Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-06-01
2004-08-17
Chowdhury, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C033S501000
Reexamination Certificate
active
06778236
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to non-destructive methods and apparatus for evaluating strain on a workpiece surface. The invention further relates to polarization-dependent reflective devices.
It is often desirable not only to measure the magnitude of an applied strain, but to know the tensor and even the precise location of the strain in the workpiece. Prior art strain gauges have been unable to provide this information in a simple and low-cost manner.
One type of fiber-optic strain gauge relies upon an optical fiber which transmits light along its length with little or no loss when straight. When deformed under an applied stress, i.e., by bending, significant attenuation of the light signal occurs. The extent of light transmission loss may be correlated to the deformation strain. J. D. Weiss in “Fiber-Optic Strain Gauge” (
J. Lightwave Tech.
7(9), September 1989) used the same principle to design an fiber-optic strain gauge using optical fibers having permanent microbends which demonstrate an increase in light transmission under applied tension. Fiber-optic strain gauges are able to identify the existence of strain, but are unable to identify its location within the article since bending (or extension of the microbend) is propagated along the entire length of the optical fiber and is not restricted to the strain site.
Fiber-optic Bragg gratings have also been described for measuring strain. Bragg gratings on an optical fiber are produced by UV exposure causing periodic changes in the index of refraction of the fiber core. A typical Bragg grating strain sensing system involves Bragg wavelength shift detection. Such sensing systems have very low reflectivity, making the sensitivity of the measurements low. Furthermore, the fragility of the optical fibers gives rise to the need to embed the fibers in a matrix so that only bulk strain is practically measured using this technique.
Photolithography has been used to form diffraction gratings in metallic strips. Applied stress causes the grating dimensions to shift which can be monitored to detect strain. The preparation and use of photolithographic diffraction gratings is awkward and computationally complex. Furthermore, the diffraction grating can not provide information regarding the location of strain in a workpiece.
Strain gauges have been described which rely upon the optical properties of thin films to detect and even quantify strain. U.S. Pat. No. 4,123,158 reports on the use of optical properties such as birefringence to detect surface strain. Photoelastic materials are used which have the optical property of polarizing light when under stress and then transmitting such light on the principle stress planes with velocities depending upon the magnitude of the applied stress. When such gauges are subjected to monochromatic polarized light, the birefringence of the photoelastic material causes the light to emerge refracted in two orthogonal planes. Because the refractive indices of light propagation are different in each direction, a phase shifting of the light waves occurs. When the waves are combined with polarizing film, regions of stress where the wave phase is canceled appear black and regions of stress where the wave phase is combined appear white. When white light is used in place of monochromatic light, the relative retardation of the photoelastic material causes the fringes to appear in colors of the spectrum.
Reflective liquid crystal displays have been developed which rely on polymer dispersed liquid crystals (PDLCs). A conventional PDLC is formed by phase separation of a liquid crystal phase from a matrix polymer phase. Photopolymerization-induced phase separation utilizes a mixture of a low molecular weight liquid crystal and a photocurable monomer. Irradiation of the homogeneous pre-polymer mixture initiates polymerization, which in turn induces a phase separation between the polymer and liquid crystal (LC). Liquid crystal droplets are formed within the sample to modulate the LC droplet density on the order of the wavelength of light.
H-PDLCs are phase separated compositions formed under holographic conditions. Holographic or optical interference preparative techniques have been used to carry out polymerization to selectively position regions of liquid crystal and polymer. Instead of random arrangement of LC droplets, the holographic exposure induces a periodic array of LC droplets and matrix polymer planes. On exposure to an optical interference pattern, typically formed by two coherent, counter-propagating lasers, polymerization is initiated in the light fringes. A monomer diffusion gradient is established as the monomer is depleted in the dark fringes, causing migration of liquid crystal from the dark fringes. The result is LC-rich areas where the dark fringes were located and essentially pure polymer regions where the light fringes were located.
Thin H-PDLC films have been incorporated into displays relying upon the optical reflective properties of the material to provide visual images. Displays incorporating these materials have been reported in “Holographically formed liquid crystal/polymer device for reflective color displays” by Tanaka et al. in
Journal of the Society for Informational Display
(
SID
), Volume 2, No. 1, 1994, pages 37-40; and also in “Optimization of Holographic PDLC of Reflective Color Display Applications” in
SID
'95
Digest
, pages 267-270 (1995). The reflective properties of such films have not been exploited in the field of strain determination.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an improved strain gauge capable of providing information regarding the existence, location and/or magnitude of strain in a workpiece.
It is a further object of the invention to provide a non-destructive strain gauge.
It is a further object of the invention to provide holographically-formed polymer dispersed liquid crystals (H-PDLCs) based devices for use in observation and measurement of strain.
It is a further object of the invention to provide polarization-sensitive reflective devices useful as strain gauges, displays or polarizing filters, and the like.
In one aspect of the invention, a reflective strain gauge includes an holographically-formed polymer dispersed liquid crystal (H-PDLC) film comprising layers of liquid crystal (LC) droplets in a polymer matrix, in which the H-PDLC film has a reflection or transmission grating capable of reflecting or transmitting light of a selected wavelength, and means for securing the film to a surface of a workpiece for monitoring the strain at said surface. Strain is observed by a change in the nature of the light reflected or transmitted from the surface of an H-PDLC-containing film.
In preferred embodiments, the film includes multiple gratings, wherein different gratings are responsive to stress applied in different directions. The multiple gratings are located within a single H-PDLC layer, or in a plurality of H-PDLC layers in which each layer has at least one grating. The grating is oriented within the H-PDLC film so that surface strain is observed as a blue shift or a red shift of the reflected light. In other embodiments, the intensity of the reflected light is polarization dependent. The LC layers may be substantially parallel, substantially perpendicular, or at an angle to the film surface.
In another preferred embodiment, the LC droplets of the H-PDLC film are oriented such that the refractive index parallel to the axis (n
e
) is greater than the refractive index perpendicular to the axis (n
o
). n
o
substantially matches the refractive index of the matrix polymer, so that light polarized perpendicular to the axis is transmitted, and light polarized parallel to the axis is reflected. Orientation may arise in the strained state such that such that in the LC droplets form ellipsoids with long axes aligned parallel to an axis of an applied force. Orientation may arise by H-PDLC formation under an orienting force.
In other embodiments, the matrix polymer is selected to have sufficient elasticity to
Bowley Christopher C.
Cairns Darran R.
Crawford Gregory P.
Danworaphong Sorasak
Faris Sadeg M.
Crispino Ralph J.
Reveo Inc.
Rude Timothy L
Scozzafava Mary Rose
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