The tensile test is the most important and most frequently used mechanical-technological test worldwide, which determines the strength and strain characteristic values for metals applications that are of crucial importance in the design and construction of components, commodities, machines, vehicles and buildings.
The test task is to determine characteristic values reliably and reproducibly and achieve international comparability.
The uni-axial tensile test is the method used to determine the characteristic values for yield point or offset yield, tensile strength and strain at break. In addition, lower yield point, yield point extension and extension at maximum force are determined.
In tensile testing on metals, the standard differentiates among four temperature ranges in which tensile tests are performed: room temperature, elevated temperature, low temperature and the temperature of liquid helium. The different temperature ranges and the liquid helium medium impose very distinct requirements on the testing systems and the test method, including the specimens to be prepared. Therefore, the international ISO standard is divided into four different parts, each of which addresses one of the above-mentioned temperature ranges:
- ISO 6892-1 test method at room temperature
- ISO 6892-2 test method at elevated temperature
- ISO 6892-3 test method at low temperatures
- ISO 6892-4 test method in liquid helium
In addition to these internationally accepted ISO standards, national standards including the American ASTM standard, the European EN standard, the Japanese JIS standard and the Chinese GB/T standard are also applied internationally. For specialized fields of application, i.e. aerospace, additional specific standards may be important or required.
The tensile test on metals or metallic materials, is mainly based on DIN EN ISO 6892-1 and ASTM E8. Both standards specify specimen shapes and their testing. The objective of the standards is to define and establish the test method in such a way, that even when different testing systems are used, the characteristic values to be determined remain reproducible and correct. This also means that the standard requirements address important influencing factors and generally formulate requirements in such a way that there is enough leeway for technical realizations and innovation.
Important characteristics of tensile testing on metals include:
- The yield point; more accurately the upper and lower yield point (ReH and ReL)
- The offset yield; generally determined as replacement yield point at 0.2 % plastic elongation (Rp0.2).
- The yield point extension; more accurately yield point extensometer extension, because it can only be determined with the use of an extensometer (Ae)
- The tensile strength (Rm)
- The uniform elongation (Ag)
- The strain at break (A), whereby the normative specifications with regard to the gauge length are of significant importance
For metallic materials with a pronounced yield point the maximum tensile force is defined as the highest reached force after the upper yield point. The maximum tensile force after exceeding the yield point can also lie below the yield point for weakly work-hardened materials, therefore the tensile strength in this case is lower than the value for the upper yield point.
The stress strain curve image shows a curve with a high level of work-hardening (1) and with a very low level of work-hardening (2) after the yield point.
For metals with yield point and subsequent stress, on the other hand, the tensile strength corresponds to the stress at the yield point.
For determination of the yield point and tensile strength, only one precise force measurement is necessary, while for all other characteristics an (automatic) strain measurement with an extensometer during the test, or manual strain measurement after removing the specimen/specimen remains, is necessary.
The most important and clearly describable requirements also relate to force measurement and the measurement of extension of the specimen under force application.
- For the force measurement the ISO 6892 series refer to ISO 7500-1 Calibration and verification of the force measuring system for tension and compression testing machines, and require Class 1 at a minimum.
- For measurement of the extension, the ISO 6892 series refer to ISO 9513 Calibration of extensometer systems used in uniaxial testing, and require Class 1 at a minimum for determination of the offset yield; for measurement of other characteristic values (with extensions larger than 5%) Class 2 may be applied.
The calibration processes, and especially the results and definitions of the classifications, are described in the standards for force measurement and extension measurement. The latter is crucial for application in the testing practice. Maximum permitted deviations and resolutions can be derived through the class affiliation for the calibrated measuring system, which have to be used for the determination of the measurement uncertainty of the measuring system.
- ASTM E8 refers to ASTM E74 for the force measurement, and
- To ASTM E83 for the extension measurement.
- The internationally applied standards are sometimes different in the structure of their content, however in their definition and requirements they are in accordance so that the relevant characteristic values derived from tensile testing do not significantly deviate from each other.
One exception to be noted is the evaluation, and with that the classification of the extensometers. While ISO 9513 refers to deviation from the set value to be reached, ASTM E83 additionally considers the ratio to the initial gauge length. An extensometer that is intended for small initial gauge lengths must meet higher measuring requirements than one for bigger initial gauge lengths.
Characteristic values, for which use of an extensometer of at least Class 1 to ISO 9513 for tensile testing of metal is required, are:
- Initial gradient of the stress-strain curve mE
- Offset yields Rp and Rt
Characteristic values, for which use of an extensometer of at least Class 2 to ISO 9513 for tensile testing of metal is required, are:
- Yield point extension Ae
- Uniform extension Ag and Agt, as well as
- Plateau range e around the tensile strength Rm or maximum tensile force Fm
- Extension after break A and At
For correct determination of yield points (ReH and ReL) and offset yields (Rp and Rt) , besides accurate force and strain measurement, the test speeds are also significant:
- Metallic materials change their characteristic values when the strain rates at which the tests are performed change.
- As a general rule, higher strain rates result in higher strength values.
- Depending on the alloy and product quality of the metallic material, the dependence on the strain rate can be very significant, and outside of the specification limits for corresponding qualities.
- This fact has lead to the introduction of an additional method to the international standard, for the adjustment of the correct test speed, at which the maintenance of specific strain rates with tighter tolerances is required.
Since 2009 both ISO and ASTM have equally adapted the so called strain rate control in their standard for tensile testing on metals, in order to improve the results reliability in the determination of yield points and offset yields.
Both standards, and in connection with them also additional national standards such as JIS Z2241 and GB/T 228, have suggested two types of implementation of this strain rate control:
- First, automatic control through use of the extensometer signal (closed loop) and
- Second, manual adjustment through preselection of a crosshead speed, at which the correct strain rate for determination of the characteristic value is then achieved (open loop).
The first method uses the modern technical options provided by drive controllers to automatically maintain the crosshead speed in the tolerance range for the strain rates specified by the standard. This method requires a control-technology-equipped testing system, however it significantly simplifies the test operation and eliminates errors in setting the crosshead speed. Therefore this control method is recommended.
The strain at break A or At is a measurement of the ductility, as well as the flow properties of a material.
The strain at break At can only be determined with extensometers, which remain on the specimen up to and including the point of break, and measure the extension of the specimen.
Strain at break A was normally measured manually, while today it is also measured with extensometers. For automatic measurements, correct determination of the point at which the specimen breaks (point of break) is therefore of significant importance.
Modern algorithms, which automatically analyze the stress-strain curve, ensure reliable specification of the break point and accurate determination of the strain at break. The break location along the specimen, more specifically along the parallel length of the specimen, is also important for reliable and accurate determination of the strain at break. If the break or point of failure is not within the gauge length of contact-type extensometers, the plastic deformation that occurs during necking and the point of failure cannot be correctly determined. Modern evaluation algorithms estimate the point of failure, or break point, relative to the measuring points of the extensometer and denote an unreliable strain at break value.
With optical, non-contact extensometers, which record the entire parallel length of the specimen, the point of break or failure can be determined. If the point of break is outside of the initial gauge length, according to ISO 6892-1:2017 Appendix I, the strain at break can nevertheless be determined, if an appropriate number of gauge marks were considered and measured during the test. The laserXtens Array as well as the videoXtens Array provide an optional solution for this task. With their use, the strain at break is automatically determined both reliably and accurately on 100% of the specimens.
The JIS Z2241 provides a classification of the break point. This is normally done manually through visual testing or by separate non-contact measurement. Both methods are personnel and time consuming. With modern optical, non-contact extensometers this task is automatically handled for tensile tests: indication of the class (depending on the break point A, B or C) is then part of the determined and recordable results.
100% reliable test results with validation to ISO 6892-1/TENSTAND.
The test results that are determined with the software to ISO 6892-1 can be verified and validated with an internationally coordinated data set and internationally coordinated test results. In a European research project with the acronym TENSTAND, raw data from tests on metals was generated and qualified. This data was used to determine and qualify test results and ranges of results. With the TENSTAND sets of data and sets of results the testing software can be quickly and reliably verified through comparison of results. The National Physical Laboratory (NPL) in London has these sets of data and sets of results available.
- The National Physical Laboratory (NPL) is the British counterpart to the German national metrology institute Physikalisch-Technische Bundesanstalt (PTB). It defines the national standards applicable in the field of physics and technology.
- Its responsibilities include the determination of fundamental and natural constants, representation, preservation and transfer of legal units of the International System of Units (SI), supplemented by services such as UKAS (United Kingdom Accreditation Service) calibration services for the legally regulated sector.