ultrasonic testing for lvl 1 & 2 vol.2

Ultrasonic TestingUltrasonic TestingPart 2Part 2

Ultrasonic Testing techniques

• Pulse Echo• Through Transmission• Transmission with Reflection

Pulse Echo Technique

• Single probe sends and receives sound

• Gives an indication of defect depth and dimensions

• Not fail safe

Defect Position

No indication from defect A (wrong orientation)

Through Transmission TechniqueTransmitting and Transmitting and receiving probes receiving probes on opposite sides on opposite sides of the specimenof the specimen

Presence of defect Presence of defect indicated by indicated by reduction in reduction in transmission signaltransmission signal

No indication of No indication of defect locationdefect location

Fail safe method

Through Transmission Technique

Advantages• Less attenuation• No probe ringing• No dead zone• Orientation does not

matter

Disadvantages• Defect not located• Defect can’t be

identified• Vertical defects

don’t show• Must be automated• Need access to both

surfaces

Transmission with ReflectionRT

Also known as:Also known as:

Tandem TechniqueTandem Technique or or

Pitch and Catch TechniquePitch and Catch Technique

Ultrasonic Pulse • A short pulse of electricity is applied to a

piezo-electric crystal• The crystal begins to vibration increases

to maximum amplitude and then decays Maximum

10% of Maximum

Pulse length

Pulse Length• The longer the pulse, the more

penetrating the sound

• The shorter the pulse the better the sensitivity and resolution

Short pulse, 1 or 2 cycles Long pulse 12 cycles

Ideal Pulse Length

5 cycles for weld testing

The Sound Beam

• Dead Zone• Near Zone or Fresnel Zone• Far Zone or Fraunhofer Zone

The Sound Beam

Distance

Intensity varies

Exponential Decay

Main Beam

Main Lobe

Side Lobes

Near Zone

Main Beam

The main beam or the centre beam has the highest intensity of sound energy

Any reflector hit by the main beam will reflect the high amount of energy

The side lobes has multi minute main beams

Two identical defects may give different amplitudes of signals

Sound BeamNear Zone• Thickness

measurement• Detection of defects• Sizing of large

defects only

Far Zone• Thickness

measurement• Defect detection• Sizing of all defects

Near zone length as small as possible

Near Zone

4Near Zone

Near Zone• What is the near zone length of a 5MHz

compression probe with a crystal diameter of 10mm in steel?

1.21000,920,54000,000,5104

Near Zone

• The bigger the diameter the bigger the near zone

• The higher the frequency the bigger the near zone

• The lower the velocity the bigger the near zone

Should large diameter crystal probes have a high or low frequency?

4Near Zone

1 M Hz 5 M Hz

1 M Hz5 M Hz

Which of the above probes has the longest Near Zone ?

Near Zone

• The bigger the diameter the bigger the near zone

• The higher the frequency the bigger the near zone

• The lower the velocity the bigger the near zone

Should large diameter crystal probes have a high or low frequency?

4Near Zone

Beam Spread• In the far zone sound pulses spread out

as they move away from the crystal

DKSine or

Beam Spread

DKSine or

Edge,K=1.2220dB,K=1.08

6dB,K=0.56

Beam axis or Main Beam

Beam Spread

• The bigger the diameter the smaller the beam spread

• The higher the frequency the smaller the beam spread

DKSine or

Which has the larger beam spread, a compression or a shear wave probe?

Beam Spread• What is the beam spread of a 10mm,5MHz

compression wave probe in steel?

DfKVSine

35.7 1278.0105000

592008.12

1 M Hz 5 M Hz

1 M Hz5 M Hz

Which of the above probes has the Largest Beam Spread ?

Beam Spread

• The bigger the diameter the smaller the beam spread

• The higher the frequency the smaller the beam spread

DKSine or

Which has the larger beam spread, a compression or a shear wave probe?

Testing close to side walls

Sound at an Interface• Sound will be either transmitted across

or reflected back

Reflected

Transmitted

Interface How much is reflected and transmitted depends upon the relative acoustic impedance of the 2 materials

The Phenomenon of Sound

REFLECTIONREFRACTION

DIFFRACTION

The Phenomenon of Sound

REFLECTIONREFRACTION

DIFFRACTION

Law of Reflection• Angle of Incidence = Angle of Reflection

60o 60o

Inclined incidence(not at 90o )Incident

Transmitted

The sound is refracted due to differences in sound velocity in the 2 DIFFERENT materials

REFRACTION• Only occurs when:The incident angle is other than 0°

Refracted

REFRACTION• Only occurs when:The incident angle is other than 0°

Refracted

The Two Materials has different VELOCITIES

No Refraction

Snell’s Law

Material 1

Material 2

2 Materialin 1 Material

VelinVel

RSineISine

Incident

Refracted

Normal

Snell’s Law

Perspex

VelinVel

RSineISine

59602730

48.3 20

SineSine

4580.04580.0

Snell’s Law

Perspex

VelinVel

RSineISine

59602730

SineSine

2730596015SinSinR

565.0SinR4.34R

Snell’s LawC

Perspex

Snell’s Law

Perspex

When an incident beam of sound approaches an interface of two different materials:REFRACTION occurs

There may be more than one waveform transmitted into the second material, example: Compression and Shear

When a waveform changes into another waveform: MODE CHANGE

Snell’s Law

Perspex

If the angle of Incident is increased the angle of refraction also increases

Up to a point where the Compression Wave is at 90° from the Normal

90° This happens at the

FIRST CRITICAL ANGLE

1st Critical Angle

C Compression wave refracted at 90 degrees

2nd Critical Angle

S (Surface Wave)90

Shear wave refracted at 90 degrees

Shear wave becomes a surface wave

1st Critical Angle Calculation

Perspex

SteelC

59602730

SineSine

59602730

458.0SinI

26.27I

190 Sin

2nd Critical Angle Calculation

Perspex

32402730

SineSine

32402730

8425.0SinI

S190 Sin

Before the 1st. Critical Angle: There are both Compression and Shear wave in the second material

At the FIRST CRITICAL ANGLE Compression wave refracted at 90°

Shear wave at 33 degrees in the material

Between the 1st. And 2nd. Critical Angle: Only SHEAR wave in the material. Compression is reflected out of the material.

At the 2nd. Critical Angle: Shear is refracted to 90° and become SURFACE wave

Beyond the 2nd. Critical Angle: All waves are reflected out of the material. NO wave in the material.

Summary• Standard angle probes between 1st and

2nd critical angles (45,60,70)• Stated angle is refracted angle in steel• No angle probe under 35, and more

than 80: to avoid being 2 waves in the same material.

One Defect Two Echoes

Snell’s Law• Calculate the 1st critical angle for a

perspex/copper interface• V Comp perspex : 2730m/sec• V Comp copper : 4700m/sec

5.355808.047002730

ultrasonic testing for lvl 1 & 2 vol.2

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