hammer blow [such as the strike of a bell]), there is
a time period between the beginning of vibration excitation,
and the moment when full resonant amplitude is reached.
During this time the amplitude is building-up or growing
(the reverse of decaying), so this phenomenon is called
reverse ring time, or RRT. For large metal structures that
are commonly stress relieved with vibration, ring or reverse
ring times (the time periods are the same, whether the
amplitude is growing or decaying), can be 20 – 40 seconds or
longer. See Figure 2.

Figure 2: Reverse ring time, or RRT, is
the time period between the start of vibration
excitation, and full resonant amplitude.[9]
-
The most frequently used method of finding the resonances of
a workpiece during vibratory stress relief is to scan
through the vibrator speed range, and record / plot the
vibration amplitude vs. the vibrator speed. The effect of
RRT, specifically the time delay between the beginning of
resonant vibration and full resonant amplitude being
achieved, dictates that the scan rate used to sweep through
the vibrator speed range be slow, in order to make an
accurate record of the resonance pattern.
-
Scanning too quickly will result in resonant peaks not being
fully depicted or being missed entirely, since the workpiece
will not have sufficient time to reach full amplitude
resonance before the vibrator speed increases (due to
scanning) beyond the resonance frequency.
-
A scan rate of 10-RPM / second has been found in practice to
result in the accurate resonant peaks recording of many
workpieces. As workpiece size increases, the scan rate might
require being decreased, in order to fully capture accurate
resonance data. See Figure 3.

Figure 3: The effects of scanning at
different scan rates: 10 and 50-RPM/sec. Peaks
that are scanned too quickly don't have enough
time to reach full resonant amplitude, due to
the RRT effect. The larger and heavier the
structure, the greater the inertia, the longer
the ring time (and reverse ring time): Thus,
larger, heavier structures might require slower
scan rates to plot accurate resonance patterns.
-
5. Tuning Vibrator Speed. The vibrator speed is then
tuned upon the resonance(s) recorded during the scan, and
the response of the workpiece to vibration is monitored.
Fine tuning of the speed, plus tight speed regulation,
enhances peak tuning and tracking capabilities. The most
common responses to treatment are:
-
Peak Growth - Typically the larger change.
-
Peak Shift, in the direction of lower RPM -
Percentage-wise, the smaller change. Typically resonance
peaks are very narrow, causing any peak shifting to
rapidly decrease the vibration amplitude, and hence,
rapidly decrease of the rate of stress relief, since
resonant amplitude is more effective in relieving
stress. Thus, any peak shifting requires fine-tuning
adjustment of the vibrator speed, in order to track the
peak to its final, stable position.
Each of these changes, which often combine,
i.e., peak growth AND shifting, is consistent with a lowering of
the rigidity of the workpiece. The workpiece rigidity is
inflated by the presence of residual stress. In the example
below, which depicts a common resonance pattern change that
occurs during vibratory stress relief, the large peak grew by
47%, while simultaneously shifting to the left 28-RPM, which is
less than 0.75%. See Figure 4.
The equipment used to perform this stress
relief had vibrator speed regulation of ± 0.02%, and speed
increment fine-tuning of 1-RPM, which allowed even subtle
shifting of the peaks to be accurately tracked to their final,
stable locale.
The pattern of change, i.e., how quickly the
peaks grow and shift, is faster at the beginning of vibration
treatment: As treatment continues, the rate of change decreases,
eventually resulting in a new, stable resonance pattern.
Stability of this new resonance pattern indicates that
dimensional stability of the workpiece has been achieved.

Figure 4: VSR Treatment Chart consists
of two plots: The upper plot is workpiece
acceleration, the lower plot is vibrator input
power, simultaneously plotted vertically vs. a
common horizontal axis of vibrator speed. Peaks
in the acceleration data depict resonances;
growth and shifting of the peaks are the
response of the workpiece to treatment.
The power plot is useful in both positioning
and orienting the vibrator, and when adjusting the vibrator
unbalance. Poor or inappropriate vibrator locations or
orientations, or excessive vibrator unbalance settings, cause
large peaks in the power plot. Use of higher-powered vibrator
motors (above 2-kW) provides more "head-room" for peaks in power
to be tolerated, and treatment to take place, which was the case
here: The power peak at ≈ 3700-RPM was only half of the vibrator
motor’s 2.3-kW power capacity (top of the power scale).
A Pre-Treatment Scan, which functions
as a base-line, is first recorded in green. The operator uses
this green data set to tune upon the resonances, and monitor the
growth and shifting of the resonance peaks. After peak growth
and shifting have subsided, a Post-Treatment Scan is made
(red). This data is superimposed on the original, green,
Pre-Treatment Scan data, documenting the changes in resonance
pattern. The stress relief treatment resulted in 47% growth of
the original, large peak, while it shifted to the left 28-RPM
(less than 0.75%).

Figure 5: Vibratory Stress Relief was
performed on this mild steel weldment weighing
almost 12 tons. Overall size was 17' × 15' × 2'
(≈ 5.2 × 5.6 × 0.6 meters). Workpiece was
supported on three, red urethane load cushions
(two of which are circled), which are positioned
far from the corners of the workpiece to
minimize damping, thus promoting resonance,
which is required for stress relief to be
achieved. The vibrator can be seen in the left,
mid-ground (circled), and the accelerometer
(vibration sensor whose output is proportional
to acceleration), can be seen in the central,
left, foreground (circled).
After stress relief treatment, the braces
(rust-colored, structural beams), which are used to maintain the
desired shape during welding, were removed. The spacing between
the two "arms" remained the same; no change was detectable
(measured to 1/32" or less than 1 mm), and the spacing remained
so throughout assembly, testing (to 60 ton test loads),
transport, and installation.
When should VSR be considered and the limits of TSR[edit]
VSR is not accepted by the Engineering
community at large as a viable method of relaxing or reducing
residual stresses in components that require it. For general
use, conventional residual stress relaxation methodologies are
recommended.[10]
Historically, the first type of stress relief
was performed on castings by storing them outside for months or
even years. This was referred to as curing, a term used
for long-term storage of freshly hewn wood. Fresh castings were
referred to as being green, meaning, they were prone to
distortion during precision machining, just as green wood
bows during cutting.
Later, thermal stress relief (TSR) was
developed to alleviate the lengthy time requirements of curing.
It has been known for many years, however, that TSR has
limitations or shortcomings, specifically:
-
-
Furnace size: workpieces can be too large to fit.
- Not effective on all
alloys, among them austenitic stainless steels.
- Should not be used on
welded structures made of low-carbon, high-strength
steels, which can suffer loss of physical properties
and/or crack initiation if thermally stress relieved.[11][12]
- Cannot be used on
workpieces that have been quenched and tempered (Q&T)
without risking loss of physical properties. Vibratory
stress relief can be successfully applied, if some level
of ductility is present after Q&T, together with
acceptable workpiece geometry (which determines resonant
vibration frequency required).
- Often not suitable for
rough-machined components, due to difficulty in removing
scale (rust-colored skin that develops on ferrous
components while in-furnace), without damaging machined
surfaces.
- Asymmetrical-shaped
workpieces, which are difficult to cool while
maintaining uniform temperature, can develop new,
unacceptably high-level, residual stress patterns during
the last stage of TSR. Cooling rates can be slowed, but
with increased costs.
Metal components, whose function would be
enhanced by stress relief, and fall into one or more of the
above categories, are strong candidates for VSR for
quality-related reasons.
Further, there is a strong economic incentive
to use vibratory stress relief on large workpieces, since stress
relief using a furnace (thermal stress relief or TSR) is highly
energy-intensive; consuming much natural gas, and hence,
producing much CO2.
The cost of TSR is approximately proportional to a metal
component’s weight or overall size, estimated to be $2,500 USD
for the structure pictured, plus transportation costs, which
might involve special transport permits, to and from a furnace.
VSR Treatment would cost a company owning appropriate equipment
less than 15% as much ( ≈ $400 ) as TSR Treatment, chiefly
amortization of equipment investment plus labor, and a modest
amount of electrical consumption, and treatment would take less
than two hours, with no transport required. However, the lack of
independent data to show that this technique is effective may
mean that even that lesser investment is not of any value, so
use of VSR should evaluated very carefully before proceeding.
References[edit]
-
Notes
-
Jump up^ [1] R.T.
McGoldrick and H. Saunders, Some Experiments in
Stress-Relieving Castings and Structures by
Vibration, Journal of the American Society of Naval
Engineer., 55, 589-609 (1943)
-
Jump up^ [2] Wiap
Stress relief since 1981
-
Jump up^ [3] R.
Dawson and D.G. Moffat, Vibratory Stress Relief:
A Fundamental Study of Effectiveness, Journal of
Engineering Material and Technology, 102,
169-176 (1980)
-
Jump up^ J.
Stubbs, "Vibratory/Thermal Stress Relief in a Weld
Joint", Case Western Reserve University, 2003.
-
Jump up^ G.
Totten et al. "ASM Handbook of Residual Stress and
Deformation in Steel", 2001 p.54-67
-
Jump up^ [4] R.
Dawson and D.G. Moffat, Vibratory Stress Relief:
A Fundamental Study of Effectiveness, Journal of
Engineering Material and Technology, 102,
169-176 (1980)
-
Jump up^ ASM
Metals Handbook, Volume 4, "Heat Treating, Cleaning
and Finishing", 1991
-
Jump up^ [5] C.A.
Walker, A.J. Waddell and D.J. Johnston, Vibratory
Stress Relief - An Investigation of the Underlying
Process, Proc. Inst. Mechanical Engineers., 209,
51-58 (1995)
-
Jump up^ [6] S.
Shakar, Vibratory Stress Relief of Mild Steel
Weldments, PhD Dissertation, Oregon Graduate
Center, U. of Oregon, 1982
-
Jump up^ ASM
Metals Handbook, Volume 4, "Heat Treating, Cleaning
and Finishing", 1991
-
Jump up^ [7] B.B.
Klauba and C.M. Adams, A Progress Report on the
Use and Understanding of Vibratory Stress Relief,
Proc. Winter Meeting of the ASME AMD 52, 47-57
(1982)
-
Jump up^ [8] W.
Hahn, Report on Vibratory Stress and
Modifications in Materials to Conserve Resources and
Prevent Pollution, Alfred University (NY),
Center for Environmental and Energy Research (CEER),
2002
-
Bibliography
PDF D. Rao, J. Ge, and L. Chen, Vibratory Stress Relief
in the Manufacturing the Rails of a Maglev System, J. of
Manufacturing Science and Engineering, 126, Issue 2, 388-391
(2004)
PDF B.B. Klauba, C.M. Adams, J.T. Berry, Vibratory Stress
Relief: Methods Used to Monitor and Document Effective
Treatment, A Survey of Users, and Directions for Further
Research, Proc. of ASM, 7th International Conference: Trends in
Welding Research 601-606 (2005)
PDF Y. Yang, G. Jung, and R. Yancey, Finite Element
Modeling of Vibratory Stress Relief after Welding, Proc of ASM,
7th International Conference; Trends in Welding Research547-552
(2005)
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