Objective 10: Acoustic Emission Testing (AET) Flashcards
How do most materials and structures respond to a sudden change or movement?
They emit energy in the form of mechanical vibrations (acoustic emissions)
What causes materials to emit mechanical vibrations?
Defect-related phenomenon, e.g. cracking or plastic deformation
What direction do acoustic emissions propogate?
From the source throughout the structure
What is the purpose of Acoustic Emission Testing?
To detect and locate defects as they occur, across the entire monitored area, providing early warning of pending failure in a timely and cost-efective manner
Do all types of structures and production processes undergo continuous loading and stressing?
Yes
What is the source of stress experienced by pipelines and vessels?
The temperature and pressure of the process itself
What is the source of stress experienced in production processes?
Machines apply stress to materials as they are being formed, shaped and joined
What is the eventual result of materials and structures being subjected to stress?
Defect growth (e.g. cracking) in weaker or fatigued areas of the structure
What distinguishes Acoustic Emission Testing from all other NDE testing?
It detects defect growth in real time, as it is occurring
What is the benefit of Acoustic Emission Testing deteting and locating flaws as early as possible?
The structure can be repaired or replaced long before a catastrophe occurs, thereby preventing loss of life, environmental damage, and costly repairs
Acoustic Emissions
Transient elastic waves, generated by the rapid release of energy from localized sources within a material
How does Acoustic Emission Testing work?
- The material defect emits acoustic waves that emanate in all directions from the source
- The sensor detects the mechanical shock wave and converts it into an electronic signal
- The electronic signal is amplified and processed by the AE instrument
What is used to sense the acoustic emissions in a material being tested?
A piezoelectric acoustic emission sensor that is in contact with the material being monitored
What is required for the production of acoustic emissions?
Mechanical stress
What are ways that mechanical stress can be produced in a material to produce acoustic emissions?
- Naturally due to normal process conditions
2. Application of external force to produce an emission
If an external force is used to produce an emission in a material being tested, what is required of that force?
- Non-destructive
2. Well below the expected defect tolerance of the material
What frequencies do AE systems operate with?
1 - 2000 Hz
What are some Background Noises that tend to mask acoustic emissions?
- Friction
- Outside Impacts
- Process generated Signals
What determines the lower frequency limit used in AE testing?
Frequency of background noises
What determines the upper frequency limit used in AE testing?
Signal attenuation
Signal Attenuation
Gradual reduction of signal strength
How is a suitable frequency range chosen for AE detection and signal processing?
- Must be above the background noises
2. Must provide the range and sensitivity to detect AE
How is a suitable frequency range achieved in AE testing?
Careful selection of AE sensors and electronic signal filtering
What is the nature of acoustic emissions?
Transient and occurring in discrete bursts
What are the AE features that can be analyzed to help identify the source of AE as noise or defect related?
- Time of hit
- Rise time
- AE amplitude
- AE counts
- Duration
- Frequency content
- The waveform itself
How complex are AE systems?
They range from simply single-channel, single purpose devices to complex multi-channel multi-processing systems
Describe the makeup of a basic AE system.
Consist of one or more AE sensors and one preamplifier per channel, connected to an AE processor
What is the role of the processor in an AE system?
It receives amplified signals from the AE sensors, as well as signals from external sensors or control inputs. It combines these inputs to form outputs indicative of the detected AE activity
What do the AE Processor output signals correlate to?
The process or stress measured. They can be pass/fail signals for:
- control purposes
- indicators and annunciators; or
- graphical outputs
How can AE Sensors determine the precise location of the signal source?
If multiple sensors are used, the time difference for the signal to arrive at each sensor can be determined and thereafter the precise location can be triangulated
How many sensors are required to determine the source location in 1 dimension (single line)?
2
How many sensors are required to determine the source location in 2 dimensions (surface or plane)?
3
How many sensors are required to determine the source location in 3 dimension?
At least 4
How can the severity of an acoustic emission source be determined?
- Monitoring multiple AE events in the same area
2. Applying cluster analysis methods
What advantage is afforded by the ability of AE testing to occur with a small number of sensors?
Very few access holes are needed for placement of AE sensors without the need to necessarily remove all of the insulation as is the case with all other NDE techniques
Which industry has one of the oldest and successful applications of AE?
Manufacturing
How can AE Testing be used to effectively perform crack detection?
AE systems can be interfaced to programmable controllers, and set to monitor cracking continuously during the high-stress manufacturing processes. When cracks occur, the AE system provides a “failure” output signal, and the part can be rejected
In which industry is AE Pressure Vessel Monitoring prominently used?
Petrochemical
How does AE Pressure Vessel Monitoring work?
- The entire pressure boundary of the vessel is monitored by an array of sensors
- Pressures 10% greater than normal operating pressure (well below MAWP) are applied incrementally to the vessel
- After each incremental pressure increase, the pressure is held for a period of time
- The AE activity during each pressure increment is monitored and recorded
How does AE testing detect leaks?
- AEs are created by turbulent flow though cracks, valves, seals, and orifices
- Acoustic energy is transmitted through the fluid, air, or structure to piezoelectric sensors
- The signal is processed, filtered, and compared to a leak profile
- The emission source is located using triangulation
How is AE testing used in the Pulp and Paper Industry?
- Locate cracks in pressure vessels and rotating equipment
2. Identify delamination of bonded materials
