ASTM_F_319_-_09_2014.pdf

Designation:F31909(Reapproved 2014)Standard Practice forPolarized Light Detection of Flaws in AerospaceTransparency Heating Elements1This standard is issued under the fixed designation F319;the number immediately following the designation indicates the year of originaladoption or,in the case of revision,the year of last revision.Anumber in parentheses indicates the year of last reapproval.Asuperscriptepsilon()indicates an editorial change since the last revision or reapproval.INTRODUCTIONElectrically conductive coatings used in aerospace transparencies for heating purposes may containflaws resulting from imperfections of materials,imperfections of manufacturing techniques,handlingdamage,or contamination.Flaws may develop before,during,or after coating and processing andusually appear as hairline cracks,scratches,or pin holes.When these flaws are of sufficient size,hotspots can occur as a result of disruption and concentration of the flow of electrical current adjacent tothe flaws.These hot spots may result in reduced service life of the transparency.Hot spot flaws in thetransparency may also produce undesirable temporary distortion of vision during powered operationof the heater and permanent vision distortion after repeated cycling of the heater.Polarized light is widely used to detect electrically conductive coating flaws during aerospacetransparency processing.1.Scope1.1 This practice covers a standard procedure for detectingflaws in the conductive coating(heater element)by theobservation of polarized light patterns.1.2 This practice applies to coatings on surfaces of mono-lithic transparencies as well as to coatings imbedded inlaminated structures.1.3 The values stated in SI units are to be regarded asstandard.No other units of measurement are included in thisstandard.1.4 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 practices and determine the applica-bility of regulatory limitations prior to use.For specificprecautionary statements see Section 6.2.Terminology2.1 Definitions:2.1.1 transparent conductive coatinga transparent thinfilm of electrically conductive material such as gold,stannousoxide,or indium oxide applied to plastic or glass which,whenbounded by connecting bus-bars energized by electricity,becomes a resistance type heating element.2.1.2 electrically conductive coating flawan electrical dis-continuity in the coating,caused generally by coating cracks,pin holes,fine threads,scratches,and so forth.3.Summary of Practice3.1 Flaws in electrically powered conductive coatings pro-duce local concentrations of current,which result in tempera-ture gradients and stresses.Since glass and plastic transparen-cies are birefringent when stressed,flaws can be detected byoptical methods,and in this case by the use of polarized light.3.2 This practice consists of directing polarized lightthrough a heated transparent test specimen and reading thetransmitted light with a polarizing screen or filter.Diffractedlight from the region of the flaw will become visible,in theform of a brighter or more intense local image,usually shapedlike a butterfly.4.Significance and Use4.1 This practice is useful as a screening basis for accep-tance or rejection of transparencies during manufacturing sothat units with identifiable flaws will not be carried to finalinspection for rejection at that time.4.2 This practice may also be employed as a go-no gotechnique for acceptance or rejection of the finished product.4.3 This practice is simple,inexpensive,and effective.Flaws identified by this practice,as with other optical methods,1This practice is under the jurisdiction of ASTM Committee F07 on Aerospaceand Aircraft and is the direct responsibility of Subcommittee F07.08 on TransparentEnclosures and Materials.Current edition approved Dec.1,2014.Published December 2014.Originallyapproved in 1977.Last previous edition approved in 2009 as F319 09.DOI:10.1520/F0319-09R14.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 are limited to those that produce temperature gradients whenelectrically powered.Any other type of flaw,such as minorscratches parallel to the direction of electrical flow,are notdetectable.5.Apparatus5.1 The elements of the apparatus are detailed below in theirphysical relationship as shown in Fig.1.The minimum sizeand spacing of the elements of the apparatus are determined bythe size and curvature of the part.The size of light source,lightdiffuser,and polarizing screen shall be large enough so thatevery portion of the electrically coated area of the testspecimen is in the light path and is uniformly back-lit.If thetest specimen is curved severely,its position may have to beadjusted during inspection so that the light path is within 20 ofnormal to the location being viewed.Since specimen size andcurvature vary con