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Dynamic Micro Testing Overcoming 5 Decades of Strain Rates

Wednesday (26.09.2018)
17:45 - 18:00 S1/01 - A03
Part of:

Miniaturization of specimens for investigating the properties of materials in smaller scales has led to the development of micro tensile testing techniques. While for quasi static and static tests some setups exist, the dynamic micro tensile testing is more challenging. The objective of the shown project was to develop a device for micro tensile tests from quasi static to very high strain rates. This setup with a linear actuator allows larger displacements to load samples until failure than piezo actuator based systems can.

The “IWM DynMicro” was verified by performing tests with a material of known properties and by interpreting the results. The setup can achieve recorded strain rates up to more than 1000/s for the given force and strain measurement techniques, depending on the used sample geometry and tested material. A wide variety of materials or material combinations can be tested and the sizes of the specimens can range from micro to semi-micro scale. Due to interchangeability of the setup, it is possible to replace components to upgrade in terms of force or velocity if needed.

With the new test setup it will be possible to test specimens from quasi static strain rates lower than 0.001/s up to high strain rates up to more than 1000 1/s and to study local rate effects. Local strain fields are analyzed by digital image correlation (DIC) with high speed video and micro optics. One example will be shown for a thin steel sample with a series of high speed pictures. The picture series was recorded at 300.000 frames per second (fps). This allowed selecting representative states and a low in-motion unsharpness due to a very short shutter time.

If the size of the sample still allows a comparable (global) material reaction, this setup allows improved material testing at higher strain rates compared to conventional setups, like servo hydraulic machines or Split Hopkinson bars. An example for 250 1/s nominal strain rate will be given for a conventional test and a micro test series. The micro testing series results range from nominal strain rates of 0.001 1/s up to 1000 1/s. The actual tests show still potential for decreasing the rate of one or two decades with additional actuator water cooling, and increasing the rate up to 4000 1/s. These are rates, which are normally achiever bar or gas gun systems. These super high rate tests need a change in the force measurement concept with local measurement on the sample. This is a topic of actual ongoing work.


Dipl.-Ing. Frank Huberth
Fraunhofer Institute for Mechanics of Materials IWM


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