Strain Gage: Materials
engineering fundamentals Strain Gage: Materials
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A typical strain gage attachment consists of three major parts: the wire, the backing, and the adhesive.

Wire

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.
  • Select Constantan (Advance, Copel) alloy wire, if
  - Applications are mainly static or quasi-static strains.
  - Applications may encounter plastic deformation (large elongation).
  - No extreme operating conditions, such as high temperatures, are present.
  - Self-temperature-compensation is preferred.
  - Cost is an issue. Constantan is the most widely used and probably the least expensive gage wire material.
  - One does not know what else to choose.  
  • Select Isoelastic alloy wire, if
  - High signal-noise ratio is needed.
  - The goal is to measure dynamic strains.
  - The application is under Fatigue (cyclic) loading.
  - Temperature is well controlled, e.g., in an air-conditioned laboratory.  
  • Select Karma alloy wire, if
  - Applications involve low temperature environment, as low as -269°C/-452°F, or temperatures that may vary during the measurement.
  - Self-temperature-compensation is preferred.
  - Soldering is NOT an issue. Karma alloy is usually more difficult to solder.  
  • Select Platinum based alloy wire, if
  - The measurements are made at high temperatures(>230°C/450°F).

Backing (Carrier) Materials

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.
  • Select polyimide backing, preferably 0.025 mm (0.001 in) or thinner, if
  - No extreme operating conditions such as high temperatures, are present.
  - Mostly static strain is encountered.
  - There are no other constraints. Polyimide is the "standard" backing material.
  • Select epoxy backing, if
  - One wishes to minimize the error induced by the backing.
  - Installation and handling are well understood. Epoxy backings are brittle and require skilled workmanship to install.
  • Select glass fiber reinforced epoxy backing, if
  - The application is under Fatigue (cyclic) loading.
  - Applications expose strain gage to moderate temperature (~400°C/750°F).
  • Select strippable backing, which will be removed during installation and the adhesive will be used as an insulator, if
  - Applications expose strain gage to high temperature.

Adhesives
Adhesives are used to secure the strain gage to the workpiece.

  • Select Cyanoacrylate cement (e.g., Eastman 910 SL), if
  - Applications are needed as soon as possible, ~ 10 min after bonding. (A gentle clamping pressure is needed for the first 1 to 2 minute.)
  - Applications do NOT last a long period of time (months).
  - One does not know what else to choose.  
  • Select epoxy, if
  - Higher bond strength is desired.
  - Higher strains at failure need to be measured.
  - 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.
  - Sufficient time is available to complete the curing process. A temperature of 120°C/250°F is applied for several hours to complete polymerization.  
  • Select ceramic cement (e.g., NBS-x-142, flame-spray BLH-H rod), if
  - High temperature environment, upto 980°C/1800°F is expected.  
  • Select cellulose nitrate cement (e.g., Duco, SR-4), if
  - Paper backed strain gages are used.
  - Temperature of 54°C/130°F, usually by blowing warm air, can be applied for curing and removing solvents.
  - The environment is fairly dry i.e. no water or high moisture is present.

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