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Serious errors are sometimes encountered in strain gage data when testing is conducted in transient heating environments. Variations in thermal gradients, temperature compensation, and monitor temperature sensor locations all contribute to the inaccuracies. The temperature of a strain gage wire is different (150 F with 100 F per sec heating rates) than that of the specimen surface under or adjacent to the gage. Strain gages respond faster to a step-function heat input than do thermocouples; resistance thermometers respond at about the same rate as strain gages. Theoretically, a quartz-compensated strain gage, when installed with adequate temperature sensors and when properly calibrated, will result in more accurate data because the strain gage output is independent of the grid wire temperature.
Johnson, Curtis E.
Research Engineer, Structures Laboratories, The Boeing Co., Seattle, Wash.