The Effect of Temperature and Strain on the Electrical Resistivity of Ni-Fe-Cr Foil

This dissertation research concentrated on two Ni-Fe-Cr alloys -- one rich in iron (36Ni-57Fe-7Cr) and one rich in nickel (70Ni-27.5Fe-2.5Cr) for application in metal foil strain gages. The motivation was to examine the high, resistance -- strain sensitivity (gage factor) characteristic of these alloys which would be attractive for improving signal-to-noise level; while being mindful of their high, resistance -- temperature sensitivity (thermal output) characteristic which could limit their range of practical application. Comparisons to other commercially available strain gage alloys were provided: 55Cu-45Ni (constantan) because of its widespread use in manufacture of metal foil strain gages and 92Pt-8W because it also possessed high gage factor and high thermal output.;The first alloy (36Ni-57Fe-7Cr) was used for over fifty years to manufacture metal foil strain gages for specific applications. Its main characteristics were high gage factor, high fatigue strength, and high thermal output. To circumvent the effect of high thermal output, typical strain gage applications involved either dynamic loading where the high gage factor and high fatigue strength could be used to advantage and the effect of thermal output was minimal; or careful exercise of Wheatstone bridge circuit cancelation of like-thermal output in adjacent arms. This research found low variation in thermal output of strain gages made with 36Ni-57Fe-7Cr foil over the temperature range of -40 to 248 °F (-40 to 120 °C). The magnitude of the thermal output was approximately 276 ppm/°F with a standard deviation of 0.1 ppm/°F. The uniformity of thermal output of 36Ni-57Fe-7Cr strain gages was attributed to the uniformity in the metal microstructure and single phase composition examined using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and x-ray diffraction (XRD). Several demonstrations were performed to show the suitability of strain gages made with 36Ni-57Fe-7Cr foil even when loading conditions were not dynamic. These demonstrations included a cantilever beam to which delta-rosette strain gages were bonded, a shear-beam load cell, and a single-point load cell to which Tee-rosette strain gages were bonded.;The second alloy (70Ni-27.5Fe-2.5Cr) was used in research studies for more than forty years concerning its ability to form an ordered crystalline structure upon application of elevated temperature and the resulting effect on a variety of physical properties such as magnetism, strength, electrical resistance, and specific heat. This dissertation research found 70Ni-27.5Fe-2.5Cr foil strain gages had high gage factor and high thermal output with good thermal output uniformity. The thermal output for strain gages made of 70Ni-27.5Fe-2.5Cr foil was about 870 ppm/oF with a standard deviation of 0.5 ppm/oF. The gage factor was nonlinear ranging in magnitude from about 5.2 to 3.0 for applied strains of 300 and 1300 muepsilon, respectively. To investigate the effect of crystalline ordering, strain gages were made with 70Ni-27.5Fe-2.5Cr foil which had been annealed under varying isochronal and isothermal conditions up to 900 °F (482 °C). The magnitude of gage factor and thermal output for strain gages made of 70Ni-27.5Fe-2.5Cr foil corresponded with transformation of the crystalline structure from a state of disorder to a state of order. Thermal output and gage factor testing revealed some minor reduction in thermal output and a significant increase in gage factor as the metal progressed to higher levels of crystalline order. These effects were attributed to differences in metal resistivity with applied strain and to the difference in mean free path of conduction electrons depending upon crystalline order.