Echometer Ultrasonic Bolt load measurement System By BOLTIGHT
Transcript of Echometer Ultrasonic Bolt load measurement System By BOLTIGHT
Echometer – Ultrasonic Bolt load measurement System
ByBOLTIGHT
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Echometer –Ultrasonic Bolt load measurement System
By
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Where it’s used
Most commonly used as load validation method.
When torqueing a joint the coefficient of friction
cannot be accurately known.
Echometer acts as an independent check on
correct joint tightening.
In service monitoring – commonly with wind
turbines.
When the original bolt load is unknown – the
Echometer can be used “backwards” to
determine the load that was removed from the
bolt.
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Benefits
• Compared to other methods of measuring
elongation the Echometer is incredibly
fast.
• Can be used to create a permanent
electronic record for bolts and joints –
especially useful for in service monitoring.
• Only minor access is required – useful for
difficult to access bolts.
• In unit measurement conversion (stress,
strain etc.) gives quick overview of what
is happening in the joint.
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• It is impossible to give an absolute accuracy value for ultrasonic elongation
measurement.
• This is due to the method having many variables from joint condition to operator
experience.
• From experience with good condition bolts an accuracy with 5% can easily be
achieved.
• This can be improved even further in lab conditions and operator experience and
care.
Accuracy
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How itworks
• The Echometer is Boltight’s ultrasonic
elongation measurement system.
• It transmits an ultrasonic pulse into a bolt and
then times how long it takes for that pulse to
reflect back to the transducer.
• The bolt is then stretched and another time
reading is taken.
• This process is shown by the animation to the
right.
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How itworks
• The change in time equates directly to the bolt
elongation after tightening.
• This can be combined with the ultrasonic
velocity and stress factor for the material to
convert this change in time into an elongation
measurement.
• This measurement can be directly converted
further into stress, strain or load using
application specific information.
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What are thelimitations• Firstly the material must conduct ultrasound – this is
not usually an issue with
bolting steels, but worth noting for exotic applications.
• Only operates at ambient temperature range (-10°C to
80°C) – Measurements at these limits should also try
to be avoided where possible.
• Must be able to take a loaded and an unloaded reading
– it doesn’t matter if the bolt is loaded to begin with,
however if the bolt cannot be loosened then it will be
impossible to get a reading different from the reference
reading.
• Thick coatings or rust create air gaps between
the measuring surface and transducer which
prevents good energy transmission of the
ultrasonic pulse preventing readings.
• Ideally the bolts should have machined ends with a
surface finish of at least 1.6 microns.
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• The following few slides refer to the effects of bolt end condition – many of these
will make reference to the reflected pulse creating noise in the bolt.
• Noise refers to random, unpredictable and undesired signals that mask
desired information content.
• With the Echometer noise affects the interpretation of the return pulse as it
causes lower amplitude signals to be received into the transducer at the
same time as the desired pulse reading.
• As shown previously when waves interact they superimpose each other which
can drastically change the shape of the return wave making it difficult to
interpret.
• Because of this noise generation should try to be avoided where possible.
Noise
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Transducers and TemperatureProbes
The size and frequency of the transducer is important.
Transducer sizes are specified based on the crystal
diameter.
Choice of frequency will vary depending on material
and bolt length.
Lower frequencies travel further whereas higher
frequencies give a better resolution.
Temperature changes both the physical length and the
ultrasonic length of a bolt.
These change at different rates to each other.
The variation in rate change is what limits the operating
temperature to ambient.
Temperature probes are recommended when
considering in-service monitoring.
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Calibration bars are used to perform a zero point
calibration.
Zero point calibration is important if multiple
Echometers are to be used to measure the same
data.
Also if in-service monitoring will last so long as to
require replacement of any of the equipment.
The zero point calibration ensures that each
Echometer, cable and transducer combination are
measuring from the same start point.
Calibration Bars
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Basic Principles -Waveform
• The image to the side shows a fundamental sinewave.
• The terminology however refers to all waveforms; simple or complex.
• The amplitude of the wave can also bedescribed as the volume of the wave or sound.
• The period can also be described as pitch,wavelength or frequency.
• With ultrasonics we are actually creating a longitudinal wave in the material as shown in the animation.
• This will be represented as a transversewave (the above picture) for clarity.
• The high density sections are the positive amplitude and the low density are the negativeamplitude.
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Basic Principles -Ultrasound
• Ultrasound is the label given to any sounds with a frequency higher
than the upper audible limit of human hearing.
• Frequency of a wave (can also be described as pitch) is measured in
Hertz (Hz).
• Hz is simply defined as number of cycles per second.
• Upper limit of human hearing is 20 kHz (20’000 Hz).
• Ultrasonic elongation measurement takes place above 2 MHz
(2’000’000 Hz).
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• A pulse (and a wave) transfers energy through a material of a
consistent density.
• The animation above depicts a pulse travelling down a dense
medium (a bolt) and transferring into a less dense medium (air).
• Note that when the pulse reaches the boundary between then air
and the bolt the energy is divided into a reflected pulse (which
remains in the bolt) and a transmitted pulse (which transfers into the
air).
• The reflected pulse has a smaller amplitude – due to the transfer of
some of the energy into the air.
Basic Principles – SoundsWave Reflection
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• The closer the densities of the materials the more energy is
transferred into the second the material and therefore the less is
reflected back up the bolt.
• Note the speed and frequency of the reflected pulse remain the
same – this is due to no change in the material density.
• The speed of the transmitted pulse increases due to the reduced
density of the air however the frequency will remain the same.
Basic Principles – SoundsWave Reflection
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Basic Principles –WaveInteraction
• Wave interaction is important to understanding
the performance of an ultrasonic transducer.
• The sound that emanates from an ultrasonic
transduce does not originate from a single
point but instead many points along the
surface of the piezoelectric crystal.
• This results in a sound field with many waves
interacting / interfering with each other.
• When waves interact they superimpose each
other as is shown in the animation to the side.
• The amplitude of the wave at any point is the
sum of the amplitudes of the individual waves.
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• Ultrasonic velocity of a material refers to the speed at which an
ultrasonic pulse will travel through the material in question.
• The ultrasonic velocity of a material is affected by interactions of a
materials microstructures – also known as the grain structure in
steels.
• As the grain structures in steels vary slightly from sample to sample
so too does the ultrasonic velocity.
• Measuring ultrasonic velocity is as simple as measuring the time it
takes for a pulse of ultrasound to travel from a known length of
material and back again.
• This technique is known as pulse echo and is suitable for
estimating acoustic velocity to 1% accuracy.
Basic Principles – UltrasonicVelocity
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Ultrasonic Length vs PhysicalLength
• When the initial reference time for a bolt is taken it is usually displayed as a
reference length.
• Note that this length is an ultrasonic length rather than a physical length and
will most likely vary slightly from the physical length of the bolt.
• The materials programmed into the Echometer initially are based on large
sample material testing to generate an average value for each material.
• In reality each sample will have a slightly different ultrasonic velocity.
• This is due to variations in the grain structure from sample to sample
mentioned previously.
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• An ideal bolt will transfer as much sonic energy as
possible from the transducer into the bolt. The
energy will be sent directly down the centre of the
bolt and back into the transducer.
• Avoid rough or irregular surface finish. While this
can be filled with couplant it still results in reduced
energy transmission and signal dispersion.
• Avoid non-perpendicular ends. If the bolt end is
not perpendicular to the bolt axis then the energy
will be transmitted toward the side wall. This will
result in the signal reflecting along the bolt and
yield poor signal quality and measurement errors.
Bolt End Condition –Transducer Placement
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Bolt End Condition –Transducer Placement
• Avoid bolt ends with raised grade marks or
indentations with a raise edge. These can cause
the transducer to be seated at an angle preventing
adequate contact.
• Avoid bolt ends with numerous or large
indentations. Small indentations can be filled with
couplant, this reduces signal strength slightly but
normal measurement should still be possible.
Many or large indentations cause too much of a
reduction in signal strength to allow accurate
results.
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Bolt End Condition –Reflection End
• Avoid rough reflecting bolt ends. Similarly to the
transducer placement if the reflecting end of the
bolt is rough or curved, most of the reflected
energy will be dispersed and a weak or distorted
echo will be received.
• Avoid reflecting bolt ends not perpendicular to the
axis of the bolt. Sonic energy will be reflected
toward the sidewall of the bolt.
• Avoid non parallel reflecting bolts ends due to
bending of the bolt.
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Transducer BeamSpread• The energy beam produced by a transducer
spreads out as it propagates through the material
– this phenomenon is referred to as beam spread
or ultrasonic diffraction.
• Beam spread occurs because as the wave
travels through the material the energy transfer is
from particle to particle – if the particles are not
directly aligned in the direction of the wave
propagation then a portion of the energy is
transferred at an angle.
• Beam spread is largely determined by the
frequency and diameter of the transducer.
2 MHz Frequency
5 MHz Frequency
9 MHz Frequency
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Transducer BeamSpread
• Spread is greater in lower frequencies than
higher frequencies.
• Additionally as the diameter of the transducer
increases beam spread is reduced.
• Beam spread is an important consideration for a
couple of reasons – firstly spread lowers the
amplitude of reflections thereby making the
return signal weaker.
• Secondly beam spread may result in difficulty
interpreting the return wave due to reflections
from the bolt sides causing noise.
2 MHz Frequency
5 MHz Frequency
9 MHz Frequency
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Transducer Selection
• Correct transducer selection is important because as we
have just covered the lower the frequency of the pulse the
greater the beam spread.
• However as sound waves travel through a medium they lose
energy to the medium and are damped.
• This is due to the molecules generating heat as they are
forced to vibrate back and forth to transmit the wave.
• Therefore a sound wave can only propagate through a
limited distance.
• Based on this lower frequency waves will travel further
because less energy is transferred to the medium.
• Note that although damped waves have decreasing
amplitudes, their frequency is unaffected.
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Temperature Compensation
• As the temperature in a material increases two things happen to that material.
• Firstly there is a physical increase in length.
• Secondly the particles within the material start to vibrate more.
• This vibration makes it harder for the ultrasonic pulse to penetrate the material – meaning the
pulse travels slower - effectively increasing the ultrasonic length.
• The increase in ultrasonic length happens at a greater rate than the physical change in
length.
• For the purposes of the Echometer these two factors are combined into a single value known
as the Temperature Coefficient.
• If the Echometer is to be used as a one time validation check then temperature compensation
does not need to be considered – this is more critical to in service monitoring.
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Thank You We look forward to further discussions on our portfolio and to serve you in
the near future.
Warm regardsMurali Narasimhan (Managing Partner)[email protected] : +91-9902783547Coretech Industrial Solutions#202A, Brigade MM Annex Block, K.R Road,7th Block Jayangar, Bengaluru -560070Website : www.coretechindustrial.in