Ultrasonic Testing

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Ultrasonic Testing

• By Chris Wu
• CEE 398
• 3/5/04

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Introduction

• High-frequency sound waves are sent out at a
  material to find material changes
• A pulser produces an electrical pulse that causes
  a piezoelectric transducer to send out a sound
  wave
• Reflected waves are transformed back into
  electrical signals by the transducer and analyzed
• Its main applications are in thickness gauging
  and flaw detection

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Background

• Originated from sonar technology, which was
  developed just before World War II
• Sonar uses technique of bouncing echoes off of
  submerged objects to detect them
• Ultrasonics, a form of nondestructive testing,
  was applied to safe life design, which ensures
  that structures dont develop macroscopic defects
  during its life, and any detection led to its
  removal

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Background (contd)

• In the early 1970s, ultrasonic testing had made
  large strides and could detect extremely small
  defects or discontinuities in metals
• An unforeseen downside was that many
  manufacturers were now discarding more metal
  parts that were deemed satisfactory earlier
• This in turn led to fracture mechanics, which
  allowed the determination of whether a crack of
  a given size would fail under a certain load if
  the fracture toughness were known (ndt-ed.org).

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Background (contd)

• Fracture mechanics allowed for the concept of
  fail safe design, which stated that structures
  could have defects as long as they would not grow
  to cause failure.
• Over the past few years, ultrasonic testing has
  become more popular due to advances in both
  computer and information technology

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Theory and Technique

• High frequency sound energy is used to find such
  things as material flaws and dimensional
  measurement
• Items needed to run an ultrasonic test
• Pulser/receiver unit
• Piezoelectric transducer
• Display devices

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The Role of Each Device

• A pulser/receiver creates a high voltage
  electrical pulse, which is sent throught the
  material in the form of propagating sound waves
  by the transducer
• Any discontinuity or flaw in the material will
  cause some energy to reflect back
• The reflected waves are converted into an
  electrical signal by the transducer and amplified
  by the receiver for signal processing

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The Role of Each Device (contd)

• The amplified electrical signal is then displayed
  on a screen
• The reflected signal strength is often displayed
  vs. time b/w signal generation and echo reception
• D vt/2 for normal beam inspection of
  discontinuities, different for angle beams
• This can lead to info on the flaws size,
  location, and orientation among other things

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Picture Gallery
Transducer
Test Specimen
Pulser/Receiver
Display Device
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Pros

• Advantages
• Sensitive to both surface and subsurface
  discontinuities
• Penetration depth is better than other NDT
  methods
• With pulse-echo, access to only one side is
  needed
• Highly accurate in regards to reflector size,
  shape, and location
• Minimal part preparation

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Cons

• Disadvantages
• Surface must be accessible to transmit ultrasound
• More training required relative to other methods
• Coupling medium is normally required to promote
  transfer of sound
• Has difficulty inspecting rough, small, or
  irregularly shaped objects
• Linear defects parallel to sound beam may go
  undetected

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Piezoelectric Transducers

• PTs contain polarized material
• When electric charge is applied, dipoles are
  induced and dimensions change
• If a force is placed on the material, it will
  change dimensions and create an electric field

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The Piezoelectric Effect
Crystal material at rest No forces applied, so
net current flow is 0
Crystal
- - -
Current Meter 0
Charges cancel each other, so no current flow
- - -
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The Piezoelectric Effect
Crystal
Crystal material with forces applied in direction
of arrows..
- - - - -
Force
Current Meter deflects in direction

Due to properties of symmetry, charges are net
on one side net - on the opposite side
crystal gets thinner and longer
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The Piezoelectric Effect
Changing the direction of the applied force..
Crystal

Current Meter deflects in - direction
Force
- - - - -
. Changes the direction of current flow, and
the crystal gets shorter and fatter.
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The electromechanical effect
When the switch is closed, and you apply the
exact amount of power to get the same current
that resulted when you squeezed the crystal, the
crystal should deform by the same amount!!
Crystal

side
power source (battery)
- side
- - - - -
. and, the crystal should get shorter and
fatter.
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Transducer types

• Contact
• Direct contact w/ specimen
• Rugged casing and plates
• Uses coupling material to remove air gaps

• Immersion
• Non-contact
• Operates in liquid

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References

• www.ndt-ed.org
• www.ndt.net
• www.ndtsupply.com
• www.qnetworld.com