The Challenges of Strain Control
High-precision device with stable feedback
Need to securely grip the specimen during the test without slippage, preferably with high stiffness. The graph shows how different types of gripping devices can effect the system stiffness, and how a machine in strain control would need to compensate.
No vibration or shock can be transmitted onto the testing system and specimen
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.
Software and Controller
Responsive control loop to maintain close tolerances on strain rate, with the ability to tune the gain settings as the stiffness changes when the specimen yields
A proportional specimen and a proportional gauge length extensometer are ideal. In reality, a specimen with good gauge length to parallel length ratio is well suited to minimize the strain seen outside of the gauge length, allowing the control to be more stable.
If your specimens vary from discontinuous yielding to continuous yielding it is important to change control methods for each type. As local yielding can occur outside of the gauge length on discontinuous yielding material, it is impossible to control from the strain feedback and should be in crosshead speed control during yield point elongation (YPE/Ae).