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
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
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).”
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
The Role of Each Device
A pulser/receiver creates a high voltage electrical pulse, which is sent through 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
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
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
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
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
The Piezoelectric Effect
Crystal material at rest: No forces applied, so net current flow is 0
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
Transducer types
Contact
Direct contact w/ specimen
Rugged casing and plates
Uses coupling material to remove air gaps
Immersion
Non-contact
Operates in liquid
Opmerkingen