Thermal Output (continued): Temperature Effects

Thermal Output (continued)

Effects of Gage Alloy
The figure below shows the variation of thermal output with temperature for a variety of strain gage alloys bonded to steel. These data are illustrative only, and not for use in making corrections. It should be noted, in fact, that the curves for constantan and Karma are for non-self-temperature-compensated alloys. With self-temperature compensation, as employed in Micro-Measurements strain gages, the thermal output characteristics of these alloys are adjusted to minimize the error over the normal range of working temperatures.

Fig.1 - Thermal output variation with temperature for several strain gage alloys (in the as-rolled metallurgical condition) bonded to steel.

As indicated by the figure above, the errors due to thermal output can become extremely large as temperatures deviate from the arbitrary reference temperature (ordinarily, room temperature) with respect to which the thermal output is measured. The illustration shows distinctly the necessity for compensation or correction if accurate static strain measurements are to be made in an environment involving temperature changes.

Isothermal Test Conditions
With respect to the latter statement, it should be remarked that if it is feasible to bring the gaged test part to the test temperature in the test environment , maintaining the test part completely free of mechanically or thermally induced stresses, and balance the strain indicator for zero strain under these conditions, no thermal output error exists when subsequent strain measurements are made at this temperature. In other words, when no temperature change occurs between the stress-free and stressed conditions, strain measurements can be made without compensating or correcting for thermal output. In practice, however, it is rare that the foregoing requirements can be satisfied, and the stress analyst ordinarily finds it necessary to take full account of thermal output effects.

Also, in the case of purely dynamic strain measurements, where there is no need to maintain a stable zero-strain reference, thermal output may be of no consequence. This is because the frequency of the dynamic strain signal is usually very high with respect to the frequency of temperature change, and the two signals are readily separable. If, however, there is combined static/dynamic strain, and the static component must also be measured, or if the frequency of temperature change is of the same order as the strain frequency, thermal output effects must again be considered.

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