Full field photoelastic stress analysis
First Claim
1. A method for determining shear stresses in a structure comprising the steps of:
- coating a structural specimen with a photoelastic material;
loading the structural specimen to produce no more than one-half of a wavelength of birefringence in the photoelastic material;
illuminating with elliptically polarized light the structural specimen coated with the photoelastic material to cause a multiplicity of discrete areas on the structure to reflect elliptically polarized light;
creating an electronic image of a portion of the structural specimen including the multiplicity of discrete areas;
analyzing the elliptically polarized light reflected from the multiplicity of discrete areas of the illuminated specimen to determine the orientation and magnitude of major and minor axes of the elliptically polarized light reflected from each of said multiplicity of specimen areas; and
processing the orientation and magnitude of major and minor axes of the elliptically polarized light for each analyzed area to produce a result indicative of shear stress magnitude and direction in the specimen at each discrete area.
1 Assignment
0 Petitions
Accused Products
Abstract
A structural specimen coated with or constructed of photoelastic material, when illuminated with circularly polarized light will, when stressed; reflect or transmit elliptically polarized light, the direction of the axes of the ellipse and variation of the elliptically light from illuminating circular light will correspond to and indicate the direction and magnitude of the shear stresses for each illuminated point on the specimen. The principles of this invention allow for several embodiments of stress analyzing apparatus, ranging from those involving multiple rotating optical elements, to those which require no moving parts at all. A simple polariscope may be constructed having two polarizing filters with a single one-quarter waveplate placed between the polarizing filters. Light is projected through the first polarizing filter and the one-quarter waveplate and is reflected from a sub-fringe birefringent coating on a structure under load. Reflected light from the structure is analyzed with a polarizing filter. The two polarizing filters and the one-quarter waveplate may be rotated together or the analyzer alone may be rotated. Computer analysis of the variation in light intensity yields shear stress magnitude and direction.
84 Citations
28 Claims
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1. A method for determining shear stresses in a structure comprising the steps of:
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coating a structural specimen with a photoelastic material; loading the structural specimen to produce no more than one-half of a wavelength of birefringence in the photoelastic material; illuminating with elliptically polarized light the structural specimen coated with the photoelastic material to cause a multiplicity of discrete areas on the structure to reflect elliptically polarized light; creating an electronic image of a portion of the structural specimen including the multiplicity of discrete areas; analyzing the elliptically polarized light reflected from the multiplicity of discrete areas of the illuminated specimen to determine the orientation and magnitude of major and minor axes of the elliptically polarized light reflected from each of said multiplicity of specimen areas; and processing the orientation and magnitude of major and minor axes of the elliptically polarized light for each analyzed area to produce a result indicative of shear stress magnitude and direction in the specimen at each discrete area. - View Dependent Claims (2, 3, 4, 5, 6, 7)
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8. A method for determining shear stresses in a structure comprising the steps of:
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coating a structural specimen with a photoelastic material; loading the structural specimen to produce no more than one-half of a wavelength of birefringence in the photoelastic material; illuminating with elliptically polarized light the structural specimen coated with the photoelastic material; analyzing the light reflected from a multiplicity of discrete areas of the illuminated specimen to determine amplitude variations in light reflected from each of said multiplicity of specimen areas and the phase relationship of the amplitude variation in light from each specimen area; and processing the amplitude variation and phase relationship for each analyzed area to produce a result indicative of shear stress magnitude and direction in the specimen at each discrete area; wherein the specimen is coated with a photoelastic material, wherein the coating is formulated to attenuate a first wavelength of light to a greater extent than it attenuates a second wavelength of light; and
further comprising the steps of;illuminating the photoelastic material coating with light of the first wavelength and light of the second wavelength and comparing the relative attenuation of the first wavelength to the second wavelength at the multiplicity of discrete areas on the surface of the specimen so as to determine the thickness of the coating at said multiplicity of discrete areas; and wherein the step of analyzing the reflected light at the multiplicity of discrete areas includes the step of correcting for the determined thickness at each of said discrete areas.
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9. A method of determining the stresses in a transparent birefringent object comprising the steps of:
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passing elliptically polarized light through a birefringent object which has been stressed to exhibit a stress induced birefringence of less than one-half wavelength of light so light passing through the birefringent object is elliptically polarized; obtaining multiple digital images of the elliptically polarized light transmitted through a multiplicity of discrete areas of the birefringent object wherein each digital image is taken through a linear polarizing filter of different orientation; analyzing the multiple digital images in a digital computer to determine variations in amplitude of the elliptically polarized light transmitted from each of the multiplicity of discrete portions of the surface of the object and the phase of said variations in amplitude of the elliptically polarized light; and calculating the stress magnitude and direction at each discrete portion of the surface from the determined variation in amplitude and phase of the elliptically polarized light transmitted from each discrete portion of the surface of the object. - View Dependent Claims (10)
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11. A method of measuring magnitude and direction of stresses in an object comprising the steps of:
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applying to a surface portion of an object a birefringent coating which attenuates light of a first wavelength and which attenuates light of a second wavelength to a greater extent than the first wavelength; determining the thickness of the coating at a multiplicity of discrete areas on the object by illuminating the coating with light composed of the first wavelength and the second wavelength and calculating thickness based on differences in attenuation of the first and second wavelengths; illuminating at least a portion of the birefringent coated portion of the surface of the object with elliptically polarized light; and determining for a multiplicity of discrete areas within the illuminated portion an amplitude variation and phase angle of said amplitude variation of elliptically polarized light reflected through the birefringent coating on each of the multiplicity of discrete areas. - View Dependent Claims (12, 13, 14, 15)
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16. A method of determining the stresses in an object comprising the steps of:
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coating a surface of an object in which the stresses are to be measured with a birefringent coating; loading the object with a stress inducing load to produce no more than one-half of a wavelength of birefringence in the coating; illuminating the coated surface with elliptically polarized light so light reflected from the surface through the coating is elliptically polarized; obtaining multiple digital images of the elliptically polarized light reflected from a multiplicity of discrete areas on the surface, wherein each digital image is taken through a linear polarizing filter of different orientation; analyzing the multiple digital images in a digital computer to determine the variation in intensity of the elliptically polarized light reflected from each of the multiplicity of discrete portions of the surface of the object and the phase angle of said intensity variations in the elliptically polarized reflected light; and calculating the stress at each discrete portion of the surface from the determined variation in intensity and phase angle of the polarized light reflected from each discrete portion of the surface of the object. - View Dependent Claims (17)
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18. A method of determining stresses in a photoelastic structure comprising the steps of:
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coating a structural specimen with a photoelastic material; illuminating the coating on the structure with non-polarized light; before loading the structure, determining for each of a multiplicity of discrete areas on the structure induced polarization of the light reflected from each discrete area followed by; illuminating the structure with elliptically polarized light; loading the structural specimen to produce no more than one-half of a wavelength of birefringence in the plastic material; creating an electronic image out of a portion of the structure including the multiplicity of discrete areas; determining for each of the multiplicity of discrete areas on the structure, the direction and magnitude of the ellipse major axis of the elliptically polarized light reflected from each of said multiplicity of discrete areas of the specimen; and combining with the direction and magnitude of the ellipse major axis, the determined induced polarization of reflected light, to produce a result indicative of shear stresses in the specimen at each discrete area. - View Dependent Claims (19)
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20. A method of determining shear stress in a specimen comprising the steps of:
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chemically altering a surface portion of an object to impart birefringent properties to said surface portion; loading the object to produce stress induced birefringence in the chemically altered surface portion of the object; illuminating the chemically altered surface with elliptically polarized light; analyzing the light reflected from the chemically altered surface portion and determining an orientation and amplitude variation of the elliptically polarized light from each of a plurality of discrete areas on the surface portion of the object. - View Dependent Claims (21, 22, 23)
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24. A method of determining stress in a specimen having an aluminum surface comprising the steps of:
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oxidizing an aluminum surface of a specimen to produce a transparent birefringent coating thereon; loading the specimen to produce birefringence in the transparent birefringent coating; illuminating the transparent birefringent coating with elliptically polarized light; and analyzing light reflected from the transparent birefringent coating and determining an orientation and amplitude variation of the elliptically polarized light from each of a plurality of discrete areas on the surface portion of the object.
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25. A method of determining stress in a specimen comprising the steps of:
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applying a birefringent coating to a surface of a specimen wherein the coating is formulated to attenuate a first wavelength of light to a greater extent than it attenuates a second wavelength of light; illuminating the birefringent coating with light of the first wavelength and light of the second wavelength and comparing the relative attenuation of the first wavelength to the second wavelength at a multiplicity of discrete areas on the surface of the specimen so as to determine the thickness of the coating at said multiplicity of discrete areas; determining the amount of induced birefringence in the coating at the multiplicity of discrete areas and correcting the observed birefringence for the determined thickness at each of said multiplicity of discrete areas; and determining shear stress in the specimen at said multiplicity of discrete areas from said corrected observed birefringence.
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26. The method of 25 wherein the coating is formulated to attenuate a third wavelength of light by an amount different from the first wavelength, and unlike the second wavelength of light, and wherein the illuminating of the birefringent coating is with light containing the third wavelength of light, and wherein the step of comparing the attenuation of the first wavelength to the second wavelength includes comparing the third wavelength.
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27. A method for determining shear stresses in a structure comprising the steps of:
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coating a structural specimen with a photoelastic material; illuminating with right-handed elliptically polarized light the coated structural specimen; illuminating with left-handed polarized light the coated specimen; adding the right-handed elliptically polarized light and the left handed elliptically polarized light reflected from the structural specimen to produce a summed light; analyzing the summed light reflected from a multiplicity of discrete areas of the illuminated specimen to determine amplitude variations in light reflected from each of said multiplicity of specimen areas and the phase relationship of the amplitude variation in light from each specimen area; and processing the amplitude variation and phase relationship for each analyzed area to produce a result indicative of shear stress magnitude and direction in the specimen at each discrete area. - View Dependent Claims (28)
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Specification