UT Testing-Section 4

8/12/2019 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



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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



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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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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

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Documents

UT Testing-Section 4