CHAPTER 6: Ultrasonic Inspection
Principle waveforms used in
Ultrasonic inspection
Compression Waves
Vibration & propagation in the same direction
Travel in solids, liquids & gases
Shear Waves
Vibration at right angle to direction of propagation
Travel in solid only
Velocity is half of the compression for the same material
Surface Waves
Elliptical vibration
Velocity 8% less than shear waves
Penetrates up to 1 wavelength deep
Lamb Waves
Propagate in thin plate material where the the thickness is equivalent wavelength
Particle motion is a complex combination of symmetrical & non-symmetrical elliptical waves
Velocity varies with the plate thickness & wavelength
Long Range Ultrasonic Testing: Wave modes in pipes
Longitudinal
Torsional
Flexural
Sound travelling through a material
loses intensity due to:
Beam Spread
Attenuation
Sound beam comparable to torch beam
Energy loss due to material
Reduction differs for small & large reflectors
Made up of absorption & scattering
Scatter
Bigger the grain size, the worse the problem
Higher the frequency of probe, worse the problem
Big grain size will give rise to the grass height
Scattering become significant as the average grain size exceeds about 1/10 of the wavelength
If the average grain size exceeds the wavelength of Ultrasonic Testing - Ultrasonic Testing is impossible to be done
If the average grain size reach 1/2 of wavelength, extremely high attenuation will occur
Sound at an interface
Sound will be either transmitted across / reflected back
How much is reflected & transmitted depends upon the relative acoustic impedance of the materials
Acoustic Impedance
Resistance to travel of sound waves within a material
Measured in kilogram per meter square second
Ultrasonic Test Methods
Pulse Echo
Through Transmission
Transmission with Reflection
Automated Inspection
Through Transmission
Testing
Advantages
Disadvantages
Less attenuation
No probe ringing
No dead zone
Orientation does not matter
Defect not located
Defect cannot be identified
Vertical defect don't shows
Must be auto
Need access for both surfaces
Pulse echo
Through transmission
Transmission with reflection
Contact scanning
Gap scanning
Immersion testing
Ultrasonic Inspection
Sensitivity
Scanning procedure
Defect sizing
The ability of an ultrasonic system to find the smallest specified defect at the maximum testing range
Depends on probe & flaw detector combination, material properties, probe frequency and signal to noise ratio
Root inspection
Side wall inspection
Weld body