|NDT.net - September 2002, Vol. 7 No.09|
If the distribution in time (according to year of publication) is analysed, it becomes evident that some organisations are virtually inactive. This can, at least for the national organisations in Europe, be explained by shifting activities to CEN or ISO. On the other hand, most organisations that promote international standards or guidelines have only recently started their activities.
It has to be noted that the status of the organisations varies from multi-national or national bodies (e.g., ISO, CEN, AFNOR), where acceptance of documents is based on corresponding treaties to industry bodies (e.g., ASTM), where acceptance is based on consensus, usually achieved by commenting and balloting, to private societies (e.g., EWGAE, DGZfP) where documents essentially represent recommendations.
The aim of the present paper is two-fold. A comparative assessment of selected documents from each of the classes (1) to (4) will indicate the state that has been achieved to date. However, the comparison will also point out areas that may need attention or become the focus of future activities. Together with an - admittedly subjective - analysis of cases where AE testing shows potential for developing new applications, these will be formulated as perspectives for future standardisation activities.
|Organisation *||Total||Class 1||Class 2||Class 3||Class 4|
|CEN||6||1||2 ***||2||1 +++|
|ASTM||30 **||1||3 +||7||17|
|Table 1: Numerical breakdown of the type of documents published or in preparation by selected standards issuing organisations (the classes are defined in the text)|
|*||ISO = International Organisation for Standardisation, CEN = Comité Européen de Normalisation
(European Standardisation Committee), ASTM = American Society for Testing and Materials,
EWGAE = European Working Group on Acoustic Emission, AFNOR = Association Française de
Normalisation (French Standardisation Society), DGZfP = Deutsche Gesellschaft für
zerstörungsfreie Prüfung (German Society for Non-destructive Testing)
||** ||ASTM E 1065 and E 1544 are not attributed to any of the four classes
||*** ||including EN 473 (CEN) on personnel qualification and certification ||+ ||including E 543 (ASTM) on standard practices for agencies performing non-destructive testing ||++ ||Committee Draft (CD) 16148 (ISO) ||+++ ||Work Item (WI) 00138076 (CEN), prEN 13445-5 contains an Annex E on Acoustic Emission but
simply refers to other documents on AE testing ||§ ||now replaced by EN 1330-9
The bulk of the ISO document is identical with that issued by ASTM with the exception of minor details in the phrasing of the definitions. The difference in the number of definitions between the two documents is due to some definitions that do not appear in the ISO standard as well as some definitions that are defined within the text of other definitions in the ASTM-document and are thus not counted separately.
The number count shows that in principle at most 44 terms could be common among these four documents in the comparison but in fact there are only 13 common terms. A listing of these is as follows: acoustic emission, signal duration, signal rise-time, (sensor) array, acoustic emission count, continuous emission, acoustic emission event, arrival time (interval), source location, zone location, acoustic emission signal, (signal) peak amplitude, and (acoustic emission) waveguide. A total of 108 different terms are defined in the four documents.
However, even the common terms differ in their definition from one document to another. Examples are "acoustic emission and Felicity effect. ISO and ASTM define acoustic emission as the class of phenomena that generates transient elastic waves by rapid release of energy from localised sources and the waves themselves, respectively. The CEN-document reserves the term acoustic emission for the transient elastic waves generated by.release of energy or by a process. The Felicity effect is defined in the ASTM-document as the presence of acoustic emission (or of detectable acoustic emission, both definitions appear independently) at a fixed sensitivity level at stress levels below those applied previously. The CEN-document requires the appearance of significant acoustic emission at loads below the previous applied maximum for the Felicity effect. The examples show that the definitions are not described at the same level of detail. In comparison, both examples, even though technically correct, somehow appear to be incomplete definitions in both documents (ASTM and ISO or CEN, respectively).
A further analysis shows that important terms are absent from the definitions in all documents. Acoustic emission intensity and severity are just two examples. Other basic terms, e.g., pencil lead break or Hsu-Nielsen source, sensor sensitivity, rearm delay time, and guard sensor are only defined in one of the four documents.
Of the different organisations, only CEN provides a document (still at draft stage) that specifically refers to general principles with respect to AE (prEN 13554). This document contains a brief summary of the properties of AE and of the advantages and limitations of the method with respect to other non-destructive test methods. The document claims to be applicable to all classes of materials (metals, composites, ceramics, concrete, etc.) but the specific, basic AE behaviour of the different classes of materials (e.g., Kaiser effect in metals and Felicity-effect in fibre-reinforced composites) is not described in detail. The information given in the document is necessarily limited to general remarks. There is, e.g., a listing of factors that affect sensor and operating frequency selection. Presented without further comments or examples, it is difficult to properly weigh these factors in the process of choosing the best solution for any given application.
It would hence be desirable to have a second level of documents, either detailing the procedures for a number of test cases or at least giving detailed examples as guidelines for making the appropriate selections. Separating these documents according to material classes with distinctly different AE behaviour, including information on the AE source mechanisms would provide starting points for AE test personnel developing applications in various areas. In particular, such documents would be helpful for people with limited experience in the specific class of materials for which the procedure shall be developed.
Sensor calibration procedures are described in sufficient detail in the documents, establishing a hierarchy of primary and secondary calibration. There are also procedures for checking the sensor response (sensor sensitivity), both at regular intervals (quality.assurance) and in test applications. For the different sensor types made of piezoelectric ceramics, these procedures are sufficient, calibration of sensors based on other principles, e.g., interferometers, are not covered by the documents. Use of these, however, is currently limited to special laboratory applications. Unless there is an increasing, routine use of non-piezoelectric sensors, there is no need for establishing the corresponding standardised procedures.
Equipment characterisation standards are based on the types of equipment commercially available at the time of the drafting of the standards or guidelines. This equipment essentially has been designed for extracting an AE signal parameter set from transient burst signals. The procedures for checking the equipment performance, therefore, specify measurements on all components of the measurement chain but emphasise the verification of the AE signal parameter measurement (e.g., EN 13477-2). The verification of AE measurement systems that use full waveform recording (e.g., with transient recorders) is not described in this document.
Of course, these developments in AE testing (waveform analysis based on transient recorders) are relatively recent but there is a need for verifying the equipment and operating characteristics, respectively. Further, fast Fourier transform analysis and pattern recognition tend to complement the AE signal parameter analysis, yielding additional information for data analysis and interpretation. Verification of the operation of both special components (e.g., programmable digital signal processors) and of the software used for this type of analysis is not at all addressed in the existing documents.
A numerical breakdown of the AE testing applications or product specific testing procedures published by the four organisations (ISO, CEN, ASTM and DGZfP) shows that a majority of the documents (8 out of 24) deals with storage tanks or pressure vessels (even if leak testing is not included in the count). A similar distribution (19 out of 46) results if additional organisations are included in the analysis. From the point of view of materials, fibre-reinforced polymer-matrix composites form the bulk of the applications in the three documents (7), followed by metals (6) and ceramics (1), and in the same order 15, 14 and 2, respectively, if additional organisations are included.
The dominance of applications on fibre-reinforced composites is quite likely due to the fact that this class of materials emits copious AE, and that early applications of these materials were in areas of critical structural significance (air/space/storage tanks in the chemical industry). It can also be noted that non-destructive testing of thick composite structures with ultrasonics is difficult because of the inherently large signal attenuation. X-ray inspection yields limited image contrast, unless low X-ray energies (typically a few kV) are applied that also have limited penetration depth. AE testing, on the other hand, is much less affected by these limitations.
There is also a number of standards and guidelines that describe AE testing of a class of test objects or AE monitoring of objects or processes. Examples are ASTM E 1932 on AE.examination of small parts, ASTM E 1139 on AE monitoring of metal pressure boundaries or ASTM E 749 and E 750 on AE monitoring of welding.
A comparison of the different AE testing applications published by ASTM shows that they differ considerably with respect to (1) the description of the test objects, (2) the stimulation to be applied, and, most important, (3) in the data analysis and evaluation. It is clear that a procedure applicable to a large range of test objects such as, e.g., small parts (ASTM E 1932) has to be written in more general terms than, e.g., a procedure covering seamless, gas-filled pressure vessels within a specified service-pressure range (ASTM E 1419). However, even small is defined as low AE signal attenuation throughout the test region. The size of relevant test objects, therefore, depends on the material, and could vary by at least one order of magnitude or more.
The stimulation is in many cases a mechanical loading of the part or structure. The load pattern as a function of time can be simple, such as, e.g., linear loading to the proof load (within a time interval specified, e.g., by the maximum allowable pressure increase), followed by unloading and linear reloading to the same load level. More complex patterns specify a so-called stair-step load with defined rise- and hold-times, unloading to a lower level (e.g., 50% or 75% of the previous load) and holding, followed by reloading to the next higher load level. Patterns of this type allow the evaluation of the Felicity-ratio of test objects made from fibre-reinforced composites. Pressure vessel testing requires pressurisation, either by the medium stored in the vessel or by water or oil, to a service load or a proof-load level, sometimes sequential loading to two different load-levels is required.
Many, but not all AE test standards describe criteria for evaluating the AE that occurs during the test. A simple example is ASTM F 1797 (AE testing of insulated digger derricks) that requires zero AE signals during the last three minutes hold time of the second load or at least a lower number of AE signals compared with the first load cycle for the test object to be acceptable. This is one of the few examples of quantitative, readily evaluated criteria. However, this criterion relies on a number of implicit assumptions. First, it is assumed that there is no or at most a constant and low level of noise signals from non-relevant sources (low compared with the signal count emitted from the relevant structural parts). Secondly, it is assumed that the AE sensors cover all crucial parts of the test object with sufficient and similar sensitivity that is constant during the test. A table in the document lists the components that must be monitored, monitoring of additional components is optional. It is also noted that significant sources of AE have to be evaluated further (with more refined AE testing or another non-destructive test method) and that both, acceptance or rejection criteria, and repair procedures for defective parts are outside the scope of the document. The definition of significant AE sources is not given in the document. Significant could be interpreted, e.g., as a number of AE signals exceeding the quantitative acceptance limit (see above) or else the clustering of a given number of AE signal sources, e.g., within an area with a defined radius.
Without a data base from testing identical or similar test objects and the experience derived from a statistical analysis of a sufficient, relatively large number of test objects it seems difficult to properly evaluate and interpret the AE. There are proprietary data bases that have been developed by some testing agencies that seem to yield reliable assessments based on AE testing. However, even if evaluation criteria are published in the procedure, the assessment of the AE can prove difficult. The distinction between relevant AE signals and noise from non-relevant sources, e.g., may affect the evaluation and yield a false assessment.
From a commercial point of view, the number of false positive calls, i.e., indications judged significant that are actually not relevant, has to be quite low. If the evaluation of the test indications leads to follow-up tests, in particular to a relatively high rate of follow-ups that result in acceptance of suspect components, the test method will not find wide-spread acceptance. In many AE test procedures, there is scant, and frequently only qualitative information on the evaluation of the AE signals. An assessment of the structural integrity and, in particular, quantitative estimates of the remaining service-life time are virtually impossible in those cases.
Another area where numerous AE applications have been published is fibre-reinforced polymer-matrix composites, in particular glass-fibre reinforced parts or structures (e.g., ASTM E 2076 for fibre-glass reinforced fan blades). Glass-fibre reinforced composites represent a large volume of the market in fibre-reinforced materials but many applications are low-cost with low or no requirements with respect to structural integrity. The increasing number of applications of these materials in automotive or, more generally, transport structures (e.g., car bodies) may lead to new AE applications requiring the development of new procedures and evaluation criteria. Air and space applications tend to prefer carbon-fibre reinforced or hybrid composites. AE from these materials is less copious compared to that from glass-fibre reinforced composites and may thus be more difficult to evaluate but the requirements on structural integrity are frequently much more stringent. The higher cost of the carbon-fibre composite materials and for their processing may make AE testing affordable compared with both, other non destructive test methods and the overall cost of the product.
There is a document on Guidance for Development of AE Applications on Composites  that has been published by the Aerospace/Advanced Composites Subcommittee of the Committee on Acoustic Emission from Reinforced Plastics (CARP) of the Society of the Plastics Industry. This guideline is still useful but the list of referenced examples should be up-dated in a revised edition.
Areas where research and development of AE applications is currently being pursued, among others, are process monitoring (with a view towards using the on-line evaluation of the AE signals for direct process control) and global or local long-term monitoring of civil-engineering structures (e.g., bridges, pipelines, off-shore platforms, etc.). In process control, AE offers the advantage that the method can be applied in-situ even in relatively harsh industrial environments and that data evaluation (looking, e.g., for specific peak amplitude levels or specific signal or power spectrum patterns) can be performed virtually on-line, i.e., with the speed needed for feedback into process control components. Long-term monitoring applications also look promising, both with on-line data transfer or temporary data storage and periodic read-out, e.g., via internet, and subsequent data analysis and evaluation in a central monitoring office.
Materials characterisation is performed in many laboratories and in some cases, AE is used as a monitoring technique. The accumulated experience is not documented in AE guidelines or standards. The proposal for an AE Atlas showing the AE activity of various classes of materials seems now to come close to realisation . An important question in that respect is the use of common standardised data formats, in particular for electronic data storage and exchange.
Documents on the AE behaviour of test objects made of glasses (brittle, amorphous), concrete (a composite), wood (a laminate or composite), to name just a few technically important material classes are lacking. While the documents on fibre-reinforced polymer-.matrix composites [e.g., 1] could yield some information for wood and concrete, glasses are not covered by any of the existing documents. Development of new materials (e.g. metal-matrix composites) may also require the development of specific guidelines for AE testing.
An area where innovative non-destructive test methods are needed is adhesive bonding of parts or structures. The application of the Acousto-Ultrasonic method to bonded metal joints is described, e.g., in ASTM E 1495 but this requires suitable access for scanning the area with emitter and sensor. Whether AE based on signal parameter analysis could be used for detecting the low-level signals emitted by thin layers of adhesives is rather questionable but waveform analysis (e.g., based on pattern recognition), maybe combined with improved source location techniques could provide a promising approach.
With respect to terms and definitions, Jollys postulate  has to be repeated here: There is still a need for international harmonisation of definitions, in the sense that (1) definitions in the different documents should become comparable and complete, and (2) the list of definitions should comprise all important basic terms.
With respect to the verification of equipment characteristic, the existing documents lag behind the technical development. The verification of the analysis tools is another area that will need attention. Developments in software quality assurance are important not only for AE testing but for other applications as well. Sensor calibration or verification of sensor response, however, is well documented by existing procedures.
In his review in 1991, Jolly  also expected a need for AE standards in the field of air/space structures and off-shore application. Even though fibre-reinforced composites represent the majority of recently published standards and guidelines, the main applications (e.g., pressure vessels, structural integrity monitoring) are not explicitly for air/space or off-shore structures.
Predictions of the need for developing new AE testing applications are inevitably prone to be wrong. However, structural integrity assessment of structures or parts made from various materials seems a promising area for AE testing. Such applications have to be developed in close collaboration between the specialists, on one hand the engineer familiar with the structural system and on the other hand the AE operator. Civil engineering structures (e.g. bridges) constitute class of test objects for which structural integrity assessment with AE testing could become a commercially successful application.
Whatever area of application of AE testing procedure, there is a need for published, well documented evaluation criteria, both for classes of test objects or for product specific analysis of AE data. Without such published criteria based on a documented data base, successful, new applications of AE analysis will not be widely applied or maybe not even developed.
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