Objective 10: Acoustic Emission Testing (AET)

Question 1

How do most materials and structures respond to a sudden change or movement?

Answer 1

They emit energy in the form of mechanical vibrations (acoustic emissions)

Question 2

What causes materials to emit mechanical vibrations?

Answer 2

Defect-related phenomenon, e.g. cracking or plastic deformation

Question 3

What direction do acoustic emissions propogate?

Answer 3

From the source throughout the structure

Question 4

What is the purpose of Acoustic Emission Testing?

Answer 4

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

Question 5

Do all types of structures and production processes undergo continuous loading and stressing?

Answer 5

Yes

Question 6

What is the source of stress experienced by pipelines and vessels?

Answer 6

The temperature and pressure of the process itself

Question 7

What is the source of stress experienced in production processes?

Answer 7

Machines apply stress to materials as they are being formed, shaped and joined

Question 8

What is the eventual result of materials and structures being subjected to stress?

Answer 8

Defect growth (e.g. cracking) in weaker or fatigued areas of the structure

Question 9

What distinguishes Acoustic Emission Testing from all other NDE testing?

Answer 9

It detects defect growth in real time, as it is occurring

Question 10

What is the benefit of Acoustic Emission Testing deteting and locating flaws as early as possible?

Answer 10

The structure can be repaired or replaced long before a catastrophe occurs, thereby preventing loss of life, environmental damage, and costly repairs

Question 11

Acoustic Emissions

Answer 11

Transient elastic waves, generated by the rapid release of energy from localized sources within a material

Question 12

How does Acoustic Emission Testing work?

Answer 12

1. The material defect emits acoustic waves that emanate in all directions from the source
2. The sensor detects the mechanical shock wave and converts it into an electronic signal
3. The electronic signal is amplified and processed by the AE instrument

Question 13

What is used to sense the acoustic emissions in a material being tested?

Answer 13

A piezoelectric acoustic emission sensor that is in contact with the material being monitored

Question 14

What is required for the production of acoustic emissions?

Answer 14

Mechanical stress

Question 15

What are ways that mechanical stress can be produced in a material to produce acoustic emissions?

Answer 15

1. Naturally due to normal process conditions

2. Application of external force to produce an emission

Question 16

If an external force is used to produce an emission in a material being tested, what is required of that force?

Answer 16

1. Non-destructive

2. Well below the expected defect tolerance of the material

Question 17

What frequencies do AE systems operate with?

Answer 17

1 - 2000 Hz

Question 18

What are some Background Noises that tend to mask acoustic emissions?

Answer 18

1. Friction
2. Outside Impacts
3. Process generated Signals

Question 19

What determines the lower frequency limit used in AE testing?

Answer 19

Frequency of background noises

Question 20

What determines the upper frequency limit used in AE testing?

Answer 20

Signal attenuation

Question 21

Signal Attenuation

Answer 21

Gradual reduction of signal strength

Question 22

How is a suitable frequency range chosen for AE detection and signal processing?

Answer 22

1. Must be above the background noises

2. Must provide the range and sensitivity to detect AE

Question 23

How is a suitable frequency range achieved in AE testing?

Answer 23

Careful selection of AE sensors and electronic signal filtering

Question 24

What is the nature of acoustic emissions?

Answer 24

Transient and occurring in discrete bursts

Question 25

What are the AE features that can be analyzed to help identify the source of AE as noise or defect related?

Answer 25

1. Time of hit 2. Rise time 3. AE amplitude 4. AE counts 5. Duration 6. Frequency content 7. The waveform itself

Question 26

How complex are AE systems?

Answer 26

They range from simply single-channel, single purpose devices to complex multi-channel multi-processing systems

Question 27

Describe the makeup of a basic AE system.

Answer 27

Consist of one or more AE sensors and one preamplifier per channel, connected to an AE processor

Question 28

What is the role of the processor in an AE system?

Answer 28

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

Question 29

What do the AE Processor output signals correlate to?

Answer 29

The process or stress measured. They can be pass/fail signals for: 1. control purposes 2. indicators and annunciators; or 3. graphical outputs

Question 30

How can AE Sensors determine the precise location of the signal source?

Answer 30

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

Question 31

How many sensors are required to determine the source location in 1 dimension (single line)?

Answer 31

2

Question 32

How many sensors are required to determine the source location in 2 dimensions (surface or plane)?

Answer 32

3

Question 33

How many sensors are required to determine the source location in 3 dimension?

Answer 33

At least 4

Question 34

How can the severity of an acoustic emission source be determined?

Answer 34

1. Monitoring multiple AE events in the same area | 2. Applying cluster analysis methods

Question 35

What advantage is afforded by the ability of AE testing to occur with a small number of sensors?

Answer 35

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

Question 36

Which industry has one of the oldest and successful applications of AE?

Answer 36

Manufacturing

Question 37

How can AE Testing be used to effectively perform crack detection?

Answer 37

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

Question 38

In which industry is AE Pressure Vessel Monitoring prominently used?

Answer 38

Petrochemical

Question 39

How does AE Pressure Vessel Monitoring work?

Answer 39

1. The entire pressure boundary of the vessel is monitored by an array of sensors 2. Pressures 10% greater than normal operating pressure (well below MAWP) are applied incrementally to the vessel 3. After each incremental pressure increase, the pressure is held for a period of time 4. The AE activity during each pressure increment is monitored and recorded

Question 40

How does AE testing detect leaks?

Answer 40

1. AEs are created by turbulent flow though cracks, valves, seals, and orifices 2. Acoustic energy is transmitted through the fluid, air, or structure to piezoelectric sensors 3. The signal is processed, filtered, and compared to a leak profile 4. The emission source is located using triangulation

Question 41

How is AE testing used in the Pulp and Paper Industry?

Answer 41

1. Locate cracks in pressure vessels and rotating equipment | 2. Identify delamination of bonded materials