T node geometry part with an edge crack

The case study aimed at verifying whether using the novel FormPlanet methodologies it could be possible to predict the edge-cracking phenomena more accurately than using current commercial code/cards.

The study was based on a T-node geometry part, aiming at representing the upper part of a car B pillar component. The shape of this component seems to be simple but often edge cracking appears.

In blue the blank shapes chosen to study the edge cracking phenomena

In this case-study, Fortiform steel and aluminium grades were tested. Edge-cracking in aluminium alloys hard to predict and, generally, when edge-cracking appears during the stamping process, it brakes completely the component.

The best blank shape to enhance the edge cracking phenomena was assessed and tested to study crack initiation and the deformation in physical stamping tests.

In parallel, virtual simulation considering different parameters and comparing the results between two models was performed. The first one (model 1) took care only about the Forming Limit Diagram (FLD) to predict the material failure, while model 2 combined FLD + the essential work of fracture (EWF) applied on the blank edge to predict the start of edge cracking. The differences between both models were mainly concerning the fracture behaviour. Model 1 showed an inner crack prediction, while in the model 2 the fracture took place at the sheet edge.

The comparison between the real stamping tests and simulation showed the improvements achieved using the failure criterion based on the Essential Work of Fracture work as the FLD criterion did not allow to predict edge fractures. From the industrial point of view, this could avoid the dies re-works due to edge cracking allowing the company to save money and time.

 Model 1 (down) and model 2 (up) simulation results

A non-standardised HET methodology has been developed through the investigation of the main parameters affecting the hole expansion ratio (HE) as clearance, tool wear, burr position and time from punch to expand. An experimental test configuration developed to objectively measure crack initiation and propagation and obtain reliable and repetitive values of HER.

The Half Specimen Dome Test (HSDT) permits to determine edge cracking behaviour during flanging in metal sheets. Compared to hole flanging tests, HSDT is a more time and cost-saving test for evaluating edge cracking. Another advantage is that is unaffected by aspects related to the tools manufacturing for cutting the holes, as the coaxiality between punch and die.

Micromechanical techniques as nanoindentation tests and Electron Back-Scatter Diffraction (EBSD) allows to assess the local damage and properties of sheared areas. The extension of local damage in sheared areas can be evaluated measuring the Young’s modulus variation by a methodology developed using the nanoindentation technique. EBSD permits to evaluate the damage by the quantification of phase transformations, grain distributions and grain size variations, recrystallised grains and texture due to the cutting and forming processes.

Fracture toughness evaluation of thin metal sheets (1-3 mm of thickness), using mechanically notched or fatigue pre-cracked DENT specimens, and thick sheets (>3mm of thickness) by bending tests of fatigue pre-cracked SENB specimens, following fast and affordable methodologies based on the Essential Work of Fracture (EWF). The EWF is the most suitable fracture parameter to rationalise crack-related problems in high-strenght metal sheets, such as edge fracture or crash behaviour.