In the late 1960s and early 1970s,research and development in acousticemission testing was undertaken in theUnited States by national laboratories andin the aviation, nuclear and petroleumindustries. In the United Kingdom, themethod was developed at ImperialCollege, London, and at the AtomicEnergy Research Establishment, Harwell.In the spring of 1967, a number ofresearchers were investigating thephenomena of acoustic emission andpublishing reports of their work butwithout a means of centralizedcommunication. There were differences interminology and experimentaltechniques, generally reflecting theresearchers’ educational background andfield of expertise. Jack C. Spannercontacted a dozen people with interest orexperience in the field of acousticemission and invited them to participatein the formation and organization of theAcoustic Emission Working Group. Thefirst meeting was held, informally,in 1967.Acoustic emission working groups werealso organized in Japan and Europe.Through the forum provided by thesegroups, an international exchange ofinformation and cooperation developedfor addressing specific areas. Informationon the history of acoustic emissiondevelopments has been published in moredetail elsewhere.14,31In the closing years of the twentiethcentury, the publication of the acousticemission volume of the NondestructiveTesting Handbookin 1987 and thecertification of inspectors togetherdemonstrated that the technology wasmature and had found widespreadacceptance by industry.424Acoustic Emission Testing
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Origin and Use of the SISystemIn 1960, the General Conference onWeights and Measures established theInternational System of Units. Le SystémeInternational d’Unités(SI) was designed sothat a single set of measurement unitscould be used by all branches of science,engineering and the general public.Without SI, the Nondestructive TestingHandbookseries would contain aconfusing mix of obsoletecentimeter-gram-second (CGS) units,imperial units and the units preferred bycertain localities or scientific specialties.SI is the modern version of the metricsystem and ends the division betweenmetric units used by scientists and metricunits used by engineers and the public.Scientists have given up their units basedon centimeter and gram and engineershave abandoned the kilogram-force infavor of the newton. Electrical engineershave retained the ampere, volt and ohmbut changed all units related tomagnetism.Table 6 lists the seven SI base units.Table 7 lists derived units with specialnames. Table 8 gives examples ofconversions to SI units. In SI, the unit oftime is the second (s) but hour (h) isrecognized for use with SI.For more information, the reader isreferred to the information availablethrough national standards organizationsand specialized information compiled bytechnical organizations.32-35MultipliersIn science and engineering, very large orvery small numbers with units areexpressed by using the SI multipliers,prefixes of 103intervals (Table 9). The
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