Test procedure determining susceptibility to edge cracks
14.10.2015 | Initiative Automotive
Test procedure determining susceptibility to edge cracks
In light of the advances towards lightweight steel construction driven by economic and environmental objectives and specifications, the steel grades in use are becoming stronger and stronger, while sheets are becoming thinner and thinner - and the component geometries are increasingly gaining in complexity at the same time. Consequently, the requirements and demands placed on the mechanical characteristics of the pre-material are continuously rising.
Major forming challenges arise, however, during the processing of the higher and ultra-high strength sheet materials that are employed. Shearing, as one of the most frequently used production processes in sheet working, is one example of such a challenge. In shearing, the material’s formability in the edge area is significantly reduced, which increases the risk of a crack at the edge. If the material cracks during the forming almost exclusively at the sheet edge, it is said to be susceptible to edge cracks. Numerous investigations have shown that a quantitative evaluation of the formability of a sheet edge produced by shearing is not possible using current test methods and parameters such as tensile tests and forming limit curves. For this reason, numerous edge crack test methods have recently been developed.
The most widespread test method that is currently standardized is the so-called hole expansion test according to ISO 16630. In this test, a hole with a diameter of dp = 10 mm is placed into the sheet sample by shearing and then expanded with a conical stamp. The operator stops the expansion as soon as he or she can see a crack running through the entire thickness of the sheet. The test result is given by the so-called hole expansion ratio, which is defined as the ratio of the hole diameter increase (Dh to D0) to the original hole diameter D0 (Figure 1).
Due to the comparatively simple performance, Salzgitter Flachstahl uses the standardized hole expansion test as a fast test and material release test. For a specific component design and to provide characteristics for the numerical forming simulation, however, information that extends beyond what can be determined with the hole expansion test according to ISO 16630 is needed. This is why additional edge crack tests, such as the so-called hole expansion test with Nakajima punch, are employed at Salzgitter Mannesmann Forschung. In the "hole expansion with Nakajima punch" test, the test setup is used to determine a forming limit curve. As in the case of the ISO 16630 hole expansion test, the trial consists of two steps. First a hole with a diameter of 20 mm is placed into a quadratic sample by punching. In the second step, the sample that has been prepared is expanded with a hemispheric punch. The test is immediately stopped as soon as a crack that runs through the entire sheet thickness can be detected. The result is the hole expansion ratio already described above. In contrast to the ISO 16630 hole expansion test, however, a stochastic pattern can be applied to the sheet surface before the forming and the ARAMIS optical measuring system from the company GOM can be deployed to perform a detailed strain analysis for the sample area near the edge. Salzgitter Mannesmann Forschung has developed an analysis macro that contains defined crack criteria. This macro can be used for the automatic detection and determination of the time of the crack and the hole expansion ratio (Figure 2).
A number of approaches can be found in the respective literature that show how to use the edge crack characteristics determined in this way in the forming simulation. A general procedure backed up by a broad database and tested on a wide range of component geometries is not yet known. Therefore a project involving the Volkswagen component tool making and group research areas, ESI GmbH, and Salzgitter Mannesmann Forschung GmbH defined a first industrially deployable procedure taking a chassis component as an example. This procedure was then applied and validated. It offers a reliable prognosis with regard to the susceptibility to edge cracking for all component edges that were not significantly formed before the cutting process and that are in the area of the one-axis pull, such as in the case of the classic collar forming. Future work will focus on validating the procedure with the help of further actual component geometries and an examination of the susceptibility to edge cracks in pre-expanded material.
Your contact at SZMF: Sebastian Westhäuser