Investigation of edge flangeability of high strength steels by hole expansion test

In the automotive industry, weight reduction and therefore the production of vehicles that consume less fuel has become one of the main design objectives. The realization of this goal without compromising mechanical performance requires the widespread use of advanced high strength steels (AHSS) with higher strength in structural elements. These steels show unique combinations of ductility and strength properties. By means of their high specific strength, they introduced as an effective solution in the automotive industry. However, as a common behavior, increasing strength levels of AHSS accompanied with decreasing ductility/formability levels. This behavior becomes more pronounced during secondary deformation applied after processes such as punching or cutting, lead to important industrial problems. To examine this concept, hole expansion test has been started to be widely used as a method that simulates the local formability of the edges after cutting operations. On the other hand, the hole expansion test is terminated when crack formation occurs throughout the thickness on the initiative of the operatör. However, since this is an operator-dependent process, it causes scattering in test results. However, this approach is an operator-dependent process, Because of that, it causes scattering in test results. Monitoring the test with a high-resolution camera is also a method used to minimize the scattering of the test results. However, this method is relatively expensive also does not perform with sufficient precision when materials of different thickness are concerned. In most cases, test systems record the forming force and punch displacement during the experiment. However, these data are generally not associated with test results at all. Similarly, it was seen that these data were not used in the decision-making mechanism regarding the termination of the test. In this study, to terminate the hole expansion test, an approach to change in force-displacement curve during the test and to match the curve characteristics with the deformation steps is presented. Within the scope of this approach, the test was terminated by matching the characteristic features of the force-displacement curves revealed in the DG tests with the deformation state. In this direction, the processes for punching and hole expansion testing of thin (1,00 mm) and thick (1,80 mm) materials belonging to DP 780 steel sheets, which have recently been widely used in the automotive industry, have been comparatively examined using experimental and computational approaches. According to the results, regardless of the thickness, it was understood that the forming of the samples during the hole expansion tests, took place at different stages under both elastic and plastic deformation. It was observed that, during the transition between these stages, the rate of change of punch force (dF / dx) with respect to the displacement exhibits significant changes. It was seen that, this change remained almost constant during the crack formation and propagation process. Based on this behavior, it has been shown that by monitoring dF / dx values during testing, the crack formation process can be determined without observing the sample.