Strain rate according to ISO 6892-1 tensile test method A
Expert article by Mr. Richter DEW – Krefeld
The Tensile test / tensile test according to DIN ISO 6892 is one of the most important tests in materials testing. Mr. Christian Richter - Deutsche Edelstahlwerke in Krefeld - has written a technical article on this topic, which we are happy to make available to you here.
The ISO 6892-1 standard strongly recommends carrying out this tensile test using Test speed as strain rate. The sample is lengthened at a defined rate per second – the Tensile testing machine lengthens (stretches) the sample according to the extension rates specified in the standard. According to method A, the methodology below should be used as a reference value
|
Speed 1: 0.00007 mm/mm/sec. |
critical materials (e.g. aircraft industry) |
|
Speed 2: 0.00025 mm/mm/sec. |
recommended (standard in the elastic range) |
|
Speed 3: 0.002 mm/mm/sec. |
critical materials (e.g. aircraft industry) |
|
Speed 4: 0.0067 mm/mm/sec. |
recommended (standard in the plastic sector) |
| strain rate according to ISO 6892-1:2017-02 | Speed index e.g.: A224 |
|
Elastic strain (in the Hooke's line / Young's modulus) |
A2XX = 0.00025 mm/mm/sec. “Closed Loop” reference variable extensometer cover Le |
|
yield point range (in the range of Lüders strain) |
AX2X = 0.00025 mm/mm/sec. “Open Loop” = reference value traverse path reference Lc |
|
plastic strain (rapid sample elongation according to Ae / Rp1,0) |
AXX4 = 0.0067 mm/mm/sec. “Open Loop” reference value traverse path reference Lc |
Unless the Tensile testing machine If it is not technically possible to use strain control using an extensometer as a control method, the standard allows other systems. These are explained in the following article.
tensile test according to DIN EN ISO 6892
Tensile testing is one of the most important test methods in materials testing. It is used to determine the strength and deformation characteristics of materials, which are an essential basis for determining the intended use or the dimensions in the design. The currently valid standard for metallic materials is DIN EN ISO 6892-1, the scope of which has now grown from 21 pages in the previous document to 81 pages. Some innovations deal with the test speed and the determination of measurement uncertainty.
Test speeds A strain rate and B stress rate
The test speed is divided into method A and method B. Method A is the strain rate, which can be used to carry out the test as an alternative to the already known stress rate (previously known as stress increase rate). The permissible range for the strain rate according to method A is greatly reduced in order to ensure the comparability of the results. Table 1 below provides an overview of the common test standards for the tensile test and the normative test speeds.
Table 1: standard test speeds
|
Standard |
Speed |
Speed |
Speed |
|
ASTM E8 |
0,00025 ± 40% |
1,15 – 11,5 |
max. 0,008 |
|
ASTM A370 |
max. 0,001 |
1,15 – 11,5 |
max. 0,008 |
|
ISO 6892-1 |
0,00025 ± 20% |
6 to 60 (steel) |
0,0067 ± 20% |
|
DIN EN 2002. |
0,00008 ± 40% |
not permitted |
max. 0,0017 |
One possibility strain-controlled speed is to use the extensometer signal (closed loop). However, this requires a modern testing machine control system. If these requirements are not met, Appendix F allows a formal estimation of the test speed according to method A using the crosshead speed. Equation 1 takes into account the stiffness of the sample and the test setup.
Formula for estimating the strain rate Method A using the crosshead speed
Cm: stiffness of the test setup
Lc: test length
m: the slope of the stress/strain curve at the moment of interest (Rp)
So: the initial cross-section
vC: traverse speed
em: strain rate
Setting the right traverse speed should be secured by validation procedures. Changing sample geometries, clamping devices or materials influence the stiffness of the system, which has an impact on the strain rate and thus the crosshead speed.
measurement uncertainty calculation tensile test
The second important addition to 6892-1 is the description of the Measurement uncertainty in Appendix J. The main focus is exclusively on the uncertainty of the test and not on the inhomogeneity of the material. For this reason, it is recommended to use certified reference material with information on the reference value and its measurement uncertainty to determine the measurement uncertainty of your own process.
In order to determine your own measurement uncertainty, the influence of the test equipment on the characteristic values must be investigated. Table J.1 in the appendix to ISO 6892-1 helps with this. The determination of the tensile strength requires the maximum force and the initial cross-section of the sample, while the determination of yield strengths the extension of the sample and the initial measuring length must also be taken into account.
The example shown on the right briefly shows how the measurement uncertainty for Rm can be determined. The uncertainty uRm results from the Gaussian error propagation.
By inserting and forming we get:
The factors of uFm and uSo are also called sensitivity coefficients cFm and cSo. This results in a combined measurement uncertainty for the tensile strength according to equation 4.
The value uFm can be taken from the calibration certificate. The error referred to as type B in the standard is determined using the rectangular distribution. For an error of 1%, the following uncertainty results according to equation 5:
In order to obtain the uncertainty for uSo, the Gaussian error propagation must be applied analogously to equation 2. Consequently, the equations for rectangular samples and round samples differ. Equation 6 describes the measurement uncertainty for round samples.
The smallest step that still has to be taken into account is the determination of the initial diameter. Its uncertainty udo results from the resolution a of the measuring tool (rectangular distribution) and the possibility of tilting during the measurement (triangular distribution).
Up to this point, only type B has been used to estimate measurement uncertainty. When examining reference material, 5 samples are usually examined. The uncertainty of repeated measurements is also called type A and is also included in the uncertainty analysis. Here, t is the Student factor for the confidence interval of 68%, s is the standard deviation of the measurements and n is the number of measured values.
This ultimately results in an estimate of the combined measurement uncertainty for Rm according to equation 9. The k-factor typically extends the confidence interval for laboratories by k=2 for 95%.
With the standard 6892-1 and the additions regarding test speed and measurement uncertainty, the requirements for the testing machine and operator have increased despite the increasing automation of testing. What is particularly important is stable, precise and correct test results, which should be ensured by analyzing and evaluating the influencing factors and restricting the permissible test parameters. Such validation processes and an assessment of the measurement uncertainty are also necessary in view of the growing need for accreditation. Testing reference material and participating in round robin tests help with this. Some descriptions in ISO 6892-1 are intended to support the operator in this. It is also to be expected that some appendices, which have so far only been informative, will take on a normative character.
