ASTM D4093-95(2005)e1

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 D 638 (Note 8) and D 882 (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 D 4000 lists the ASTM materials standards that currently exist.
SCOPE
1.1 This test method covers measurements of direction of principal strains, 1  and 2, and the photoelastic retardation, , 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.
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: 30-Jun-2005
• ICS: 83.080.01 - Plastics in general
• Technical Committee: D20 - Plastics
• Drafting Committee: D20.10 - Mechanical Properties

Relations

• Replaces: ASTM D4093-95(2001)e1 - Standard Test Method for Photoelastic Measurements of Birefringence and Residual Strains in Transparent or Translucent Plastic Materials
• Effective Date: 01-Jul-2005
• Replaced By: ASTM D4093-95(2010) - 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-Jul-2005

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
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Designation: D4093 – 95 (Reapproved 2005)
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.
´ NOTE—Editorial changes were made throughout in July 2005.
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 ´
i o ij j
Usingphotoelastictechniques(initiallydevelopedtomeasurestressesintransparentmodels)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(whereprincipalstrains ´ and ´ arecontainedintheplane
1 2
perpendicular to the direction of light propagation (see Fig. 1), the incoming vibration splits into two
wavesvibratinginplanesof ´ and ´ .Thedifferencebetweentheindexesofrefraction n = c/v and
1 2 1 1
n = c/v (or birefringence) is:
2 2
n − n =(A − B)(´ − ´ )= k(´ − ´ )
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, d, given by:
d =(n − n )t = kt(´ − ´ )
1 2 1 2
where t is the thickness of material crossed by the light. A similar equation, relating d to the
difference of principal stresses, s and s , can be written:
1 2
d =(n − n )t = Ct(s − s )
1 2 1 2
The objective of photoelastic investigation is to measure: (a) the azimuth, or direction of principal
strains, ´ and ´ (or stresses s and s ), and (b) the retardation, d, used to determine the magnitude
1 2 1 2
of strains. A complete theory of photoelastic effect can be found in the abundant literature on the
subject (an extensive bibliography is provided in Appendix X2).
1. Scope using a compensator, for the purpose of analyzing strains in
transparent or translucent plastic materials. This test method
1.1 This test method covers measurements of direction of
can be used to measure birefringence and to determine the
principal strains, ´ and ´ , and the photoelastic retardation, d,
1 2
difference of principal strains or normal strains when the
principaldirectionsdonotchangesubstantiallywithinthelight
ThistestmethodisunderthejurisdictionofASTMCommitteeD20onPlastics
path.
and is the direct responsibility of Subcommittee D20.10 on Mechanical Properties.
Current edition approved July 1, 2005. Published October 2005. Originally
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approved in 1982. Last previous edition approved in 2001 as D4093-95 (2001) .
DOI: 10.1520/D4093-95R05E01.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
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D4093 – 95 (2005)
FIG. 1 Propagation of Light in a Strained Transparent Material
1.2 Inadditiontothemethodusingacompensatordescribed 3.1.1 compensator—an optical device used to measure re-
in this test method, other methods are in use, such as the tardation in transparent birefringent materials.
goniometric method (using rotation of the analyzer) mostly
3.1.2 polarizer—polarizing element transmitting light vi-
applied for measuring small retardation, and expressing it as a
brating in one plane only.
fraction of a wavelength. Nonvisual methods employing spec-
3.1.3 quarter-wave plate—a transparent filter providing a
trophotometricmeasurementsandeliminatingthehumanjudg- 1
relative retardation of ⁄4 wavelength throughout the transmit-
ment factor are also possible.
ting area.
1.3 Test data obtained by this test method is relevant and
3.2 Definitions of Terms Specific to This Standard:
appropriate for use in engineering design.
3.2.1 birefringence—retardation per unit thickness, d/t.
1.4 The values stated in either SI units or inch-pound units
3.2.2 retardation, d—distance (nm) between two wave
aretoberegardedasstandard.Thevaluesstatedineachsystem
fronts resulting from passage of light through a birefringent
may not be exact equivalents; therefore, each system shall be
material. (Also called “relative retardations.”)
used independently of the other. Combining values from the
3.2.3 strain, ´-strain (or deformation per unit length)—
two systems may result in nonconformance with the standard.
could be permanent, plastic strain introduced in manufacturing
1.5 This standard does not purport to address all of the
process, or elastic strain related to the existing state of stress.
safety concerns, if any, associated with its use. It is the
Both types of strains will produce strain-birefringence in most
responsibility of the user of this standard to establish appro-
polymers.Birefringencecanalsoresultfromopticalanisotropy
priate safety and health practices and determine the applica-
due to crystalline orientation.
bility of regulatory limitations prior to use.
3.2.4 strain-optical constant, k—material property, relating
NOTE 1—There is no known ISO equivalent to this test method.
the strains to changes of index of refraction (dimensionless).
2. Referenced Documents k 5 ~n 2 n !/~´ 2´ !
1 2 1 2
2.1 ASTM Standards:
3.2.5 stress-optical constant, C—material property relating
D618 Practice for Conditioning Plastics for Testing
the stresses to change in index of refraction. C is expressed in
2 −12 2
D638 Test Method for Tensile Properties of Plastics
m /N or Brewsters (10 m /N). C is usually temperature-
D882 Test Method for Tensile Properties of Thin Plastic
dependent.
Sheeting
C 5 ~n 2 n !/~s 2s !
1 2 1 2
D4000 Classification System for Specifying Plastic Materi-
als
4. Summary of Test Method
E691 Practice for Conducting an Interlaboratory Study to
Determine the Precision of a Test Method 4.1 To analyze strains photoelastically, two quantities are
measured: (a) the directions of principal strains and (b) the
3. Terminology
retardation, d, using light paths crossing the investigated
material in normal or angular incidence.
3.1 Definitions:
4.2 The investigated specimen or sample is introduced
between the polarizers (see Fig. 2 and Fig. 3). A synchronous
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
rotationofpolarizersfollowsuntillightintensitybecomeszero
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
at the observed location. The axes of the polarizers are then
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. parallel to direction of strains, revealing these directions.
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D4093 – 95 (2005)
FIG. 2 Transmission Set-up of Polariscope
FIG. 3 Reflection Set-up of Polariscope
4.3 To suppress the directional sensitivity of the apparatus, Such measurements can lead to elimination of defective parts,
thesetupischanged,introducingadditionalfilters.Acalibrated process improvement, control of annealing operation, etc.
compensator is introduced and its setting adjusted until light
5.3 When testing for physical properties, polariscopic ex-
intensity becomes zero at the observed location. The retarda-
amination of specimens is required, to eliminate those speci-
tioninthecalibratedcompensatoristhenequalandoppositein
mensexhibitingabnormalinternalstrainlevel(ordefects).For
signtotheretardationintheinvestigatedspecimen(seeFig.4).
example: Test Methods D638 (Note 8) and D882 (Note 11)
recommend a polariscopic examination.
5. Significance and Use
5.4 Thebirefringenceoforientedpolymerscanberelatedto
orientation, shrinkage, etc.The measurements of birefringence
5.1 Theobservationandmeasurementofstrainsintranspar-
aid in characterization of these polymers.
ent or translucent materials is extensively used in various
modeling techniques of experimental stress analysis. 5.5 For many materials, there may be a specification that
5.2 Internalstrainsinducedinmanufacturingprocessessuch requires the use of this test method, but with some procedural
ascasting,molding,welding,extrusion,andpolymerstretching modifications that take precedence when adhering to the
canbeassessedandpartexhibitingexcessivestrainsidentified. specification. Therefore, it is advisable to refer to that material
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D4093 – 95 (2005)
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
difference in retardation between the two quarter-wave plates
6. Apparatus
should not exceed 65 nm.
6.1 The apparatus used to measure strains is shown sche-
6.1.3.2 The quarter-wave plates will be indexed, to permit
matically in Fig. 4. It consists of the following items:
theirinsertioninthefieldoftheapparatuswiththeiraxesat45°
6.1.1 Light Source:
to the polarizers direction. The two quarter-wave plates shall
6.1.1.1 Transmitted-Light Set-Up—An incandescent lamp
havetheiraxescrossed(thatis,theiropticalaxesperpendicular
or properly spaced fluorescent tubes covered with a diffuser
toeachother),thusinsuringthatthefieldremainsatmaximum
should provide a uniformly diffused light. To ensure adequate
darkness when both quarter-wave plates are inserted (see Fig.
brightness, minimum illumination required is 0.3 W/in.
5).
(0.0465 W/cm ). Maximum light source power is limited to
6.1.4 Compensator—The compensator is the essential
ensure that the specimen temperature will not change more
means of measuring retardation. The following types of com-
than 2°C during the test. The incandescent lamp must be
pensators can be used:
selected to provide a color temperature no lower than 3150 K.
6.1.4.1 Linear Compensator —In the linear compensator
Thereshouldbenovisiblenonuniformity,darkorbrightspots
the retardation in the compensator is linearly variable along its
on the diffuser surface, when no specimen is inserted in the
length.Agraduated scale shall be attached to the compensator
apparatus.
body in such a manner that slippage cannot occur. The
6.1.1.2 Reflection-Light Source—For the reflection set-up
calibration characteristic of the compensator shall include the
an incandescent, reflector-equipped projection lamp is re-
position along its length (as indicated by the scale) of the line
quired. The lamp shall be equipped with proper lenses to
where the retardation is zero and the number of divisions d per
ensure uniform illumination of the investigated object. At a
unit retardation (usually one wavelength). (The retardation per
distanceof2ft(610mm)fromthelampanareaof1ft (0.093
division is D= l/d.) The scale density shall be sufficient to
m )shouldbeilluminated,withnovisibledarkorbrightspots.
provideclearvisibilityforobserving1%oftheusefulrangeof
The lamp power should be at least 150 W.
the compensator.
6.1.2 Polarizer—The polarizing element shall be kept 4
6.1.4.2 Uniform Field Compensator —The uniform field
clean. The ratio of the transmittance of polarizers with their
compensator is usually constructed from two optical wedges
axes parallel, to the transmittance of the polarizers with their
movedbymeansofaleadscrew,theamountofrelativemotion
axes perpendicular to each other (or in crossed position),
being linearly related to the total thickness and the retardation.
should not be less than 500.Aglass-laminated construction of
The lead screw motion shall be controlled by a dial drum or
polarizers is recommended. The polarizers must be mechani-
counter. Calibration of this compensator shall include the
cally or electrically coupled to insure their mutually perpen-
dicular setting while rotated together to measure directions. A
graduated scale must be incorporated to indicate the common
Also known as “Babinet” compensator.
rotation of polarizers to a fixed reference mark. Also known as “Babinet-Soleil” compensator.
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D4093 – 95 (2005)
FIG. 4 Apparatus
FIG. 5 Direction Measuring Set-up
position, as indicated by the drum or counter, where the 7. Test Specimen
retardation is zero and the number of division of drum or
7.1 Sheet,film,ormoregenerally,aconstant-thicknessitem
counter d per unit of retardation. (The retardation per division
can be examined using a transmission set-up. For use in
isD= l/d.)
reflection, a reflecting surface must be provided. This can be
6.1.4.3 Compensators have a limited range of measured
accomplished by painting one side of the specimen with
retardation. In case the retardation in the sample exceeds the
aluminum paint. Alternatively, it is possible to place the
rangeofthecompensatorused,insertionofanoffsetretarderis
examined sheet specimen against a clean metal surface (pref-
needed. The offset retarder must be calibrated and positioned
erably aluminum) or an aluminum-painted surface.
along the axes of the compensator, between the analyzer and
7.2 Examination of complex surfaces or shapes sometimes
the sample.
requires the use of an immersion liquid. The examined item is
6.1.5 Filter—Monochromatic light is required to perform
placed inside a tank containing a liquid selected to exhibit
various operations in photoelasticity and some operations
approximately the same index of refraction as the tested item.
cannotbesuccessfullyaccomplishedusingwhitelight.Inthose
This technique is commonly used to examine three-
instances a monochromatic light can be obtained introducing
dimensional shapes.
within the light path, a filter transmitting only light of the
7.3 If conditioning is required, Procedure A of Practice
desired wave length. To best correlate with observation in
D618 shall be used.
white light, a narrow band-pass filter with peak transmittance
at 570 6 6 nm and a maximum transmitted band-width (at
half-peak point) of 10 nm should be used. Krylon aluminum aerosol can spray paint was found satisfactory.
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