ASTM_D_4093_-_95_2014.pdf

Designation:D409395(Reapproved 2014)Standard Test Method forPhotoelastic Measurements of Birefringence and ResidualStrains in Transparent or Translucent Plastic Materials1This standard is issued under the fixed designation D4093;the number immediately following the designation indicates the year oforiginal adoption or,in the case of revision,the year of last revision.A number in parentheses indicates the year of last reapproval.Asuperscript epsilon()indicates an editorial change since the last revision or reapproval.INTRODUCTIONLight propagates in transparent materials at a speed,v,that is lower than its speed in vacuum,c.Inisotropic unstrained materials the index of refraction,n=c v,is independent of the orientation of theplane of vibration of light.Transparent materials,when strained,become optically anisotropic and theindex of refraction becomes directional.The change in index of refraction is related to strains.If nois the refractive index of unstrained material,the three principal indices of refraction,ni,become linearfunctions of strain:ni no=AijjUsing photoelastic techniques(initially developed to measure stresses in transparent models)strains in plasticscan be assessed.In isotropic materials,two material constants,A and B,are sufficient to describe theiroptomechanical behavior:Aij=A when i=j,andAij=B when i fij.When light propagates through a region(where principal strains 1and 2are contained in the plane perpendicularto the direction of light propagation(see Fig.1),the incoming vibration splits into two waves vibrating in planes of1and 2.The difference between the indexes of refraction n1=c v1and n2=c v2(or birefringence)is:n1 n2=(A B)(1 1)=k(1 2)where k is a material property called the strain-optical constant.As a result of their velocity difference,the wavesvibrating along the two principal planes will emerge out of phase,their relative distance,or retardation,given by:=(n1 n2)t=kt(1 2)where t is the thickness of material crossed by the light.Asimilar equation,relating to the difference of principalstresses,1and 2,can be written:=(n1 n2)t=Ct(1 2)The objective of photoelastic investigation is to measure:(a)the azimuth,or direction of principal strains,1and2(or stresses 1and 2),and(b)the retardation,used to determine the magnitude of strains.A complete theoryof photoelastic effect can be found in the abundant literature on the subject(an extensive bibliography is providedin Appendix X2).1.Scope1.1 This test method covers measurements of directionofprincipal strains,1and 2,and the photoelastic retardation,using a compensator,for the purpose of analyzing strains intransparent or translucent plastic materials.This test methodcan be used to measure birefringence and to determine thedifference of principal strains or normal strains when theprincipal directions do not change substantially within the lightpath.1.2 In addition to the method using a compensator describedin this test method,other methods are in use,such as thegoniometric method(using rotation of the analyzer)mostlyapplied for measuring small retardation,and expressing it as afraction of a wavelength.Nonvisual methods employing spec-trophotometric measurements and eliminating the human judg-ment factor are also possible.1This test method is under the jurisdiction of ASTM Committee D20 on Plasticsand is the direct responsibility of Subcommittee D20.10 on Mechanical Properties.Current edition approved Dec.1,2014.Published December 2014.Originallyapproved in 1982.Last previous edition approved in 2010 as D4093-95(2010).DOI:10.1520/D4093-95R14.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 1.3 Test data obtained by this test method is relevant andappropriate for use in engineering design.1.4 The values stated in either SI units or inch-pound unitsare to be regarded as standard.The values stated in each systemmay not be exact equivalents;therefore,each system shall beused independently of the other.Combining values from thetwo systems may result in nonconformance with the standard.1.5 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.NOTE1There is no known ISO equivalent to this test method.2.Referenced Documents2.1 ASTM Standards:2D618 Practice for Conditioning Plastics for TestingD638 Test Method for Tensile Properties of PlasticsD882 Test Method for Tensile Properties of Thin PlasticSheetingD4000 Classification System for Specifying Plastic Materi-alsE691 Practice for Conducting an Interlaboratory Study toDetermine the Precision of a Test Method3.Terminology3.1 Definitions:3.1.1 compensatoran optical device used to measure re-tardation in transparent birefringent materials.3.1.2 polarizerpolarizing element transmitting light vi-brating in one plane only.3.1.