Development of a holistic approach for force and strain measurements in high-speed tensile tests

Crash loads with high strain rates can often be found in automotive and aerospace industries, so that a material characterization on dynamic basis is required to guarantee safety properties of highly loaded components. Most exterior parts nowadays are made of fiber-reinforced polymers (FRP), which show strain rate dependent properties. For this purpose, the determination of characteristic parameters in highspeed tensile tests is a common method. Since FRP show an increasing ultimate tensile strength with higher strain rates, a high potential regarding material savings and lightweight design is enabled. Consequently, a comprehensive understanding of the data, which is measured in high-speed tensile tests, is necessary. Typically, the test force is provided by a piezo-electric load cell, whereas the piston movement is recorded by a capacitive sensor. This is also how it is realized with the highspeed impact testing machine HITS-TX. Furthermore, HITS-TX provides an additional grip displacement sensor (GDS) for measuring the specimen movement just inside the grips and therefore excluding influences of additional parts in the load train. This is in accordance with existing knowledge about oscillations and stress waves in load train, influencing recorded force and displacement signals. A general challenge of high-speed material testing is the impact occurring between the approach jig and actuator piston at test beginning. An approach device is usually used in high-speed systems in order to accelerate the piston to the desired test speed, before the specimen is loaded. As a result of the impact inside this device, oscillations, which are travelling as elastic stress waves, are transmitted to the load train. These waves are reflected in the load cell at the bottom of the test system and, as a consequence, influencing force and displacement measurements. In order to avoid or at least to reduce these oscillations, it is essential to install a mechanism for damping the force application. For this purpose, different materials or geometries at the contact surfaces can be used, as well as installing damping elements. However, the effect of damping should no longer have an influence on the measurement at a load of 25% of the yield strength.

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Application News
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Impact Testing, High-Speed Video Camera
force and strain measurements, high-speed tensile tests, DIC-analysis, Materials/Automotive, High Speed Mechanical Testing and Imaging, HITS-X Series High Speed Impact Testing Machines, Hyper Vision HPV-X2 High-Speed Video Camera
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