Adhesively Bonded Strain Gages for Extended Use at 315°C on Polymers

A unique strain gage and adhesive system was developed for mechanical property measurement of polymers and polymer composites at elevated temperatures. This system overcomes some of the problems encountered in using commercial strain gages and adhesives. An important limitation with typical commercial strain gage adhesives is that they require a post-cure at temperatures substantially higher than the maximum test temperature. The exposure of the specimen to this temperature may affect subsequent results, and in some cases may be higher than the glass transition temperature of the polymer. In addition, while typical commercial strain gages can be used for short times at temperatures up to 370°C, their long-term use is limited to 230°C. This precludes their use for testing some high-temperature polyimides near their maximum-temperature capability. The system that was developed consists of a nonencapsulated, unbacked gage grid that is bonded directly to the polymer using a specially formulated adhesive. The gage is applied after the specimen has been cured, but prior to the normal post-cure cycle. The adhesive was formulated to cure under the specimen post-cure conditions. Special handling, mounting, and electrical connection procedures were developed, and a fixture was designed to calibrate each strain gage after application on the specimen. A variety of tests was conducted to determine the performance characteristics of the gages at elevated temperatures on PMR-15 neat resin and titanium specimens. These tests included static tension, thermal exposure, and creep tests. The gage and adhesive system performed within normal strain gage specifications at 315°C under these conditions.