CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Nonpro visional Patent

Application No. 14/712,602 which was filed on May 14, 2015, the contents of which is hereby incorporated by reference herein.

FIELD OF INVENTION

[0002] The present invention relates generally to strain gages, and, more particularly to metal resistance strain gages having a high gage factor, which sometimes is referred to resistance-strain sensitivity.

BACKGROUND

[0003] Metal strain gages are used for sensing strain at a surface to which the strain gage is mounted. The amount of strain is determined on basis of the change in the electrical resistance of an electrical circuit in the strain gage. Typically, the circuit is formed by a thin metal foil or a thin metal conductor arranged in a serpentine pattern. As the surface under investigation is strained, deformation in the strain gage causes a change in the electrical resistance of the circuit. Strain gages are said to have a strain factor, resistance-strain sensitivity or a gage factor (GF).

[0004] The GF is the ratio of fractional change in electrical resistance to the mechanical strain. The GF can be represented by:

GF = (AR/R) / 8

where AR is the change in resistance caused by the strain sensed by the strain gage, R is the resistance of the unstrained strain gage, and 8 is the strain.

Additional details about the GF are available in ASTM E251.

[0005] For many metal resistance strain gages, the GF is no more than

4, and in many cases is 3 or less. In some applications, it is desirable to have a metal resistance strain gage with a GF greater than 5. Some of the benefits of a strain gage with a high GF are an enhanced signal to noise ratio and the ability to measure low levels of strain.

[0006] Among the known metal resistance strain gages, gages made from copper, nickel and manganese (Cu-44Ni-2Mn) have a GF of approximately 2; gages made from iron, nickel, and chromium (Fe-36Ni-7Cr) have a GF of approximately 3; and, gages made from platinum and tungsten (Pt-8W) have a GF of approximately 4. Metal resistance strain gages with a GF greater than about 4 have not been available; however, there are non- metal resistance strain gages available with a GF greater than 4, but they have disadvantages such as high resistance-temperature sensitivity (thermal output) and non-uniform properties from gage-to-gage. Furthermore these non-metal resistance strain gages are brittle and must be handled carefully to avoid breakage.

[0007] Accordingly, a need exists for a metal resistance strain gage with a GF of 5 or greater.

SUMMARY

[0008] The present invention provides a high gage factor strain gage with a GF of at least 5. The strain sensitive metallic element of the gage has a composition, on a weight basis, in the range of approximately 63% to 84% Ni and approximately 16% to 37% Fe. A preferred composition is 75% Ni and 25% Fe (a stoichiometric composition); and a more preferred composition of the Ni-Fe alloy corresponds with the binary NieFe composition found in the LI2 region of the NiFe phase diagram illustrated in Figure 2.

[0009] In some embodiments, an alloying component is present in an amount that is less than about 10% of the chemical composition of the strain sensitive metallic element on a weight basis. The alloying component is preferably selected from among the group consisting of manganese, tungsten, molybdenum, chromium, and combinations thereof. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Embodiments of the present invention are illustrated in the appended drawings wherein:

[0011] Figure 1 illustrated a metal resistance strain gage in a configuration compatible with the present invention;

[0012] Figure 2 is a phase diagram for a nickel iron alloy suitable for use with the present invention; and,

[0013] Figure 3 illustrated a face-centered-cubic lattice structure suitable for use with the present invention.

DETAILED DESCRIPTION

[0014] Figure 1 illustrates a strain gage 100 in a configuration suitable for use with the present inventions. The strain gage 100 includes a serpentine strain sensitive metallic element 102 on an optional backing 104. The strain gage 100 includes connecting pads 106a, 106b for electrically the strain gage 100 to first ends of electrical leads 108a, 108b. Second ends of the electrical leads 110a, 110b are connected to known measurement instrumentation which applies an input signal to the strain gage and receives an output signal from the strain gage that corresponds with the strain induced in the strain gage 100.

[0015] The metallic element 102 of gage 100 may be formed as a wire, a foil and etched or cut into the desired shape, or a metal applied to a backing as a thin film, for example by thin film deposition. In each of these examples, the metallic element 102 is a metal with a chemical composition on a weight basis of approximately 63% to 84% Ni and approximately 16% to 37% Fe. The nominal chemical composition is 75% Ni and 25% Fe on a weight basis (a stoichiometric composition).

[0016] In a preferred embodiment, the Ni-Fe alloy corresponds with the binary NieFe composition found in the LI2 region of the NiFe phase diagram 200 illustrated in Figure 2. With reference to Figures 2 and 3, the LI2 region 204 in Figure 2 is a Ni-Fe alloy that forms a face-centered-cubic crystal lattice 300 with nickel atoms 302 occupying face positions and iron atoms 304 occupying the corner positions as illustrated in Fig. 3. Some characteristics of this alloy, such as, strength, magnetism, corrosion resistance, and electrical resistance are well known. A commercially available alloy comprising approximately 70% Ni and 30% Fe is used for resistance temperature detectors (RTDs) because of its predictable change in electrical resistance as a function of temperature. However, such alloys had not been considered for use as a strain sensitive element in an electrical resistance strain gage.

[0017] As a result of the inventors' investigation and experimentation,

Ni-Fe alloys were found to have characteristics, for example resistance-strain sensitivity (i.e., GF), that made them desirable for use in a strain gage and were unknown prior to the inventors' work. The inventors found that alloys with a nominal composition by weight of 75% Ni and 25% Fe could provide strain gages with a GF higher than that of known metal resistance strain gages. The inventors also established that binary Ni-Fe alloys within the LI2 region 204 in figure 2 provided resistance strain gages with higher gage factors (i.e., a GF greater than 5).

[0018] By way of an example, it was found that cold working of the alloy, such as by rolhng, followed by annealing increased the GF of the material to a value above 5. Rolling an alloy containing ranges of approximately 63% and 84% Ni and approximately 16% and 37% Fe by weight into a foil using known processes for foil gages and annealing at a temperature range between 600 and 900 °F for a time in the range of 1 and 16 hours yielded a strain gage with a GF that was increased over the GF of known metal resistance strain gages. In all embodiments, the GF increased to at least 5, and some embodiments had GF increase to 10 and 20. As noted earlier, the strain gage may be formed by drawing the alloy into a wire or metal film vapor deposition of the alloy in accordance with known techniques for forming strain gages.

[0019] Based upon their work, the inventors theorized that cold working and annealing facilitate the ordering of the Ni and Fe atoms in the alloy so that the Ni atoms occupy the face positions and the Fe atoms occupy the corner positions of a face-centered-cubic crystal lattice, as illustrated in Figure 3.

[0020] It was also determined that beneficial results were observed by the addition of not more than 10% by weight of the ternary or higher order alloy of one of more alloying components. By way of an example, an alloy of between approximately 63% and 84% Ni and approximately 16% and 37% Fe (both by weight percent) with additions of manganese (Mn), tungsten (W), molybdenum (Mo), chromium (Cr), or combinations thereof, not exceeding 10% by weight of the alloy, provided at least one characteristic beneficial to the processing of the alloy into an electrical resistance strain gage. The individual additions of l%-5% Mn, 0.5%-2% W, l%-5% Mo, and l%-5% Cr were observed to improve processing of the alloy into a high gage factor material. Again, it is theorized that at least some of the alloying additions facilitated ordering of the Ni and Fe atoms into a face-centered-cubic lattice 300 illustrated in Figure 3.

[0021] In addition to providing an electrical resistance strain gage with a gage factor (GF) greater than 5, these strain gages have an enhanced signal to noise ratio, which may advantageously enhance the ability to measure low levels of strain previously difficult to measure with an electrical resistance strain gage. Additionally, the high gage factor (GF) makes it possible to achieve the same level of fractional resistance change (AR/R) as a conventional prior art strain gage such as Cu-44Ni-2Mn but at substantially lower applied strain level (ε). This can provide several advantages including robustness and stability of the measurement system further comprised of the transducer spring element to which the high gage factor strain gage is attached.

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