Metals test type only. Straining rate is a specified rate of strain of the parallel section of the specimen. It is run in extension control. The specified straining rate converts into an equivalent crosshead speed by multiplying the strain rate by the gauge (parallel) length of the specimen. For example, a rate of 1% per minute on a specimen having 100mm parallel length results in a crosshead speed of 1mm/min. The Metals test type calculates an appropriate position rate that is in close proximity to the target strain rate and controls the crosshead movement at that rate throughout the ramp in which it applies. The actual strain rate that is achieved is affected by the specimen compliance. The calculation for straining rate is: Strain rate * Parallel length = Position rate
This calculation is only valid in the plastic region (or yielding region) of the stress-strain curve, where the majority of crosshead displacement translates into permanent specimen deformation. Because the position rate is fixed for the entire ramp and is calculated without accounting for possible compliance factors, it is not recommended for the elastic portion of the test. Since straining rate is only appropriate in the plastic region of a test, it should only be selected as the control mode in either Ramp 2 or Ramp 3.
If you use straining rate during the elastic portion of the test, it is strongly recommended that you instead use strain rate (Adaptive) with an extensometer on electromechanical systems or use strain control on servohydraulic systems. The main advantage of the straining rate is that gain value settings are unnecessary since it is essentially in position control and it does not require an extensometer. However, if you need very accurate strain rates, you should use either Strain Rate (Adaptive) or Strain 1 / Strain 2 control. Both of these control modes rely on feedback from an extensometer. Note: Since the test is in extension control, the gauge length used to calculate the strain is the parallel length of the specimen and not the gauge length of the extensometer.