ACOUSTIC EMISSION TECHNIQUE
APPLICATION TO PRESSURE VESSEL TESTING

ABSTRACT：Acoustic emission (AE) technique (AET) is a progressively mature non-destructive testing method which was begun in 1960s. AET has been widely used in materials testing, pressure vessel testing, aging aircraft testing, bridge testing and crane testing etc. and their structural integrity evaluation. This paper introduces the principles and application field of AET. The AE testing method of pressure vessels are given. The progress of AE instruments, pressure vessel AE testing standards and application is reviewed. At the last, this paper prospects that the future developing direction of pressure vessel AE testing is on-line monitoring and pattern recognition and artificial neural network pattern recognition analysis of AE signals.

Key words: Acoustic Emission, Pressure Vessel, Testing, Review


1. Introduction
     Acoustic emission and microseismic activity are naturally occurring phenomena. Although it is not known exactly when the first acoustic emissions were heard, fracture processes such as the snapping of twigs, the cracking of rocks and the breaking of bones were probably among the earliest. The first acoustic emission used by an artisan may well have been in making pottery[1].
Acoustic Emission is the elastic energy that is spontaneously released by materials when they undergo deformation. In the early 1960s, a new nondestructive testing technology was born when it was recognized that growing cracks and discontinuities in pressure vessels could be detected by monitoring their acoustic emission signals[2]. Although acoustic emission is the most widely used term for this phenomenon, it has also been called stress wave emission, stress waves, microseism, microseismic activity and rock noise.
     Formally defined, acoustic emission is "the class of phenomena where transient elastic waves are generated by the rapid release of energy from localized sources within a material, or the transient ealstic waves so generated." [3] This is a definition embracing both the process of wave generation and the wave itself.
     Acoustic emission differs from most other nondestructive methods in two significant respects. First, the energy that is detected is released from within the test object rather than being supplied by the nondestructive method, as in ultrasonics or radiography. Second, the acoustic emission method is capable of detecting the dynamic processes associated with the degradation of structural integrity. Crack growth and plastic deformation are major sources of acoustic emission. Latent discontinuities that enlarge under load and are active sources of acoustic emission by virtue of their size, location or orientation are also the most likely to be significant in terms of structural integrity.
     Usually, certain areas within a structural system will develop local instabilities long before the structure fails. These instabilities result in minute dynamic movements such as plastic deformation, slip or crack initiation and propagation. Although the stresses in a metal part may be well below the elastic design limit, the region near a crack tip may undergo plastic deformation as a result of high local stresses. In this situation, the propagating discontinuity acts as a source of stress waves and becomes an active acoustic emission source.
     Acoustic emission examination is nondirectional. Most acoustic emission sources appear to function as point source emitters that radiate energy in spherical wavefronts. Often, a sensor located anywhere in the vicinity of an acoustic emission source can detect the resulting acoustic emission.
     The acoustic emission method offers the following advantages over other nondestructive testing methods:

1. Acoustic emission is a dynamic inspection method in that it provides a response to discontinuity growth under an imposed structural stress; static discontinuities will not generate acoustic emission signals.

2. Acoustic emission can detect and evaluate the significance of discontinuities throughout an entire structure during a single test.

3. Since only limited access is required, discontinuities may be detected that are inaccessible to the more traditional nondestructive methods.

4. Vessels and other pressure systems can often be requalified during an in-service inspection that requires little or no downtime.

5. The acoustic emission method may be used to prevent catastrophic failure of systems with unknown discontinuities, and to limit the maximum pressure during containment system tests.

2. Application of Acoustic Emission Tests
     Acoustic emission examination is a rapidly maturing nondestructive testing method with demonstrated capabilities for monitoring structural, detecting leaks and incipient failures in mechanical equipment, and for characterizing materials behavior. The first documented application of acoustic emission to an engineering structure was published in 1964 and all of the available industrial application experience has been accumulated in the comparatively short time since then.
     A classification of the functional categories of acoustic emission applications is given below:

1. mechanical property testing and characterization;
2. preservice proof testing;
3. in-service (requalification) testing;
4. on-line monitoring;
5. in-process weld monitoring;
6. mechanical signature analysis;
7. leak detection and location; and
8. geological application.

     By definition, on-line monitoring may be continuous or intermittent, and may involve the entire structure or a limited zone only. Although leak detection and acoustic signature analysis do not involve acoustic emission in the strictest sense of the term, acoustic emission techniques and equipment are used for these applications.
     Up today, acoustic emission technique has been applied to a wide variety of research, civil and industrial fields. Next is the summary of acoustic emission applications:

1. Material testing: crack testing of materials, fatigue testing of metal and alloy materials, corrosion detection of metal, hydrogen embrittlement monitoring in high strength steels, integrity testing of composite materials, reinforced plastics testing, ceramic material testing, and tribology testing of materials.
2. Chemical and petroleum industries: integrity testing of pressure vessels, spherical tank testing, cryogenic tank testing, cooldown testing for hot reactors, towers, columns and hot piping systems, tank bottom testing, real time corrosion detection, FRP tank and pipe line testing, leak detection of gas valves, leak detection of buried pipes, sand detection of offshore pipelines, and offshore platform integrity monitoring.
3. Electric and power plant industries: power plant monitoring and diagnostics, high pressure vessels and steam chests testing, steam line testing and continuous monitoring, partial discharge testing of transformers, quantitative steam loss evaluation of valves, general AE testing in intrinsically safe environments, bucket truck testing, continuous monitoring of furnace, continuous leak detection of hot reheat pipe lines, boiler leak detection, turbine blade testing, bearing condition monitoring of turbines, loose particle and touch detection of turbine blade.
4. Aircraft and aerospace industries: aircraft proof testing (both military and commercial), aging aircraft testing, fatigue testing of complete structures/aircraft, incoming inspection of advanced composites for new aircraft, corrosion detection under the skin of wings, in-situ monitoring of landing gears, wind turbine blade and helicopter blade testing, on-board continuous aircraft monitoring, crack detection of fuselage, pivot bearing and skin lap joints testing, in-process monitoring of transmission of gear box of helicopter, shuttle composite fuel tanks and explosive bolt testing, structure proof testing of aerospace launcher.
5. Metalworking industries: Tool wear and breakage detection, touch detection of grinding wheel/dresser and workpiece, dressing verification, quality control of metalworking processes, chatter detection, forging press testing, crash detection and prevention of manufacturing process.
6. Civil engineering: structure testing of concrete buildings, bridges testing, tunnel testing, dam testing, continuous surveillance of flaw or crack propagation of concrete structures, crane testing.
7. Transportation application: detect and locate flaws and their sizes for tube trailers, railroad cars and tank trucks, crack detection in railway materials and structures, integrity testing of bridges and tunnels, condition monitoring of ball bearing and journal bearing of trucks and trains, crack detection of wheels and shafts of trains.
8. Other application: pinpoint trouble spot in welds, head disk interference detection, integrity testing of pressure flasks, drought stress monitoring of crops and woods, wear and friction testing, rock detection, geological and seismological applications, engine status monitoring, on-line process monitoring of rotating machinery, crack detection of steel roller, automobile shaft strengthening process monitoring, investment casting process monitoring, charge and discharge monitoring of Li/MnO2 battery cells.

3. Principle of Acoustic Emission Testing
     Figure 1 is the schematic diagram of acoustic emission testing principle. The acoustic emission source emits elastic waves into material. The waves propagate in the material. When an acoustic emission wavefront impinges on the surface of a test object, very minute movements of the surface molecules occur. A sensor's function is to detect this mechanical movement and convert it into a specific, usable electric signal. Preamplifiers amplify electric signals with 20~40dB and send them to the mainframe. The mainframe carry on the acquisition, display, save and analysis for the acoustic emission signals.

Display


Figure 1. Schematic Diagram of Acoustic Emission Testing Principle

     Equipment for processing acoustic emission signals is available in a variety of forms ranging from small portable instruments to large multichannel systems. Components common to all systems are sensors, preamplifiers, filters, signal acquisition and analysis system to make the signal measurable. Methods used for measurement, display and storage vary more widely according to the demands of the application.
     The sensors used for acoustic emission testing often resemble an ultrasonic search unit in configuration and generally utilize a piezoelectric transducer as the electromechanical conversion device. The sensors may be resonant or broadband. The main considerations in sensor selection are (1) operating frequency; (2) sensitivity; and (3) environmental and physical characteristics. For high temperature tests, waveguides may be used to isolate the sensor from the environment. Waveguides have also been used to precondition the acoustic emission signals as an interpretation aid.
     The preamplifier must be located close to the sensor. Sometimes it is actually incorporated into the sensor housing. The preamplifier provides required filtering, gain (most commonly 40 dB) and cable drive capability. Filtering in the preamplifier (together with sensor selection) is the primary means of defining the monitoring frequency for the acoustic emission test. This may be supplemented by additional filtering at the mainframe.
     Choosing the monitoring frequency is an operator function, since the acoustic emission source is essentially wide band. Reported frequencies range from audible clicks and squeaks up to 50MHz. Although not always fully appreciated by operators, the observed frequency spectrum of acoustic emission signals is significantly influenced by the resonance and transmission characteristics of both the specimen (geometry as well as acoustic properties) and the sensor. In practice, the lower frequency limit is governed by background noise; it is unusual to go below 10 kHz except in microseismic work. The upper frequency limit is governed by wave attenuation that restricts the useful detection range; it is unusual to go above 1 MHz. The single most common frequency range for acoustic emission testing is 100 to 300 kHz.
     Sources of acoustic emission include many different mechanisms of deformation and fracture. Earthquakes and rockbursts in mines are the largest naturally occurring emission sources. Sources that have been identified in metals include crack growth, moving dislocations, slip, twinning, grain boundary sliding and the fracture and decohesion of inclusions. In composite materials, sources include matrix cracking and the debonding and fracture of fibers. These mechanisms typify the classical response of materials to applied load. Other mechanisms fall within the definition and are detectable with acoustic emission equipment. These include leaks and cavitation; friction (as in rotating bearings)


4. Acquisition and Analysis Methods of Acoustic Emission Signals
     Acoustic emission signals possess very large dynamic scope. The amplitude of displacements are from 10-15m to 10-9m. In addition, the emitting rates of acoustic emission sources also are very changeable. Acoustic emission signals are artificially classified into burst emission and continuous emission. The amplitudes of burst signals are much higher than the background noise and the occurrences of individual signals are well separated in time. The pulse of continuous signals can not be resolved.
     The objective of an acoustic emission test is to detect the presence of emission sources and to provide as much information as possible about the source. The technology for detecting and locating sources is well established and acoustic emission signals can provide a large amount of information about the source of the emission and the material and structure under examination. The purpose of source characterization is to use the sensor output waveform to identify the sources and to evaluate their significance. There is thus a qualitative (source identification) and a quantitative (source intensity or severity) aspect to characterization.
     The signal waveform is affected by (1) characteristics of the source; (2) the path taken from the source to the sensors; (3) the sensor's characteristics; and (4) the measuring system. Generally the waveforms are complex and using them to characterize the source can be difficult. Information is extracted by methods ranging from simple waveform parameter measurements to artificial intelligence (pattern recognition) approaches. The former often suffices for simple preservice and in-service tests. The latter may be required for on-line monitoring of complex systems.
     In recently, the acquisition, record and analysis methods of acoustic emission signals basically include two classes. One is to acquire, save and analyze the waveform of acoustic emission signals. Another is to acquire, save and analyze the simple waveform parameters of acoustic emission signals.
4.1 Waveform Analysis Methods of Acoustic Emission Signals
     Figure 2 shows the waveforms of typical acoustic emission signals produced by lead breaking and surface crack growing on a weld seam of a steel pressure vessel. Acoustic emission analysis methods based on waveforms include in Fast Fourier Transform Algorithm (FFT), classical frequency spectrum analysis, modern frequency spectrum analysis, wavelet analysis, pattern recognition, artificial neural network pattern recognition and so on.

(a)0.5mmHB lead breaking	(b)Surface crack growing

Figure 2. Typical waveforms of acoustic emission signals of steel pressure vessel

     The basic condition adopting waveform analysis methods is to acquire and save the whole waveforms of acoustic emission signals by using wide-band sensors. It is easy to satisfy this condition in laboratory. But there are two factors to limit the use of this method in the filed test for large size pressure vessels. One is that the sensitivity of wide-band sensor is much less than the resonant sensor. A complete waveform can not be acquired when the distance between sensor and acoustic emission source is larger than 1 meter for steel pressure vessel. Another factor is that acquiring and saving all waveforms of acoustic emission signals of all channels are very difficult for the whole testing period for a large pressure vessel. It is impossible to analyze the acoustic emission signals and display the location of acoustic emission sources on real time. Until today, the waveform analysis methods have not commonly applied to acoustic emission test for pressure vessel.

4.2 Simple Waveform Parameters Analysis Methods of Acoustic Emission Signals
     The method recording simple waveform parameters of acoustic emission signals has been adopted more than 30 years by acoustic emission instruments. Figure 3 shows the definitions of simple waveform parameters of acoustic emission signals. Using this model, the acoustic emission event, count (ringdown count), event energy, signal amplitude, duration and rise time can be defined. Cumulative representations of these parameters can be defined as a function of time or test parameter (such as pressure or temperature), including: (1) total event; (2) amplitude distribution; and (3) accumulated energy. Once a specific parameter is selected, rate functions may be defined as a function of time or test parameter: event rate; count rate; and energy rate.


Figure 3. Definition of simple waveform parameters

     Comparing to record the waveform of acoustic emission signals, simple waveform parameter method can record much more signals and perform real time data analysis and display. This method is commonly using in acoustic emission test for pressure vessel. Table 1 lists the simple waveform parameter data of acoustic emission signals produced by crack growth on the weld seam of a steel pressure vessel. The classical analysis methods include in historical variety, distribution and correlation for simple waveform parameters. Figure 4 to 6 are typical examples. The intensity and activity of acoustic emission source can be obtained by use of these classical analysis methods. Some advanced analysis methods such as pattern recognition, artificial neural network pattern recognition, grey correlation analysis, and fuzzy analysis have been adopted to simple waveform parameters[4-6]. Some results can give the quality and dangerous degree of acoustic emission sources.
Table 1. Simple Waveform Parameters of Acoustic Emission Signals

4.3 Location of Acoustic Emission Sources
     Acoustic emission location methods include zone, one-dimensional linear, two-dimensional planar, and three-dimensional location. Due to most acoustic emission sources appear to function as point source emitters that radiate energy in spherical waveforms, a sensor located anywhere in the vicinity of an acoustic emission source can detect the resulting acoustic emission. One sensor testing can not locate the direction of acoustic emission signals. In addition, one sensor only can monitor a limited zone due to the attenuation of wave propagation for large size structures. Two sensors can be used as linear location of acoustic emission signals through measuring the time difference. Pressure piping test generally adopts linear location. At least three sensors can be used as planar location of acoustic emission signals through measuring their arrival time differences. Due to most of pressure vessels are thin wall vessels, acoustic emission tests of pressure vessels commonly use planar location method. The distances between sensors are usually from 3 to 5 meters for large pressure vessels. Acoustic emission test needs 16 channels for a 100m3 LPG tank and 32 channels for a 1000m3 LPG sphere. Figure 7 shows the sensor array of acoustic emission test for a 50m3 LPG sphere. Three-dimensional source location at least needs 4 sensors and measures 3 time differences. This method is very suitable for acoustic emission test of ultra-high pressure thick wall vessels.

Figure 7. sensor array of acoustic emission test for a 50m3 LPG sphere

5. Acoustic Emission Testing Methods of Pressure Vessels
      The periodic inspection of in-service pressure vessel normally adopts magnetic particle testing (MT), dye penetration testing (PT), ultrasonic testing (UT) or radiographic testing methods spot or 100% to test the welding seam. It was found that there are some disadvantages for these methods as below:

1. If there are many welding defects in the pressure vessel, these methods can not distinguish which defects are dangerous and which defects are stable.
2. For most of pressure vessels, after finished the periodic inspection, no any defect was found. But stopping operation of pressure vessels has resulted in economic lost for the user of pressure vessel.
3. These methods can not carry out on-line monitoring and safety evaluation for pressure vessel.

However, the advantages of acoustic emission testing method just make up the last disadvantages of normal nondestructive methods. The acoustic emission tests of pressure vessels can be classified into preservice proof testing, in-service testing and on-line monitoring. The details will be given below.
5.1 Preservice Acoustic Emission Proof Testing
     Preservice acoustic emission testing is performed during the proof hydrostatic pressure test. Due to there is no any special requirement for this test, the fabrication time of pressure vessel will not be extended. The advantages of this test are as follows:

1. Due to the whole shell of pressure vessel is real time monitored during loading, the leakage and catastrophic failure resulting from crack growing can be prevented.
2. The integrity of whole pressure vessel can be evaluated.
3. The active acoustic emission sources can be discovered and located. After performing re-testing by use of other nondestructive methods, the lost or new cracks can be found and repaired. The safety and reliability of pressure vessel is further improved.
4. The distribution and releasing of residual stress of pressure vessel can be indicated.

5.2 Acoustic Emission Testing and Defect Assessment of In-service Pressure Vessel
     The procedure of acoustic emission test and defect assessment of in-service pressure vessel is as follows:

1. Stop operating and empty the pressure vessel of operating media.
2. Perform pressure testing (with water, liquid or gaseous media) and acoustic emission testing. The highest testing pressure should be larger than 110% of highest operating pressure.
3. Analyze acoustic emission data and give the locations of significant active acoustic emission sources.
4. Adopt visual inspection, MT or PT, UT or RT to retest the acoustic emission sources, find and repair the active defects.
5. Sometimes perform 100% MT for the surface of welding seam of pressure vessel to find and eliminate the non-active surface cracks during pressure test.
6. Pneumatic test.
7. Resume operation.

    Comparing with normal nondestructive testing methods and defects assessment by use of fracture mechanics, the advantages adopting acoustic emission testing is listed below:

1. Due to the re-test ratio by use of normal nondestructive testing methods is very little, the inspection time is greatly shortened.
2. The whole pressure vessel test covers the shortage of random spot check. The safety of pressure vessel is further raised.
3. If there are many code-reject weld defects in a pressure vessel, fracture mechanics assessment needs 100% ultrasonic test to measure the geometric size of these defects. The stop operating time and cost will be greatly increased. However, adopting acoustic emission test can enormously shorten the stop operating time and easily find the active defects.

5.3 On-line Monitoring and Safety Evaluation of Pressure Vessels
     Due to the requirement of produce, some pressure vessels that have arrived at the periodic inspection date still can not stop operating. Acoustic emission on-line monitoring is employed to evaluate the safety of these pressure vessels. Followings are the test procedure:

1. Reduce the operating pressure to 90% highest operating pressure.
2. Pressurizing with the operating media and perform acoustic emission on-line monitoring until the pressure arrive at 110% highest operating pressure.
3. Analyze the acoustic emission data, give the safe operating pressure and extending operation time of pressure vessels.

    Acoustic emission on-line monitoring not only affects the produce, but also insures the safe operation and extends the using period of pressure vessels. It takes a huge economic benefits for the user of pressure vessels.


6. The Progress of Acoustic Emission Testing for Pressure Vessel
6.1 Standards
     Up today, many countries have laid down and issued a lot of acoustic emission standards. United States possess 43 standards issued by ASTM, ASME, SPI, DOT, AAR, SAE, USNRC, CGA, API AND ASNT. The main standards for pressure vessel tests are as follows:

1. ASTM E569-91 Standard practice for acoustic emission monitoring of structures during controlled stimulation.
2. ASTM E1139-92 Standard practice for continuous monitoring of acoustic emission from metal pressure boundaries.
3. ASTM E1419-91 Test method for examination of seamless, gas filled, pressure vessels using acoustic emission.
4. ASTM 403-90-1 Standard test method for examination of liquid filled atmospheric and low pressure metal storage tanks using AE.
5. ASME "Acoustic emission examination of fiber-reinforced plastic vessels", Article 11, Subsection A, Section V, Boiler and Pressure Vessel Code (1983 and later editions).
6. ASME "Acoustic emission examination of metallic vessels during pressure testing", Article 12, Subsection A, Section V, Boiler and Pressure Vessel Code (1988 addendum and later editions).
7. ASME "Acoustic emission for continuous monitoring of pressure vessels", Article 13, Subsection A, Section V, Boiler and Pressure Vessel Code.
8. DOT-E 8944 Code variance (Jumbo tube trailer testing), 1983 and subsequent renewals and issuance to other licensees.
9. AAR "Procedure for acoustic emission evaluation of tank cars and IM-101 tanks", Issue 1, January, 1991 and annex Z thereto.
10. CGA C-18 Methods for acoustic emission re-qualification of seamless steel compress gas tubes, 1995.
11. ASNT CARP Recommended practice for acoustic emission testing of pressurized highway tanks made of fiberglass reinforced plastic with bolsa cores.

6.2 Instruments
     Most of multichannel acoustic emission systems application to pressure vessel test are SPARTAN serious instruments made in Physical Acoustics Corporation (PAC), USA. These instruments are analogous and can only record the arrival time and 7 simple waveform parameters of acoustic emission signals. In recent years, PAC, Digital Waves Corporation and Vallen Systeme Corporation have all developed digital acoustic emission instruments that can acquire and record the waveform of acoustic emission signals. Due to the prices are too high, these digital instruments are still not commonly applied to the pressure vessel testing.
The first single channel AE instrument was developed by Shenyang Computer Research Institute in China in 1976. From then two channels and four channels AE instruments were manufactured and sold in the 1980s. The first multi-channel (8-64 channels) AE instrument was developed by CBPVI in 1997. A serious of commercial acoustic emission instruments have been developed and sold in China. The first multi-channel digital AE instrument has been developed by Tsinghua University in 1999.

6.3 Pressure Vessel Testing
     According to the report[7-9], acoustic emission testing of pressure vessel is generally used in United States. Mansanto Chemical Corporation has successfully tested several thousands of pressure vessels. Many of them are on-line monitoring. This method also has been successfully used in Japan, Italy, Australia and so on[10-13].
     The most important activities in AE testing in China have been run in the scope of pressure vessel safety research and evaluation since 1980. Now more than 30 companies are engaged in pressure vessel testing with AE. Almost all the multi-channel AE instruments are used as pressure vessel testing. More than 100 large size pressure vessels are tested every year. CBPVI is the largest inspection organization for pressure vessel[14-21]. Up today, CBPVI has successfully tested more than 600 large pressure vessels by use of acoustic emission technique.

7. Prospects for the Future
     In recent years, due to the design, fabrication, inspection and use of pressure vessel are strictly controlled, it is very rare to find serious original welding defects in pressure vessels. How to extend the operating period and short the periodic inspection time are the matters of interest to numerous users of pressure vessels. The advantages of acoustic emission technique determine that on-line monitoring of pressure vessels will emerge a huge market in future. Due to the re-test adopting normal nondestructive testing methods can not be performed for most of on-line operating pressure vessels, the investigation of qualitative distinguished technique and determination of dangerous degree of acoustic emission sources should be the developing direction of acoustic emission testing of pressure vessels.

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