Wöhler fatigue test

Fatigue Tests: determining the vibration resistance of materials and components using optical 3D metrology

Fatigue tests are separated into different categories:

Low Cycle Fatigue Tests according to ISO 12106 und ASTM E606

High Cycle Fatigue Tests according to DIN 50100, ASTM E466-15 or ISO 1099

The High Cycle Fatigue Test (HCF), also referred to as S-N test, Woehler 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. The knowledge about the fatigue behavior of materials and components ensures that no critical material damage or sudden fatigue failure occurs during the end product’s life cycle.

Stress Concentrations and Notch Effect

The High Cycle Fatigue Test (HCF) 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 High Cycle Fatigue Test (or Woehler fatigue test) is an integral part of prototype testing, where it can be used to assess the component design or calculate the durability, 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 compared to the stress concentration on standardized specimen geometries in materials testing. As a consequence, the individual design characteristics significantly affect the fatigue behavior of the components and possibly accelerate failure. This phenomenon is called notch effect (or stress concentration effect) in technical literature.

How does the High Cylce Fatigue Test (HCF) 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 Woehler fatigue test, the mean stress remains constant. The samples of a test series are alternately loaded by the stress deflection (amplitude) on both sides of the mean stress level until a predefined failure criterion occurs, e.g.:

  • The High Cycle Fatigue Test (HCF) proceeds until the sample fails (or a clearly defined failure criterion occurs, e.g. fracture or cracl).
  • A threshold value in number of cycles is defined beforehand.The High Cylce Fatigue Test (S-N test) ends when the specimen or component reaches the threshold number of load cycles without showing any visible failure criterion. In this case the specimen or component under test is said to be fatigue resistant.
  • Scientists and test engineers always carry out several High Cycle Fatigue Tests (S-N tests) on identical samples one after the other. The stress amplitude 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 threshold number of load cycles is reached. In general, at least three tests are carried out per load amplitude to statistically verify the values.
The results of all the runs of a Woehler fatigue test series are finally entered into a diagram: the Woehler diagram. This diagram shows the dependence of stress amplitudes (Y-axis) and their respective threshold number of load cycles (X-axis). This results in the Woehler curve (synonym: Woehler line).  

How can deformation be measured in the High Cycle Fatigue Test ?

A classical measuring device for the High Cycle Fatigue Test is the strain gauge (or gage), whose resistance value changes when the object surface is strained or compressed. Strain gauges/gages  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 and connected to an amplifying device or a so-called data acquisition system (DAQ) via cables. 

What sounds simple turns out to be more complex in practice: The local application of the strain gauge/gage  represents a physical intervention in the composition of the sample surface. Even if the adhesive layer of the strain gauge/gage 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/gage, which falsifies the test. In addition, the use of strain gauges/gages entails a second problem: Not only does the material tested fatigue, the material of the strain gauge/gage is also subject to fatigue. Especially in high-tech composites, the material fatigue of the strain gauge/gage may occur earlier than the fatigue of the material to be tested. Consequently, the High Cycle Fatigue Test (Woehler fatigue test) may have to be aborted earlier than actually intended, i.e. already with the failure of the strain gauge/gage. 

A useful alternative or supplement to strain gauges/gages is optical 3D metrology: Camera-based measuring systems track the test sequence in real time (in multi-sensor setups from different perspectives simultaneously), and allow for non-contact acquisition of measuring data. The captured measuring values of strains and 3D displacements provide provide clear information about the deformation of the test specimen. The measured data is automatically transferred to the measurement software, allowing various evaluations (e.g. comparison of the measuring data with the simulation data). 

Which GOM measuring system is suitable for the High Cylce Fatigue Test (HCF)?

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 of interest. The measuring area of the ARAMIS system can be flexibly adapted to the test specimen. No matter if it is a small component or special construction of several meters’ length, the ARAMIS sensors always cover the complete test setup. In contrast to conventional strain gauges/gages, the system records the measuring data completely in a non-contact way. If required, the user can also apply virtual strain gauges/gages to the component via the connected ARAMIS Professional software without having to worry in advance about where the highest deformation 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 point-wise 3D displacements up to the creation of meaningful measuring 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.

 

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