ASTM D4093-95(2010)

Standard Test Method for Photoelastic Measurements of Birefringence and Residual Strains in Transparent or Translucent Plastic Materials

SIGNIFICANCE AND USE
The observation and measurement of strains in transparent or translucent materials is extensively used in various modeling techniques of experimental stress analysis.
Internal strains induced in manufacturing processes such as casting, molding, welding, extrusion, and polymer stretching can be assessed and part exhibiting excessive strains identified. Such measurements can lead to elimination of defective parts, process improvement, control of annealing operation, etc.
When testing for physical properties, polariscopic examination of specimens is required, to eliminate those specimens exhibiting abnormal internal strain level (or defects). For example: Test Methods D638 (Note 8) and D882 (Note 11) recommend a polariscopic examination.
The birefringence of oriented polymers can be related to orientation, shrinkage, etc. The measurements of birefringence aid in characterization of these polymers.
For many materials, there may be a specification that requires the use of this test method, but with some procedural modifications that take precedence when adhering to the specification. Therefore, it is advisable to refer to that material specification before using this test method. Table 1 of Classification System D4000 lists the ASTM materials standards that currently exist.
SCOPE
1.1 This test method covers measurements of direction ofprincipal strains, ε1  and ε2, and the photoelastic retardation, δ, using a compensator, for the purpose of analyzing strains in transparent or translucent plastic materials. This test method can be used to measure birefringence and to determine the difference of principal strains or normal strains when the principal directions do not change substantially within the light path.
1.2 In addition to the method using a compensator described in this test method, other methods are in use, such as the goniometric method (using rotation of the analyzer) mostly applied for measuring small retardation, and expressing it as a fraction of a wavelength. Nonvisual methods employing spectrophotometric measurements and eliminating the human judgment factor are also possible.
1.3 Test data obtained by this test method is relevant and appropriate for use in engineering design.
1.4 The values stated in either SI units or inch-pound units are to be regarded as standard. The values stated in each system may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from the two systems may result in nonconformance with the standard.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
Note 1—There is no known ISO equivalent to this test method.

General Information

• Status: Historical
• Publication Date: 31-Oct-2010
• ICS: 83.080.01 - Plastics in general
• Technical Committee: D20 - Plastics
• Drafting Committee: D20.10 - Mechanical Properties

Relations

• Replaces: ASTM D4093-95(2005)e1 - Standard Test Method for Photoelastic Measurements of Birefringence and Residual Strains in Transparent or Translucent Plastic Materials
• Effective Date: 01-Nov-2010
• Referred By: ASTM C1901-21e2 - Standard Test Method for Measuring Optical Retardation in Flat Architectural Glass
• Effective Date: 01-Nov-2010

Standards Content (Sample)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: D4093 − 95(Reapproved 2010)
Standard Test Method for
Photoelastic Measurements of Birefringence and Residual
Strains in Transparent or Translucent Plastic Materials
This standard is issued under the fixed designation D4093; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
Light propagates in transparent materials at a speed, v, that is lower than its speed in vacuum, c. In
isotropic unstrained materials the index of refraction,n=c⁄v, is independent of the orientation of the
planeofvibrationoflight.Transparentmaterials,whenstrained,becomeopticallyanisotropicandthe
index of refraction becomes directional. The change in index of refraction is related to strains. If n
o
istherefractiveindexofunstrainedmaterial,thethreeprincipalindicesofrefraction, n,becomelinear
i
functions of strain:
n − n =^ A ε
i o ij j
Using photoelastic techniques (initially developed to measure stresses in transparent models) strains in plastics
can be assessed. In isotropic materials, two material constants, A and B, are sufficient to describe their
optomechanical behavior:
A = A when i = j, and
ij
A = B when i fi j.
ij
Whenlightpropagatesthrougharegion(whereprincipalstrainsε andε arecontainedintheplaneperpendicular
1 2
to the direction of light propagation (see Fig. 1), the incoming vibration splits into two waves vibrating in planes of
ε and ε . The difference between the indexes of refraction n =c⁄v and n =c⁄v (or birefringence) is:
1 2 1 1 2 2
n − n =(A − B)(ε − ε )= k(ε − ε )
1 2 1 1 1 2
where k is a material property called the strain-optical constant.As a result of their velocity difference, the waves
vibrating along the two principal planes will emerge out of phase, their relative distance, or retardation, δ, given by:
δ =(n − n )t = kt(ε − ε )
1 2 1 2
where tisthethicknessofmaterialcrossedbythelight.Asimilarequation,relatingδtothedifferenceofprincipal
stresses, σ and σ , can be written:
1 2
δ =(n − n )t = Ct(σ − σ )
1 2 1 2
The objective of photoelastic investigation is to measure: (a) the azimuth, or direction of principal strains, ε and
ε (or stresses σ and σ ), and (b) the retardation, δ, used to determine the magnitude of strains.Acomplete theory
2 1 2
of photoelastic effect can be found in the abundant literature on the subject (an extensive bibliography is provided
in Appendix X2).
1. Scope can be used to measure birefringence and to determine the
difference of principal strains or normal strains when the
1.1 This test method covers measurements of direction
principaldirectionsdonotchangesubstantiallywithinthelight
ofprincipal strains, ε and ε , and the photoelastic retardation,
1 2
path.
δ, using a compensator, for the purpose of analyzing strains in
transparent or translucent plastic materials. This test method
1.2 Inadditiontothemethodusingacompensatordescribed
in this test method, other methods are in use, such as the
goniometric method (using rotation of the analyzer) mostly
ThistestmethodisunderthejurisdictionofASTMCommitteeD20onPlastics
applied for measuring small retardation, and expressing it as a
and is the direct responsibility of Subcommittee D20.10 on Mechanical Properties.
fraction of a wavelength. Nonvisual methods employing spec-
Current edition approved Nov. 1, 2010. Published March 2011. Originally
ε1
trophotometricmeasurementsandeliminatingthehumanjudg-
approved in 1982. Last previous edition approved in 2005 as D4093-95 (2005) .
DOI: 10.1520/D4093-95R10. ment factor are also possible.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D4093 − 95 (2010)
FIG. 1 Propagation of Light in a Strained Transparent Material
1.3 Test data obtained by this test method is relevant and 3.1.3 quarter-wave plate—a transparent filter providing a
appropriate for use in engineering design. relative retardation of ⁄4 wavelength throughout the transmit-
ting area.
1.4 The values stated in either SI units or inch-pound units
aretoberegardedasstandard.Thevaluesstatedineachsystem 3.2 Definitions of Terms Specific to This Standard:
may not be exact equivalents; therefore, each system shall be 3.2.1 birefringence—retardation per unit thickness, δ/t.
used independently of the other. Combining values from the
3.2.2 retardation, δ—distance (nm) between two wave
two systems may result in nonconformance with the standard.
fronts resulting from passage of light through a birefringent
1.5 This standard does not purport to address all of the material. (Also called “relative retardations.”)
safety concerns, if any, associated with its use. It is the
3.2.3 strain, ε-strain (or deformation per unit length)—
responsibility of the user of this standard to establish appro-
could be permanent, plastic strain introduced in manufacturing
priate safety and health practices and determine the applica-
process, or elastic strain related to the existing state of stress.
bility of regulatory limitations prior to use.
Both types of strains will produce strain-birefringence in most
polymers.Birefringencecanalsoresultfromopticalanisotropy
NOTE 1—There is no known ISO equivalent to this test method.
due to crystalline orientation.
2. Referenced Documents
3.2.4 strain-optical constant, k—material property, relating
the strains to changes of index of refraction (dimensionless).
2.1 ASTM Standards:
D618Practice for Conditioning Plastics for Testing
k 5 ~n 2 n !/~ε 2 ε !
1 2 1 2
D638Test Method for Tensile Properties of Plastics
3.2.5 stress-optical constant, C—material property relating
D882Test Method for Tensile Properties of Thin Plastic
the stresses to change in index of refraction. C is expressed in
Sheeting
2 −12 2
m /N or Brewsters (10 m /N). C is usually temperature-
D4000Classification System for Specifying Plastic Materi-
dependent.
als
C 5 ~n 2 n !/~σ 2 σ !
E691Practice for Conducting an Interlaboratory Study to 1 2 1 2
Determine the Precision of a Test Method
4. Summary of Test Method
3. Terminology
4.1 To analyze strains photoelastically, two quantities are
measured: (a) the directions of principal strains and (b) the
3.1 Definitions:
retardation, δ, using light paths crossing the investigated
3.1.1 compensator—an optical device used to measure re-
material in normal or angular incidence.
tardation in transparent birefringent materials.
3.1.2 polarizer—polarizing element transmitting light vi- 4.2 The investigated specimen or sample is introduced
brating in one plane only. between the polarizers (see Fig. 2 and Fig. 3). A synchronous
rotationofpolarizersfollowsuntillightintensitybecomeszero
at the observed location. The axes of the polarizers are then
For referenced ASTM standards, visit the ASTM website, www.astm.org, or parallel to direction of strains, revealing these directions.
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
4.3 To suppress the directional sensitivity of the apparatus,
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. thesetupischanged,introducingadditionalfilters.Acalibrated
D4093 − 95 (2010)
FIG. 2 Transmission Set-up of Polariscope
FIG. 3 Reflection Set-up of Polariscope
compensator is introduced and its setting adjusted until light 5.2 Internalstrainsinducedinmanufacturingprocessessuch
intensity becomes zero at the observed location. The retarda-
ascasting,molding,welding,extrusion,andpolymerstretching
tioninthecalibratedcompensatoristhenequalandoppositein
canbeassessedandpartexhibitingexcessivestrainsidentified.
signtotheretardationintheinvestigatedspecimen(seeFig.4).
Such measurements can lead to elimination of defective parts,
process improvement, control of annealing operation, etc.
5. Significance and Use
5.3 When testing for physical properties, polariscopic ex-
5.1 Theobservationandmeasurementofstrainsintranspar-
amination of specimens is required, to eliminate those speci-
ent or translucent materials is extensively used in various
mensexhibitingabnormalinternalstrainlevel(ordefects).For
modeling techniques of experimental stress analysis.
D4093 − 95 (2010)
FIG. 4 Apparatus
example: Test Methods D638 (Note 8) and D882 (Note 11) 6.1.2 Polarizer—Thepolarizingelementshallbekeptclean.
recommend a polariscopic examination. The ratio of the transmittance of polarizers with their axes
parallel, to the transmittance of the polarizers with their axes
5.4 Thebirefringenceoforientedpolymerscanberelatedto
perpendicular to each other (or in crossed position), should not
orientation, shrinkage, etc.The measurements of birefringence
belessthan500.Aglass-laminatedconstructionofpolarizersis
aid in characterization of these polymers.
recommended.The polarizers must be mechanically or electri-
5.5 For many materials, there may be a specification that
cally coupled to insure their mutually perpendicular setting
requires the use of this test method, but with some procedural
whilerotatedtogethertomeasuredirections.Agraduatedscale
modifications that take precedence when adhering to the
must be incorporated to indicate the common rotation of
specification. Therefore, it is advisable to refer to that material
polarizers to a fixed reference mark.
specification before using this test method. Table 1 of Classi-
6.1.3 Quarter-Wave Plates—Two quarter-wave plates are
ficationSystemD4000liststheASTMmaterialsstandardsthat
required in the procedure described below (see 9.2):
currently exist.
6.1.3.1 The retardation of each quarter-wave plate shall be
142 6 15 nm, uniform throughout its transmission area. The
6. Apparatus
difference in retardation between the two quarter-wave plates
6.1 The apparatus used to measure strains is shown sche-
should not exceed 65 nm.
matically in Fig. 4. It consists of the following items:
6.1.3.2 The quarter-wave plates will be indexed, to permit
6.1.1 Light Source:
theirinsertioninthefieldoftheapparatuswiththeiraxesat45°
6.1.1.1 Transmitted-LightSet-Up—Anincandescentlampor
to the polarizers direction. The two quarter-wave plates shall
properly spaced fluorescent tubes covered with a diffuser
havetheiraxescrossed(thatis,theiropticalaxesperpendicular
should provide a uniformly diffused light. To ensure adequate
toeachother),thusinsuringthatthefieldremainsatmaximum
brightness, minimum illumination required is 0.3 W/in.
darkness when both quarter-wave plates are inserted (see Fig.
(0.0465 W/cm ). Maximum light source power is limited to
5).
ensure that the specimen temperature will not change more
6.1.4 Compensator—The compensator is the essential
than 2°C during the test. The incandescent lamp must be
means of measuring retardation. The following types of com-
selected to provide a color temperature no lower than 3150 K.
pensators can be used:
There should be no visible nonuniformity, dark or bright spots
6.1.4.1 Linear Compensator —In the linear compensator
on the diffuser surface, when no specimen is inserted in the
the retardation in the compensator is linearly variable along its
apparatus.
length.Agraduated scale shall be attached to the compensator
6.1.1.2 Reflection-Light Source—Forthereflectionset-upan
body in such a manner that slippage cannot occur. The
incandescent, reflector-equipped projection lamp is required.
calibration characteristic of the compensator shall include the
The lamp shall be equipped with proper lenses to ensure
position along its length (as indicated by the scale) of the line
uniformilluminationoftheinvestigatedobject.Atadistanceof
where the retardation is zero and the number of divisions d per
2 2
2ft(610mm)fromthelampanareaof1ft (0.093m )should
be illuminated, with no visible dark or bright spots. The lamp
power should be at least 150 W. Also known as “Babinet” compensator.
D4093 − 95 (2010)
FIG. 5 Direction Measuring Set-up
unit retardation (usually one wavelength). (The retardation per 7.2 Examination of complex surfaces or shapes sometimes
division is D= λ⁄d.) The scale density shall be sufficient to requires the use of an immersion liquid. The examined item is
provideclearvisibilityforobserving1%oftheusefulrangeof placed inside a tank containing a liquid selected to exhibit
the compensator. approximately the same index of refraction as the tested item.
6.1.4.2 Uniform Field Compensator —The uniform field This technique is commonly used to examine three-
compensator is usually constructed from two optical wedges dimensional shapes.
movedbymeansofaleadscrew,theamountofrelativemotion
7.3 If conditioning is required, Procedure A of Practice
being linearly related to the total thickness and the retardation.
D618 shall be used.
The lead screw motion shall be controlled by a dial drum or
counter. Calibration of this compensator shall include the
8. Calibration and Standardization
position, as indicated by the drum or counter, where the
8.1 A periodic verification (every 6 months) is required to
retardation is zero and the number of division of drum or
ensure that the apparatus is properly calibrated. The following
counter d per unit of retardation. (The retardation per division
points require verification:
isD= λ⁄d. )
8.1.1 Verification of Polariscope:
6.1.4.3 Compensators have a limited range of measured
8.1.1.1 Verify that the polarizers remain in “crossed” posi-
retardation. In case the retardation in the sample exceeds the
tion. A small deviation of one of the polarizers produces an
rangeofthecompensatorused,insertionofanoffsetretarderis
increase in the light intensity transmitted.
needed. The offset retarder must be calibrated and positioned
8.1.1.2 Verify that the quarter-wave plates are properly
along the axes of the compensator, between the analyzer and
crossed. A small deviation of one quarter-wave plate from its
the sample.
“indexed” position will produce an increase in the light
6.1.5 Filter—Monochromatic light is required to perform
intensity transmitted.
various operations in photoelasticity and some operations
8.1.2 Verification of the Compensator:
cannotbesuccessfullyaccomplishedusingwhitelight.Inthose
8.1.2.1 Examine the compensator in the polariscope and
instances a monochromatic light can be obtained introducing
verify that its δ=0 point coincides with the calibration
within the light path, a filter transmitting only light of the
reported.
desired wave length. To best correlate with observation in
8.1.2.2 Using monochromatic light (filter), verify that the
white light, a narrow b
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