Fine blanking is a shear cutting manufacturing process capable of producing sheet metal parts with completely smooth cutting surfaces, free of cracks and fracture-breaks. Additionally, this manufacturing process provides superior dimensional accuracy and surface finish compared to the conventional shearing process. However, for optimum results, together with the tool and the press, the material properties need to be considered as a component of a complete system.
In this context, a study of fracture behavior was carried out by ARANIA and Eurecat Technology Center on C45 steel grade for fine blanking applications. The aim of this study case was to validate an additional test capable of discriminating adequate (OK) from not adequate (NOK) material when standard tensile tests are not suitable for it. The chosen complimentary methodology was the Essential Work of Fracture (EWF) combined with a metallographic analysis, both optical and Scanning Electron Microscopy (SEM), to better understand the obtained results.
The EWF method consists of applying a constant displacement to a fatigue pre-notched specimen and recording the obtained load-line displacement curves for different ligament-length set of samples. The main advantage of the EWF method is its relative simplicity compared to standard testing procedures and has shown to be suitable to readily measure the fracture toughness of high strength sheet materials and better understand their cracking behavior during forming or in crash situations.
Figure 1. (a) Load vs. displacement curves of specimens with different ligament lengths; (b) lineal regression to obtain Essential Work of Fracture.
In order to run the EWF tests, Single Edge Notched Bending (SENB) specimens were obtained both from OK and NOK C45 material grade. These specimens have a 4mm notch machined by EDM at 90º respect to the rolling direction as a fatigue crack initiation point. Additionally, a fatigue pre-crack was nucleated in the notch edge of the specimens by a RUMUL resonant machine to achieve 5 different ligament lengths.
Figure 2. Single Edge Notch Bending (SENB) Specimen.
With the above-mentioned specimens, the EWF tests were run. From these tests, the load vs. displacement curves and fracture work vs. ligament length regression were obtained.
Load vs. displacement curves for C45 OK material showed how the height of the curve escalates as the ligament length increases. However, for C45 NOK, despite having a similar behaviour, for half of the specimens, a sudden drop on the load was observed after de maximum load.t
Figure 3. Load vs. displacement curves of OK and NOK samples.
ARANIA
ARANIA S.A., located in Amorebieta (Bizkaia) and founded in 1940, is a leading manufacturer in Southern Europe for cold rolled high and low carbon steel, as well as high-yield strength alloyed and microalloyed steel sectors. The company counts with the most advanced technology currently available on the market to provide a competitive product which is adapted to its customer’s needs, a working force of around 170 people and a maximum production capacity of 140,000 tons per year. The company has a high export rate, reaching 65% in recent years, and is present in more than 20 countries, mainly in Europe but also in America or Eastern Europe.
