Ultrasonic testing (UT) is a family of non-destructive testing techniques based on the propagation of ultrasonic waves in the object or material tested. In most common UT applications, very short ultrasonic pulse-waves with center frequencies ranging from 0.1-15 MHz, and occasionally up to 50 MHz, are transmitted into materials to detect internal flaws or to characterize materials.
Ultrasonic Testing Terms:
Sound is basically vibrations of atoms or particles traveling in a medium.
Frequency (f) of sound means the number of vibration cycles per second. Unit: Hertz (Hz)
Velocity (V) of Sound is given by distance traveled by the time taken
Sound vibrations having a frequency between 20 to 20,000 Hz is called audible sound waves and can be heard by humans.
Ultrasound is sound waves having frequency more than 20,000 Hz and can’t be heard by humans. Ultrasonic is a study and application of ultrasound and Ultrasonic testing means testing of materials using Ultrasound.
Types of Ultrasonic waves :
- Longitudinal waves or Compression Waves: Propagation of waves in a straight direction. They Propagate in solid, liquid and gas.
- Transverse Waves or Shear Waves: particle vibrations are at a right angle to the direction of wave propagation. Propagates in solid only. These waves have a velocity approximate 50% of longitudinal waves.
- Surface Wave or Rayleigh Waves: these waves propagate region lesser than one wavelength beneath the surface of material following an elliptical orbit having velocity 90% compared to transverse waves. Such waves propagate only in Solids.
- Lamb Wave or Plate wave: travels in a solid material having a thickness equal to or less than its three-wavelength.
Basic Components of Ultrasonic Test Equipment.
Probe or Transducer: converts electric energy into ultrasound energy and vice versa utilizing a phenomenon known as the piezoelectric effect. Some naturally occurring piezoelectric materials are quartz, tourmaline, lithium sulfate, cadmium sulfide, and zinc oxide.
Basic types of Probe -:
Couplant: It is used to eliminate air between the probe and the specimen surface used in liquid or paste form. commonly used couplants are glycerin, water, oils, petroleum greases, silicone greases, etc.
Ultrasonic flaw detector (UFD): It is the main controller of the whole testing process it activates the transducer using pulse control. Receives reflected waves from the material and converts received vibrations in echo using sweep control and Display unit. Hence making it possible to inspect the material soundness.
How it works
When the transducer is in contact with the test piece with no air gap between the UFD shows Initial and back wall echo. Now on scanning the test piece using a probe , when the probe comes over a discontinuity a separate echo appears in between Initial echo and Back wall echo causing the back wall echo to reduce in amplitude or diminish completely indicating the presence of a discontinuity.
Applications of Ultrasonic Testing:
Used to detect inherent, processing, service discontinuities in metals, nonmetals, ceramics, and plastics.
- Laminations, stringers, seams in Forgings,
- Slag, porosity, lack of fusion and penetration in welds,
- Inclusions, Shrinkages, Gas in castings,
Some of the advantages of ultrasonic inspection include:
- It is a volumetric testing method means it is able to detect both surface and subsurface discontinuities.
- flaw detection or measurement is superior to other NDT methods.
- Equipment used for testing is portable and easy to operate in different locations.
- highly accurate in determining discontinuity position and estimating size and shape.
- Minimal part preparation is required.
- It can also be used for the thickness measurement of the specimen.
The ultrasonic inspection also has its limitations, which include:
- Direct contact is required to inspect the test specimen.
- Skill and training are more extensive than with some other methods.
- The coupling medium required.
- Materials that are rough, irregular in shape, very small, exceptionally thin or not homogeneous are difficult to inspect.
- 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 the characterization of flaws