Young’s Modulus is the slope of the Stress v Strain curve measured from the origin of the curve. The initial strain in the test sample is very small so that any movement in the machine itself, in the load cell or any “take up” movement in the grips is large in comparison to the initial strain in the test sample. Crosshead movement, which is derived from an optical encoder on the motor shaft, includes all these movements which although small, are relatively large in comparison to the initial strain in the test sample. Measuring Modulus or Proof Stress using crosshead movement will result in extremely large errors. An extensometer must be used. This is attached to the test sample and measures the strain solely in the test sample itself.

Proof Stress is also derived from the initial slope of the stress strain curve displaced by typically 0.1% or 0.2% along the x axis. This means that if the initial slope of the stress strain curve is wrong, the Proof Stress measurement will also be wrong.

For accurate Flexure or Compressive Modulus measurements, especially on rigid materials a flexure or compression unit incorporating an LVDT (Linear variable differential transformer) is recommended for the reasons mentioned above.

Please Note

The above notes are intended for student use and are written in simple easy to read style. They are not intended to be text book definitions which readers may wish to refer to for further information.

Tinius Olsen/Hounsfield QMat Users

Qmat software has built in protection to prevent users attempting to generate a test to measure Young’s Modulus or Proof Stress using crosshead movement. However the system may be “unlocked” by clicking on the padlock icon in the top right hand corner of the test generator screen. A warning will automatically come up warning the user of the risks in proceeding. Using crosshead to measure Young’s Modulus/Proof Stress is only useful as a comparison between test samples rather than an absolute/accurate measurement.