1、Designation: E 2479 06Standard Practice forMeasuring the Ultrasonic Velocity in Polyethylene TankWalls Using Lateral Longitudinal (LCR) Waves1This standard is issued under the fixed designation E 2479; the number immediately following the designation indicates the year oforiginal adoption or, in the
2、 case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers a procedure for measuring theultrasonic velocities in the outer w
3、all of polyethylene storagetanks.An angle beam lateral longitudinal (LCR) wave is excitedwith wedges along a circumferential chord of the tank wall. Adigital ultrasonic flaw detector is used with sending-receivingsearch units in through transmission mode. The observedvelocity is temperature correcte
4、d and compared to the expectedvelocity for a new, unexposed sample of material which is thesame as the material being evaluated. The difference betweenthe observed and temperature corrected velocities determinesthe degree of UV exposure of the tank.1.2 The practice is intended for application to the
5、 outersurfaces of the wall of polyethylene tanks. Degradation typi-cally occurs in an outer layer approximately 3.2-mm 0.125-in. thick. Since the technique does not interrogate the insidewall of the tank, wall thickness is not a consideration other thanto be aware of possible guided (Lamb) wave effe
6、cts orreflections off of the inner tank wall. No special surfacepreparation is necessary beyond wiping the area with a cleanrag. Inside wall properties are not important since the longitu-dinal wave does not strike this surface. The excitation of Lambwaves must be avoided by choosing an excitation f
7、requencysuch that the ratio of wavelength to wall thickness is one fifthor less.1.3 UV degradation on the outer surface causes a stiffeningof the material and an increase in Youngs modulus and thelongitudinal wave velocity.2. Referenced Documents2.1 ASTM Standards:2E 494 Practice for Measuring Ultra
8、sonic Velocity in Mate-rialsE 543 Specification for Agencies Performing Nondestruc-tive TestingE 1316 Terminology for Nondestructive ExaminationsE 2373 Practice for Use of the Ultrasonic Time of FlightDiffraction (TOFD) Technique2.2 ASNT Documents:3SNT-TC-1A Recommended Practice for NondestructiveTe
9、sting Personnel Qualification and CertificationANSI/ASNT CP-189 ASNT Standard for Qualification andCertification of Nondestructive Testing Personnel2.3 AIA Document:4NAS-410 Certification and Qualification of NondestructiveTesting Personnel3. Terminology3.1 DefinitionsFor definitions of terms used i
10、n this prac-tice, see Terminology E 1316.4. Summary of Practice4.1 The lateral longitudinal wave (henceforth called the LCRwave) used in this practice is selected because it is the fastestwave in the tank wall, and, therefore its arrival at the receiverlocation is free from surrounding spurious indi
11、cations comingthrough the tank wall. The typical setup is shown in Fig. 1where the sending and receiving transducers are connectedwith a link through a pivot joint. The frequency selected is suchthat the wavelength is short compared to the wall thickness,assuring bulk wave velocity. Moreover, since
12、it is a bulk wavethe propagation is not affected by variations in the inside tankwall. Therefore, the velocity measured in the outer tank wall isindicative of the material properties of that region, and notaffected by the inner tank wall conditions.5. Apparatus5.1 The ultrasonic system to be used in
13、 this practice shallinclude the following:5.1.1 Test InstrumentAn ultrasonic instrument comprisinga time base, pulser and receiver and A-scan display showingfull wave (RF) signals with gates such that arrival times can bedetermined with a resolution of 10 ns or better. A required1This practice is un
14、der the jurisdiction of ASTM Committee E07 on Nonde-structive Testing and is the direct responsibility of Subcommittee E07.06 onUltrasonic Method.Current edition approved Feb. 15, 2006. Published March 2006.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer
15、 Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3Available fromAmerican Society for Nondestructive Testing (ASNT), P.O. Box28518, 1711 Arlingate Ln., Columbus, OH 43228-0518.4Available from Aerospace In
16、dustries Association of America, Inc. (AIA), 1250Eye St., NW, Washington, DC 20005.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.feature is the ability to freeze the signal and manipulate andzoom the gate so that the appropriate pe
17、ak or zero crossing maybe identified with satisfactory resolution. The proper arrivaltime is either the first significant peak or the preceding positive(upward) zero crossing. Zero offset is used to standardize theobserved velocity with the expected velocity in a referencestandard. Further, the inst
18、rument must be capable of commu-nicating with a laptop computer or other digital signal-processing device and sending arrival waveforms as well asother pertinent data for processing and storage. The ultrasonicand computer functions may be incorporated in a single unit.The receiving amplifier must be
19、 capable of displaying at fullscreen height the signals arriving at the receiver search unit forall tank conditions.5.1.2 Search UnitThe dual longitudinal angle beam (LCR)search unit propagates waves across the chord of the tank wall.The LCRwave is excited at an incident angle slightly past thefirst
20、 critical angle. A typical transducer has a 25-mm 1-in.diameter element, with low damping and narrow bandwidth inorder to maximize the signal strength. The wedge has a lowspeed material column for energy transmission to provide aSnells law match with the polyethylene tank wall. Typicaltransducer fre
21、quencies range from 0.5 MHz to 2.25 MHz, Thefrequency must be high enough to assure that no Lamb wavesare excited in the tank wall. Search unit separation must begreater than the near field estimated experimentally using thestandardization block and must be such that the longitudinalwave travels acr
22、oss the chord of the tank wall and does notstrike the inside wall. A typical distance is 47 mm 1.85 in.,but may be adjusted to other spacing to accommodate exami-nation in moderate and low loss polymers and different tankwall thicknesses.5.1.3 CouplantStandard ultrasonic gel type couplants areprefer
23、red. The couplant must adhere to the sidewall of the tankand not run off, yet it must be easily wiped off when theexaminations are completed, leaving no significant residue. Itmust be fully compatible with the polyethylene tank material.5.1.4 ComputerThe computer supporting this examina-tion should
24、be able to store full site and tank detail information.Further, it should be able to calculate the true travel path basedon probe separation and tank curvature. It should be able tocalculate expected velocity at the wall temperature during thetest. The difference between the expected speed for newma
25、terial at the test temperature and the observed speed is theparameter used to evaluate tank wall condition. Manual dataentry in a spreadsheet must be possible if the computer is notavailable, or its use is inconvenient. The calculations describedabove may be accomplished in the spreadsheet or by han
26、dcalculations.5.1.5 Reference BlocksA small section of material is usedfor standardization. This section should be the same typematerial as the tank being examined, and should be flat.Initially, it should have experienced no significant UV expo-sure and it should be protected from long-term exposure
27、 duringits use. First, the search units need to be checked to assure theintegrity of the travel path in the wedge, and that a strong LCRsignal is being generated. Secondly, the standardization of thezero offset on the ultrasonic unit requires that the arrival timebe adjusted to give an observed velo
28、city equal to the expectedvelocity for the sample being examined. The procedure forstandardization is given in more detail in the following and inAppendix X2.6. Practice6.1 Standard practice is to take readings at locations ap-proximately 30 cm 1 ft and 90 cm 3 ft from the base(bottom) of the tank.
29、These readings should be taken at aminimum of two different N-E-S-W directions on the tank.Their relation to some notable location on the tank, forexample, the tank manhole, should be recorded since tanksmay be moved and turned during their life. The surface shouldbe clean and not have undue surface
30、 fluctuations. The impor-tant thing is that a spot gives good readings and that the samelocation is investigated from year to year. The location shouldbe marked on the tank or designated on the record so that futuredata are collected at the same place.FIG. 1 Dual Search Unit Examination Setup Using
31、LCRWaves on Tank WallE24790626.1.1 For a typical examination, connect the sending trans-ducer to the BNC OUT terminal and the receiving transducer tothe BNC IN terminal.6.1.2 Place a generous amount of couplant on both of thesearch unit faces (a dollop about 25 mm 1 in. in diameter). Itis required s
32、ince there is some initial priming of the surfaceneeded for full transmission into the material. Place the searchunit on the area of the tank to be examined. When looking atthe dual search unit, the two search units should be in acircumferential arrangement. The dual search unit assembly isspring lo
33、aded. Manipulate the search unit assembly until agood signal is visible on the screen. Repeat couplant applica-tion if needed for additional surface priming. Once a goodsignal has been found, the signal should be frozen with theultrasonic unit for further analysis. The gate may be moved tothe approp
34、riate point on the wave as discussed above. The tankwall velocity may then be calculated.6.1.3 For maximum confidence, the practice of full removal,wiping and reapplication of the couplant should be repeatedseveral times at each search unit position. Since there willalways be some scatter in the dat
35、a, a minimum of three valuesshould be obtained at each location of interest.6.1.4 The LCRwave traversing the chord of the tank wallshould appear as the first arriving signal on the flaw detectorscreen after the initial pulse. A short gate (time less than onewavelength) is used to identify the arriva
36、l time of the LCRwave. Fig. 2 shows a typical signal identified by the gatesetting. Here the cursor is on the first peak of the wave. Theultrasonic unit should be set to display the time associated withthe zero crossing of the earliest peak in the gate. It is easilydistinguishable from the preceding
37、 portions and from thefollowing wave that goes through the interior of the material.Since the refracted beams of the two search units are approxi-mately parallel to the surface of the tank wall, there is no beamintersection point as defined by Practice E 2373.6.1.5 Velocity values based on previousl
38、y measured tanks inthe field are available as an aid in isolating the LCRwave(Appendix X3). Using the prediction curve based on the yearsof service for the tank being examined can narrow down theapproximate location of the LCRwave arrival.6.1.6 Occasionally, a complex waveform will occur makingthe L
39、CRdifficult to isolate. This is often due to a combinationof conditions such as incomplete contact, high instrument gainused in these lossy materials and the parallel sound paths(crosstalk) that occur in the search units and in the air. In thewave shown in Fig. 3, the LCRwave is less distinguishable
40、.This arrival was taken from the same location as the wave inFig. 2. There appears to be a wave that looks similar to the LCRwave in front of the cursor. This, however, comes from parallelsignal transmission in the higher speed wedge material andshould not be evaluated. In addition, the wave behind
41、the LCRwave is commingled. Without experience and the predictedvelocity table, one could easily take the wrong reading in thiscase.6.1.7 The characteristics to look for in finding the LCRwaves are: a sharply rising peak immediately after a significanttrough and a slight separation between the wave a
42、nd thefollowing wave. This appearance may depend on the specifictransducer properties. The height of the wave may be similar tothe amplitude of the following wave. The cross talk waveformis usually smaller than the following waves. Most importantly,the operator should use the predicted velocities fo
43、r tanks of thatage group value as a guide to narrow down a region of interest.6.1.8 Temperature for establishing the temperature cor-rected velocity may be added to the data set either manuallyfrom an external device such as an infrared indicator, or,directly into the computer through an input devic
44、e.6.2 Consistency of DataThe predicted values may beaffected by the frequency of the dual search units as well as thespacing since this may affect the travel path or region in thetank wall the wave encounters. For newer tanks an adepttechnician can use experience to find the correct waveform. Forold
45、er tanks, the attenuation is likely to be very high, and theprediction values will be necessary.6.3 StandardizationStandardizing the ultrasonic systemand the search unit assembly is extremely important, forFIG. 2 Typical Signal with LCRPeak Located Within the GateE2479063several reasons. First, the
46、working condition of the search unitmust be assured before beginning a test. Secondly, the operatormust reassure that the instrument settings are correct. Thesearch unit standardization block shown in the appendix shouldbe used to assure that the search unit is properly designed andconstructed and t
47、hat the LCRwave is properly excited. Thesystem is standardized using the system standardization blocksalso described in the appendix. Both blocks should be madefrom polyethylene material as used in the tanks being exam-ined. For the system standardization, the technician should usethe system standar
48、dization block and find the correct peak onthe LCRwave and adjust the zero offset until the laptopcomputer reads a difference in predicted and measured wavespeed that is close to zero. A close value is desired, but 63 (asshould suffice. The zero offset is a function of the delay in thesearch unit, a
49、nd is a function of temperature as well as travelpath in the wedge. A nominal zero offset of 14 to 15) is usedprovided that the standardization and corrected velocities arecalculated later. The exact value is dependent on the wedge andtransmission column of the sender and receiver.7. Basis of Application7.1 The following items are subject to contractual agree-ment between the parties using or referencing this standard.7.1.1 Personnel QualificationPersonnel performing ex-aminations to this standard shall be qualified in accordancewith a nationally or international
