4: Specimen and Sound Beam Flashcards
The near zone is also called the what zone? And how does the sound act?
Also called Fresnel.
Sound intensity fluctuates.
The far zone is also called the what? And how does the sound act?
Also called the Fraunhofer.
Sound diverges predictably.
The length of the near field is directly proportional to what?
The diameter and frequency of the transducer.
Why is it important to know how far the near zone extends into the part?
The signal amplitude (height) cannot be trusted to represent the size of the reflector.
Where is the maximum sound intensity located?
End of the near field - beginning of far field
As the sound transitions from the near zone to the far zone, the sound pressure builds and waves become uniform.. what are the waves now referred to?
Plane wave
What is it referred to when the sound moves into the far zone and begins to spread in a cone shape?
Beam spread/beam divergence
How can beam spread be altered?
By changing the diameter and/or frequency of the transducer.
What happens to the beam spread when testing a narrow specimen?
The sound will hit the sides of the specimen and produce mode conversion.
The angle of divergence is inversely proportional to what?
The diameter and frequency of the transducer.
An increase in diameter/frequency will produce a smaller angle of divergence.
Where is the sound intensity greatest?
Along the centreline (100%) and weaker towards the edges
When a defect is at its maximum amplitude, where does that mean it will be in relation to transducer?
On the centreline
What formula is used to calculate beam spread at 0% intensity?
Sin theta = 1.22V/fD
What formula is used to calculate intensity at 10% edge of beam?
Sin theta = 1.08wavelength/D
What formula is used to calculate intensity at 50% edge of beam?
Sin theta = 0.56wavelength/D
What is attenuation?
The loss of energy of the sound beam from the time it is sent by the crystal to the time it returns to the crystal.
What are dislocations?
Crystalline defects
When does scattering occur?
When the material contains reflectors that are large in relation to the half wavelength.
How is the length of near field affected by an increase in frequency?
Length of near field is directly proportional to frequency.
As frequency increases = length of near field increases.
How is the beam spread affected by an increase in probe diameter?
Beam spread decreases with an increase in diameter.
What is the problem with inspecting in the near zone?
Reflectors can’t be easily evaluated due to pressure interference.
What are the 3 factors that affect beam spread?
Frequency and diameter of transducer, velocity in material.
What are the 2 main characteristics that will cause attenuation in the material?
Absorption and scattering.
What could cause attenuation by absorption?
Absorption is caused by friction… which produce heat and reduce sound energy and crystalline dislocations… which reduce inelasticity of the material.
