Wöhler experiment
The Wöhler test or fatigue test is an experiment to determine the fatigue strength of a material or a component. The results of several Wöhler test runs can be found in the Wöhler curve . In mechanical engineering , the Wöhler test is an important aid for the calculation and verification of fatigue strength or operational strength. It is named after August Wöhler , who developed the test as a reaction to the Timelkam railway accident .
Wöhler experiment
With the Wöhler tests the fatigue strength , more precisely the fatigue strength and fatigue strength of materials or components (component Wöhler test) is determined. For this purpose, several test bodies are loaded cyclically, usually with a sinusoidal load-time function. Depending on how the experiment is carried out, the stress can arise from tensile / compressive loads, bending , torsion or shear forces.
To determine the Wöhler curve, different test bodies are tested on different load horizons. Each Wöhler test runs until a defined failure of the specimen (fracture, crack) occurs or a specified number of vibrations (including the number of limit cycles) is survived. Test specimens that reach the limit number of cycles without recognizable failure are referred to as runners. For each Wöhler test, the mean stress, high stress and undervoltage of the cyclic load are constant. Either only the mean voltage or only the ratio between upper and lower voltage is varied between the tests of the same Wöhler curve.
The spread of the measurement results of the Wöhler tests is strikingly large. It comes only slightly from inadequacies in the tests, but mainly from diverging material properties within the components. It obeys the extreme value theory of W. Weibull and EJ Gumbel, namely the distribution of the smallest strengths of the volume elements ( Weibull distribution ). The statistical influence of size also follows from the extreme value theory : Small components have a greater fatigue strength on average than large components made of identical material.
Wöhler curve
The results of the tests are entered in a diagram. In the Wöhler diagram, the nominal stress amplitude S a is usually plotted linearly or logarithmically over the logarithmically represented, tolerable number of cycles. The resulting curve is called the Wöhler curve or Wöhler line . The three areas K, Z and D are entered in the adjacent Wöhler curve: short-term strength, fatigue strength and fatigue strength.
| No. | Stress deflection in N / mm² | Load change until break |
|---|---|---|
| 1. | ± 350 | 4252 |
| 2. | ± 300 | 8384 |
| 3. | ± 250 | 21987 |
| 4th | ± 200 | 70355 |
| 5. | ± 180 | 108664 |
| 6th | ± 160 | 10 million without a break |
With body-centered cubic materials, the Wöhler curve often changes into a horizontal line with decreasing stress amplitude, i.e. H. there is an assignable fatigue strength. In the case of face-centered cubic metals, the Wöhler curve generally drops continuously, so that these materials can usually only be assigned one fatigue strength.
Web links
- Summary of the fatigue strength test (PDF; 41 kB)
literature
- German Institute for Standardization : DIN 50100 .
- Josef Köhler: Try or calculate. Fatigue strength and zero break line . In: Materialwissenschaft und Werkstofftechnik , Vol. 41 (2010), No. 3, Wiley-VCH Verlag, Weinheim ISSN 0933-5137
- Josef Köhler: Relative fatigue strength , De Gruyter Oldenbourg Verlag, 2014
Individual evidence
- ^ Georg Jacobs: Machine design . Mainz Verlag, Aachen 2015, ISBN 3-86130-748-0 , p. 19-21 .
- ↑ a b Christoph Broeckmann, Paul Bite: Materials Science I . Mainz Verlag, Aachen 2016, p. 40-53 .
- ↑ Standard DIN 50100: 2016-12 Fatigue test - implementation and evaluation of cyclical tests with constant load amplitude for metallic material samples and components
- ↑ tec-science: fatigue test. In: tec-science. July 13, 2018, accessed on October 25, 2019 (German).