The electrical resistance of a length of wire varies in direct proportion to the change in any strain applied to it. That’s the principle upon which the strain gauge works.
The most accurate way to measure this change in resistance is by using the Wheatstone bridge. This is a balanced electrical circuit which displays any change on an indicator or feeds it into a process.
The main components of a strain gauge is a strain sensitive alloy. The most common is constantan at a thickness of 0.0001 inch, which is used in the foil grid. Constantan also has the best combination of properties necessary for many strain gauge applications.
The grid consists of a photo-etched pattern mounted on a very thin backing made from a plastic such as polyimide, epoxy or glass-fibre reinforced epoxy-phenolic approximately 0.001 inch thick.
This backing allows the strain gauge to be handled during installation.
It also provides a ready-to-bond surface for cementing the gauge to the specimen, and electrical insulation between the metal foil and the test piece.
Depending on the particular application, there’s a wide range of foils and backings to choose from.
There are also many factors to consider when selecting a gauge.
These include temperature range, test frequency, elongation, environment, resistance, and so on.
Because there are literally thousands of different strain gauge patterns available, it is vital for an experienced strain gauge engineer to select the most suitable to make sure you get the optimum performance from your installation.
WHERE CAN I USE STRAIN GAUGES?
Strain gauges are used in an increasing number of applications in all kinds of industry world-wide. For example:
- In load cells for weighbridges, scales, hoppers, vehicles and in medical and educational applications.
- For monitoring structures such as bridges and buildings.
- In research and development applications, including automotive, aerospace, medical, process, oil and gas, and power generation.
- And in virtually every other sector of industry.
Strain gauges can be installed in extreme conditions:
- Temperatures from cryogenic (-269°C) to 1300°C.
- Radiation areas.
- Areas of extreme vibration.
- Applications involving high elongation.
So there are very few locations where a strain gauge can’t be used. But the selection of the strain gauge and its consumables is critical for successful long term testing.
INTRODUCTION TO STRAIN GAUGING
Choosing the strain gauges and consumables is just the start. Next comes installation, which is labour-intensive and depends on the skill of the operator.
The whole process can be broken down into a number of basic stages:
- Inspecting the test specimen, especially the strain gauge location and surface finish.
- Ensuring that the operator has a full set of installation requirements, a job file and an understanding of the test specimen limitations, such as temperature limits.
- Preparing the strain gauge and organising bonding clamps, if necessary.
- Preparing the test specimen for surface cleansing.
- Marking out the strain gauge position.
- Chemically cleaning the surface of the test specimen.
- Positioning the strain gauge on the test specimen.
- Applying adhesive and applying clamping pressure if a heat curing adhesive is used.
- Unclamping and inspecting the installation and, if necessary, post curing the installation.
- Inspecting the installation again and beginning wiring.
- Inspecting the wiring and checking for resistance, leakage and micro strain out of balance.
- Applying the coatings, ensuring that the correct cure times are used.
- Completing the strain gauge installation sheet. Click here to see an example.
This is only a brief run through the installation process, but it gives a flavour of what is involved.
Tip: It’s always useful to take photographs during installation for the job file. Not only does this help to complete the report, it also provides some dimensional detail.