The imminent release of ISO 6892-1 :2009 signifies a new level of testing machine control 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 whilst recording data for results in the elastic and elastic-plastic transition zones. This affects results such as proof stress (e.g. 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, response of drive system, control electronics and specimen stiffness contributed to differences in results between different testing machines, and inter-laboratory tests around the world. Closed loop strain control is now internationally agreed to be the better type of machine control to obtain results with a high level of confidence.
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 to a time base, and the drive controller constantly adjusts the speed of the crosshead in order to maintain the required strain rate.
The new standard requires the strain rates to be controlled to a value of ±20% which translates to ±4µm/s at strain rate of 0.025%/s based on 80mm gage length. All of this can now be achieved with a few mouse clicks using Zwick’s testXpert® software. The benefit to users is that it will now be possible to obtain more reliable and reproducible test results especially for materials which 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 straight-forward, whilst for ReH testing machine manufacturers have had to implement complex algorithms which can handle the inherently unstable stress / strain data as the specimen transits from the elastic to plastic zone (or behavior). 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 in order 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 in order 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 and 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 0.01µm 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 to 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.
As mentioned earlier in this article 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 is based on decades of testing experience and is able to handle multiple data acquisition channels as well as control multiple drive units, for example when adding torsion applications to an normal test machine. Its onboard firmware simultaneously handles complex real-time tasks such as strain control, synchronized data acquisition, and the monitoring of all safety systems.
The requirements of the upcoming Standard are already totally 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 preselections as the software automatically searches for the appropriate test material characteristic, slope of elastic part, Proof Stress, or Yield Stress, and controls the machine accordingly 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 industry leading roboTest® testing systems as these robotic testing 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.
During the last twenty five years most organizations carrying out tensile tests on metals use digital extensometers which can measure both elastic and plastics strain up to specimen failure. The thousands of these devices in the market allow for gage lengths from 10 to 200mm, and with high accuracy and measurement resolutions of up to 0.12µm, achieve Class 0.5 according to ISO 9513 or ASTM E 83 class B-1. The benefit of these devices is that no operator involvement is necessary and this improves the reproducibility of test results. With the advent of the new Standard these extensometers facilitate better strain control due to better and more consistent alignment as well as their measurement precision. An additional benefit is that time is also saved which improves the test throughput.
Specimen Gripping Systems
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.
In summary, Zwick believes that its presence on the International Standard Committee enables it to remain ahead of the market, and that it is the first materials testing producer to integrate the requirements of ISO 6892-1:2009 into its products. It offers its customers the opportunity to lead in their own market sectors and begin working to the new standard as soon as it is released.