There are two ways to ensure your strain results are accurate: measuring with crosshead movement or measuring with system compliance.
Crosshead movement is measured using a high-resolution encoder. When you move the crosshead with no specimen installed, the reported measurement of that movement is often more accurate than for many extensometers.
However, when you install a specimen and apply a tensile or compressive load, the accuracy of the measurement of crosshead movement becomes dependent upon the “system compliance.”
Compliance refers to the tendency of the various components of a testing system to deflect under load. Consider every component in a testing system as equivalent to a very stiff spring. When you apply a load to that component, even a major item such as a crosshead, it will deflect, either bending, stretching, or compressing. If it is a very stiff spring the deflection is tiny, but still measurable. Compliance is the inverse of stiffness; the more stiff, the less compliant.
There are three sources of compliance in a system: the load frame, the load string components, and the specimen itself.
• The load frame is designed with a very high stiffness. We measure the stiffness at a particular load and publish that figure as part of the specifications of the load frame.
• Load string compliance is usually not known. There may be few or many components in a load string: grips or fixtures, couplings, or one or more load cells. Many components do not have published stiffness values.
• The specimen compliance is usually what you are trying to measure.
As a rule of thumb, if the compliance of your specimen is around 100 times greater than the compliance of the load frame and the load string components, you can assume that the reported crosshead movement is equivalent to the deflection experienced by the specimen. However, if you are testing a very stiff specimen, you should always use an extensometer to measure specimen deflection.
If it is not possible for you to use an extensometer, then you should evaluate the system compliance before the test. Either install an extremely stiff specimen and apply a tensile force, or install compression platens and apply a compressive force with the platens touching each other. The resulting deflection measurement gives a close indication of the system compliance. When you test the specimen, you can remove this value from the result.
Are you experiencing issues when measuring strain? Or do you still have questions? Leave us a comment ....
Crosshead movement is measured using a high-resolution encoder. When you move the crosshead with no specimen installed, the reported measurement of that movement is often more accurate than for many extensometers.
However, when you install a specimen and apply a tensile or compressive load, the accuracy of the measurement of crosshead movement becomes dependent upon the “system compliance.”
Compliance refers to the tendency of the various components of a testing system to deflect under load. Consider every component in a testing system as equivalent to a very stiff spring. When you apply a load to that component, even a major item such as a crosshead, it will deflect, either bending, stretching, or compressing. If it is a very stiff spring the deflection is tiny, but still measurable. Compliance is the inverse of stiffness; the more stiff, the less compliant.
There are three sources of compliance in a system: the load frame, the load string components, and the specimen itself.
• The load frame is designed with a very high stiffness. We measure the stiffness at a particular load and publish that figure as part of the specifications of the load frame.
• Load string compliance is usually not known. There may be few or many components in a load string: grips or fixtures, couplings, or one or more load cells. Many components do not have published stiffness values.
• The specimen compliance is usually what you are trying to measure.
As a rule of thumb, if the compliance of your specimen is around 100 times greater than the compliance of the load frame and the load string components, you can assume that the reported crosshead movement is equivalent to the deflection experienced by the specimen. However, if you are testing a very stiff specimen, you should always use an extensometer to measure specimen deflection.
If it is not possible for you to use an extensometer, then you should evaluate the system compliance before the test. Either install an extremely stiff specimen and apply a tensile force, or install compression platens and apply a compressive force with the platens touching each other. The resulting deflection measurement gives a close indication of the system compliance. When you test the specimen, you can remove this value from the result.
Are you experiencing issues when measuring strain? Or do you still have questions? Leave us a comment ....
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