1、Designation: E 1814 96 (Reapproved 2007)Standard Practice forComputed Tomographic (CT) Examination of Castings1This standard is issued under the fixed designation E 1814; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of
2、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 uniform procedure for the exami-nation of castings by the computed tomography (CT) tech-nique.
3、 The requirements expressed in this practice are intendedto control the quality of the nondestructive examination by CTand are not intended for controlling the acceptability or qualityof the castings. This practice implicitly suggests the use ofpenetrating radiation, specifically X rays and gamma ra
4、ys.1.2 This practice provides a uniform procedure for a CTexamination of castings for one or more of the followingpurposes:1.2.1 Examining for discontinuities, such as porosity, inclu-sions, cracks, and shrink;1.2.2 Performing metrological measurements and determin-ing dimensional conformance; and1.
5、2.3 Determining reverse engineering data, that is, creatingcomputer-aided design (CAD) data files.1.3 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health
6、 practices and determine the applica-bility of regulatory limitations prior to use. For specific safetystatements, see Section 7, NBS Handbook 114,2and21 CFR 1020.40 and 29 CFR 1910.96.2. Referenced Documents2.1 ASTM Standards:3E 543 Specification for Agencies Performing Nondestruc-tive TestingE 131
7、6 Terminology for Nondestructive ExaminationsE 1441 Guide for Computed Tomography (CT) ImagingE 1570 Practice for Computed Tomographic (CT) Exami-nationE 1672 Guide for Computed Tomography (CT) SystemSelectionE 1695 Test Method for Measurement of Computed Tomog-raphy (CT) System Performance2.2 ASNT
8、Documents:4SNT-TC-1A Recommended Practice for Personnel Qualifi-cation and Certification in Nondestructive TestingANSI/ASNT CP-189 Standard for Personnel Qualificationand Certification of Nondestructive Testing Personnel2.3 Military Standards:5MIL-STD-410 Nondestructive Testing Personnel Qualifica-t
9、ion and CertificationNAS 410 Certification and Qualification of NondestructiveTest Personnel2.4 Code of Federal Regulations:521 CFR 1020.40 Safety Requirements of Cabinet X RaySystems29 CFR 1910.96 Ionizing Radiation3. Terminology3.1 DefinitionsDefinitions of terms applicable to thispractice may be
10、found in Terminology E 1316 and GuideE 1441.3.2 Definitions of Terms Specific to This Standard:3.2.1 fixturingthe mounting hardware used to place theobject in the CT system.3.2.2 representative quality indicator (RQI)a real part, ora fabrication of similar geometry in radiologically similarmaterial
11、to a real part, that has features of known characteris-tics that represent all of the features for which the parts arebeing examined.3.2.3 scan planscan locations and the system configura-tion parameters for a specific part examination.3.2.4 objecta part or specimen being subjected to CTexamination.
12、4. Significance and Use4.1 CT may be performed on an object when it is in theas-cast, intermediate, or final machined condition. A CT1This practice is under the jurisdiction of ASTM Committee E07 on Nonde-structive Testing and is the direct responsibility of Subcommittee E07.01 onRadiology (X and Ga
13、mma) Method.Current edition approved July 1, 2007. Published July 2007. Originally approvedin 1996. Last previous edition approved in 2002 as E 1814 - 96(2002).2NBS Handbook 114, General Safety Standard for Installations, Using Non-Medical X-Ray and Sealed Gamma-Ray Sources, Energies Up to 10 MeV, N
14、ationalInstitute of Standards and Technology (NIST), Gaithersburg, MD.3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM
15、 website.4Available fromAmerican Society for Nondestructive Testing (ASNT), P.O. Box28518, 1711 Arlingate Ln., Columbus, OH 43228-0518, http:/www.asnt.org.5Available from Standardization Documents Order Desk, DODSSP, Bldg. 4,Section D, 700 Robbins Ave., Philadelphia, PA 19111-5098, http:/www.dodssp.
16、daps.mil.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.examination can be used as a design tool to improve wax formsand moldings, establish process parameters, randomly checkprocess control, perform final quality control (QC) exami
17、na-tion for the acceptance or rejection of parts, and analyzefailures and extend component lifetimes.4.2 The most common applications of CT for castings arefor the following: locating and characterizing discontinuities,such as porosity, inclusions, cracks, and shrink; measuringas-cast part dimension
18、s for comparison with design dimen-sions; and extracting dimensional measurements for reverseengineering.4.3 The extent to which a CT image reproduces an object ora feature within an object is dictated largely by the competinginfluences of spatial resolution, contrast discrimination, andartifacts of
19、 the imaging system. Operating parameters strike anoverall balance between image quality, examination time, andcost.4.4 Artifacts are often the limiting factor in CT imagequality. (See Practice E 1570 for an in-depth discussion ofartifacts.) Artifacts are reproducible features in an image thatare no
20、t related to actual features in the object. Artifacts can beconsidered correlated noise because they form repeatable fixedpatterns under given conditions yet carry no object informa-tion. For castings, it is imperative to recognize what is and isnot an artifact since an artifact can obscure or masqu
21、erade as adiscontinuity.Artifacts are most prevalent in castings with longstraight edges or complex geometries, or both.5. Basis of Application5.1 The following items shall be agreed upon between thepurchaser and the supplier and specified in the contract or joborder:5.1.1 Nondestructive Testing Age
22、ncy EvaluationThe useof a nondestructive testing (NDT) agency, as defined inPractice E 543. If a systematic assessment of the capability ofthe agency is specified, a documented procedure, such as thatdescribed in Practice E 543, should be used as the basis forevaluation.5.1.2 Personnel Qualification
23、sAll CT examination per-sonnel shall be qualified and certified in accordance with awritten procedure conforming to ANSI/ASNT CP-189, SNT-TC-1A, MIL-STD-410, NAS 410, or a similar document. Thewritten procedure shall include training that addresses CTissues specifically.5.1.3 General RequirementsGen
24、eral requirements shallbe specified in accordance with Section 8:(1) written proce-dure, 8.1; and (2) CT system validation measurements, 8.3.5.1.3.1 Specific requirements regarding preparation and ap-proval of the written procedures should be agreed upon inadvance by the purchaser and the supplier.5
25、.1.4 FixturingThe object fixturing shall be determinedby agreement between the purchaser and the supplier inaccordance with 9.2.5.1.5 Image ProcessingImage processing routines used inanalysis of the CT data shall be specified in accordance with6.2:(1) dimensional measurements, 6.2.1; and (2) discont
26、inuitycharacterization, 6.2.2.5.1.6 Discontinuity TypesA listing of the expected kindsof discontinuities shall be provided or referenced, and theacceptance and rejection criteria shall be stipulated.5.1.7 RecordsRecords requirements shall be specified inaccordance with Section 10.6. Apparatus6.1 The
27、 success of the CT application depends on theoverall system configuration and the selection of appropriatesubsystem components. Guidance on the selection of sub-system components and the overall system configuration isprovided in Guide E 1672. Guidance on the initial qualificationand periodic requal
28、ification of the CT system is provided inTest Method E 1695. The suitability of the CT system shall bedemonstrated by attainment of the required image quality andcompliance with all other requirements stipulated herein.6.2 Computer/Image Processing SystemImage processingsystems may be used for image
29、 enhancement operations thatwill facilitate dimensional measurements and discontinuitydetection or characterization.6.2.1 Dimensional measurements, with tolerance, can beobtained from the CT image. There is a degree of blurring inthe CT image that makes sharp boundaries indistinct. Acommon approach
30、for on-screen dimensional measurements isto generate a density profile along a straight line between thepoints in the image representing the distance to be measured.The end points of the measurement are generally taken to bethe density profile values located at the half maximum valuepoint on each sl
31、ope. This is called the full-width-at-half-maximum (FWHM) method. This method or various othertechniques, that is, the area under the curve or determiningcontours for CAD output, can be generalized for wall thick-ness, hole diameter, and crack width measurements.6.2.2 Each dimensional measurement te
32、chnique has its ownprecision, and for its determination, the creation of the CTimage must be understood thoroughly. Due to the finite spotsize of the source, and the finite aperture size of the detector, apoint-like object will not appear in an image as a sharp point.Instead, the “true” image will b
33、e convolved with a Gaussiandistribution-like function called the point spread function(PSF). Therefore, when looking at a density profile along a linein a CT image, an abrupt density change (that is, from materialto air) will not appear as a step but as a curve. See GuideE 1441 and Sections 5, 8, an
34、d 9 for further discussion.6.2.3 Some tools require the availability of an object thatcan be scanned and then dissected (destructive evaluation) forcomparison with actual dimensional measurements. The CTsystem can be “standardized” for a specific object from thiscomparison data.6.2.4 Various types o
35、f density analysis tools may be neededfor discontinuity characterization, such as tools for measuringlow-density indications, missing mass, area, and shape.6.3 Purchasers are cautioned to test thoroughly, or haveprior experience with, the proposed image processing param-eters before authorizing rout
36、ine use. For example, some spatialfilter functions produce directional results and may suppressdesired image information. Other spatial filters can introduceartifacts into the image.E 1814 96 (2007)27. Safety7.1 The CT procedures shall comply with applicable local,state, and federal safety regulatio
37、ns.8. Requirements8.1 Written ProcedureThe CT examination should beperformed in accordance with a written procedure. The proce-dure should address all applicable portions of this practice andshould be available for review during interpretation of theimages.8.1.1 Practice E 1570 (System Configuration
38、) details a listof the variables that can affect the examination outcome for aselected system configuration. The values used for thesevariables should be documented in the written procedure.8.1.2 The written procedure or scan plan should also includethe following:8.1.2.1 Description of the object an
39、d engineering drawings.8.1.2.2 Fixturing requirements and instructions.8.1.2.3 Handling requirements and instructions.8.1.2.4 RQI to be used.8.1.2.5 Algorithms or methods to be used for measuringdimensions and discontinuities.8.1.2.6 Scan LocationsFor example, scan locations can bespecified by sketc
40、h, photograph, or drawing or determined bya digital radiograph of the object from the same CT scanner.8.2 CT System ValidationThe CT system performanceparameters must be determined initially and monitored regu-larly to ensure consistent results. The CT performance levelmay be specified in terms of o
41、ne or both of the following:(1) Detectability of Features in an RQIThe use of anactual or simulated object has two benefits. First, CT scansthrough a dimensionally known object can help to standardizethe CT system when taking dimensional measurements. Sec-ond, an RQI demonstrates that the required s
42、ystem perfor-mance can be achieved, in both the CT examination and theinterpretation and analysis.(2) Performance Measurements, Using a Disk Phantom, asOutlined in Test Method E 1695The performance measure-ments determined using the disk phantom are the CT systemspatial resolution and contrast sensi
43、tivity at specified scanparameters. These performance measurements are required tomonitor the overall system performance and check for varia-tions over time, as well as to determine the performance of theequipment prior to CT examination of the objects.8.3 Validation Measurement IntervalsSystem stab
44、ilityover time must be demonstrated to the purchaser. There are twoapproaches, as follows:8.3.1 System performance measurements, using the diskphantom, can be taken periodically to check the ratio of signalto noise levels at disk center, spatial resolution, and contrastresolution. These data will be
45、 made available to the purchaserfor review, upon request.8.3.2 A disk phantom or RQI, or both, can be scannedbefore and after CT examination of the casting(s). If a variationexists, explanations and corrections will need to be made andvalidated.9. Procedure9.1 Handling and PreparationFollow the hand
46、ling in-structions as documented in the written procedure. While nosurface preparation is required for CT, the removal of surfaceblemishes and debris that could confuse the image is recom-mended.9.2 Object FixturingFixturing is typically incidental tothe examination of castings since CT will image a
47、ny hardwarefor fixturing along with the object. However, precision fixtur-ing must be used when orientation or repeatability, or both, areof high importance. The fixtures usually vary from part to partand examination to examination. If possible, the fixtures shouldnot be in the scan plane.9.2.1 Prec
48、ision FixturingPrecision fixturing will be nec-essary if any of the following requirements pertain:9.2.1.1 Dimensional measurements in a precise location orrelative to a precise plane, or both, are specified.9.2.1.2 Dimensional measurements are to be registered withother types of measurements, such
49、as, ultrasonic.9.2.1.3 Part to part comparisons are to be made.9.2.2 Precision Fixturing ProcedureThe fixturing proce-dure will include the type and amounts of hardware, thelocations of the hardware relative to the object and the CTsystem, and how to mount and dismount the object. Theprecision fixturing should be: (1) specific to the casting to beexamined; (2) out of the scan plane, whenever possible; and (3)made of material less dense than the casting, to reduce or avoidartifacts, if in the scan plane.9.3 Data CollectionData shall be collected in accordancewi
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