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Products and solutions, 2006-10-24, 09:06 AM

Accelerated time to production with TEM

The path from part design to stable production is a long one. But thanks to TEM (ThyssenKrupp Steel Extrapolation Method) this path is now a little shorter, at least in the area of sheet metal forming. TEM is a new computational model which can greatly reduce the amount of testing required to determine material characteristics. At the same time it offers significantly more realistic data than previous commonly used formulae. TEM was developed by the Auto division of ThyssenKrupp Steel AG. The model is to be made available to automotive manufacturers and suppliers as a software module to prepare material data for their forming simulation programs.

The auto industry uses these programs to simulate the behavior of steel materials in the forming press for the production of a newly developed part, for example to check whether cracking or undesired thinning could occur. Using these virtual systems, parts and production methods are optimized to the stage where it can be assumed that production will present no problems. But the simulation results are only as good as the material characteristics fed into the programs.

Anyone wishing to predict how steel will behave during the forming process mainly requires data on the work-hardening behavior of the material and on the forming limit curve, which describes the maximum attainable elongation. These two values together describe the forming and stress combinations under which a specific steel will lose its elasticity, harden and then crack or tear under excessive stress.

Several test methods are available for determining the hardening behavior of materials, the most common being the one-dimensional tensile test. This method, in which a roughly 250 millimeter long, dogbone-shaped specimen is clamped in a tensile testing device and elongated, is in widespread use as it is fast, cheap and extensively standardized. The problem is that the data provided are not sufficient for use in simulations because the values recorded only represent a small elongation range of 80 millimeters. Most real-life forming operations go well beyond this range.

Although there are formulae which can be used to extrapolate these data for higher elongations, they have not always proved reliable in practice. That's why the major auto manufacturers carry out additional material tests to obtain more reliable input data for their forming simulations. One of the best is the bulge test, in which a circular specimen roughly 160 millimeters in diameter is clamped in a device and then subjected to hydraulic pressure from below. Significantly higher elongations are achieved by this method than by tensile testing. The disadvantage is that it is around seven times more expensive than tensile testing, not universally available, takes a long time and has not been internationally standardized. The tests used to determine forming limit curves are also very complex and have not yet been fully standardized. It currently takes around a week to determine a single curve.

Applications engineers at ThyssenKrupp Steel have now found a new, less expensive way of determining hardening behavior and forming limit curves which also delivers more reliable results. They carried out a series of bulge tests for each of the cold-rolled steels from the ThyssenKrupp Steel range and additionally checked their own findings by having them tested in external laboratories. The data acquired in this way were then correlated with the characteristics measured for the same materials in the one-dimensional tensile test. It proved possible to describe these correlations in mathematical formulae. A similar method was used for the forming limit curves.

Using these formulae, it is now possible - based only on the quick, low-cost tensile test - to make reliable predictions of material behavior even for elongation ranges which are not covered by the tensile test. This means that in the future users can dispense with the need for complex additional tests; the tensile test is enough, and TEM supplies reliable curves for forming simulation at the push of a button. The software module is initially available for all cold-rolled products from ThyssenKrupp Steel, but there are plans to extend TEM to hot-rolled products in the future.

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