NDT Ultrasonic Testing Principles 2 — Questions and Answers
Question 1: What is the acoustic impedance of a material defined as?
- The ratio of stress to strain in the material
- The product of material density and ultrasonic wave velocity (Correct answer)
- The resistance of a material to crack propagation
- The attenuation coefficient divided by frequency
Correct answer: The product of material density and ultrasonic wave velocity
Acoustic impedance (Z) equals material density (ρ) multiplied by the ultrasonic wave velocity (v), expressed as Z = ρv.
Question 2: When an ultrasonic wave travels from steel into water, what primarily occurs at the interface?
- Total internal reflection because steel has higher impedance
- Partial reflection and partial transmission due to impedance mismatch (Correct answer)
- Complete transmission with no reflection
- The wave converts entirely to heat energy
Correct answer: Partial reflection and partial transmission due to impedance mismatch
At any interface between materials of different acoustic impedances, the wave is partly reflected and partly transmitted; the proportions depend on the impedance difference.
Question 3: Which type of ultrasonic wave motion involves particles oscillating perpendicular to the direction of wave propagation?
- Longitudinal (compression) wave
- Surface (Rayleigh) wave
- Shear (transverse) wave (Correct answer)
- Lamb wave
Correct answer: Shear (transverse) wave
Shear waves, also called transverse waves, have particle motion perpendicular to the direction of wave travel, unlike longitudinal waves where motion is parallel.
Question 4: What is the primary purpose of couplant in contact ultrasonic testing?
- To increase the frequency of the transducer
- To eliminate the air gap and improve acoustic energy transfer between transducer and part (Correct answer)
- To slow down the ultrasonic wave for easier measurement
- To magnetize the test surface for better defect detection
Correct answer: To eliminate the air gap and improve acoustic energy transfer between transducer and part
Couplant fills the microscopic air gap between the transducer and test surface, since air has very low acoustic impedance and would cause nearly total reflection of ultrasonic energy.
Question 5: The near field (Fresnel zone) of an ultrasonic transducer is characterized by:
- Uniform, predictable beam intensity that decreases with distance
- Highly irregular intensity variations making flaw sizing unreliable (Correct answer)
- Maximum beam divergence and spreading
- Absence of any side lobes or acoustic artifacts
Correct answer: Highly irregular intensity variations making flaw sizing unreliable
The near field has complex interference patterns causing irregular amplitude variations, making flaw sizing and detection unreliable in this zone.
Question 6: Snell's Law in ultrasonics describes:
- The relationship between frequency and wavelength in a given material
- The attenuation of sound as it travels through a medium
- The refraction of ultrasonic waves at the interface between two materials (Correct answer)
- The minimum detectable flaw size based on transducer frequency
Correct answer: The refraction of ultrasonic waves at the interface between two materials
Snell's Law defines how the angle of an ultrasonic beam changes (refracts) when it crosses from one medium to another, based on the ratio of wave velocities in each material.
Question 7: What effect does increasing ultrasonic test frequency have on resolution and penetration?
- Increases both resolution and penetration equally
- Improves resolution but reduces penetration depth (Correct answer)
- Reduces resolution but increases penetration depth
- Has no effect on either resolution or penetration
Correct answer: Improves resolution but reduces penetration depth
Higher frequency produces shorter wavelengths, improving resolution and sensitivity to small defects, but attenuation increases with frequency, reducing maximum penetration depth.
NDT Ultrasonic Testing Principles 2
What is the acoustic impedance of a material defined as?