Scientific publications

In this page you will be able to find the scientific publications related to ToughSteel project published by partners

Stating failure modelling limitations of high strength sheets: implications to sheet metal forming

Journal of Materials Processing Technology. Materials 2021, 14(24), 7821 Special Issue Metal Forming and Forging.​

Olle Sandin, Pär Jonsén, David Frómeta and Daniel Casellas

This article discusses the fracture modelling accuracy of strain-driven ductile fracture models when introducing damage of high strength sheet steel. Numerical modelling of well-known fracture mechanical tests was conducted using a failure and damage model to control damage and fracture evolution. A thorough validation of the simulation results was conducted against results from laboratory testing. Such validations show that the damage and failure model is suited for modelling of material failure and fracture evolution of specimens without damage. However, pre-damaged specimens show less correlation as the damage and failure model over-predicts the displacement at crack initiation with an average of 28%. Consequently, the results in this article show the need for an extension of the damage and failure model that accounts for the fracture mechanisms at the crack tip. Such extension would aid in the improvement of fracture mechanical testing procedures and the modelling of high strength sheet metal manufacturing, as several sheet manufacturing processes are defined by material fracture


Stretch flangeability of AHSS automotive grades versus cutting tool clearance, wear, angle and radial strain gradients

IOP Conf. Series: Materials Science and Engineering (2022): 1238 (012041).

P Larour; J Hinterdorfer; L Wagner; J Freudenthaler; A Grünsteidl; M Kerschbaum

Stretch flangeability tests are performed on cold rolled AHSS automotive steel grades for crack sensitivity evaluation. The ISO16630 tests procedure is used with different cutting edge conditions including cutting clearance, cutting tool wear, cutting tool angle and hole expansion punch tool geometry. Among all influence parameters, the cutting clearance is considered to be the most critical to control. Low (<10%) and high (>20%) clearances are significantly detrimental to hole expansion ratio (HER). An optimum in HER is reached around 15% clearance. Both cutting punch wear and cutting die wear each affect negatively the HER values in comparison to new sharp cutting tools. Varying the cutting angle with concave and convex hole punching tool geometries instead of orthogonal cutting has a rather negative effect on stretch flangeability due to irregular cut edge quality along hole perimeter and excessive tool wear in the concave configuration. The influence of hole expansion punch geometry (flat R25, biaxial Nakajima R25 vs. ISO16630 60° conical) on the HER values is also investigated. The (true) HER values are proportional to the FE simulated logarithmic radial strain gradient at fracture.


Prediction of sheared edge characteristics of advanced high strength steel

IOP Conf. Series: Materials Science and Engineering (2022): 1238 (012041).

Olle Sandin; Samuel Hammarberg; Sergi Parareda; David Frómeta; Daniel Casellas

In the present work, numerical models are developed for the shearing and cutting process of advanced high strength steel-blanks which can predict the edge morphology in the shear effected zone. A damage model, based on the modified Mohr-Coulomb fracture surface, is calibrated. To increase the predictability of the numerical models, the fracture surface is fine-tuned in areas corresponding to the stress-state of cutting, a methodology called Local calibration of Fracture Surface (LCFS). Four cutting cases with varying clearance are simulated and verified with experimental tests, showing good agreement. It is thus found that the suggested methodology can simulate cutting with adequate accuracy. Furthermore, it is found that solely using plane-stress tensile specimens for calibrating the fracture surface is not enough to obtain numerical models with adequate accuracy.


Sheared edge formability characterization of cold-rolled advanced high strength steels for automotive applications

IOP Conf. Series: Materials Science and Engineering (2022): 1238 (012041).

Antoni Lara; David Frómeta; Sergi Parareda; Daniel Casellas; Patrick Larour; J Hinterdorfer; Eisso Atzema; Martin Heuse

Edge cracking has become a limiting factor in the use of some advanced high strength steels (AHSS) for high-performance automotive applications. This fact has motivated the development of a multitude of experimental tests for edge formability prediction over the last years. In this sense, the Hole Expansion Test (HET) according to ISO16630 has been established in the automotive industry as a standard procedure for edge cracking sensitivity ranking. However, whereas it may be useful for rapid material screening, the results are often not accurate and reliable enough. Consequently, alternative methods based on Digital Image Correlation (DIC) have been proposed aimed at improving the prediction of edge cracking occurrence during forming and obtaining useful strain data that can be implemented in forming simulations. This paper explores the applicability of different DIC-based methods, such as Half-Specimen Dome Tests, Sheared Edge Tensile Tests, and KWI hole expansion tests with a flat nosed punch, for characterizing the edge formability of three cold-rolled AHSS sheets. The results obtained from the different testing methods are compared and validated with a laboratory-scale demonstrator. Finally, the limitations and advantages of the different methods are discussed.


A new cracking resistance index based on fracture mechanics for high strength sheet metal ranking

IOP Conf. Series: Materials Science and Engineering (2021): 1157 (012094).

David Frómeta; Sergi Parareda; Antoni Lara; Laura Grifé; Ilef Tarhouni; Daniel Casellas

Driven by current safety and weight reduction policies in the automotive sector, the development of new high strength sheet metal products has experienced unprecedented growth in the last years. With the emergence of these high strength materials, new challenges related to their limited ductility and higher cracking susceptibility have also raised. Accordingly, the development of new fracture criteria accounting for the material’s cracking resistance has become unavoidable. In this work, a new cracking resistance index (CRI) based on fracture mechanics is proposed to classify the crack propagation resistance (i.e. the fracture toughness) of high strength metal sheets. The index is based on the fracture energy obtained from tensile tests with sharp-notched specimens. The procedure is very fast and simple, comparable to a conventional tensile test, and it may be used as routine testing for quality control and material selection. The CRI is investigated for several advanced high strength steel (AHSS) sheets of 0.8-1.6 mm thickness with tensile strengths between 800 and 1800 MPa. The results show that the proposed index is suitable to rank high strength steel sheets according to their crack propagation resistance and it can be correlated to the material’s crashworthiness and edge cracking resistance.


Trending topics in edge ductility research

Forming technology forum 2022, Enschede, Netherlands, 22/09/2022

Eisso H. Atzema

Advanced High Strength Steels (AHSS) often exhibit issues in so-called edge ductility. The residual deformation available in a cut edge is limited by the cutting operation. And although better edge preparation methods are available they are not economically viable and hence shearing/cutting remains the preferred option. After years of research many influences on cut edge formability are known. For instance that the optimal clearance in AHSS is larger than that prescribed in the ISO TS16630 test standard. That sharp tools make a difference. It is also still seen that the test shows considerable scatter and difference between labs.

But there are also a number of lesser known phenomena. The delay time between cutting and forming sometimes makes a difference. The speed of cutting can make a difference. A second cut of only a fraction of material (so-called shaving) can make a significant difference but the difference depends strongly on sharpness of tools. The gradient of strain in radial direction is thought to have an influence on the results. Alternative tests to HEC may yield different ranking of materials. Links are being found between fracture toughness and HEC, and between local ductility and HEC. These links, however, do not necessarily apply to all materials.

This paper aims to give an overview of the newer findings from projects that the author is involved in as well as recent literature that has come to the attention of the author.