The apparent strain stability and repeatability of a BCL3 resistance strain gage

Experiments were conducted at NASA-Lewis to study the effect of microstructural instability on the apparent strain stability and reproducibility of a BCL3 resistance strain gage. The resistance drift of the gage at various temperatures in the phase transition temperature range (PTTR) was measured. The effects of the heating and cooling rates with which the gage passed through the PTTR on the apparent strain characteristics of the gage were also studied. BCL3 gage, like other Fe-Cr-Al based gages, exhibited apparent strain instability in the temperature range of 700 to 1100 F due to the reversible microstructural transition the gage materials experienced in this temperature range. The BCL3 gage had a maximum apparent strain drift in the neighborhood of 770 F with an average drift rate of approx. -440 microstrain/hr in 2 hrs. The use of the BCL3 gage as well as other Fe-Cl-Al based gages for static strain measurements within the PTTR should be avoided unless the time durations in the PTTR are small enough to introduce a neglible drift. The microstructure transition that the BCL3 gage underwent occurred in the temperature range of 750 to 1050 F during heating and around 1000 to 800 F during cooling. The heating rate, and, in particular, the cooling rate with which the gage passed through the PTTR affected the shape and the repeatability of the apparent strain curve of the gage.