Please enable JavaScript.
Coggle requires JavaScript to display documents.
Chapter 6: Methods of NDT 5) Ultrasonic Testing - Coggle Diagram
Chapter 6: Methods of NDT
5) Ultrasonic Testing
Part 1: Principles of Sound
a)
Sound
: is a vibrations that travel through the air or another medium as an audible mechanical wave. It is produced from a vibrating body. The vibrating body causes the medium (water, air, etc.) around it to vibrate thus producing sound.
b) Properties of Sound Wave
Velocity (speed of wave), v
The distance travelled by a wave in one second
S.I unit of v: metres per second (m/s or ms-1).
Wavelength, λ
The minimum distance in which a sound wave repeats itself is called its.
That is it is the length of one complete wave.
S.I unit for λ: metre (m)
Frequency
The number of complete waves or cycles produced in one second.
S.I unit of frequency: hertz or Hz
Let v = λ / T
where T= 1/f
Thus, v = f X λ
c) Principle waveforms used in UT inspections
Principle: Whenever there is a change in the medium, the ultrasonic waves are reflected. Thus, from the intensity of the reflected echoes, the flaws are detected without destroying material
d) Types of Ultrasonic waves
Compression (longitudinal wave)
: The particle vibrate in parallel to wave direction
Shear (transverse wave
): The particle vibrate in perpendicular to wave direction
Surface or Rayleigh waves
: The particle vibrate in elliptical orbit
Lamb or plate waves
: The particle vibrate in component perpendicular to surface
e) Methods of sound wave travel through a material
Beam Spread
Sound beam comparable to torch beam
Reduction differs from small and large reflectors
Attenuation
Energy loss due to type of material
made up of absorption and scattering
Part 2: Ultrasonic Test Methods
a) Three Basic Ultrasonic Inspection Methods
Pulse Echo
Through Transmission
Transmission with Reflection (Resonance)
:star: Through Transmission
A techniques that used two aligned transducers located in opposite sides of the part
One transducer act as transmitter and the other is receiving probes on opposite sides of the specimen.
Presence of defect indicated by reduction in transmission signal,
No indication of defect location
Advantages
Less attenuation
No probe ringing
No dead zone
Orientation does not matter
Disadvantages
Defect not located.
Defect cannot be defected.
Vertical defect don't shows
Must be automatic
Need access for both surfaces
:star: Transmission with reflection (pitch-catch technique)
Pitch-catch technique: A test with a transmitter and a receiver transducer where the path of the ultrasonic beam is not straight line but follows a complex path (the beam is reflected one or more times before reaching the receiver)
a) Direct pitch-catch technique b) Indirect pitch-catch technique
:star: Pulse Echo
Based on mechanical waves (ultrasound) generated by piezo-magnetically excited element at a frequency range (Between 2 and 5Mhz)
Single probe send and receives sound
Gives an indication of defect depth and dimensions.
Transmitted wave beam is reflected within the play and the flaws, then returns to the flaw that can be both transmitter and receiver.
Applications
Identification of internal defects in welds, metal, plastics, ceramics, glass.
Examination of plates, castings and forgings.
Types of examinations
Pulse-echo ultrasound examination using longitudinal and transverse waves.
Advantages
High sensitivity and high mobility.
Not limited only to magnetic materials (as with magnetic particles) nor to the discontinuities open to the surface (such as dye penetrant) and to the scan, it does not require any kind of radiation protection.
Mobility of the equipment.
b) Sound at an Interface
Sound will be either transmitted across or reflected back
c) Snell's law
Describes the relationship between the angles and the velocities of the waves.
Formula
Sin Incident/ Sin reflected = velocity in material 1/ velocity in material 2
d) Data presentation
Ultrasonic data can be collected and displayed in a number of different formats. the three most common formats are;
A-scan
B-scan:
C-scan
D-scan
f) Probe Design
Compression wave
Shear wave
Twin crystal
Part 3: UT
a) Sensitivity: smallest specified defect at a maximum testing range Sensitivity depends on:
Probes and flaw detector combination
Material properties
Probe frequency
Signal to noise ratio
b) Advantages of Ultrasonic testing
Sensitive to small discontinuities both surface and subsurface
Depth of penetration for flaw detection or measurement is superior to other methods,
Only single-sided access is needed when pulse-echo technique is used.
High accuracy in determining reflector position and estimating size and shape
Minimal part preparation required.
Electronic equipment provides instantaneous results.
Detailed images can be produced with automated systems.
Has other uses such as thickness measurements, in addition to flaw detection.
c) Limitations of Ultrasonic testing
Surface must be accessible to transmit ultrasound.
Skill and training is more extensive than with some other methods.
normally requires a coupling medium to promote transfer of sound energy into test specimens.
Materials that are rough, irregular in shape, very small, exceptionally thin or not homogenous are difficult to inspect.
Cast iron and other coarse grained materials are difficult to inspect due to low sound transmission and high signal noise.
Linear defects oriented parallel to the sound beam may go undetected.
Reference standards are required for both equipment calibration, and characterization of flaws
