Strain, a measure of deformation, is typically measured using devices that quantify how much an object changes shape when subjected to a force. Here's a breakdown of how it's measured using traditional methods, focusing on insights from the provided reference:
Traditional Methods of Strain Measurement
The primary traditional methods for measuring strain are extensometers and strain gauges. Both of these methods have the common requirement of needing physical contact with the material being tested. This contact is necessary to accurately measure the deformation.
Extensometers
- Function: Extensometers measure the change in length over a specific distance. They are essentially precise measuring devices designed to track how much a material stretches or compresses.
- Measurement Length: According to the provided reference, extensometers usually measure strain over a relatively long section of material, typically up to 25 mm or even more. This means they provide an average strain value over that length.
- Directionality: Extensometers are directional, meaning they measure strain only along the direction in which they are applied. If the material deforms in a different direction, the extensometer will not measure that change accurately.
- Example: If you stretch a rubber band and want to know how much it lengthened, you could attach an extensometer to it to measure the change in length.
Strain Gauges
- Function: Strain gauges are small, thin devices that are attached directly to the material. They work based on the principle that the electrical resistance of a wire changes when it is stretched or compressed.
- Measurement: As the material deforms, the strain gauge deforms with it, and this deformation changes its electrical resistance, which is measured. This change is then converted into a strain value.
- Directionality: Like extensometers, strain gauges are also directional, measuring strain only along the direction of their alignment.
- Example: Strain gauges are widely used in civil engineering to monitor the stress and strain on bridges and other structures. They are also used in various engineering applications where precise deformation measurement is crucial.
Comparison of Traditional Methods
| Feature | Extensometer | Strain Gauge |
|---|---|---|
| Contact | Requires physical contact | Requires physical contact |
| Direction | Directional | Directional |
| Length | Measures over a larger length (up to 25 mm or more) | Measures locally at the point of contact |
| Method | Measures change in length directly | Measures change in electrical resistance |
Challenges of Traditional Methods
- Physical Contact: Both extensometers and strain gauges require physical contact with the specimen, which can sometimes be an issue, for example when measuring the strain of very small specimens or where contact is not possible.
- Directionality: The directional nature of these tools requires precise alignment and placement to measure strain in the desired direction.
- Local vs. Average Strain: Extensometers measure strain over an average length, while strain gauges measure strain locally. This must be considered when choosing the appropriate measurement method.
