It is not hard to imagine that a lot of factors influence the hardening behaviour of metals. Questions like; How does a material react to a strain path change? or What is the influence of the temperature on the material behaviour?, are questions we’re trying to answer. Strain rate, strain direction, temperature, anisotropy in the material, etc. all play a role in the stress-strain curves. Tensile tests, shear tests and other destructive experiments are used to determine the influence, both qualitative and quantitative, of these parameters.

In the industry, a need for improved models has developed. On the one hand this stems from the lack of accuracy in the classical models because they take in account just a few parameters. They are outdated and for that reason more sophisticated models have to be used to get the desired results. On the other hand, improved materials (e.g. TRIP steel) cannot be described with the `classical’ models, see also the figure on the right. Metal alloys are developed to meet certain requirements and were not available in the past; however, the available material models fit the regular metals but not the modern alloys.

TRIP (TRansformation Induced Plasticity) is one of these new products. Under plastic deformation the crystal lattice changes from austenite to martensite. This property can be very desirable; the martensite lattice is stronger then the austenite lattice. However, the problem with the transformation is that currently it is not known when and how the transformation takes place. Therefore metallurgists try to find out what is actually happening within the material. A fortunate property of the martensite lattice is the magnetism it shows, and hereby making it detectable during tests.

Within our group a biaxial test facility is attended, that can load a sheet metal specimen on any arbitrarily combination of shear and tensile deformation. It is designed by our staff and unique in the world. In the figure on the right the set-up is depicted. It consists of a computer, a camera with light source and the actual frame where the sample is tested. The computer controls both the testing device and the camera. The camera determines the strain in the sample by means of dots that are applied to it. The magnetic sensor is positioned on the rear side of the sample.







Another phenomenon characterising material behaviour is strain rate. When a car crashes, plastic deformation takes place very rapidly. Within microseconds you’re valuable car has decreased 1 m in length. The hardening behaviour of metals under these circumstances is different from the regular deformation speed, e.g. when the car was fabricated. For this purpose tests have been developed that reproduce the situation during crash. Use is made of rectangular cases on which heavy loads are dropped. The result of such a test is visible in the figure on the left. Instead of bending, the bar crumples and hereby absorbs a lot of energy. The data from these tests are used to determine the parameters in crash models.

The experimental facilities are mostly quite expensive and labour-intensive to maintain. Universities can not afford to have all the different tests in their own laboratory and that’s why a lot of this work is done in co-operation with other universities (TU/e, TUD, TU Aken) and with the industry (Corus, Philips, Boal).

Anthology of finished graduate projects:

• M. van Riel, Determination of the transverse stress in a combined tensile—shear test
  

Industrial partners:

University partners: