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Tensile test according to ISO 6892
The two methods of the standard
The test standard ISO 6892-1 is related to metallic materials. It is similar to ASTM E8/E8M but not identical. It describes three different test procedures:
Method A describes the strain control mode.
A1 is in closed loop control,
A2 is in open loop mode.
Method B describes the stress control mode.
ISO 6892-1 deals with the tensile test at room temperature
ISO 6892-2 deals with the tensile test at elevated temperature
Tensile test according to ISO 6892-1
Method A (strain-controlled test procedure)
Method A1: ‘closed loop’
A closed loop is tested here. An extensometer is required for this. The strain rate of the extensometer signal is controlled.
Method A2: ‘open loop’
Testing is performed with an open loop.
The strain rate is estimated using the parallel specimen length:
Traverse speed v = Lc * dε/dt
The following applies from the start to the fulfilment of ReH, Rp and Rt:
Strain control with dε/dt = 0.000 07 s-1 ±20%
Then (ReL, Ae, Rm, Ag, Agt, A, At, Z) applies:
Strain control with dε/dt = 0.00 2 s-1 ±20%
The AGX-V universal testing machines from Shimadzu are ideally suited for this test method.
Method B ( stress-controlled execution of the experiment)
In method B, the initial test speed is made dependent on its elasticity.
Until Rp0.2 is determined, the following applies:
- When E < 150 GPa: –> 2 to 20 MPa/s
- When E ≥ 150 GPa: –> 6 to 60 MPa/s
From Rp0.2 applies:
- Strain rate < 0.008 s−1.
For this method, it is best to use the AGS-X or EZ-X material testing machines from Shimadzu.
And what about the displacement regulation?
The conventional displacement control in mm/min does not directly correspond to methods A or B of ISO 6892-1, as this standard primarily bases the control of the test speed on strain and stress rates.
However, it is most similar to method B (stress-controlled method). In this mode, the test speed is indirectly controlled by the stress rate, which in turn can be accompanied by a constant crosshead speed.
Strain control explained in 7 steps:
Step 1:
Elongation = change in length / initial length.
If the initial length L0 = 100mm and the change in length = 1mm, then the elongation is ε= 1/100 = 0.01
Step 2:
Strain rate dε/dt = change in strain / change in time.
If the test is run at a constant strain rate of 0.01 s-1 , this corresponds to 1mm/s (=60mm/min) at this moment.
Step 3:
If the sample is 10% elongated, then L=110mm.
At this point, 1mm elongation is still 1/110.
The elongation rate is therefore dε/dt= 1/110 = 0.00909 s-1
Conclusion: The test speed becomes slower.
Step 4:
However, since you want to get faster and faster during the test, you define constant strain rates in ISO 6892, e.g. 0.00025 s-1 .
Step 5:
If at L=100mm dε/dt = 0.00025 s-1,
then the test speed is v = 100 mm * 0.00025 s-1 = 0.025mm/s =1.5mm/min.
And with L=110mm dε/dt = 0.00025,
the test speed v = 110 mm * 0.00025 s-1 =1.65mm/min.
Step 6:
As this speed is relatively slow, the standard allows you to accelerate to a higher speed (0.0067 s-1 ) after determining the elasticity parameters.
Step 7:
Since the elastic range is usually not that large, let’s assume that the sample is still about the same length after the elastic characteristic values, i.e. 100mm. Then dε/dt = 0.0067 s-1 corresponds to a speed of approx. v=40mm/min.
If the sample length becomes L=150mm at some point, then v = 60mm/min.
So the traverse speed becomes faster and faster.
Discussion
Method
The test speed is a long discussed topic of this standard. Whereas in earlier standards it was simple speeds in mm/min, in ISO 6892 it is the strain rates. The strain rate is the strain per unit of time. The standard contains a large number of conditions for which different elongation rates are to be applied. A detailed reproduction of these conditions is omitted here and reference is made to the standard.
However, we want to shed light on a few practical aspects: At the beginning of the tensile test, characteristic values such as modulus of elasticity, yield strength and proportional limit are determined. Especially with hard materials with a high modulus of elasticity, a considerable level of force is achieved very quickly. In order to obtain a sufficient number of measuring points, the tensile test is run slowly in this area. So that a test does not take long, it is accelerated afterwards. Now it depends on the material whether you see this transition on the stress-strain curve. If the material is strain rate sensitive and you switch speeds quickly, you will see a kink in the curve. This is unsightly for the curve, but provides information on the strain rate sensitivity of the material. With a slower speed change, this kink can be made to disappear and the curve looks “nice”.
The question of right or wrong swings to the application of the measurement results. What are they used for? What strain rate is present at the location for which you want to make a statement with the measured value?
This question leads us to another topic, standards in general. ISO 6892 is a rather general standard that does not deal with the intended use of the determined values. It is focused on minimising the scatter of measurement results. Standards, on the other hand, which are written by the car or aircraft industry, for example, are more application-oriented.
Universal testing machines offered by TeMeCo:
Universal testing machines AGX-V
Shimadzu EZ-X materials testing machine
Tensile testing machine Compression testing machine AGS-X
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