Needs a precise and stable drive system with high stiffness. Below shows a stress strain curve on nominally similar materials, one tested on a high stiffness frame and the other on a lower stiffness frame. Using an estimated strain method calculation, both tests are run at a constant crosshead speed of 2.25 mm/min. There was a 21% difference in ‘specimen speed’ (expressed in mm/min), which lead to a 5% difference in the yield result.
The graph below shows the specimen speed difference between a ‘stiff’ system and a less stiff system. At the start of the test it is shown that the crosshead movement is transferred into strain on the specimen very quickly, whereas on the weaker system it takes longer. This is due to the machine/load cell/grips deflection, which causes that movement to not be transferred to the specimen. If both machines where using strain control then the results would be much more comparable, but the control will likely be more challenging on the weaker system.