Southwest Research Institute® (SwRI®) News

Elastomer Strain Gage

San Antonio, Texas -- Sept. 18, 1975 -- Industrial Research Magazine has given an IR 100 Award designation to Southwest Research Institute® (SwRI®), San Antonio, for an invention. Cited as one of the top 100 technological products developed during the past year, the device is a new type gage for measuring strains in soft, flexible materials.

In the past, the determination of strains on such materials as rubber, plastic or living tissue has been limited to visual or photographic observation of pattern and color changes in coated materials under stress in polarized light.

Now it is possible to use a gage which utilizes the same electrical resistance changes as does the foil grid strain gage which is commonly used in industry. While the latter is usually restricted to rigid structures experiencing less than one percent strain, the SwRI elastomer gage performs on structures undergoing up to fifty-percent strains.

In the foil grid strain gage, strains effect changes in the length of the wire to produce current resistance differentials. In the SwRI gage, the sensing element is a one-fifth mil (.0002 inch) capillary tube filled with a mercury alloy. It is encapsulated in a supple polyurethane U-shaped structure. When the unit is enclosed in or attached to the specimen, the mercury alloy column behaves in the same way as the wire in the foil grid strain gage. The strain can be measured at a remote station with a simple electrical resistance circuit as a function of compression or decompression of the mercury column.

The device has applications over a broad industrial area. It can be used for experimental analysis of tires as well as tire-like structures (bags, hoses). It also shows promise in determining heavy strains on such special-use flexible materials as skirts for hovercraft, etc. It should prove valuable in the studies of the solid propellants and other plastic materials which are subject to large deformations. It can also be used in studies of bones, muscles and other body tissues.

The device was developed by a team headed by Jarvis Michie, manager of an SwRI structural systems section (and includes Eugene Anderson, Leonard Rastrelli and Albert Reichert). The team has already furnished some gages for industrial use.

The award will be received by Dr. Robert C. DeHart, vice president of SwRI, at a banquet in the Great Hall of Chicago's Museum of Science and Industry Thursday night. The gage will be displayed in a museum exhibit for the next month.

For more information about the Elastomer Strain Gage, contact Joe Fohn, Communications Department, Southwest Research Institute, P.O. Drawer 28510, San Antonio, Texas, 78228-0510, Phone (210) 522-4630, Fax (210) 522-3547.

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