Metals tensile test to ISO 6892-1 and ASTM E8/E8M: Understanding the differences and benefiting from them
Steel manufacturers, aluminum manufacturers, OEMs and testing laboratories typically perform metal tensile tests as part of quality assurance. Material properties that are crucial for the strength, safety, and reliability of components are tested. At the same time, it must be ensured that the respective test is performed according to standards – depending on customer requirements or market access, to ISO 6892-1, ASTM E8/E8M, or both standards. This is because both standards are important worldwide and are therefore used in parallel in many laboratories.
The goal is the same in all cases: The tensile test should provide consistent and reliable results that are traceable and reproducible. However, this is exactly where the challenge begins, especially for laboratories that regularly perform tests according to both standards. This raises the question: Can results to ISO 6892-1 and ASTM E8/E8M be compared with each other – and if so, under what conditions?
Two standards, one testing objective – but not always the same results
Some laboratories that test according to both standards deliberately take advantage of the similarities between the standards: They define parameters identically where both standards allow it, thereby creating a better basis for reproducible results and more efficient processes.
Because those who know the essential differences can gain real advantages in everyday laboratory work. Results can be better classified, possible deviations become more understandable, and test sequences can be designed more deliberately. Above all, however, it becomes clear where parameters can be defined in the same way to improve reproducibility – and where caution is required because the standards use different procedures or definitions.
Two standardization systems Differences Testing solutions
The two standards at a glance
This article focuses on tensile testing of metals at room temperature according to two key standards:
- ISO 6892-1: Metallic materials – Tensile testing – Part 1: Method of test at room temperature
- ASTM E8/E8M: Standard test methods for tension testing of metallic materials
The standard parts for elevated or low temperatures are not considered here.
Our experts are represented on the relevant ISO and ASTM standardization committees.
Why are there ISO and ASTM standards?
The differences between ISO 6892-1 and ASTM E8/E8M also have to do with the organizations from which these standards originated. ISO is an international organization based in Switzerland in which national standardization organizations of the member countries are represented. ASTM is based in the US and was originally strongly influenced by the US, but today it is also used globally and brings together members from industry, science and public authorities.
For historical reasons, the two organizations differ in their structures, working methods, updating processes, and priorities. This is also reflected in the standards: Both provide a standard for the metal tensile test, but sometimes place different emphases and levels of detail.
Given the global importance of both standards, the push for harmonization was – and remains – obvious. There are already numerous similarities: The basic process of the metal tensile test is the same according to both standards; both use reproducible test systems and equipment such as specimen grips, and with ASTM E8M, the metric variant with SI units was established. A common specimen geometry for flat specimens is also a result of the harmonization.
The testing process itself follows the same logic in both standards: Starting from the semi-finished product, a specimen is prepared, the geometry is measured, the tensile test is performed, and the data is recorded and evaluated. It is precisely this common basic structure that provides a good starting point for further steps towards harmonization.
An overview of the main differences
Even though the basic testing process is similar, ISO 6892-1 and ASTM E8/E8M differ in several points that are relevant in everyday laboratory work. These include, in particular, the specimen geometries, the specimen dimension measurement or determining the initial cross-section, testing system requirements, result definitions, as well as test speeds and control methods.
These differences are not all equally critical. Some mainly concern the designation, others have a direct influence on the determined values. Therefore, it is not only decisive that a difference exists, but also what this difference means for the result.
Specimen geometries Cross-section measurement Testing system requirements Test result Test speed
1. Specimen geometry
For the flat specimen, there is a predefined geometry that is consistent with ISO 6892-1 and ASTM E8/E8M. This common geometry was part of the harmonization and makes it possible to better compare tests under certain conditions.
| Parameter | ISO 6892-1 | ASTM E8/E8M |
|---|---|---|
| Specimen type | B1 | Flat specimen (sheet type) |
| Width | 12.5 mm | 12.5 mm |
| Gauge length | 50 mm | 50 mm |
| Thickness | up to 3 mm | 0.13 to 5 mm |
The overlapping range is therefore for sheet thicknesses from 0.13 mm to 3 mm. In this range, the common sheet type geometry can be used to make test conditions as similar as possible. However, this does not automatically mean that all results are directly reproducible – but it reduces an important influencing factor.
The situation is different for round specimens. Both standards define cylindrical round specimens, and in both cases the gauge length is defined in relation to the diameter. However, ASTM E8 differentiates according to diameters, while ISO 6892-1 takes material groups into account. There is no predefined common round specimen geometry. If comparable conditions are to be created, the gauge length and diameter must be deliberately coordinated with each other.
For special products such as screws, pipes or similar products, separate sections or annexes are sometimes relevant and must be considered.
As soon as different specimen geometries are used, the results are not reproducible – not even if the values are related to the cross-section. Therefore: In order to obtain a reproducible stress-strain behavior, the gauge length and cross-section must be identical.
2. Specimen dimension measurement / determining the initial cross-section
The influence of a cross-section measurement is often underestimated in practice. The initial cross-section has a direct effect on all calculated stress values. If the cross-section is determined differently, the results can also differ – even if the actual test was performed properly.
For ISO 6892-1, measurements are taken at several points along the parallel specimen length to determine the cross-section. The cross-section values are then averaged. An accuracy of ±2% is required for the thickness measurement.
The ASTM E8/E8M takes a different approach here. The measurement is taken at the narrowest point of the parallel specimen length. In addition, absolute values are specified for the accuracy of the thickness measurement: 2 µm for sheet thicknesses less than 2.5 mm, 10 µm for thicknesses from 2.5 mm to 5 mm, and 20 µm for thicknesses greater than 5 mm.
| ISO 6892-1 | ASTM E8/E8M | |
|---|---|---|
| Determination | Measurement at several points of the parallel specimen length and averaging of the cross-sectional values | The measurement at the narrowest point of the parallel specimen length |
| Thickness measurement accuracy | ±2% | Depending on the sheet thickness (< 2.5 mm, 2.5 - 5 mm / > 5 mm): 2 µm / 10 µm / 20 µm |
The difference between the percentage and absolute requirement for the thickness measurement can be significant depending on the sheet thickness. Example: For a sheet thickness of 3 mm, a deviation of up to 60 µm is possible according to ISO 6892-1, while ASTM E8M only allows 10 µm. Such differences can become relevant in audits, for customer inquiries or if results are close to the limit values.
3. Testing system requirements
The basic test arrangement is reproducible according to ISO 6892-1 and ASTM E8/E8M:: A suitable load frame including measurement and control electronics, specimen grips, load cell, extensometer, and software for data evaluation, storage, and analysis are required. This means that the technical basis for tests according to both standards is fundamentally similar and there is no need to use two completely different test systems in the laboratory.
However, there are differences in the required accuracy classes for load cells and extensometers. In addition, the accuracy classes are defined differently, because they are based on different standards: For ISO 6892-1, ISO 7500-1 (force measurement) or ISO 9513 (extensometers), and for ASTM E8, ASTM E4 (force measurement) and ASTM E83 (extensometers).
In our consultations, we consider the specific applications and check which requirements from ISO 6892-1 and ASTM E8/E8M are higher in each case. On this basis, the test equipment is designed in such a way that a testing system reliably covers both standards as far as possible.
4. Test results
There are differences in terminology and determination method for the test results. Many values are very similar and differ only in their designation.
However, caution is advised with respect to elongation at maximum force and elongation at break. The determination of these characteristic values is different.
| ISO 6892-1 | ASTM E8 | |
|---|---|---|
| Gradient of the elastic part | mE | - |
| Yield (elongation) point at 0.2% plastic elongation | Rp0.2 | Yield point (offset = 0.2%) |
| Upper Yield Point | ReH | UYS |
| Lower Yield Point | ReL | LYS |
| Yield point extension | AL | YPE |
| Tensile Strength | Rm | TS |
| Percentage plastic extension at maximum force | Ag | - |
| Percentage total extension at maximum force | Agt | Elu - uniform elongation |
| Strain at break | A - manual and with software | Strain at fracture (manual) |
| Percentage total elongation at fracture | At | Strain at break |
Uniform elongation or elongation at maximum force
The name is almost identical, it is also the same characteristic value, and yet the results are not directly reproducible. This is because the determination of the elongation at maximum force, also known as uniform elongation, is different according to ISO and ASTM.
According to ISO 6892-1, the percentage total extension at maximum force Agtis determined at the point of maximum force.
With ASTM E8, the total elongation at maximum force Elu is determined differently: A horizontal line is drawn 0.5% below the maximum force, and the elongation is read in the middle of this line.
Depending on the material, this can lead to relevant differences in the result.
Another difference: ISO 6892-1 additionally defines the percentage plastic extension at maximum force (Ag), where the elastic elongation is subtracted from the total value. The ASTM does not use this characteristic value.
Strain at break
The total elongation at the time of fracture is determined at the last measuring point before the reduction in force, since this marks the fracture. In ISO 6892-1, the characteristic value is called percentage total elongation at fracture At, ASTM E8 refers to it as elongation at break.
For the elongation at break A to ISO 6892-1, the elastic extension is subtracted so that only the remaining extension is output. Here, the ASTM E8 makes the essential difference, because the elongation after fracture is determined manually, reassembling and measuring the two halves of the specimen after fracture. For this purpose, markers are applied to the specimen before the test.
Since the results between manual and automatic measurement usually vary greatly, manual calculation could also be used for comparability with ISO. This is also possible according to ISO 6892-1, but is used less and less due to the greater effort and the greater dispersion of the values.
There is one more point to consider when comparing the strain at break values: The test speed in the plastic deformation range is defined differently in the two standards and can vary here. However, the test speeds significantly influence the results, which becomes even clearer in the next section.
5. Test speed and control methods
Both standards define three different control methods for the test speed until the yield point is reached. The methods are functionally comparable, but the designations differ – and this is precisely what often leads to confusion in everyday laboratory work.
Results obtained using different control methods are not reproducible – a critical point.
| ISO 6892-1:2020 | Description of test method | ASTM E8/E8M-22 | |
|---|---|---|---|
| Method A1 | Closed-loop strain rate control, e.g., 0.00025 s-1 ±20% | Method B | Closed-loop strain rate control, e.g., 0.00025 s-1 ±40% |
| Method A2 | Open-loop strain rate control, e.g., 0.00025 s-1 ±20% | Method C | Crosshead speed e.g. 0.00025 s-1 ±20 % |
| Method B | Stress speed e.g. 6/60 MPa/s for steel or 2/20 MPa/s for aluminum | Method A | Stress rate 1.15 … 11.5 MPa/s |
| Test speed (after exceeding the yield point), e.g., to determine the tensile strength | z.B. 0.0067 s-1 ±20 % (or 0.00025 s-1 or 0.002 s-1) | Test speed (after exceeding the yield point), e.g., to determine the tensile strength | e.g. 0.00083 … 0.0083 s-1 |
The specifications for the control of stress speed vary. While in ISO 6892-1 the stress speed depends on the modulus of elasticity of the material, in ASTM E8 it is independent of it.
On the other hand, the methods with strain rate control until the yield point is reached are reproducible. Here, the same test speeds were defined in both standards. Only in the "closed loop" variant does ASTM E8 allow a higher tolerance.
Metallic materials are sensitive to strain rates. A higher test speed leads, for example, to higher yield points and thus has a major influence on the test results.
The best reproducibility of test values is achieved with the closed-loop method. This guarantees identical strain rates when determining the yield point. This method places higher demands on the equipment, but can pay off with more reliable results and less variation.
Conclusion: Know the differences, use the similarities efficiently
ISO 6892-1 and ASTM E8/E8M are both recognized worldwide and established in practice. Those who understand the differences can correctly classify results, avoid misinterpretations, and work more efficiently. Because the two standards are harmonized in some areas, similarities can be used in a targeted manner: for example, through the same specimen shapes, settings, procedures, and overall reproducible test conditions.
In everyday laboratory work, a testing system that covers both standards without complex modifications makes switching between ISO 6892-1 and ASTM E8/E8M considerably easier. In addition to the appropriate test equipment, the software plays a decisive role: The standard test programs from testXpert already contain all relevant standard requirements and guarantee a test that conforms to the standards.
Machine settings can also be saved in the testing software and automatically reproduced. This makes it possible to switch between tests according to ISO 6892-1 and ASTM E8/E8M in a safe, reproducible manner and with little effort.
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