1. Ultrasonic Testing
By: Chris Wu
CEE 398
3/5/04

2. 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

3. 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 don’t develop macroscopic defects during its life, and any detection led to its removal

4. Background (cont’d)
In the early 1970’s, 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).”
Other laws were developed to predict the rate of growth of cracks under cyclic loading

5. Background (cont’d)
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
For example,

6. 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

7. 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

8. The Role of Each Device (cont’d)
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 flaw’s size, location, and orientation among other things

9. Picture Gallery Transducer Test Specimen Pulser/Receiver
Display Device

10. 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

11. 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

12. Piezoelectric Transducers
PT’s 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

13. 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

14. 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

15. The Piezoelectric Effect
Changing the direction of the
applied force………..
Crystal
Force
Current Meter
deflects in -
direction
…. Changes the direction of
current flow, and the crystal gets
shorter and fatter.

16. 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.

17. Transducer types Contact Immersion Direct contact w/ specimen
Rugged casing and plates
Uses coupling material to remove air gaps
Immersion
Non-contact
Operates in liquid

18. References www.ndt-ed.org www.ndt.net www.ndtsupply.com