Determination of deformation properties using the Aramis system (GOM)

Modern steels used in automotive manufacture feature very high strengths and good deformation properties. In addition, these materials demonstrate particularly good hardening properties at the high deformation speeds that occur in a crash. High-speed tensile tests provide the parameters for new steel materials that are used in crash simulations.

Unlike quasi-static tensile tests, high-speed tensile tests place higher requirements on the feasible take-off speeds of the traction device and its clamping system. At the point of force transfer, vibrations occur that cause the whole machine – particularly the powertrain – to vibrate. Measurements are taken as close to the sample as possible, or even on the sample itself, in order for the force and path data to be as free as possible from vibration information noise. Such measurements are generally taken with strain gauges. The strain gauges are calibrated in a quasi-static pretest and bonded either to the powertrain or directly onto the sample. Bonding the strain gauge obstructs the tensile properties of the measured part. This can distort the measurement result.
Non-contact, optical systems have now become an established means of determining the local and global deformation properties of materials and components.

The ARAMIS system by the Gesellschaft für optische Messtechnik mbH (GOM) in Braunschweig is one such system. ARAMIS works with high-resolution, high-speed cameras, an adjustable camera mounting with lighting elements, as well as with the relevant hardware and software for data capture and evaluation. A stochastic object grid is applied manually to the material sample and forms a unique pattern on the sample. During testing, the camera takes a series of images of the deformation process.

The sample surface is broken down into so-called facets and this forms the basis for evaluating the images. Each facet features a unique pattern and can be accurately matched to all images in the series. Based on the series of images (Figure 1), the path information (displacement) is determined by correlation with the initial state of each facet. This provides measurement data of the elongation in the sample without influencing it. The data obtained and the force values are used to calculate the stress-strain curve.

The ARAMIS system is used at Salzgitter Mannesmann Forschung GmbH to determine parameters for the crash properties of steels, which the customer then uses for component calculation and design.