The imminent release of ISO 6892-1:2009 signifies a new level of testing-machine control and is intended to improve the reproducibility of test results obtained from a tensile test on metal.
This new specification focuses particularly on closed-loop strain control of the testing machine while recording data for results in the elastic and elastic-plastic transition zones.
This affects results such as proof stress (for example Rp0.2), also known as offset yield, and upper yield (ReH).
The accuracy of the determination of ReH and Rp0.2 has a direct bearing on the calculations used by structural engineers involved in the building industry or engineers working in collaboration with testing institutes.
Until now, variables such as different specimen grip-types, testing-machine load-frame compliance, drive-system response, control electronics and specimen stiffness, all contributed to differences in results between different testing machines and inter-laboratory tests around the world.
Closed-loop strain control is recognised as the best type of machine control to obtain results.
The closed-loop strain-control system of a testing machine is complex and requires synergy between all major system components and in some cases the skill of the machine operator.
It requires the testing system to monitor the strain signal, which comes from the extensometer.
This device measures the increase in gage length of the specimen as it is subjected to load.
This signal is then compared with a time base and the drive controller constantly adjusts the speed of the crosshead to maintain the required strain rate.
The new standard requires the strain rates to be controlled to a value of +/-20 per cent, which translates to +/-4um/s at strain rate of 0.025 per cent/s based on 80mm gage length.
Zwick's Testxpert software can achieve all of this with a few mouse clicks.
This allows users to obtain more reliable and reproducible test results, especially for materials that are strain-rate sensitive.
The standard explicitly states that an extensometer must be used to measure the specimen strain and the resulting strain rate must be controlled and maintained up to and around the characteristic being measured.
For Rp0.2 this is relatively straightforward, while for ReH testing, machine manufacturers have had to implement complex algorithms that can handle the inherently unstable stress / strain data as the specimen transits from the elastic to plastic zone (or behaviour).
Failure to correctly recognise this transition point introduces errors in ReH.
For testing machines where closed-loop strain control is not possible, the standard allows a position-controlled variant where the crosshead speed must be pre-selected to achieve the desired strain rate.
However, this is time-consuming and requires the determination of the system stiffness and specimen stiffness at the characteristic point to be measured.
As a result it requires a number of pre-tests and additional specimens to set up the machine-control parameters.
The following topics are also prerequisites for carrying out tests correctly to the new ISO standard: the testing-machine drive units must have a high-resolution positioning technology so that small displacements can be traversed slowly and smoothly.
To achieve this requires high resolution control and AC motors without gears or brushes, offering the additional benefit of being wear-free and avoiding commutation effects or torque ripple at very low speeds.
The wide speed range of AC motors, typically achieving crosshead speeds from 0.01um up to 2000mm/min or more, also allows high-speed crosshead return or high speeds for other tests carried out on the same machine.
Compared with older technology systems, the performance of the latest technology enables materials-testing machines to cycle continuously at maximum speed over the full load range of the testing machine without overheating.
Closed-loop control requires perfect synergy between all components of the testing machine, and the controller is no exception as it forms the important link between the mechanical components of the test frame and the control-software algorithms.
Zwick's Testcontrol system can handle multiple data-acquisition channels as well as control multiple drive units, for example when adding torsion applications to a normal test machine.
Its onboard firmware simultaneously handles complex real-time tasks such as strain control, synchronised data-acquisition and the monitoring of all safety systems.
The requirements of the upcoming standard are already integrated into Zwick's Testxpert II software.
Easily activated, if required, it means that customers can switch to the new algorithm as soon as the standard is released.
Customers with alternative specifications can use the many other control functions built into the system, for example position control, or load / stress control.
Once configured for the spectrum of specimen to be tested, there is no need for the operator to make pre-selections, as the software automatically searches for the appropriate test-material characteristic, slope of elastic part, proof stress or yield stress, and controls the machine until the desired result has been achieved.
After that, the speed is automatically switched to that specified in the next test phase of the test.
The software has been developed for use with Zwick's Robotest testing systems as these systems must be able to make real-time decisions during a testing sequence.
The adaptive controller adjusts the response of the drive system according to the required strain-rate setting, specimen geometry and load-frame compliance.
The Testxpert II software also includes the international Tenstand algorithm-validation system.
The recommended gripping solution for metals testing using closed-loop strain-control is to use parallel-acting hydraulic grips.
The benefits include: high clamping forces at the start of the test, no slip-stick effects as with poorly maintained wedge grips and good specimen clamping and alignment throughout the entire test.
