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A typical strain gage attachment consists of three major parts: the wire, the backing, and the adhesive.
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| Wire |
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| The material construction of the wire directly affects the sensitivity of the strain gage. Further discussion on sensitivity can be found in the strain gage sensitivity page. |
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Select Constantan (Advance, Copel) alloy wire, if |
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Applications are mainly static or quasi-static strains. |
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Applications may encounter plastic deformation (large elongation). |
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No extreme operating conditions, such as high temperatures, are present. |
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Self-temperature-compensation is preferred. |
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Cost is an issue. Constantan is the most widely used and probably the least expensive gage wire material. |
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One does not know what else to choose. |
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Select Isoelastic alloy wire, if |
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High signal-noise ratio is needed. |
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The goal is to measure dynamic strains. |
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The application is under Fatigue (cyclic) loading. |
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Temperature is well controlled, e.g., in an air-conditioned laboratory. |
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Select Karma alloy wire, if |
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Applications involve low temperature environment, as low as -269°C/-452°F, or temperatures that may vary during the measurement. |
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Self-temperature-compensation is preferred. |
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Soldering is NOT an issue. Karma alloy is usually more difficult to solder. |
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Select Platinum based alloy wire, if |
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The measurements are made at high temperatures(>230°C/450°F). |
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| Backing (Carrier) Materials |
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| Backing (carrier) materials are needed, because strain gage wires are very fragile and difficult to handle. The backing is usually made of a dielectric (usually plastic) which provides a good electrical insulation between the wires of the strain gage and the specimen. Polyimide is the prevalent backing material, and is thus the default standard. |
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Select polyimide backing, preferably 0.025 mm (0.001 in) or thinner, if |
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No extreme operating conditions such as high temperatures, are present. |
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Mostly static strain is encountered. |
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There are no other constraints. Polyimide is the "standard" backing material. |
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Select epoxy backing, if |
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One wishes to minimize the error induced by the backing. |
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Installation and handling are well understood. Epoxy backings are brittle and require skilled workmanship to install. |
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Select glass fiber reinforced epoxy backing, if |
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The application is under Fatigue (cyclic) loading. |
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Applications expose strain gage to moderate temperature (~400°C/750°F). |
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Select strippable backing, which will be removed during installation and the adhesive will be used as an insulator, if |
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Applications expose strain gage to high temperature. |
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| Adhesives |
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| Adhesives are used to secure the strain gage to the workpiece. |
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Select Cyanoacrylate cement (e.g., Eastman 910 SL), if |
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Applications are needed as soon as possible, ~ 10 min after bonding. (A gentle clamping pressure is needed for the first 1 to 2 minute.) |
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Applications do NOT last a long period of time (months). |
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One does not know what else to choose. |
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Select epoxy, if |
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Higher bond strength is desired. |
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Higher strains at failure need to be measured. |
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A clamping pressure, 350 kPa (50 psi) for thin bond lines is used. A clamping pressure of 35-140 kPa (5-20 psi) in general, can be applied during the curing process. |
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Sufficient time is available to complete the curing process. A temperature of 120°C/250°F is applied for several hours to complete polymerization. |
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Select ceramic cement (e.g., NBS-x-142, flame-spray BLH-H rod), if |
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High temperature environment, upto 980°C/1800°F is expected. |
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Select cellulose nitrate cement (e.g., Duco, SR-4), if |
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Paper backed strain gages are used. |
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Temperature of 54°C/130°F, usually by blowing warm air, can be applied for curing and removing solvents. |
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The environment is fairy dry i.e. no water or high moisture is present. |
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