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Sheet metal High Speed tensile test

ARAMIS and the detection of flow behaviour for sheet metal material in high speed tensile test

Measuring Systems: ARAMIS

Keywords: Flow curve, high speed camera

Fig. 1: ARAMIS set up
Specimen during a measurement Fig. 2: Specimen during a measurement

Information about the flow behaviour of material is important for crash- and forming simulations. The flow behaviour can be described by the true stress strain curve.

Mechanical sensors are not suited for determination of the flow behaviour above uniform extension because they provide only an integral value. Further are mechanical sensors not suitable for high speed tensile tests because of the occurring accelerations.

The high data point density qualifies the ARAMIS system to determine the true stress strain behaviour in the case of necking with more than a hundred data points. In addition it is possible to receive the plain strain tensor (e.g. longitudinal, transversal) and the thickness reduction for each measurement point. Because ARAMIS uses a non-contact measuring method high speed tensile tests are possible by using high speed cameras.

Fig. 1 shows the set up of an ARAMIS-3d-system installed on a ZWICK testing machine and a flat specimen.

Fig. 2 shows a sequence of pictures during a measurement. The detected distribution of longitudinal and transversal strain and thickness reduction can be seen in the fig. 3, 4 and 5 for a measurement with low haul-off speed.

Longditudinal strain Fig. 3: Longditudinal strain
Longditudinal strain Fig. 4: Transversal strain
Thickness reduction Fig. 5: Thickness reduction
Animated longditudinal strain Fig. 6: Animated longditudinal strain
 
 
Longditudinal and transversal Fig. 7: Longditudinal and transversal strain

High-speed cameras allow measurements with more than thousand pictures taken per second. Longitudinal strain as measured during a high-speed tensile test (haul-off speed 23m/s) is shown in fig.6.

The longitudinal and transversal strain along a longitudinal section for different stress can be seen in fig. 7. Up to a longitudinal strain of 20% a uniform extension can be determined. For higher values necking forms out and reaches values up to 80% until fracture.

Graph of longditudinal strain Fig. 8: Graph of longditudinal strain

In this experiment the pictures are taken with a rate of 18´000 per second. Fig. 8 displays the longitudinal strain along a section. Only every second load state is shown. The maximal strain reaches a value of 100%. In this experiment the cameras accomplish a strain resolution of approx. 0.8% in the sector of uniform extension. The strain accuracy is 0.1%.

By courtesy of DaimlerChrysler and the LWF Universität Paderborn.