Creep test

The creep test ( English stress rupture test is) a material testing methods to determine the material behavior at a constant test temperature above room temperature and after prolonged action of a constant tensile force . A distinction is made between the interrupted and the uninterrupted creep test.

need

The necessity of the creep test is due to the fact that materials behave differently under long-term static loads at elevated temperatures than under the same load at room temperature. Is observed at these elevated temperatures already at voltages below the yield point without load increase after a certain time a slow but steady irreversible, plastic deformation, which with creep ( English creep ) is designated and, after a sufficiently long exposure time leads to rupture of the sample. This behavior is due to the fact that all structural mechanisms are thermally activated when exposed to high temperatures.

In general, this effect occurs at temperatures up to about 30% of the melting temperature T _s . If the temperature rises above about 40% T _s , the metal yields plastically under constant stress.

Until the 1930s, it was assumed that there is a minimum strength limit (creep strength) at elevated temperatures, similar to fatigue strength . In 1941 A. Thum and K. Richard (Institute for Material Science (IfW), Technische Hochschule Darmstadt) proved that there is no fatigue strength at high temperatures and that short-term tests are not suitable for describing the material behavior under high temperature loads. This was the starting point for extensive and systematic long-term investigations and for the development of an experimental method for the determination of long-term parameters of materials under high temperature stress. This resulted in the creep test, which today is one of the most important experiments for describing the high temperature behavior of materials and is standardized in accordance with DIN EN ISO 204.

Time expansion curves

The elongation ε in a creep test can usually be divided into three areas:

primary creep
secondary creep
tertiary creep

With primary creep there is a decreasing, with secondary a constant and with tertiary creep an increasing creep speed έ.

Experimental technology

For the design and operation of high-temperature components such as aviation turbine blades or steam turbine rotors, long-term material parameters are required. Therefore, high-temperature materials have to be tested in creep tests for up to 100,000 h (approx. 12 years) and longer, which is a great test-technical challenge, since all test parameters must be kept precisely constant over many years. Special testing machines were therefore developed for creep testing (see picture).

Single and multiple sample testing machine for creep tests

In the creep test, a sample is statically loaded (at constant tension ) at a constant high temperature and the elongation is measured over the exposure time. The measured elongation shows a characteristic curve called the creep curve (see creep (materials) ). As a result, the following characteristic values are determined in the creep test:

Exposure time to break ( ) ${\ displaystyle t_ {u}}$
Creep strength ( ) ${\ displaystyle R_ {u, t, T}}$
Time expansion limits ( ) ${\ displaystyle R_ {p \ varepsilon}}$
Amounts of high temperature expansion (for example creep , permanent , plastic initial , anelastic return ) ${\ displaystyle \ varepsilon _ {f}}$ ${\ displaystyle \ varepsilon _ {per}}$ ${\ displaystyle \ varepsilon _ {i}}$ ${\ displaystyle \ varepsilon _ {k}}$

The creep test is carried out in the following steps:

The sample is heated to the test temperature: the sample, clamping and test parts and displacement transducers must be brought into thermal equilibrium,
Alignment of the distance measurement,
Application of the test load,
Carry out the test until the end of the test at constant test force and constant temperature.

The fracture of the specimen usually marks the end of the test. However, reaching a certain creep strain or a certain stress time can also be the aim of the creep test. In this case, the sample is relieved and removed after the test objective has been achieved.

One or two samples can be tested at the same time in individual testing machines. The strain measurement can, however, be carried out continuously with special strain sensors, which on the one hand allows high precision of the strain measurement values and on the other hand allows a high measuring rate with high data coverage of the creep curve. The disadvantage here, however, is the high technical effort involved in long-term tests, in which a single sample occupies an individual testing machine for years. This uninterrupted test technique is therefore generally used for short-term tests of up to approx. 1,000 hours of exposure to determine the primary creep area.

In multi-sample testing machines, up to 60 samples and more can be tested simultaneously at one temperature, which is a significant cost advantage. The samples are summarized here in test strings that are loaded with a defined test force. With the different test diameters of the creep samples, different test voltages can also be tested in one test string. The strain measurement in multi-sample testing machines cannot be carried out with strain sensors, so that a continuous strain measurement is not possible. For this reason, creep tests are carried out in multi-sample testing machines using the interrupted test technique, in which at certain times (e.g. after 1,000 hours) a complete test string is pulled from the multi-sample testing machine, which is then dismantled after cooling to room temperature. The corresponding creep tests are then optically measured with a measuring microscope. The test string is then reassembled, inserted into the multi-sample testing machine and loaded with the test force again after the test temperature has been reached. The interrupted test technique therefore provides far fewer strain measurement points than the uninterrupted test technique in individual testing machines. The interrupted test technology is therefore used for long-term tests of 1,000 to 200,000 hours (24 years).

By combining the two test techniques (starting the creep test in individual testing machines until the secondary creep range is reached and then converting and continuing the test in the multi-sample testing machine), a good compromise can be achieved between high data usage on the one hand and cost-effective performance of long-term tests on the other.

The implementation of standard-compliant creep tests requires a great deal of know-how and experience. This applies above all to the measurement and control of the test temperature, which is the most important factor influencing the measurement result. A quality-assured calibration procedure for thermocouples and temperature measuring circuits is essential for this. A maximum temperature difference of 3 K is permitted at a temperature below 600 ° C, and a maximum temperature difference of 5 K is permitted at a temperature above 800 ° C. Furthermore, extensive experience with long-term testing technology, the influencing variables of the sample geometry, the loading procedures, the determination of measurement uncertainties and, last but not least, the material behavior under high temperature stress are essential for the determination of reliable creep parameters. The creep tests are carried out in special high-temperature laboratories that have the appropriate equipment, calibration facilities and know-how. These high-temperature laboratories are set up in research facilities and at material and component manufacturers.

While the stress is kept constant in the creep test and the change in elongation is measured, the related relaxation test takes place at constant elongation and determines the change in tension.

Application, guidelines and recommendations

The results from the creep tests are usually particularly valuable because the creep tests often take many years and therefore only a limited number of test results can be determined in a certain period of time, such as the stress time until breakage or up to a defined yield point . On the other hand, long-term tests are associated with high technical and therefore financial expense. The results of these tests can be shown in diagrams such as the creep diagram or the time expansion diagram. These diagrams can now be taken from tables for many materials and are the basis for high-temperature component design.

Since high-temperature components have to meet the highest safety requirements, the quality assurance of creep test technology plays a special role. In addition to the DIN EN ISO 204 test standard, guidelines and recommendations for creep tests have been developed at national and international level, which on the one hand ensure comparable and reliable test results in creep laboratories and on the other hand are intended to support the evaluation and evaluation of the test results for the determination of material properties. At national level, guidelines for the implementation and evaluation of creep tests have been drawn up within the working groups of heat-resistant steels (AGW) and high-temperature materials (AGHT), which represent an association of material manufacturers , system manufacturers , research institutions and professional associations . The working groups AGW and AGHT were the first institutions worldwide for joint research in the field of high-temperature materials and have been determining particularly long-term creep data for decades. Within the framework of the European working group ECCC, recommendations for the implementation and evaluation of creep tests have been developed. The experience of the AGW / AGHT working groups has also been incorporated. Both the work of the AGW / AGHT working groups and that of the European ECCC working group are taken into account in the standardization organizations .

literature

A. Thum, K. Richard: Embrittlement and damage to heat-resistant steels under long-term stress . In: Arch. Fd Eisenhüttenw. , 15, 1941, pp. 33-45

KH Kloos, J. Granacher, A. Scholz, R. Tscheuschner: Testing of metallic materials at high temperatures ,
Part 1: Hot tensile test and creep test in single and multiple sample testing machines . In: Materialprüf. , 30, 1988, pp. 93/98.
Part 2: Special problems of the creep test and relaxation test . In: Materialprüf. , 30, 1988, pp. 151/55.

J. Granacher: For the transfer of high temperature values to components . In: VDI Reports , No. 852, 1991, pp. 325/52.

A. Scholz, M. Schwienheer, C. Berger: High-temperature testing of metallic materials - testing technology, standardization, data management and evaluation , lecture at the 25th lecture event of the working group for heat-resistant steels and high-temperature materials on November 22, 2002 in Düsseldorf, conference proceedings, publisher: VDEh Düsseldorf, 2002, pp. 77/90.

A. Scholz, M. Schwienheer, F. Müller, S. Linn, M. Schein, C. Walther, C. Berger: High temperature testing - a contribution to material development and qualification as well as simulation of component stress . In: Mat.-wiss. u. Material techn. , 2007, 38, No. 5, p. 372/78

Individual evidence

↑ ^a ^b ^c ^d Christoph Broeckmann, Paul Bite: Materials Science I . Institute for Material Applications in Mechanical Engineering at RWTH Aachen University , Aachen 2016, pp. 30–39.

[broeckmann-1] Christoph Broeckmann, Paul Bite: Materials Science I . Institute for Material Applications in Mechanical Engineering at RWTH Aachen University , Aachen 2016, pp. 30–39.