Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
  • G01B9/00—Measuring instruments characterised by the use of optical techniques
  • G01B9/02—Interferometers
  • G01B9/021—Interferometers using holographic techniques
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
  • G01B11/00—Measuring arrangements characterised by the use of optical techniques
  • G01B11/16—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge
  • G01B11/161—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge by interferometric means
  • G01B11/164—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge by interferometric means by holographic interferometry

Definitions

• This invention relates to apparatus for subjecting a test object to two differnt ambient pressures so that an interferometric analysis may be performed of the deformation of the object surface resulting from the pressure change, and more particularly to such apparatus employing a pressure chamber designed so that no net forces are exerted on the support mechanism for the camera or test object as a result of the pressure change.
• a common method of non-destructively testing an object involves the interferometric comparison of images of the object surface taken before the change in the ambient pressure on the object.
• the interferometric analysis may be performed using Holographic apparatus as disclosed in Principles of Holography, Smith, Wiley-Interocience N.Y. , 1969 pg. 193-197, or an electric camera as disclosed in U.S. Patent 3,816,649.
• the test object and the image recording apparatus are disposed within a pressure chamber and the object is illuminated with coherent light derived from a laser which is typically supported externally of the chamber and introduces its beam into the chamber through a train of optical elements.
• An image is first recorded of a coherently illuminated section of the surface of the object and then the pressure within the chamber is changed to impose stressing forces on the object.
• a second exposure of the same object sur ⁇ - face is then made and is interferometrically compared with the first record to detect deformation of the object surface as a result of the pressure change.
• the pressure chamber has been formed by placing a semi-spherical dome over the rigid base plate that supports the object and the image forming apparatus.
• the dome has been supported by a crane or derrick so that it may be lifted away from the machine to allow insertion and removal of the test object and access tothe camera or other image forming apparatus.
• the free edge of the dome has been formed with, a resilient seal so that when the dome is lowered into contact with the base plate the volume bounded by the interior of the dome and the base plate has formed a sealed pressure chamber.
• a vacuum pump draws air out of this chamber to control the ambient pressure on the object.
• the present invention is accordingly directed toward a novel design for a pressure chamber useful in performing double exposure interferometric analysis which substantially eliminates movement of the interfering elements resulting from stresses imposed on the structure as a result of pressure changes within the chamber.
• the pressure chamber of the present invention provides a base plate having a horizontal upper surface for supporting the test object and the image forming elements.
• a semi- spherical dome is supported for movement so that it may be positioned with its free edge in contact with the top of the base, or may be removed to allow access to the base.
• a manifold is formed on the underside of the base plate extending over at least a projection of that area on the top side of the base plate forming part of the pressure chamber. This manifold includes a bottom plate spaced from the base plate and parallel to it. The volume of
• the manifold is connected to the volume on the upper side of the base plate, preferably by apertures formed through- the base plate, so that when the dome overlies the top of the base plate and a vacuum is drawn within the resulting pressure chamber, the pressure in the manifold, on the underside of the base plate, is also modified.
• the pressures on the upper and lower sides of the base plate are equalized and no net forces are induced on the base plate when the pres- sure within the chamber is changed.
• the bottom plate of the manifold will deform a& a result of the pressure change but the connection between this bottom plate of the manifold and the base plate includes resilient ele- ⁇ ments which, prevent this deformation from exerting any substantial forces on the base plate.
• the base plate itself can be formed in a simpler less expen ⁇ sive manner than the base plates of the prior art and the present design accordingly results in lower cost and more effective pressure chamber system.
• FIGURE 1 is an elevational sectional view through an interferometric pressure chamber formed in accordance with the prior art
• FIGURE 2 is an elevational sectional view through an interferometric pressure chamber formed in accordance with, the present invention.
• the chamber includes a hase, generally indicated at 10.
• the base 10 may typically be formed out of steel and may consist of a dis - ⁇ shaped upper plate 12, a disc-shaped lower plate 14, a vertical side plate 16 and interior ribs 18 extending between the top and bottom plates 12 and 14 to form a truss-like grid. Typically the elements will be welded to one another to form a highly rigid assembly.
• the base 10 is typically supported above a rigid foundation 20, preferably of concrete or the like, by vibration isoloation mounts 22.
• the upper side of the base plate 10 acts as a support for a schematically illustrated test object 24 and schematically illustrated recording apparatus 26.
• the recording apparatus will include means for coherently illuminating the test object and photographic or electronic means for recording images.
• the dome 28 carries a resilient ring 32 on its free edge and the crane can position the dome with the resilient ring in contact with the upper surface of the base plate, it can move the dome 23 out of position to allo access to the top of the base plate.
• a vacuum pump 34 can control the pressure within the chamber.
• a relatively low vacuum is drawn, a first exposure of the object surface is made, and then the vacuum is increased and a second exposure of the object surface is made.
• the base plate 10 forms one wall of the pressure chamber, forces will be exerted on the base plate, causing it to move upwardly toward the dome as the pressure within the chamber is reduced.
• the dotted lines 36 in FIGURE 1 illustrate, in exaggerated form, the change in position of the surfaces of the upper plate 12 and lower plate 14 of the base as the pressure within the dome is reduced. This motion tends to distort the record of the interferometric analysis and the truss-like form of the base 10 is intended to provide it with a rigidity that will minimize this undesirable motion.
• FIGURE 2 This chamber employs a horizontal steel base plate 40, usually disc-shaped in plan view.
• the plate 40 is supported above the rigid foundation 20 on vibration isolating mounts 42, of conventional design.
• the upper surface of the base plate 40 supports a schematically illustrated test object 24 and the recording and illumi ⁇ nating apparatus 26.
• a dome 44 having a resilient ring 46 affixed to its free edge may be positioned on top of the base plate 40 by a crane arm 48.
• a rigid steel ring 50 having a diameter at least equal to the diameter of the free end of the dome 44 is welded to the underside of the base plate 40 to encompass an area on the underside of the plate 40 that is a projectio of the area encompassed by the dome on the upper side of the plate.
• the ring 40 has a resilient gasket 52 fixed to its underside. The gasket in turn supports a relatively flexible disc-shaped steel manifold flex plate 54.
• the plate 54 along with the rigid ring 50 and the gasket 52 define a manifold volume on the underside of the base plate 40. This volume is connected to the pressure chamber on the upper side of the base plate by apertures 56 formed through, the base plate.
• a va,cuum pump 58 connects to the manifold volume and controls the vacuum within the manifold and the test vacuum chamber when the dome 44 is positioned in con ⁇ tact with the upper side of the base plate, fhen the dome is so positioned and a vacuum is drawn in the manifold area and the test chamber by the pumps 58, the decrease in pressure causes the manifold bottom plate 54 to flex to the position illustrated in phantom lines designated 54a; however, the pressure an the base plate is equal on both sides so no net forces are exerted on the base plate. Accordingly, no change in position is induced on the test object 24 or the recording apparatus 26 as a result of the changes in pressure within the test chamber. Since the base plate 40 forms a neutral axis with respect to the pressure changes, it is unaffected by the pressure change.

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Investigating Strength Of Materials By Application Of Mechanical Stress (AREA)
• Instruments For Measurement Of Length By Optical Means (AREA)
• Length Measuring Devices By Optical Means (AREA)

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• 1984
  • 1984-01-05 WO PCT/US1984/000002 patent/WO1985003123A1/en not_active Ceased
  • 1984-01-05 JP JP59500648A patent/JPS61501046A/ja active Granted
  • 1984-01-05 EP EP19840900655 patent/EP0167530A1/en not_active Withdrawn

Non-Patent Citations (1)

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