The ability of alloy 70Ni-27.5Fe-2.5Cr to form an ordered crystalline structure upon application of elevated temperature and the resulting effects on a variety of physical properties such as magnetism, strength, electrical resistance, and specific heat are well known. This paper demonstrates that strain gages made with 70Ni-27.5Fe-2.5Cr foil with an ordered crystalline structure have both high gage factors and high thermal outputs. The thermal output for strain gages made of 70Ni-27.5Fe-2.5Cr foil is demonstrated to be about 1,374 μΩ/Ω/K. The gage factor is nonlinear ranging in magnitude from about 5.0 to 3.6 for applied strains of 300 and 1,300 μE, respectively. The magnitude of gage factor and thermal output correspond with transformation of the crystalline structure from a state of disorder to a state of order. Comparisons to strain gages made with 36Ni-57Fe-7Cr foil are provided because of its use in the manufacturing of metal foil strain gages and their demonstrated high gage factors and high thermal outputs. Practical application for 70Ni-27.5Fe-2.5Cr strain gages is demonstrated by dead-weight loading of shear-beam load cells at low applied strain levels to minimise the effect of nonlinear gage factor; and Wheatstone bridge cancellation of high thermal output. KEYWORDS gage factor, metal foil strain gages, Ni 3 FeCr, short range order, thermal output | INTRODUCTIONCrystalline order in Ni 3 Fe binary and several Ni 3 Fe ternary alloys has been reported extensively in the literature, primarily on two types of topics: the effect of crystalline order on material property; and the method by which crystalline order is detected. Material properties have included hardness and work hardening, [1] elastic constants and strength, [2] thermal conductivity, electrical resistivity and Seebeck coefficient, [3] specific heat, [4] and magnetic permeability.[5] The material property may actually be quite sensitive to crystalline order as reported by Binnatov et al. [4] where specific heat appeared to be more sensitive than Mossbauer spectroscopy in detecting crystalline order. Regarding the specific strain gage properties of thermal output and gage factor in Ni 3 Fe ternary alloys, no prior research relating these to crystalline order was found. Detection of crystalline order has typically been by indirect measurement, most notably electrical resistivity. As such, the bonded electrical resistance strain gage may be ideally suited for detection of crystalline order since thermal output and gage factor involve change in electrical resistivity with the former being from change in temperature and the latter being from change in applied strain.

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This study presents the fatigue response of 304 stainless steel foil, cold-rolled to a thickness of 3.2 µm with 87 percent cold work at orientations of 0, 45, and 90 degrees to the direction of rolling. Fatigue specimens were fabricated by laminating a supportive layer of 20-µm polyimide film to one side of the foil and patterning 242 crack initiation features by photolithographic process. Progression of fatigue damage was determined through electrical resistance measurement. The fatigue response was demonstrated to be largely affected by anisotropy existing between the rolling direction and the off-axis orientations. Fatigue cracks that traveled in a direction parallel to the elongated grains (cyclic loads applied at 90-degree orientation to foil rolling direction) had the most fatigue response (undesirable characteristic). The construction of the specimens with thin foil supported by a film backing contributed to high fatigue threshold.

This paper describes the development of a tool designed to spot weld Ailtech strain gages to the inside surface of small diameter tubing. Fundamental spot welding techniques are discussed with emphasis placed upon the adaptations that were necessary to attach a gage that was neither visually nor physically accessible to the installer. A special electrode was made that enables a pair of welds to be made simultaneously, one on either side of the strain tube. The installation tool described has been used successfully to spot weld four strain gages spaced 90 degrees (1.57 radians) apart to the inside surface of 0.552 inch (14.02 millimeter) diameter tubing to depths of 26 inches (660 millimeters).