NDT Acoustic Emission Testing 2 — Questions and Answers
Question 1: What is the primary function of a preamplifier in an acoustic emission testing system?
- To filter out high-frequency noise above 1 MHz
- To amplify the small AE sensor signal before transmission to the main amplifier (Correct answer)
- To digitize the AE waveform at the point of detection
- To provide electrical isolation between the sensor and the data acquisition unit
Correct answer: To amplify the small AE sensor signal before transmission to the main amplifier
The preamplifier amplifies the low-voltage AE signal (typically microvolts to millivolts) close to the sensor to improve the signal-to-noise ratio before the signal travels through cables to the main system.
Question 2: The Hit Definition Time (HDT) parameter in AE data acquisition is used to:
- Set the maximum allowable signal duration before the system resets
- Determine when an AE hit has ended by timing from the last threshold crossing (Correct answer)
- Define the time window used for source location calculations
- Control the delay between successive pencil lead break calibrations
Correct answer: Determine when an AE hit has ended by timing from the last threshold crossing
HDT (also called hit lockout time) defines how long after the last threshold crossing the system waits before declaring a hit complete, preventing a single emission from being split into multiple hits.
Question 3: For linear source location in acoustic emission testing, what is the minimum number of sensors required?
- One
- Two (Correct answer)
- Three
- Four
Correct answer: Two
Linear (1D) source location requires at least two sensors; the source position along a line is calculated from the difference in arrival times (delta-T) between the two sensors.
Question 4: Which of the following best describes signal attenuation in acoustic emission testing?
- The increase in AE signal frequency as it travels away from the source
- The loss of AE signal amplitude as the wave propagates through the material (Correct answer)
- The reflection of AE waves at the back wall of the test specimen
- The conversion of longitudinal AE waves into shear waves at boundaries
Correct answer: The loss of AE signal amplitude as the wave propagates through the material
Attenuation in AE testing refers to the reduction of signal amplitude with increasing distance from the source due to geometric spreading, material damping, and scattering.
Question 5: The detection threshold in an AE system is set to:
- Match the resonant frequency of the piezoelectric sensor
- Exclude background electromagnetic noise while capturing true AE signals (Correct answer)
- Limit the maximum amplitude of signals stored in the database
- Define the upper boundary of the frequency bandpass filter
Correct answer: Exclude background electromagnetic noise while capturing true AE signals
The detection threshold is a voltage level set above the background noise floor so that only genuine AE signals exceeding it are recorded, preventing false hits from electrical or mechanical noise.
Question 6: A couplant is used between an AE sensor and the test surface primarily to:
- Reduce the electrical resistance between the sensor and the component
- Ensure efficient transfer of acoustic waves from the surface into the sensor (Correct answer)
- Protect the sensor from high-temperature environments during testing
- Increase the resonant frequency response of the piezoelectric element
Correct answer: Ensure efficient transfer of acoustic waves from the surface into the sensor
Couplant fills air gaps between the sensor face and the test surface, which would otherwise severely impede the transmission of high-frequency stress waves into the sensor.
Question 7: In acoustic emission testing, the term 'event' (as opposed to 'hit') refers to:
- Any signal that exceeds the preset amplitude threshold
- A single source occurrence detected by two or more sensors nearly simultaneously (Correct answer)
- A burst emission with duration greater than 1 millisecond
- The electrical noise spike generated when the preamplifier saturates
Correct answer: A single source occurrence detected by two or more sensors nearly simultaneously
An AE event is a single physical source occurrence (e.g., a micro-crack) that may generate hits on multiple sensors; grouping hits from the same source into one event allows for source location.
NDT Acoustic Emission Testing 2
What is the primary function of a preamplifier in an acoustic emission testing system?