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Transcript of UT Testing-Section 4
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Section 4: Measurement Techniques
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Content: Section 4: Measurement Techniques
4.1: Normal Beam Inspection
4.2: Angle Beams
4.3: Reflector Sizing
4.4: Automated Scanning
4.5: Precision Velocity Measurements
4.6: Attenuation Measurements
4.7: Spread Spectrum Ultrasonics
4.8: Signal Processing Techniques
4.9: Flaw Reconstruction Techniques
4.10: Scanning Methods
4.11: Scanning Patterns
4.12: Pulse Repetition Rate and Penetration
4.13: Interferences & Non Relevant Indications
4.14: Exercises
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Expert at works
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4.1: Normal Beam Inspection
Pulse-echo ultrasonic measurements can determine the location of adiscontinuity in a part or structure by accurately measuring the time required
for a short ultrasonic pulse generated by a transducer to travel through a
thickness of material, reflect from the back or the surface of a discontinuity,
and be returned to the transducer. In most applications, this time interval is afew microseconds or less. The two-way transit time measured is divided by
two to account for the down-and-back travel path and multiplied by the
velocity of sound in the test material. The result is expressed in the well-
known relationship:
d = vt/2 or v = 2d/t
where d is the distance from the surface to the discontinuity in the test piece,
v is the velocity of sound waves in the material, and t is the measured
round-trip transit time.
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A-Scan
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A Scan
http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/MeasurementTech/applet_4_1/applet_4_1.htm
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Precision ultrasonic thickness gages usually operate at frequencies between
500 kHz and 100 MHz, by means of piezoelectric transducers that generate
bursts of sound waves when excited by electrical pulses. A wide variety oftransducers with various acoustic characteristics have been developed to
meet the needs of industrial applications. Typically,
1. lower frequencies are used to optimize penetration when measuring thick,highly attenuating or highly scattering materials,
2. while higher frequencies will be recommended to optimize resolution in
thinner, non-attenuating, non-scattering materials.
0.5 MHz ~ 100 MHz
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In thickness gauging, ultrasonic techniques permit quick and reliable
measurement of thickness without requiring access to both sides of a part.
Accuracy's as high as1 micron or0.0001 inch can be achieved in someapplications. It is possible to measure most engineering materials
ultrasonically, including metals, plastic, ceramics, composites, epoxies, and
glass as well as liquid levels and the thickness of certain biological specimens.
On-line or in-process measurement of extruded plastics or rolled metal oftenis possible, as is measurements of single layers or coatings in multilayer
materials. Modern handheld gages are simple to use and very reliable.
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4.2: Angle Beams I
Angle Beam Transducers and wedges are typically used to introduce arefracted shear wave into the test material. An angled sound path allows the
sound beam to come in from the side, thereby improving detectability of flaws
in and around welded areas.
= Angle of reflection, T=Material thickness, S= Sound path,
Surface distance = Sin x S, Depth= Cos x S
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A-Scan
http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/MeasurementTech/applet_4_2/applet_4_2.htm
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Angle Beam Transducers and wedges are typically used to introduce a
refracted shear wave into the test material. The geometry of the sample
below allows the sound beam to be reflected from the back wall to improvedetectability of flaws in and around welded areas.
= Angle of reflection, T=Material thickness, S= Sound path,
Skip = 2(T x Tan), Leg = T/Cos, V Path = 2 x Leg
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A-Scan
http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/MeasurementTech/applet_4_3/applet_4_3.htm
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Flaw Location and Echo Display
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Flaw Location and Echo Display
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Flaw Location and Echo Display
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Flaw Location and Echo Display
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Flaw Location and Echo Display
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Dead Zone
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Near Surface Detectability with Angle Beam Transducer
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Flaw Location
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Flaw Location with Angle Beam Transducer
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Flaw Location with Angle Beam Transducer
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Flaw Location with Angle Beam Transducer
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Flaw Location with Angle Beam Transducer
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Why angle beam assemblies are used
Cracks or other discontinuities perpendicular to the surface of a test piece, or
tilted with respect to that surface, are usually invisible with straight beam testtechniques because of their orientation with respect to the sound beam.
Perpendicular cracks do not reflect any significant amount of sound energy
from a straight beam because the beam is looking at a thin edge that is much
smaller than the wavelength, and tilted cracks may not reflect any energyback in the direction of the transducer. This situation can occur in many types
of welds, in structural metal parts, and in many other critical components. An
angle beam assembly directs sound energy into the test piece at a selected
angle. A perpendicular crack will reflect angled sound energy along a paththat is commonly referred to as a corner trap, as seen in the illustration below.
http://www.olympus-ims.com/en/applications/angle-beam-transducers/
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The angled sound beam is highly sensitive to cracks perpendicular to the far
surface of the test piece (first leg test) or, after bouncing off the far side, to
cracks perpendicular to the coupling surface (second leg test). A variety ofspecific beam angles and probe positions are used to accommodate different
part geometries and flaw types. In the case of angled discontinuities, a
properly selected angle beam assembly can direct sound at a favorable angle
for reflection back to the transducer.
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http://www.olympus-ims.com/en/applications/angle-beam-transducers/
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How they work -- Snell's Law
A sound beam that hits a surface at perpendicular incidence will reflect
straight back. A sound beam that hits a surface at an angle will reflect forwardat the same angle.
S d th t i t itt d f t i l t th b d i
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Sound energy that is transmitted from one material to another bends in
accordance with Snell's Law of refraction. Refraction is the bending of a
sound beam (or any other wave) when it passes through a boundary betweentwo materials of different velocities. A beam that is traveling straight will
continue in a straight direction, but a beam that strikes a boundary at an angle
will be bent according to the formula:
Typical angle beam assemblies make use of mode conversion and Snell's
Law to generate a shear wave at a selected angle (most commonly 30, 45,
60, or 70 degrees) in the test piece. As the angle of an incident
longitudinal wave with respect to a surface increases, an increasingportion of the sound energy is converted to a shear wave in the second
material, and if the angle is high enough, all of the energy in the second
material will be in the form of shear waves.
Th t d t t d i i l b t t k
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There are two advantages to designing common angle beams to take
advantage of this mode conversion phenomenon:
(1) First, energy transfer is more efficient at the incident angles that
generate shear waves in steel and similar materials.
(2) Second, minimum flaw size resolution is improved through the use ofshear waves, since at a given frequency, the wavelength of a shear
wave is approximately 60% the wavelength of a comparable longitudinal
wave, and minimum flaw size resolution increases as the wavelength of
a sound beam gets smaller.
S l ti th i ht l b bl
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Selecting the right angle beam assembly
The parameters that affect angle beam performance include not only the
(1)beam angle generated by the wedge, but also (2) transducer frequencyand (3) element size. The optimum beam angle will generally be governed
by the geometry of the test piece and the orientation of the discontinuities
that the test is intended to find. Transducer frequency affects penetration
and flaw resolution:
1. As frequency increases, the distance the sound wave will travel in a given
material decreases, but resolution of small discontinuities improves.
2. As frequency decreases, the distance the sound wave will travel increasesbut the minimum detectable flaw size will become larger.
3. Similarly, larger element sizes may decrease inspection time by increasing
coverage area, but the reflected echo amplitude from small discontinuities
will decrease. Smaller element sizes will increase reflection amplitude from
small discontinuities, but the inspection may take longer because the
smaller beam covers less area.
These conflicting factors must be balanced in any given application, based on
specific test requirements.
Contoured wedges
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Contoured wedges
The IIW recommends the use of a contoured wedge whenever the gap
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The IIW recommends the use of a contoured wedge whenever the gap
between the wedge and the test surface exceeds 0.5 mm (approximately
0.020 in.). Under this guideline, a contoured wedge should be used wheneverpart radius is less than the square of a wedge dimension (length or width)
divided by four:
whereR = radius of test surface
W = width of wedge if testing in axial orientation, length of wedge if testing in
circumferential orientation
Of course switching to a small wedge, if possible within the parameters ofinspection requirements, will improve coupling on curved surfaces. As a
practical matter, contouring should be considered whenever signal strength
diminishes or couplant noise increases to a point where the reliability of an
inspection is impaired.
Focused dual element angle beams
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Focused dual element angle beams
The vast majority of angle beam assemblies use single element, unfocused
transducers. However, in some tests involving highly attenuating or scatteringmaterials such as coarse grain cast stainless steel, focused dual element
angle beams are useful. Because they have separate transmitting and
receiving elements, dual element transducers can typically be driven at higher
excitation energies without noise problems associated with ringdown orwedge noise. Focusing permits a higher concentration of sound energy at a
selected depth within the test piece, increasing sensitivity to discontinuities in
that region.
High temperature wedges
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High temperature wedges
Standard angle beam assemblies are designed for use at normal
environmental temperatures only. For situations where metal must beinspeced at elevated temperature, special high temperature wedges are
available. Some of these wedges will tolerate brief contact with surfaces as
hot as 480 C or 900 F. However, it is important to note that high
temperature wedges require special attention with regard to the sound paththey generate. With any high temperature wedge, sound velocity in the wedge
material will decrease as it heats up, and thus the refracted angle in metals
will increase as the wedge heats up. If this is of concern in a given test,
refracted angle should be verified at actual operating temperature. As a
practical matter, thermal variations during testing will often make precise
determination of the actual refracted angle difficult.
Surfaces as hot as 480C / 900F
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snap-in
threaded
steel with a shear wave velocity of approximately 3,250 M/S or 0.1280 in/uS.
4 3: Reflector Sizing
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4.3: Reflector Sizing
There are many sizing methods, these include:
4.3.1 Crack Tip Diffraction
When the geometry of the part is relatively uncomplicated and the orientation
of a flaw is well known, the length (a) of a crack can be determined by a
technique known as tip diffraction. One common application of the tipdiffraction technique is to determine the length of a crack originating from on
the backside of a flat plate as shown below. In this case, when an angle beam
transducer is scanned over the area of the flaw, the principle echo comes
from the base of the crack to locate the position of the flaw (Image 1). Asecond, much weaker echo comes from the tip of the crack and since the
distance traveled by the ultrasound is less, the second signal appears earlier
in time on the scope (Image 2).
Crack Tip Diffraction Methods
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p
No animation.
Crack height (a) is a function of the ultrasound velocity (v) in the material, the
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g ( ) y ( ) ,
incident angle (Q2) and the difference in arrival times between the two signal
(dt). Since the incident angle and the thickness of the material is the same inboth measurements, two similar right triangle are formed such that one can
be overlayed on the other. A third similar right triangle is made, which is
comprised on the crack, the length dt and the angle Q2. The variable dt is
really the difference in time but can easily be converted to a distance by
dividing the time in half (to get the one-way travel time) and multiplying this
value by the velocity of the sound in the material. Using trigonometry an
equation for estimating crack height from these variables can be derived as
shown below.
Crack Tip Diffraction Method
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p
The equation is complete once
distance dt is calculated by dividing
the difference in time between the
two signals (dt) by two and
multiplying this value by the sound
velocity.
4.3.2 6 dB Drop Sizing-
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For Large Reflector (greater than beam width), i.e. there is no BWE.
6 dB Drop Method
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6 dB Drop Method
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6 dB Drop Method
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www.youtube.com/embed/hsR17WA3nHg
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4.3.3 The 20 dB drop sizing method
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We can use a beam plot to find the edge of a defect by using the edge of
the sound beam.
If we know the width of a beam at a certain distance from the crystal, we
can mark the distance across a defect from where the extreme edges of
the beam touch each end of the defect and then subtract the beam width toget the defect size.
When the signal from the defect drops by 20dB from its peak, we judge
that the edge of the beam is just touching the end of the defect. We canfind the width of the sound beam at that range by consulting the beam plot
that we have made
Note: The peak of the defect is normally taken as being the last peak onthe screen before the probe goes off the end of the defect, not necessarily
the maximum signal from a defect.
20 dB Drop Method
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20 dB Drop Sizing- For Small Reflector (smaller than beam width).
To use this method the transducer beam width need to be first determined
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To use this method the transducer beam width need to be first determined.
Construction of a beam edge plot -20dB Normal Beam
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Find the hole at a depth of 13mm on an IOW block with a 0 degree probe and
maximise the signal. Move the probe until you get the highest signal youcan from the hole, then turn the signal to FSH using gain. Mark the position
of the middle of the probe on the side of the block.
Move the probe to one side until the signal drops to 10%FSH (-20dB) and
mark the centre of the probe on the side of the block.
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Move the probe to the other side of the hole until the signal drops to
10%FSH (-20dB) and mark the centre of the probe on the block.
Use the distances between the marks on the block to plot the beam on a
piece of graph paper. Measure 13mm depth on the paper then mark the
distances of the probe centre at -20dB from the beam centre at 100%FSH
on either side.
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Now find the 25mm hole and maximise the signal, turning it to 100%FSH.
Move the probe to either side of the hole marking the centre of the probe
on the side of the block where the signal drops by 20dB.
Measure 25mm on the paper and use the distances on the block to plot the
beam dimensions at 25mm.
Repeat using the 32mm hole. Join up the points marking the probe centre
at 20dB to obtain a beam plot.
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Note that we have only drawn the beam width in one plane, so the probe
must be marked accordingly and used to measure defects in this plane.
We use knowledge of the beam spread to size defects, find the edges and
hence their width, length and sometimes orientation.
Construction of a beam edge plot -20dB Angle Beam
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4.3.4 Equalization Back Wall Sizing- The probe moving off the edges of
the reflector until the amplitude is equal to the rising BWE
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the reflector until the amplitude is equal to the rising BWE
4.3.5 Maximum Amplitude Techniques
The technique is used for small reflector. The probe moving off the edges of
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e tec que s used o s a e ecto e p obe o g o t e edges o
the reflector until the amplitude is maximum and the line joining the boundary
is the size of reflector cluster.
4.3.6 The DGS Method
Distance Gain Size Method. The technique is used to find the equivalent
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q q
reflector size by comparing the gain between the flaw and the known size
reflector.
4.4: Automated Scanning
Ult i i t d f t t d d t i iti d
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Ultrasonic scanning systems are used for automated data acquisition and
imaging. They typically integrate a ultrasonic instrumentation, a scanningbridge, and computer controls. The signal strength and/or the time-of-flight of
the signal is measured for every point in the scan plan. The value of the data
is plotted using colors or shades of gray to produce detailed images of the
surface or internal features of a component. Systems are usually capable ofdisplaying the data in A-, B- and C-scan modes simultaneously. With any
ultrasonic scanning system there are two factors to consider:
how to generate and receive the ultrasound. how to scan the transducer(s) with respect to the part being inspected.
Automatic Scanning
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It is often desirable to eliminate the need for the water coupling and a number
of state-of-the-art UT scanning systems have done this. Laser ultrasonic
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systems use laser beams to generate the ultrasound and collect the resulting
signals in an noncontact mode. Advances in transducer technology has lead
to the development of an inspection technique known as air-coupled
ultrasonic inspection. These systems are capable of sending ultrasonic
energy through air and getting enough energy into the part to have a useable
signal. These system typically use a through-transmission technique sincereflected energy from discontinuities are too weak to detect.
The second major consideration is how to scan the transducer(s) with respect
to the part being inspected. When the sample being inspected has a flat
f i l b f d If h l i li d i l
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surface, a simple raster-scan can be performed. If the sample is cylindrical, a
turntable can be used to turn the sample while the transducer is held
stationary or scanned in the axial direction of the cylinder. When the sample
is irregular shaped, scanning becomes more difficult. As illustrated in the
beam modeling animation, curved surface can steer, focus and defocus the
ultrasonic beam. For inspection applications involving parts having complexcurvatures, scanning systems capable of performing contour following are
usually necessary.
http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Graphics/Flash/AppleScan/Apple2.swf
4.5: Precision Velocity Measurements
Changes in ultrasonic wave propagation speed along with energy losses
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Changes in ultrasonic wave propagation speed, along with energy losses,
from interactions with a materials microstructures are often used tonondestructively gain information about a material's properties.
Measurements of sound velocity and ultrasonic wave attenuation can be
related to the elastic properties that can be used to characterize the texture of
polycrystalline metals. These measurements enable industry to replacedestructive microscopic inspections with nondestructive methods.
Of interest in velocity measurements are longitudinal wave, which propagate
in gases, liquids, and solids. In solids, also of interest are transverse (shear)
waves. The longitudinal velocity is independent of sample geometry when thedimensions at right angles to the beam are large compared to the beam area
and wavelength. The transverse velocity is affected little by the physical
dimensions of the sample.
Pulse-Echo and Pulse-Echo-Overlap Methods
Rough ultrasonic velocity measurements are as simple as measuring the time
it t k f l f lt d t t l f t d t th
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it takes for a pulse of ultrasound to travel from one transducer to another
(pitch-catch) or return to the same transducer (pulse-echo). Another methodis to compare the phase of the detected sound wave with a reference signal:
slight changes in the transducer separation are seen as slight phase changes,
from which the sound velocity can be calculated. These methods are suitable
for estimating acoustic velocity to about 1 part in 100. Standard practice formeasuring velocity in materials is detailed inASTM E494.
ASTM E494 - 10
Measuring Ultrasonic Velocity in Materials
Active Standard ASTM E494 | Developed by Subcommittee: E07.06
Book of Standards Volume: 03.03
Precision Velocity Measurements (using EMATs)
Electromagnetic-acoustic transducers (EMAT) generate ultrasound in the
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g ( ) g
material being investigated. When a wire or coil is placed near to the surfaceof an electrically conducting object and is driven by a current at the desired
ultrasonic frequency, eddy currents will be induced in a near surface region. If
a static magnetic field is also present, these currents will experience Lorentz
forces of the formF = J x B
where F is a body force per unit volume, J is the induced dynamic current
density, and B is the static magnetic induction.
EMATs
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http://www.resonic.com/emar_how_it_works.html
http://www.resonic.com/error%20scan.swfhttp://www.resonic.com/scan2.swf
The most important application of EMATs has been in nondestructive
evaluation (NDE) applications such as flaw detection or material property
characterization Couplant free transduction allows operation without contact
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characterization. Couplant free transduction allows operation without contact
at elevated temperatures and in remote locations. The coil and magnetstructure can also be designed to excite complex wave patterns and
polarizations that would be difficult to realize with fluid coupled piezoelectric
probes. In the inference of material properties from precise velocity or
attenuation measurements, use of EMATs can eliminate errors associatedwith couplant variation, particularly in contact measurements.
Differential velocity is measured using a T1-T2---R fixed array of EMAT
transducer at 0, 45, 90 or 0, 90 relative rotational directions depending on
device configuration:
EMAT Driver Frequency: 450-600 KHz (nominal)
Sampling Period: 100 ns
Time Measurement Accuracy:
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Time Measurement Accuracy:
-- Resolution 0.1 ns-- Accuracy required for less than 2 KSI Stress Measurements:
Variance 2.47 ns
-- Accuracy required for texture: Variance 10.0 Ns
------ W440 < 3.72E-5------ W420 < 1.47E-4
------ W400 < 2.38E-4
Time Measurement Technique
Fourier Transform-Phase-Slope determination of delta time between received
RF bursts (T2-R) - (T1-R) where T2 and T1 EMATs are driven in series to
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RF bursts (T2-R) - (T1-R), where T2 and T1 EMATs are driven in series to
eliminate differential phase shift due to probe liftoff.
Slope of the phase is determined by linear regression of weighted data points
within the signal bandwidth and a weighted y-intercept. The accuracy obtained
with this method can exceed one part in one hundred thousand (1:100,000).
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Relative measurements such as the change of attenuation and simple
qualitative tests are easier to make than absolute measure. Relative
attenuation measurements can be made by examining the exponential decay
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attenuation measurements can be made by examining the exponential decay
of multiple back surface reflections. However, significant variations inmicrostructural characteristics and mechanical properties often produce only
a relatively small change in wave velocity and attenuation. Absolute
measurements of attenuation are very difficult to obtain because the echo
amplitude depends on factors in addition to amplitude.
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Attenuation:
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AUt
Ao
Attenuation:
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Section of bi-phase modulated spread spectrum ultrasonic waveform
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Multiple probes may be used to ensure that acoustic energy is propagated
through all critical volumes of the structure. Triangulation may be incorporated
with multiple probes to locate regions of detected distress. Spread spectrum
ultrasonics can achieve very high sensitivity to acoustic propagation changeswith a low level of energy.
Spread Spectrum UT
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Two significant applications of Spread Spectrum Ultrasonics are:
1. Large Structures that allow ultrasonic transducers to be "permanently"
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affixed to the structures, eliminating variations in transducer registrationand couplant. Comparisons with subsequent acoustic correlation
signatures can be used to monitor critical structures such as fracture
critical bridge girders. In environments where structures experience a
great many variables such as temperature, load, vibration, orenvironmental coupling, it is necessary to filter out these effects to obtain
the correct measurements of defects.
In the example below, simulated defects were created by setting a couple ofsteel blocks on the top of the bridge girder.
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2. Piece-part assembly line environments where transducers and couplant
may be precisely controlled, eliminating significant variations in transducer
registration and couplant. Acoustic correlation signatures may be statistically
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compared to an ensemble of known "good" parts for sorting oraccepting/rejecting criteria in a piece-part assembly line environment.
Impurities in the incoming steel used to forge piece parts may result in sulfite
stringer inclusions. In this next example simulated defects were created by
placing a magnetized steel wire on the surface of a small steel cylindricalpiston used in hydraulic transmissions.
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EMATs with Spread Spectrum Ultrasonic
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http://www.resonic.com/emar_how_it_works.html
http://www.resonic.com/error%20scan.swfhttp://www.resonic.com/scan2.swf
4.8: Signal Processing Techniques
Signal processing involves techniques that improve our understanding of
i f ti t i d i i d lt i d t N ll h i l i
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information contained in received ultrasonic data. Normally, when a signal ismeasured with an oscilloscope, it is viewed in the time domain (vertical axis is
amplitude or voltage and the horizontal axis is time). For many signals, this is
the most logical and intuitive way to view them. Simple signal
processing often involves the use of gates to isolate the signal of interest orfrequency filters to smooth or reject unwanted frequencies.
When the frequency content of the signal is of interest, it makes sense to view
the signal graph in the frequency domain. In the frequency domain, the
vertical axis is still voltage but the horizontal axis is frequency.
Display
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Time/Magnitude
domain
Frequency
/Magnitude domain
The frequency domain display shows how much of the signal's energy is
present as a function of frequency. For a simple signal such as a sine wave,
the frequency domain representation does not usually show us much
dditi l i f ti H ith l i l h th
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additional information. However, with more complex signals, such as theresponse of a broad bandwidth transducer, the frequency domain gives a
more useful view of the signal.
Fourier theory says that any complex periodic waveform can be decomposedinto a set of sinusoids with different amplitudes, frequencies and phases. The
process of doing this is called Fourier Analysis, and the result is a set of
amplitudes, phases, and frequencies for each of the sinusoids that makes up
the complex waveform. Adding these sinusoids together again will reproduceexactly the original waveform. A plot of the frequency or phase of a sinusoid
against amplitude is called a spectrum.
Fourier Analysis
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Fourier Analysis
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Fourier Analysis
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Exercise: Try replicating time domain signal in the upper left box with a
pattern similar to the image on the right. Note the resulting bandwidth in the
frequency domain (magnitude) in the lower left box. Next try changing the
magnitude perhaps more of a "mountain" shape tapering to zero Note that
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magnitude, perhaps more of a mountain shape tapering to zero. Note that"narrowing" the magnitude, results in more cycles in the time domain signal.
4.9: Flaw Reconstruction Techniques
In nondestructive evaluation of structural material defects, the size, shape,
and orientation are important flaw parameters in structural integrityt T ill t t fl t ti lti i i lt i
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and orientation are important flaw parameters in structural integrityassessment. To illustrate flaw reconstruction, a multiviewing ultrasonic
transducer system is shown below. A single probe moved sequentially to
achieve different perspectives would work equally as well. The apparatus and
the signal-processing algorithms were specifically designed at the Center forNondestructive Evaluation to make use of the theoretical developments in
elastic wave scattering in the long and intermediate wavelength regime.
4.10: Scanning Methods
4.10.1 Pulse Echo Method
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Pulse Echo Method
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Amplitude loss: Inverse Square Law
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Influence of Shadow on axial defects
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Influence of reflector orientation on signal
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Influence of reflector size on signal
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Pitch-Catch Methods- Tandem
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Pitch-Catch Methods- Tandem
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Pitch-Catch Methods- Through Transmission
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Video on Through Transmission Methods
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www.youtube.com/embed/bRgCLb2cDU4?list=UUSOUDD4-FPV4tzqvUnquwXQ
4.10.3 Immersion Methods
For immersion testing of steel and aluminum in water, the water path shall be
at least 1 for every 4 thickness of the specimen (or of specimen thickness
minimum). If the transducer is too close, the 2nd front reflection will appeared
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between the 1st front reflection and the 1st backwall echo and this may be
wrong interpreted as discontinuity.
Immersion Methods- The water path shall be of specimen thickness
minimum.
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Minimum + [ (?)]
Q. In immersion testing, to remove the second water reflection (2nd entry
surface signal) from between the entry surface signal and the first back
reflection, you should:
a) Increase repetition rate
b) D f
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b) Decrease frequency
c) Decrease sweep length
d) Increase water depth
Immersion Methods- The water path shall be of specimen thickness
minimum. (plus 6mm)
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Minimum + [ (?)]
Modified Immersion Methods- Bubbler Chamber
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Angle Beam Immersion Methods
Note the small front surface reflection. This due to the inclined incident angle
reflected away from the transducer.
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Other Reading (Olympus)- Angle Beam Immersion Methods
Immersion transducers offer three major advantages over contact transducers:
1. Uniform coupling reduces sensitivity variations.2. Reduction in scan time due to automated scanning.
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3. Focusing of immersion transducers increases sensitivity to small reflectors.
Focusing ConfigurationsImmersion transducers are available in three different configurations:
unfocused (flat),
spherically (spot) focused, and cylindrically (line) focused.
Focusing is accomplished by either the addition of a lens or by
curving the element itself. The addition of a lens is the most
common way to focus a transducer.
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Unfocused transducer
By definition, the focal length of a transducer is the distance from the face
of the transducer to the point in the sound field where the signal with the
maximum amplitude is located. In an unfocused transducer, this occurs at adistance from the face of the transducer which is approximately equivalent
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d sta ce o t e ace o t e t a sduce c s app o ate y equ a e t
to the transducers near field length. Because the last signal maximum occurs
at a distance equivalent to the near field, a transducer, by definition, can not
be acoustically focused at a distance greater than its near field.
Focus may be designated in three ways:
FPF (Flat Plate Focus) - For an FPF focus, the lens is designed to produce
a maximum pulse/echo response from a flat plate target at the distance
indicated by the focal length
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PTF (Point Target Focus) - For a PTF focus, the lens is designed to produce
a maximum pulse/echo response from a small ball target at the distance
indicated by the focal length
OLF (Optical Limit Focus) - The OLF designation indicates that the lens is
designed according to the lens makers formula from physical
optics and without reference to any operational definition offocal length. The OLF designation describes the lens and
ignores diffraction effects.
Video on Immersion Testing
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www.youtube.com/embed/W07-Z9at=UUSOUDD4-FPV4tzqvUnquwXQ
Q1: Which of the following scanning methods could be classified as an
immersion type test?
A. Tank in which the transducer and test piece are immersed
B. Squirter bubbler method in which the sound is transmitted in a column offlowing water
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C. Scanning with a wheel-type transducer with the transducer inside a liquid
filled tire
D. All of the above
Q2: In an immersion test of a piece of steel or aluminum, the water distance
appears on the display as a fairly wide space between the initial pulse and
the front surface reflection because of:
A. Reduced velocity of sound in water as compared to test specimen
B. Increased velocity of sound in water as compared to test specimen
C. Temperature of the waterD. All of the above
4.11: Scanning Patterns
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4.12: Pulse Repetition Rate and Penetration
The energy of the generated sound depend on the pulse repetition rate, the
higher the repetition rate the higher the energy and the sound able to
penetrate thicker material. However if the PRR is excessive, ghost signal
may formed, this is due to the fact that the next sequence of pulse is
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y q p
generated before the expected returning signal reaching the receiver.
1. The pulse repetition frequency or pulse repetition rate PRR:is the number of pulse of ultrasonic energy that leave the probe in a given
time (per second). Each pulse of energy that leave the probe must return
before the next pulse leave, otherwise they will collide causing ghost
echoes.
2. Transit time: The time taken for the pulse to travel from the probe and
return
3. Clock interval: The time between pulse leaving the probe.
The transit time must be shorter than the Clock interval else, ghost signal may
formed. Typically the Clock interval should be 5 time the transit time.
PRR- Pulse Repetitive Frequency/Rate and Maximum Testable Thickness
Clock interval = 1/PRR
When Transit time = Clock intervalFor pulse echo method:
M i t t bl l th V l it Cl k i t l
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Maximum testable length = x Velocity x Clock interval
Typically the Clock interval should be 5 time the transit time, i.e. the soundpath should travel 5 times the maximum testable length. (1st BWE, 2nd BWE,
3rd BWE, 4th BWE to 5th BWE.)
Note: The Clock interval has neglected the time occupied by each pulse.
Pulse Repetition Rate and Penetration
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Pulse Repetition Rate and Penetration
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Pulse Repetition Rate and Near Surface Sensit ivity
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4.13: Interferences & Non Relevant Indications
Following are signal interferences that may produce non-relevant UT
indications:
1. Electrical interference
2. Transducer interference
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3. Test specimen geometric interference
4. Test specimen surface interferences5. Test material structure interferences
6. Test material internal mode conversion interference
7. UT techniques induced interferences (In correct PRR/ Band width/
Frequency selection/ Excessive Beam Spread/ etc.)
Transducer Interference- Transducer internal reflections & Mode conversion
may cause interference
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Specimen Surface Interference- You can determined whether the signal is
from the surface wave or the refracted wave simply by touching the surface
ahead of the wave (assuming the velocity of surface wave at 0.9 of the shear
wave)
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Mode Conversion Interference
The mode conversion interference during testing of long cylindrical specimen
with longitudinal wave often appeared after the first back wall echo. The
signal can be easily distinguished and ignore.
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Material Geometric Interference
False signals may generated due to the test specimen structural
configurations resulting in spurious signals.
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Non Relevant Indications
Transducer with Excessive Beam Spread may generate signal, usually after
the 1st BWE. The example below the convex surface defocused the beam
and lead to excessive beam spread, using a proper contoured probe mayeliminate the problem. However excessive contour may results in generation
of surface wave.
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Non Relevant Indications
The geometric abnormalities at root penetration and weld surface (crown)
may reflect the sound path, returning to the receiver as signals. To
distinguished the non relevant indications, finger touching will damped thesignals. Further testing may be necessary to ensure the signals were not from
the surface defects like surface crack. Any near surface indication that are
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unusually consistent could be a non relevant indication.
4.14: Exercises
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4.14-1: Compared 6 dB Drop Sizing with Equalization Technique
The 6 dB MethodFor Large Reflector (greater than beam width), i.e. there is no BWE.
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Compared 6 dB Drop Sizing with Equalization Technique
The Equalization Back Wall Sizing- The probe moving off the edges of the
reflector until the amplitude is equal to the rising BWE
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Q1 What is the correct water path between the transducer and the steel front
surface to focused a transducer for a area of interest at below a steel
surface?
Given that:
Focal length of transducer in water = 6
Velocity of sound in water= 1484 m/s
Velocity of sound in steel = 5920 m/s
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y
Equivalent depth in water for steel depth = 4x = 2
The water path= 6- 2 = 4
Break Time
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mms://a588.l3944020587.c39440.g.lm.akamaistream.net/D/588/
39440/v0001/reflector:20587?BBC-
UID=e5203c9d59fef1a79c12d8c601e839f58db16f7d5d6448f556
74c540f1856834&SSO2-UID=