Wöhler fatigue test
Wöhler fatigue test: determining the vibration resistance of materials and components using optical 3D metrology
The Wöhler fatigue test, also referred to as dynamic fatigue test or continuous vibration test, is a cyclic load test to determine the fatigue behavior of materials and components. The fatigue behavior or vibration resistance provides information on the deformation and failure behavior of a material or component under dynamic oscillating load. The test results play an important role for using the materials and components in practice, as cyclic mechanical load is often the cause of component failure. Knowing the fatigue behavior allows for precise conclusions on the finite-life fatigue strength and fatigue limit of a material or component, ensuring that no critical material damage or sudden fatigue failure occurs during the intended use.
The Wöhler fatigue test can be used for basic research, for instance, when it comes to novel fiber-reinforced composites in fields such as automotive engineering, aerospace or biomechanics. Besides, the Wöhler fatigue test is an integral part of prototype testing, where it can be used to assess the component design or calculate the product life, for example. Designers cannot rely on the general material characteristics during product development without testing, as the characteristics cannot be transferred 1:1 to any component. The reason is that bore holes, the component size and shape as well as other design characteristics lead to a changed stress concentration on the component. As a consequence, these characteristics significantly affect the fatigue behavior of the components and possibly accelerate
How does the Wöhler fatigue test work?
The test specimen is placed in a test stand and subjected to cyclic loading (tension, compression, bending, torsion or shear) typically using a sinusoidal load-time function. During the Wöhler fatigue test, the medium stress remains constant. The samples of a test series are alternately loaded by the stress deflection (amplitude) on both sides of the medium stress until a predefined event occurs, e.g.:
- The Wöhler fatigue test proceeds until the sample fails (or a clearly defined failure criterion occurs, e.g. fracture).
- The Wöhler fatigue test ends after X cycles.
- The Wöhler fatigue test ends when the limiting oscillation frequency is reached.
Scientists and test engineers always carry out several tests on identical samples one after the other. The stress deflection from sample to sample is gradually reduced (staircase method) until the predefined event (e.g. the fracture of the test specimen) no longer occurs or the limiting oscillation frequency is reached. In general, at least three tests are carried out per load amplitude to statistically verify the values.
How can deformation be measured in the Wöhler fatigue test?
A classical measuring tool for the Wöhler fatigue test is the strain gauge, whose resistance value changes when the object surface is stretched or compressed. Strain gauges are available on the market in a wide variety of materials and shapes, so that suitable strain gauges are available for every standard test. In order to record the deformation of the material or component to be tested, one or more strain gauges are manually applied to the sample.
What sounds simple turns out to be more complex in practice: The local application of the strain gauge represents a physical intervention in the composition of the sample surface. Even if the adhesive layer of the strain gauge is very thin, a local notch effect can be observed. The resulting small surface defects can lead to unwanted fractures in the area of the strain gauge, which falsify the test. In addition, the use of strain gauges entails a second problem: Not only does the material tested fatigue, the material of the strain gauge is also subject to fatigue. Especially in high-tech composites, the material fatigue of the strain gauge may occur earlier than the fatigue of the material to be tested. Consequently, the Wöhler fatigue test may have to be aborted earlier than actually desired, i.e. already with the failure of the strain gauge.
A useful alternative or supplement to strain gauges is optical 3D metrology: Camera-based measuring systems track the test sequence in real time from different perspectives, allow for non-contact acquisition of measuring data and provide clear information about the deformation of the test specimen. The measured data is automatically transferred to the connected measurement software, allowing various evaluations (e.g. comparison of the measuring data with the simulation data).
Which GOM measuring system is suitable for the Wöhler fatigue test?
The ARAMIS optical 3D measuring system records with high precision 3D coordinates, 3D displacements and 2D surface strains both over the entire surface and at specific points. The measuring area of the ARAMIS system can be flexibly adapted to the test field. No matter if it is a small component or special construction of several meters length, ARAMIS sensors always cover the complete test setup. In contrast to conventional strain gauges, the system records the measuring data completely without contact. If required, the user can also apply virtual strain gauges to the component via the connected ARAMIS Professional software without having to worry in advance about where the highest load will occur. The software guides the user through the complete measuring procedure: starting with the acquisition of measuring data through the analysis of surface deformations or individual points up to the creation of meaningful reports that are easy to understand and interpret even for users who do not have any experience in measurement technology (e.g. cooperation partners or customers). The extent of the deformation of the test specimen can be visualized in a color deviation representation, for example.