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Test Systems Basically two Test systems available Contact

Transducer in direct contact with specimen through Plastic wedges, wear plates or flexible membranes

A thin layer of liquid as couplant applied in between part surface and transducer to remove air

Immersion Entire part and transducer submerged in a tank of water Bubbler or squirter technique, sound beam propagated through a

column of flowing water impinging on the test surface, part not necessarily immersed in water

In both Water acts as a couplant between part surface and transducer

The immersion system lends itself for automated, mass production process

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Test Methods Both systems have common test methods

Pulse echo Through-transmission Resonance

Pulse-echo & Through transmission methods employ same three techniques St.beam Angle-beam Surface

Resonance method employs St.beam technique only

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COMMON NDT TECHNIQUES

Pulse echo - Dual ProbesPulse echo Single crystal Transceiver

Through Transmission-two separate probes

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Pulse-echo A single probe probe emits short-

pulses of sound and receive reflected pulses from any boundary, surfaces, discontinuities in the interval between emitting pulses.

The received reflections displayed on a CRT

The same probe acts both as transmitter & receiver

The position of echoes indicate depth of discontinuities from where the probe is positioned or thickness of part

The height of echo indicates severity of flaw area that is impinged and returned to the transducer

Suitable for st.beam,angle beam & surface wave techniques

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Through transmission

Two separate transducers are utilised each on opposite sides of part geometrically aligned

One transducer transmits and the other receives. If a discontinuity lies in the sound path, the receiver

transducer obtains decreased signal amplitude Larger the area of discontinuity in the beam path larger

the reduction in received echo height, while there is no display of position of discontinuity

Limited to applications where both sides of part are accessible

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Resonance

Requires a single transducer, tunable by a continuous wave oscillator

Continuous longitudinal waves are transmitted into the material The transmitted frequency is varied till standing waves are set-

up Standing waves causes the specimen to vibrate with greater

amplitude Frequency at which resonance occurs is indicated as a display

in oscilloscope, deflection on a meter or a read-out display Unaccountable change in frequency means discontinuity Equipments fall into two category

Thickness meters used for gauging, corrosion monitoring & gross discontinuity

Bond testers used to determine lack of bond

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St.Beam

Contact, Single crystal, Longitudinal wave probe:

used for detection of essentially planar discontinuities major axis parallel to the surface

limitations are on near surface resolution due to

“Dead Zone” “Near Field” effects.

If thin materials are to be inspected or if flaws are sought close the surface, a delay line probe or a twin crystal probe should be used.

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St.Beam Indications from discontinuities

just below the entry surface within crystal ringing & entry surface will not be detected

The thickness of material represented by the timebase distance covered by these 3 signals is called the “Dead Zone”

for flaw detection St.beam Probes with high “sensitivity” have low damping.

Probes with high resolution will have high damping and used for thickness measurements.

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Angle beam Single crystal probe St.beam probe with the exception of

the wear plate being replaced , but mounted on an angled plastic wedge.

The angle of the wedge is carefully calculated so that when placed on a particular material the longitudinal wave generated experiences “mode conversion” and refraction to produce a shear wave of the required angle to detect obliquely oriented discontinuities.

Shear wave probes are available with fixed (integral) wedges or removable wedges enabling one probe to be used for several different shear wave angles.

As they do not have a wear plate the crystal will be easily damaged if used without a wedge

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surface wave probe plastic wedges are machined to

produce the 2nd critical angle, shear wave probe with refracted shear wave angle 90º and becomes a Surface wave probe.

Also known as Rayleigh waves. Surface waves have a velocity,

which is approximately 90% of a shear wave (or 45% of a longitudinal wave).

will be attenuated or reflected back by surface contamination (paint, liquids, etc.) or small surface breaking discontinuities.

suitable for applications where the surface is not readily accessible for other inspection means

care exercised when applying couplant.

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Surface wave Testing Surface waves closely follow the surface

of the material and have an elliptical particle displacement

will travel around complex geometry‟s over long distances if the surface condition allows it.

will be stopped almost immediately and may be reflected by any surface contamination, such as water, oil, grease or paint.

also reflect from a sharp corner. Skin and stiffeners are machined from

solid plate. Inaccessible cracks are present running

from fuel transfer holes to edge of ribs. This is a sealed tank with no internal access. Except from the outside of the lower wing skin.

A shear wave goes through mode conversion as it reaches the inner surface of the hole.

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St-beam Delay tip probe

For detection of flaws close to the sound entry surface

consists of a plastic cylinder screwed into the crystal as shown below.

wave generated by the crystal passes through the plastic before it reaches test specimen.

A large portion of the near field used up in the plastic cylinder.

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St-beam Delay tip probe

Only the time base beyond the plastic delay tip is monitored

Combines crystal ringing & electric zero signals are effectively eliminated.

The interface between the delay tip and the specimen will still produce a signal that will conceal a portion of the time base close to the entry surface.

The amount of hidden material “Dead Zone” will be considerably less than with a regular contact, longitudinal probe.

When calculating delay tip thickness required, 3 mm should be added

Delay tip probes also be used when inspecting high temperature

components .The delay tip acts as an insulator for the crystal.

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St-beam Dual Probes used for thin materials or for

finding flaws close to the entry surface.

probe has 2 separate crystals mounted as shown

Through transmission mode is selected.

L-wave passes through a wedge on a slight angle and into the material where it refracts slightly, but remains a longitudinal wave because the angle of incidence is below the first critical angle.

When the sound wave bounces off the back wall of the material it is received by the second crystal

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St-beam Dual Probes

Least dead zone with dual crystal probes

best for near surface inspection or thin material inspections.

Limitations on thickness range The signal returning from the back

wall must be receiving crystal If the material is too thin, the

signal will not reach crystal after just one bounce.

If the material is too thick, the returning signal will miss the probe completely.

When inspecting tubular products the acoustic barrier axis should be at 90º to the tube axis as shown below

.

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Plate (Lamb) wave Testing The whole plate flexes with

the wave propagation. Plates up to 3 wavelengths

thick can be inspected. Lamb waves are also

useful for inspecting inaccessible areas.

The velocity is frequency dependent

There are 2 modes of Lamb wave propagation, Symmetrical and Asymmetrical.

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Multiple Element probes (Mosaic) Any probe that contains more than 2

crystals (elements) is called a mosaic probe.

some utilize multiple elements to shape the beam by having concentric elements fired sequentially.

Others have a long paintbrush type arrangement consisting of many elements (sometimes several hundred), each fired sequentially to build up a “B” scan image of the entire area beneath the probe instantly. This is a development of the principles used for medical ultrasound.

The probe are up to 100 mm long and if swept sideways, can inspect a 100 mm wide strip very quickly.

These types of probes are ideally suited to carbon fibre composite inspections and for in-situ inspection of aircraft wing skin to spar bond lines.

This inspection was previously done using ¼" diameter delay tip transducers in an overlapping raster –scan pattern

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Immersion technique Purest form of immersion testing both the part and the probe are immersed in a

fluid (usually water, with additives to combat corrosion, fungi and suspended air bubbles).

water acts as couplant As there is no contact between the probe and

the part, no wear plate is needed and high-scanning speeds can be achieved.

thinner crystals can be used, which ultimately means higher frequency.

The probe is scanned over the component in an X – Y configuration to produce a raster Scan pattern by the combined effects of the “Bridge” (Which travels along the tank) and the “Carriage” (Which travels across the bridge).

probe / component, water path distance can be adjusted by raising or lowering the “column” as required.

The angle of the probe can be adjusted using the “Manipulator” (Angulator) ,varied to the extent of producing shear waves in the part.

Some tanks also have a turntable fitted which allows circumferential components to be scanned

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Immersion Testing Advantages

Speed of inspection Ability to control & direct sound beam Adaptability for automated scanning High frequency transducers practicable that improve

sensitivity of flaw detection Only a single crystal transducer is required for

inspection The longitudinal wave beam directed into part

through water column Either 0º Longitudinal wave or desired shear waves

(45º through 70º) are angles are introduced into materials through water

The water path is kept ¼ th of part thickness plus 6 mm

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Immersion technique Immersion tank systems allow use of

“focused beam transducers”. The beam is focused to a point at a particular

distance called the “focal point”. This concentrates the energy at this point

rather than having it disperse (spread) as is the case with unfocused probes.

Another way of preventing the beam from spreading is to use a collimator, which is basically a tube attached to the probe to channel the beam

Focused probes acts as a spherical lens, and focuses the sound at the centre of its radius.

If a component is inspected for flaws in specific area of its cross section, that area can be gated. e.g. for bonded components, the “bond line” would be gated.

Any signals above a pre-determined threshold will trigger the alarm or send a proportional voltage signal to the recording device

part is submerged in a bath of water and a submerged probe is scanned across it in a raster scan pattern using pulse echo or through transmission techniques to detect, size and evaluate flaws.

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Acoustic lenses Straight (flat) crystals with

a focusing lens attached and causes the sound waves to refract to a focal point.

referred to as “Acoustic Lenses”

Formula for calculating focal length n = ratio of velocity

between lens material and liquid velocity.

V1-lens material V2 –Water

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Bubbler, Squirter

Bubbler & Squirter Sound beam projected through a nozzle on the probe. Suitable water column required for testing is permanently

maintained underneath the probe Sound beam directed perpendicular or at an angle to surface Can be adopted for through transmission, two separarate

probes used Bubbler is a modification of squinter Probe is immersed in water with inlet & outlet for flow of

water and probe is adjusted to maintain distance to part Applications

High speed scanning of Plate, sheet, strip, cylindrical forms & regularly shaped parts

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Wheel type transducers

Wheel Transducer Transducer mounted inside a water filled tyre Mounted on the axle & held in a fixed position

while the tire rotates Thhe wheel is is run across surfaces of parts or

held stationary while the part moves past the wheel

Applications Inspection of plate for laminations Billets for primary & secondary pipe Angle beam (45º) inspection

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Squirter Probes In a squirter system the probe is

mounted in a nozzle assembly. Water is fed to the nozzle through

plastic pipes. Inside the nozzle the water passes by

the front face of the probe then passes out of the nozzle through a small diameter hole to produce a jet of water, jet of water acts as a couplant

usually used in pairs to produce a through transmission arrangement.

pairs of probes kept in alignment and usually achieved by mounting them on a moveable gantry.

The component is mounted rigidly while the pair of probes on the gantry performs the scanning action.

Alternatively, the probes may be fixed and the component passed between the probes.

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Bubbler Probes Bubbler Probes Low water pressure single

probe systems used The component rests on

the top of the probe nozzle assembly

water provides a couplant cushion as the components are manually passed over the probe assembly.

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Wheel probes

rare today. an old concept using large diameter

(150 mm – 300 mm) hollow rubber wheels which have probes mounted inside and are filled with glycol as a coupling medium as shown below.

this is a through transmission arrangement, there will be no depth information provided on any flaws found.

ultrasound travels through the liquid glycol, through the rubber tire, through the component, through the tire of the second wheel, through the glycol to the Receiving probe.

pair of wheels are held in alignment by a calliper type gantry that is manipulated up and down as the gantry is also incremented along the length of the part.

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ADVANCED DIGITAL ULTRASONIC FLAW DETECTOR

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In Pulse–echo systems at regular intervals electronic clock triggers pulser circuit

Pulser imposes a short interval of high frequency ac or unipolar negative spike on transducer

Clock simultaneously triggers sweep circuit A constant interval between pulses are

automatically incorporated in equipments based on selected range

Pulses are repeated 50 to 2000 times/sec The equipment-control that effects constant

interval is PRF/PRR/PPS

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Theoretically maximum depth of examination is controlled by PRF

Practical limit depends on sound attenuation in examination material, test frequency & system sensitivity

The PRR rate is chosen so that one pulse travels the test specimen enough number of times to dissipate sound energy to a non-noticeable level, before next pulse is triggered

Time interval between pulses is 60 times that of operating time Ta

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Calculation of PRF *Eg: 100 mm steel ; Velocity in Steel= 6mm/μsec Ultrasonic beam travel:100 ÷ 6=17; for to & fro 17+ 17=34 μsec For shear wave testing : Ta becomes Ta ÷055=1.8Ta

Range Operating Time-Ta

Pulse spacing

60Ta

PRF

106 ÷ 60Ta

mm μsec μsec Hz

10 3.4 204 5000

100 34*60x34=

2040500

500 170 10000 100