Ultrasound NDT

Question 1

What are the principles of ultrasound NDT?

Answer 1

• The propagation of ultrasonic elastic waves through the solid object to be examined
• Monitoring of the signal caused by the interaction between the ultrasound beam with any discontinuity encountered in its path within the specimen

Question 2

What are ultrasonic waves?

Answer 2

Mechanical elastic waves that are transmitted in materials by elastic oscillations of atoms and molecules around their original equilibrium position

Question 3

What happens when a US beam encounters a defect?

Answer 3

The US beam can be reflected, attenuated or diffracted, according to rules of propagation of waves

Question 4

How are US beams transferred into the material?

Answer 4

Directly by contact, placing the probe/transducer near the surface of the component, often with a conduction (couplant) medium

Question 5

What happens when the frequency of the US beam is icreased?

Answer 5

Increases both sensitivity and resolution for defect detection. Because:
- US tends to be reflected and diffused from coarse grains or small discontinuities

But the maximum depth at which it is possible to detect a defect, is greatly reduced by increased frequency

Question 6

What is the general rule about the size of the US beam?

Answer 6

The minimum detectable defect size is at least equal half the wavelength of the US beam

Question 7

Why is there a need for a couplant medium between the probe and the part?

Answer 7

• To avoid the presence of air bubbles, or of any other uncontrolled matter at the interface between the probe and the object to be tested
• Strong acoustic impedance difference between air and solids would cause the nearly complete reflection of the beam back to the probe

Question 8

What are the different ways to represent the US signals?

Answer 8

• A-scan system (amplitude)
• B-scan system (brightness)
• C-scan system (contrast)
• D-scan system (depth)

Question 9

What are the advantages of ultrasound NDT?

Answer 9

• High sensitivity of detection, so that very small flaws can be detected under optimal conditions
• Good penetration depth that makes the method applicable for thick components
• It is possible to accurately estimate the defect position and size
• It requires accessibility only from one side of the component to be inspected

Question 10

What are the limitations of ultrasonic NDT?

Answer 10

• Cannot be applicable to materials with high acoustic attenuation or with very complex geometries
• The surfaces to be contacted with the probes have to be preferably smooth to avoid excessive scatter of the signal because of roughness
• Skilled operators are required