Does the component deliver what the characteristic promises?

22.06.2016 | Initiative Automotive

Does the component deliver what the characteristic promises?

How customized testing methods help with material selection.
Figure 1: Chassis component and derivation of the specimen geometry: a) Semi-finished product; b) Forming operation; c) Stamping, trimming operation; d) Cyclic inspection

Characteristics play a crucial role during component development. Characteristics of standardized test specimens are often used as input data during process optimization and component development. Classic examples are tensile tests and Wöhler fatigue tests on tensile specimens. A comparison of such specimens with the later component clearly shows that they do not have much in common. In the case of the classic specimens, the component's overall production process is not taken into consideration when determining the characteristic. At the same time, it is obvious that forming (work hardening) and trimming (notching) clearly influence the steel. This fact can ultimately lead to the actual component performance being completely different than the determined specimen characteristics would lead to believe.
 
In order to be able to offer our customers high-performance products it is necessary to take the influencing factors described above into consideration when determining the characteristics. Therefore a bowl-shaped specimen form was developed at Salzgitter Mannesmann Forschung GmbH that can be tested under laboratory conditions and that is complex enough to take production-related influencing factors into consideration (Figure 1).

Figure 2: Test concept
Figure 2: Test concept for bowl-like hot-rolled strip specimen

The specimen consists of a strip-shaped semi-finished product that runs through a production process that is similar to that of the future steel component: First a bowl shape is deep drawn and then a hole is stamped into the middle of this bowl. The specimen produced in this way is checked in a Wöhler test. This involves placing the specimen into a clamping device and then subjecting it to cyclic loading at a controlled force with a semi-spherical stamp that lies on the cut edge of the stamped hole (Figure 2).
 

Figure 3:Comparison of S355MC, DP600 and SZBS800
Figure 3: Comparison of the hot-rolled strip steels S355MC, DP600 and SZBS800 in the Wöhler test

The following Wöhler crack lines for three steel grades are shown by way of example for results with the component-like bowl specimen (Figure 3). The steels S355 (t = 3.6 mm), DP600 (t = 3.6 mm) and bainitic SZBS800 (t = 3.5 mm) were used. The determined cycles-to-cracking values are a characteristic for the future component's service life.
 

These results are determined under conditions that are close to those of actual use. They are necessary to allow a meaningful comparison of the various steel grades. It can be seen that the Wöhler lines for the S355 and DP600 steels in the machined stated (meaning formed and trimmed) do not differ. Compared to these conventional hot-rolled strip steels, the SZBS800 bainitic steel offers clear advantages because when the same load is used it achieves higher cycles-to-cracking values and consequently a potentially longer component service life.
 
Does the component deliver what the characteristic promises? This question can be unambiguously answered with "yes" if the right tests have been conducted and the right characteristics have been taken into consideration when selecting the material. The presented specimen and the results achieved with it support us in working with the customer to select the right materials for demanding applications.