Devices for testing fatigue under alternating or pulsating tensile or bending load are at our service which are based on one of the two patents DE10204258 or DE102019001241. For testing of purely alternating bending load, we even use the first natural frequency of the unsupported speciemen in the structure mechanical sense of a beam in bending. When indicated the specimen is prolonged at its ends, preferably monolithically or by adding parts, such that the desired natural frequency is achieved.
Application cases
Determination of fatigue data by test may be needed for example, if
- these data are required to support an analysis model
- demonstration of minimum fatigue strength is requested by the customer
- the root cause of a failure case needs to be identified
- the lot dependency of the dynamic strength of a product needs to be documented
In order to limit the effort of testing per lot, one can benefit from the simple ways of testing under alternating bending load without bending preload. There are analytical approaches for transformation of this specific fatigue property considering other load types or ranges. These approaches are independent from the tested lot. Relative comparison of the fatigue due to bending would be sufficient anyway, if the transformation is linear. Otherwise, the transformation can be performed after the test and the lots can be compared then.
Principle of resonance driven testing devices
By exciting a structure with its natural frequency, the maximum amplitude is achieved. According to this principle low priced testing devices can be designed which allow for fatigue testing of light to heavy specimen at frequencies from a few Hz up to 1 kHz.
A specific mode shape is related to oscillation at natural frequency. Therefore, stressing of the specimen is equally explicit. When supporting the device at its nodal points it is fully decoupled from the environment avoiding any kind of interface reactions.
Fatigue under torsional loading can also be tested according to first of above-mentioned patents.
Testing devices at our service
One device was developed particularly for testing according to EN 6072 of flat specimen of type Tmini from Aluminium at 110 Hz. With modified adapters it is however suitable for other materials and shapes of specimen with equivalent cross-sectional area.
Another device was developed particularly for testing according to DIN 969 respectively DIN ISO 3800 of bolt-nut-connections up to thread size M14 at about 250 Hz. This device is also suitable for other specimen of comparable cross-sectional area or alternating rather than pulsating loads.
A device for the alternating or pulsating bending load at 80 Hz to 100 Hz is applicable for flat bending specimen up to 5 mm thickness.
The test for purely alternating bending load can be set-up for both, flat and cylindrical specimen up to a size which yet allows for handling by man power. The measurement and control devices are available as well as several electro magnets of different power range. The structural parts which are required beside the specimen, are so easy to manufacture that they are usually available when the specimens are.
Devices for testing under torsional or rotating bending load can also be developed according to the presented principles. For that or for building larger devices or devices for other parameters reasonable lead time needs to be allowed for.
With all mentioned devices the load signal (full Wheatstone-bridge of strain gauges or accelerometer) will be related per type of specimen to signal of stressing of a calibration-specimen. The calibration of the calibration specimen will be documented. Each test is performed until a chosen number of cycles which indicates fatigue resistance is achieved or a crack is initiated. A drop in frequency clearly identifies the initiation of a crack.
Services
- Definition of test plan
- Performance of tests, processing of measurement data and documentation of results
- Fatigue limit
- Determination of Wöhler lines
- Determination of the fatigue strength
- Assessment of test results
- Consideration of the test results in the Structural analysis
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