Measuring and testing – Specimen stress or strain – or testing by stress or strain... – By loading of specimen
Patent
1995-12-01
1997-05-27
Chilcot, Richard
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
By loading of specimen
73788, 356 355, G01L 1124
Patent
active
056334672
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a method for non-destructive testing of structures, and apparatus for the carrying out of that method.
Many structures are subject to deterioration over time, with an adverse effect on their load bearing ability. Accordingly, it is necessary to test the structures periodically to determine whether a dangerous condition exists. Structures which require such periodic testing include concrete structures such as building floors, bridges and the like, and metal structures such as aircraft parts.
In some cases, structural testing involves the application of fairly large loads, such as filled water tanks, to provide significant deflection of the structure, and measuring that deflection. This can be dangerous if the load bearing capability of the structure has degraded seriously, as the structure may fail catastrophically under such a large load.
Other methods of testing structures require damaging the structure. For example, if an aircraft section is to be examined for cracks, paint must be removed from the area to be tested. If a concrete structure which is post tensioned by the use of tensioning cables is to be tested, concrete can be removed to expose a portion of the cable, either for optical examination or for testing by passing high frequency radio signals through the cable.
It would therefore be advantageous to have a method and apparatus for the measurement of the integrity of structures without the necessity of some destruction of the structure or of putting a load large enough to damage the structure on it.
For this reason, techniques such as interferometry have been proposed to determine surface deformations. For example, several patents have proposed putting diffraction gratings on an object to be tested, and measuring the deformation of such gratings by interferometry. Examples are found in U.S. Pat. No. 3,985,444 (Takashima) and U.S. Pat. No. 4,850,693 (Deason). Other patents have proposed taking holograms of a surface before stress, and after stress. An example is shown in U.S. Pat. No. 3,911,733 (Bhuta). Still other patents use variations of speckle interferometry, such as U.S. Pat. No. 4,591,996 (Vachon) and U.S. Pat. No. 4,436,419 (Stetson), or shearing interferometry, as shown in U.S. Pat. No. 5,005,345 (Hung).
It has also been taught to use a system with two interferometers, both having the same target points and vibrating a structure while measuring with the two interferometers, as shown in U.S. Pat. No. 4,526,465 (Corty). Other designs require reflection from a prism or the like attached to the structure to be tested, as is shown for example in U.S. Pat. No. 4,815,855 (Dixon).
In general, the prior art methods using interferometry are fairly cumbersome. In those methods which require comparison of holograms or other patterns taken at different times, care must be taken to position the device in exactly the same spot, so that comparable figures can be obtained. In many cases, complicated pieces of equipment are required, and complicated processing is necessary to get a useful result.
DESCRIPTION OF THE INVENTION
The invention provides a local measurement of the apparent stiffness or rigidity of a structure. One or more supports are placed on a surface of a structure to be tested, so that they transmit movements of that structure. The supports in turn support a beam, and transmit to it the movements of the structure which pass through the supports. At a portion of the beam which is unsupported by the supports, for example at a point roughly midway between two supports, a measurement is made of a quantity which varies with distance between the surface of the structure and the beam. As will be seen below, this need not be the actual distance, but only some quantity which will vary if the distance changes. This measurement is made under several different conditions, such as a lapse of considerable time or a variation of a deflecting force applied to the structure, and the variation in the measured quantity is noted. Because the variation is expected to be small, the m
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Chilcot Richard
Noori Max H.
Pure Technologies Inc.
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