Acoustic Emission (AE) Testing


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Acoustic Emission (AE) refers to the generation of transient elastic waves produced by a sudden redistribution of stress in a material. When a structure is subjected to an external stimulus (change in pressure, load, or temperature), localized sources trigger the release of energy, in the form of stress waves, which propagate to the surface and are recorded by sensors. With the right equipment and setup, motions on the order of picometers (10 -12 m) can be identified. Sources of AE vary from natural events like earthquakes and rockbursts to the initiation and growth of cracks, slip and dislocation movements, melting, twinning, and phase transformations in metals. In composites, matrix cracking and fiber breakage and debonding contribute to acoustic emissions. AE’s have also been measured and recorded in polymers, wood, and concrete, among other materials.

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Acoustic Emission (AE) Testing

  2. 2. PRINCIPLE Acoustic Emission may be defined as a transient elastic wave generated by the rapid release of energy within a material. When a structure is subjected to an external stimulus (change in pressure, load, or temperature), localized sources trigger the release of energy, in the form of stress waves, which propagate to the surface and are recorded by sensors. This occurs when a small surface displacement of a material is produced.
  3. 3. HISTORY & FACTS Probably the first practical use of AE was by pottery makers. As early as 6,500 BC, potters were known to listen for audible sounds during the cooling of their ceramics, to asses the quality of there products. Probably the first observation of AE in metal was during twinning of pure tin as early as 3700 B.C. The first documented observations of AE appear to have been made in the 8th century by Arabian alchemist Jabir ibn Hayyan. He described the “harsh sound or crashing noise” emitted from tin. He also describes iron as “sounding much” during forging. In 1936, Friedrich Forster and Erich Scheil (Germany) conducted experiments that measured small voltage and resistance variations caused by sudden strain movements caused by martensitic transformations. Today, AE Non-Destructive Testing used practically in all industries around the world for different types of structures and materials.
  4. 4. TESTING PROCESS Materials "talk" when they are in trouble: with Acoustic Emission equipment you can "listen" to the sounds ofcracks growing, fibers breaking and many other modes of active damage in the stressed material.
  5. 5. 1. DETECTION OF AE Sources of AE include many different mechanisms of deformations and fracture whilst the detection process remains the same. As a crack grows a number of emissions are released. When the AE wave front arrives at the surface of a test specimen minute movements of the surface molecules occur. The function of AE sensors is to detect this mechanical movement and convert it into a useable electric signal.
  6. 6. 2. PROCESSING OF AE SIGNALS The small voltage generated by the sensor is amplified and the raw radio frequency (RF) signal is transferred to the computer. Based on user defined characteristics, the RF signal is split into discrete waveforms. These waveforms are then prescribed by characteristics such as amplitude, rise time, absolute energy based on a user defined threshold.
  7. 7. 3. DISPLAYING AE SIGNALS The collected waveforms can then be displayed in two ways. One, function of waveform parameters. Two, as the collected waveform itself. Most AE tests currently only record the waveform parameters and ignore the collected waveform, mainly due to the large amount of computing memory it uses.
  8. 8. 4. LOCATING AE SIGNALS The automated source location capability of AE is perhaps its most significant attraction as a non- destructive testing (NDT) technique. The predominant method of source location is based on the measurement of time difference between the arrival of individual AE signals at different sensors in an array.
  9. 9. DIFFERENT FROM OTHER NDTTECHNIQUES The first difference pertains to the origin of the signal. Instead of supplying energy to the object under examination, AET simply listens for the energy released by the object. AE tests are often performed on structures while in operation, as this provides adequate loading for propagating defects and triggering acoustic emissions. The second difference is that AET deals with dynamic processes, or changes, in a material. This is particularly meaningful because only active features (e.g. crack growth) are highlighted. The ability to discern between developing and stagnant defects is significant. However, it is possible for flaws to go undetected altogether if the loading is not high enough to cause an acoustic event. Furthermore, AE testing usually provides an immediate indication relating to the strength or risk of failure of a component. Other advantages of AET include fast and complete volumetric inspection using multiple sensors, permanent sensor mounting for process control, and no need to disassemble and clean a specimen.
  10. 10. AE SOURCE LOCATION TECHNIQUES Locating the source of significant acoustic emissions is often the main goal of an inspection.
  11. 11. 1. MULTIPLE CHANNEL SOURCE LOCATION TECHNIQUE AE systems are capable of using multiple sensors/channels during testing, allowing them to record a hit from a single AE event. As hits are recorded by each sensor/channel, the source can be located by knowing the velocity of the wave in the material and the difference in hit arrival times among the sensors, as measured by hardware circuitry or computer software. Source location techniques assume that AE waves travel at a constant velocity in a material. By properly spacing the sensors in this manner, it is possible to inspect an entire structure with relatively few sensors.
  12. 12. 2. LINEAR LOCATION TECHNIQUE One of the commonly used computed-source location techniques is the linear location principle Linear location is often used to evaluate struts on truss bridges. When the source is located at the midpoint, the time of arrival difference for the wave at the two sensors is zero. Whether the location lies to the right or left of the midpoint is determined by which sensor first records the hit. This is a linear relationship and applies to any event sources between the sensors.
  13. 13. 3. ZONAL LOCATION TECHNIQUE As the name implies, zonal location aims to trace the waves to a specific zone or region around a sensor. Zones can be lengths, areas or volumes depending on the dimensions of the array. The source can be assumed to be within the region and less than halfway between sensors. When additional sensors are applied, arrival times and amplitudes help pinpoint the source zone.
  14. 14. 4. POINT LOCATION In order for point location to be justified, signals must be detected in a minimum number of sensors: two for linear, three for planar, four for volumetric. Accurate arrival times must also be available. The velocity of wave propagation and exact position of the sensors are the necessary criteria. Equations can then be derived using sensor array geometry or more complex algebra to locate more specific points of interest.
  15. 15. INSTRUMENTATION Typical AE apparatus consist of the following components: Sensors used to detect AE events. Preamplifiers amplifies initial signal. Typical amplification gain is 40 or 60 dB. Cables transfer signals on distances up to 200m to AE devices. Cables are typically of coaxial type. Data acquisition device performs filtration, signals’ parameters evaluation, data analysis and charting.
  16. 16. SENSOR AE sensors respond with amazing sensitivity to motion in the low ultrasonic frequency range (10 kHz - 2000 kHz). Motions as small as 10-12 inches and less can be detected. These sensors can hear the breaking of a single grain in a metal, a single fiber in a fiber-reinforced composite, and a tiny gas bubble from a pinhole leak as it arrives at the liquid surface. The transducer element in an AE sensor is almost always a piezoelectric crystal, which is commonly made from a ceramic such as lead zirconate titanate (PZT).
  17. 17. APPLICATIONS1. WELD MONITORING During the welding process, temperature changes induce stresses between the weld and the base metal. These stresses are often relieved by heat treating the weld.
  18. 18. 2. BUCKET TRUCK INTEGRITY EVALUATION Accidents, overloads and fatigue can all occur when operating bucket trucks or other aerial equipment. If a mechanical or structural defect is ignored, serious injury or fatality can result.
  19. 19. 3. BRIDGES Bridges contain many welds, joints and connections, and a combination of load and environmental factors heavily influence damage mechanisms such as fatigue cracking and metal thinning due to corrosion. Acoustic Emission is increasingly being used for bridge monitoring applications because it can continuously gather data and detect changes that may be due to damage without requiring lane closures or bridge shutdown. In fact, traffic flow is commonly used to load or stress the bridge for the AE testing.
  20. 20. 4. AEROSPACE STRUCTURES Most aerospace structures consist of complex assemblies of components that have been design to carry significant loads while being as light as possible. AET has been used in laboratory structural tests, as well as in flight test applications. NASAs Wing Leading Edge Impact Detection System is partially based on AE technology.
  21. 21. OTHER APPLICATIONS Petrochemical and chemical: storage tanks, reactor vessels, offshore platforms, drill pipe, pipelines, valves, hydro-treater etc. Electric utilities: nuclear reactor vessels, piping, steam generators, ceramic insulators, transformers, aerial devices. Fiber-reinforced polymer-matrix composites, in particular glass-fiber reinforced parts or structures (e.g. fan blades) Material research (e.g. investigation of material properties, breakdown mechanisms, and damage behavior) Real-time leakage test and location within various components (small valves, steam lines, tank bottoms)
  22. 22. ADVANTAGES  High sensitivity.  Early and rapid detection of defects, flaws, cracks etc.  Real time monitoring  Cost Reduction  Minimization of plant downtime for inspection, no need for scanning the whole structural surface.
  23. 23. STANDARDSASME - American Society of Mechanical Engineers• Acoustic Emission Examination of Fiber-Reinforced Plastic Vessels, Article 11, Subsection A, Section V, Boiler and Pressure Vessel Code• Acoustic Emission Examination of Metallic Vessels During Pressure Testing, Article 12, Subsection A, Section V, Boiler and Pressure Vessel Code• Continuous Acoustic Emission Monitoring, Article 13 Section VASTM - American Society for Testing and Materials• E569-97 Standard Practice for Acoustic Emission Monitoring of Structures During Controlled Stimulation• E650-97 Standard Guide for Mounting Piezoelectric Acoustic Emission Sensors• E749-96 Standard Practice for Acoustic Emission Monitoring During Continuous Welding• E750-98 Standard Practice for Characterizing Acoustic Emission Instrumentation• E976-00 Standard Guide for Determining the Reproducibility of Acoustic Emission Sensor Response• E1067-96 Standard Practice for Acoustic Emission Examination of Fiberglass Reinforced Plastic Resin (FRP) Tanks/Vessels• E1106-86(1997) Standard Method for Primary Calibration of Acoustic Emission Sensors• E1118-95 Standard Practice for Acoustic Emission Examination of Reinforced Thermosetting Resin Pipe (RTRP)• E1139-97 Standard Practice for Continuous Monitoring of Acoustic Emission from Metal Pressure Boundaries• E1211-97 Standard Practice for Leak Detection and Location Using Surface-Mounted Acoustic Emission Sensors• E1316-00 Standard Terminology for Nondestructive Examinations• E1419-00 Standard Test Method for Examination of Seamless, Gas-Filled, Pressure Vessels Using Acoustic Emission• E1781-98 Standard Practice for Secondary Calibration of Acoustic Emission Sensors• E1932-97 Standard Guide for Acoustic Emission Examination of Small Parts• E1930-97 Standard Test Method for Examination of Liquid Filled Atmospheric and Low Pressure Metal Storage Tanks Using Acoustic Emission• E2075-00 Standard Practice for Verifying the Consistency of AE-Sensor Response Using an Acrylic Rod• E2076-00 Standard Test Method for Examination of Fiberglass Reinforced Plastic Fan Blades Using Acoustic Emission
  24. 24. ASNT - American Society for Nondestructive Testing• ANSI/ASNT CP-189, ASNT Standard for Qualification and Certification of Nondestructive Testing Personnel.• CARP Recommended Practice for Acoustic Emission Testing of Pressurized Highway Tankers Made of Fiberglass reinforced with Balsa Cores.• Recommended Practice No. SNT-TC-1A.Association of American Railroads• Procedure for Acoustic Emission Evaluation of Tank Cars and IM-101 tanks, Issue 1, and Annex Z thereto, “ Test Methods to Meet FRA Request for Draft Sill Inspection program, docket T79.20-90 (BRW) ,” Preliminary 2Compressed Gas Association• C-1, Methods for Acoustic Emission Requalification of Seamless Steel Compressed Gas Tubes.European Committee for Standardization• DIN EN 14584, Non-Destructive Testing – Acoustic Emission – Examination of Metallic Pressure Equipment during Proof Testing; Planar Location of AE Sources.• EN 1330-9, Non-Destructive Testing – Terminology – Part 9, Terms Used in Acoustic Emission Testing.• EN 13477-1, Non-Destructive Testing – Acoustic Emission – Equipment Characterization – Part 1, Equipment Description.• EN 13477-2, Non-Destructive Testing – Acoustic Emission – Equipment Characterization – Part 2, Verification of Operating Characteristics.• EN 13554, Non-Destructive Testing – Acoustic Emission – General Principles.Institute of Electrical and Electronics Engineers• IEEE C57.127, Trial-Use guide for the Detection of Acoustic Emission from Partial Discharges in Oil- Immersed Power Transformers.
  25. 25. THANK YOUAET 08.11.2011 OLK