Gauge length measurement depends on the material being tested and the relevant ASTM or ISO standard. Here’s a breakdown of how it's typically determined:
Gauge Length Measurement Methods
Essentially, gauge length is the original length of the specimen over which you're measuring changes in length (strain) during a mechanical test. According to the provided information, there are two primary methods used:
1. Using the Distance Between Grips of the Universal Testing Machine
- In this approach, the gauge length is defined as the distance between the grips that hold the specimen in the testing machine.
- This method is straightforward and suitable for many types of tests, especially when the strain is expected to be relatively uniform across the entire length of the specimen between the grips.
- Practical Insights: This technique is often used in tensile testing of materials where the overall deformation of the specimen is of interest.
2. Using Strain Measurement Devices
- Here, the gauge length is defined by the portion of the specimen being measured by a specific strain-measuring device.
- Examples of such devices:
- Non-contacting video extensometers are common. They use cameras and image processing to track marks on the specimen, allowing for highly accurate strain measurement without touching the material, which is crucial for fragile materials or during high-temperature testing.
- Clip-on extensometers which use physical contact with the specimen and may induce error or limit the testing of some specimens.
- Why use a strain device? They offer precise localized strain measurements, especially when deformation is not uniform across the specimen's length. This is important for tests where localized stress/strain is expected, like when testing for material properties or fracture mechanics.
Table Summarizing Gauge Length Measurement Methods
| Method | Gauge Length Definition | Typical Use Cases | Advantages | Disadvantages |
|---|---|---|---|---|
| Distance Between Grips | Length between grips of testing machine | General tensile testing, when strain is relatively uniform across the specimen. | Simpler to implement | Less accurate for localized measurements; may include deformation within the grips |
| Strain Device (e.g., video extensometer) | Length measured by the strain device | Measuring strain with high precision, particularly for localized deformation, fragile materials, high-temperature tests | High accuracy, non-contact measurement, good for non-uniform deformation, localized strain measurement | Can be more expensive and complex to implement, requires careful setup and calibration |
Conclusion
Understanding how gauge length is measured is crucial for accurate materials testing. Depending on the standard and test requirement, the gauge length can be defined either by the distance between the grips or by the measurement area of a strain measuring device. The method chosen significantly affects the test results.
