In the recent years, it has increased the importance of lightweighting in the automotive industry due to the CO2 emission regulations. Generally, the vehicle trend is to increase the weight because more equipment is onboard than the previous model to increase the safety and the comfort during the usage (navigation systems, connectivity, ADAS, etc.).
In the automotive industry, there is an urgent need to work on the reduction of CO2 to be in accordance with greenhouse emissions regulations. One of the key factors to pursuit this purpose is the use of new advanced high-strength steels (AHSS) or aluminium grades in order to reduce the weight of the cars and, consequently, the fuel consumption and CO2 emissions.
Figure 1: Evolution of CO2 regulation through the years 2000-2025. Source: ICCT.
Edge-cracking, the main drawback of the use of AHSS for lightweighting in the automotive industry
Typical metal sheet forming processes for manufacturing AHSS parts includes cutting operations like shearing, blanking or punching. It is well known that such operations produce a shear-affected zone in the cut edge, with material deformation and damage (work hardening, residual stresses, voids, microcracks or cracks). Such edge damage degrades the edge mechanical properties, which reduces local formability (needed for post-cutting forming operations).
In materials with limited ductility as AHSS, edge damage may trigger edge cracks in sheared areas that expand during forming operations involving stretch flanging or hole expansion. Typical examples of stretch flanges in the automotive industry include cut-outs in automotive inner panels and corners of window panels, hub-holes of wheel discs, hidden joints, etc. This cracking phenomenon is known in the automotive industry as edge-cracking.
Edge-cracking is one of the main drawbacks in the application of AHSS as lightweight solutions. It compromises part quality because it cannot be predicted from the part design stage and cracks could appear in part production when cutting parameters or coil properties change.
Promising results in accurately predicting edge-cracking in AHSS and aluminium components
The newest AHSS family in the market is the Trip-Bainitic Steel, which combines high mechanical properties (1180 MPa of tensile strength) with better formability than conventional Dual Phase Steel. The extra formability in Trip-Bainitic Steel is provided by the retained austenite that during the deformation is transformed into martensite. This material grade is under study in the FormPlanet research project by Centro Ricerche Fiat (CRF).
Together with Eurecat, CRF is responsible for analysing the correlation between the physical behaviour of the material during the stamping phase and the virtual one. During the virtual simulation were taken in consideration and compared 3 different models. The first one (model 2) takes care only about the Forming Limit Diagram (FLD) to predict the material failure, the model 3.1 combine FLD + the essential work of fracture (EWF) applied on the blank edge to predict the edge cracking starting, the last, model 3.2, the more completed, combines the FLD + EWF + UF a parameter that describe the crack propagation evolution.
Figure 2: Models adopted during the stamping virtual simulation
This research aims at predicting with higher accuracy the behaviour or the material and component during the stamping phase and forecast when and where the edge-cracking phenomena could appear.
The study includes, from one hand, Trip-Bainitic Steels, and, from another hand aluminium grades as in this material edge-cracking is extremely hard to predict and, generally, when edge-cracking appears during the stamping process, it brakes completely the component.
Eurecat has provided CRF with blank shapes for T node geometry (that aims to simulate the upper side of the B pillar reinforcement) to perform the stamping test. Hollow shapes help accentuate the edge-cracking behaviour. CRF has used a hydraulic press to perform physical tests to control in detail the deformation, the blank holder force was setup to 200 bars limiting the material flow under the die.
Figure 3: In blue, the blank shapes chosen to study the edge crack phenomena
The results are very promising as they indicate a good correlation between the physical and virtual simulation method. We could observe that the crack initiated in the same place predicted by the simulation virtual tool.
Further investigation will be carried out during the project to refine the simulation method and increase the accuracy of the correlation between what can be predicted via virtual simulation and what happens to the component during stamping process.
Our goal is to reach a 100% accuracy through virtual simulation order to be able to predict edge-cracking and reduce costs during automotive components development phases.
Figure 4: Comparison between physical and virtual simulation.
Michele Maria Tedesco
Master’s degree in Mechanical Engineering from the Politecnico of Turin. He started working on the Sheet Metal Department at Centro Ricerche Fiat (CRF) to become today the responsible of the Metals Department, including sheets, bulk, failure analysis and fatigue in its competences.