Full field photoelastic stress analysis

• US 6,219,139 B1
• Filed: 10/05/1999
• Issued: 04/17/2001
• Est. Priority Date: 06/02/1997
• Status: Expired due to Fees

• Current Assignee: Stress Photonics Incorporated
• Original Assignee: Stress Photonics Incorporated
• Inventors: Lesniak, Jon R.
• Primary Examiner(s): Pham, Hoa Q.
• Application Number:
  US09/412,254
• Time in Patent Office: 560 Days
• Field of Search: 356/33, 356/35, 356/32, 356/364-369, 738/626.24, 737/05, 738/00, 738/50, 250/225
• US Class Current: 356/366
• CPC Class Codes: G01L 1/241 by photoelastic stress analysis

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.

Claims

1. An apparatus for determining shear stresses in a birefringent material comprising:

an elliptically polarized light source for projecting light on to a birefringent material;

an optical element aligned with the light source so as to receive light from the light source after the light has passed through the birefringent material;

a beam splitter in light-receiving relation with the optical element, the beam splitter providing a plurality of light paths to a plurality of imaging sensor arrays; and

a means disposed along each of said plurality of light paths, for linearly polarizing with different orientation the light passing along each of said paths.

2. The apparatus of claim 1 wherein the elliptically polarized light source provides circularly polarized light.

3. The apparatus of claim 1 wherein the optical element both projects light from the source and receives light from a birefringent material.

4. The apparatus of claim 1 wherein the beam splitter comprises a first beam splitter in light providing relationship with a first polarizing beam splitter, and a one-half wave plate followed in light receiving relationship by a second polarizing beam splitter, and wherein the first polarizing beam splitter provides two light paths, and the second polarizing beam splitter provides two light paths and the first and second polarizing beam splitters provide the means for polarizing light.

5. An apparatus for measuring stresses in an object comprising:

a source of circularly polarized light;

an object having a birefringent coating intimately attached on a surface of the object;

the surface being illuminated by the source of circularly polarized light;

an analyzer system positioned to receive light from the source reflected off the object and passing through the birefringent coating on the object, the analyzer having a beam splitter and a plurality of cameras each receiving similar images of the object, wherein each camera has a digital array on which the image of the surface of the object is projected, each digital array having an associated linear polarizing filter of a different orientation positioned in front of said digital array; and

a computer receiving data from each digital array and processing said data to determine orientation and magnitude of stresses in the object.

6. A stress analyzing instrument comprising:

a housing;

a light source mounted to the housing;

a first lens which projects light from the light source in a beam of parallel rays;

a first plane polarizer mounted to the housing and receiving said beam of parallel rays;

a first one-quarter waveplate mounted to the housing to receive the beam parallel rays passing through the first plane polarizer to create a beam of circularly polarized light;

a non-polarizing first beam splitter mounted to the housing to receive the beam of circularly polarized light and to direct a projected beam which is emitted from the housing and onto a specimen, wherein a reflected light beam returned by the specimen enters the housing and passes through the first beam splitter;

a second beam splitter mounted to the housing to receive the reflected beam passing through the non-polarizing first beam splitter, wherein the second beam splitter splits said reflected beam of light into a first split beam and a second split beam;

a first one-half waveplate mounted to the housing which receives said first split beam;

a first polarizing beam splitter mounted to the housing to receive the first split beam exiting the first one-half waveplate, being thereby split into a first and a second orthogonal polarized beam;

a first camera positioned to image said first polarized beam;

a second camera positioned to image said second polarized beam;

a second polarizing beam splitter mounted to the housing to receive the second split beam from the second beam splitter, the second polarizing beam splitter producing a third and a fourth orthogonal polarized beams;

a third camera mounted to the housing to image said third orthogonal polarized beam;

a fourth camera mounted to the housing to image said fourth orthogonal polarized beam, the four cameras simultaneously viewing said specimen; and

a controller receiving digital images from said first, second, third, and fourth cameras, and processing said images to generate an image of the stresses in the specimen.

7. The stress analyzing instrument of claim 6 further comprising:

a second lens mounted to the housing to receive the reflected beam exiting the first beam splitter; and

an aperture stop mounted to the housing to receive the reflected beam exiting the second lens ahead of the second beam splitter.

8. The stress analyzing instrument of claim 6 further comprising plane polarizers positioned in front of each camera to clean up the light passing through each side of the beam splitters.

9. The stress analyzing instrument of claim 6, further comprising an optical element mounted to the housing through which the projected beam is directed from the housing onto an analyzed object.

10. A method of determining the stresses in a transparent birefringent object comprising the steps of:

passing elliptically polarized light through a birefringent object;

obtaining multiple digital images of the elliptically polarized light transmitted through the 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 the variation of elliptically polarized light transmitted through each of a multiplicity of discrete portions of the object and the phase of said transmitted light; and

calculating the stress at each discrete portion of the surface from the determined variation and phase of said transmitted light from each discrete portion of the surface of the object.

11. The method of claim 10 wherein the elliptically polarized light is circularly polarized.

12. A method of determining weak birefringence in glass comprising the steps of:

passing elliptically polarized light through glass;

obtaining multiple digital images of the elliptically polarized light transmitted through a multiplicity of discrete areas of the glass 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

providing a display of the stresses within the specimen.

13. The method of claim 12 wherein:

the glass is an automobile window, and wherein residual stresses arising from the manufacture of the automobile window is detected.

14. A stress analyzing instrument comprising:

a light source, producing a beam of light;

a first polarizer;

a second polarizer;

a means for rotating at least the second polarizer at a selected angular rate;

one and only one one-quarter wave plate positioned between the first polarizer and the second;

a camera with an array of light detecting sensors;

wherein the beam of light is directed through the first polarizer and directly or after reflection passes through the second polarizer and the one-quarter wave plate and onto the array of light detecting sensors; and

a means for analyzing the light detected by each sensor in the sensor array to determine amplitude variations in light and phase relationship of the amplitude variations in received light in relation to the selected angular rate of the second polarizer.