What is Ultrasonic Testing

What is Ultrasonic Testing- A NDT method to see inside materials

Ultrasonic Testing or also called UT is a volumetric Non-Destructive Testing (NDT) method. Unlike surface inspection methods, UT makes it possible to find flaws inside the material. High-frequency sound waves are sent into the material with an ultrasonic transducer. The ultrasound that reflects off defects in the material is made visible in a graph. Ultrasonic Testing gives insight in the depth, size, nature and orientation of the detected indications. The thickness of the material, such as wall thickness of pipes, can also be measured. 

Ut machine

How does Ultrasonic Testing work?

In ultrasonic testing, an ultrasound transducer connected to a diagnostic machine is passed over the object being inspected. The transducer is typically separated from the test object by a couplant (such as oil) or by water, as in immersion testing.

However, when ultrasonic testing is conducted with an Electromagnetic Acoustic Transducer (EMAT) the use of couplant is not required.

Principle of Ultrasonic Testing (UT)

As we see in below figure, left side : A UT probe sends a sound wave (Ultrasonic waves)  into the material. There are two indications, one from the initial pulse of the probe, and the second due to the back wall echo generated by the probe. In the presence of a defect which fall in the sound waves sent by the probe (As shown in below figure, right side) a defect creates a third indication and simultaneously reduces the amplitude of the back wall indication. The depth of the defect is determined by the ratio D/Ep. Here Ep is the material thickness, and its corresponding thickness is shown in the UT machine display as we can see in the below picture.

Principle of Ultrasonic Testing (UT)

Types of Ultrasonic waves

Sound waves used in UT have four main types :

1. Longitudinal waves oscillate in the same direction as the propagation of the wave. The density of these waves fluctuates as they move. These waves can travel through liquids, gases and solids though movements of compression and expansion.

2. Shear waves are also called transverse waves. In this type, the particles vibrate and move in a right angle to the direction of the wave’s propagation. These waves are stronger in solids than in other states of matter, though they are generally weaker when compared to longitudinal waves.

3.Surface waves are also called Rayleigh waves. The particle vibration of these waves is in the form of an elliptical orbit. These waves are extremely sensitive to surface irregularities and defects. They are also good at following curves and are useful in places where other waves cannot reach.

4. Lamb waves are also called plate waves. They are similar to Rayleigh waves, but cannot be generated in pieces that are thick. They require flat pieces that are only a few wavelengths thick.

Types of Scan in Ultrasonic Testing

Ultrasonic data can be collected and displayed in a number of different formats. The three most common formats are know in the NDT world as A-scanB-scan and C-scan presentations. Each presentation mode provides a different way of looking at and evaluating the region of material being inspected. Modern computerized ultrasonic scanning systems can display data in all three presentation forms simultaneously.

Advantages & Limitations of Ultrasonic Testing

Advantages of Ultrasonic Testing:

  1. High penetrating power
  2. High sensitivity
  3. Usually only one surface needs to be accessible
  4. Insight in size, orientation, shape and nature of defects
  5. Non-hazardous to operations or nearby personnel
  6. Portable equipment
  7. Can be used in automated setup
  8. Immediate test results

Limitations of Ultrasonic Testing:

  1. Surface must be accessible to transmit ultrasound
  2. Usually a couplant (gel or oil) is needed to enable interference-free transfer of the ultrasound
  3. Objects that have a rough surface or are very small, thin or not homogenous are difficult to inspect
  4. Cast iron and other coarse grained materials are difficult to inspect due to low sound transmission and high signal noise
  5. Linear flaws oriented parallel to the direction of the sound beam may go undetected
  6. Reference standards are required for equipment calibration and for the characterization of flaws

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