Towards the accurate prediction of the plastic deformation and fracture of titanium alloy Gr5

Significance 

The design of lightweight and high-performance components used in numerous applications requires accurate prediction of the mechanical behavior of titanium Ti6A14V alloys. Generally, the mechanical behavior of these alloys exhibits complex features due to strength differential effects and distortional hardening. Considerable efforts, focusing mainly on modeling a part until fracture, have been devoted to advance the available models for finite element simulation of titanium alloys. These approaches take into account the sensitivity of the alloy yield strengths to both the temperature and strain rates. According to the existing literature, constitutive models can be identified based on uniaxial loading and/ or biaxial loading. Nonetheless, accurate prediction of large deformation behavior in titanium alloys have remained a challenge.

In recent research, Professor Víctor Tuninetti from Universidad de La Frontera, Dr. Gaëtan Gilles from Siemens Company, Prof. Paulo Flores from Universidad de Concepción, Gonzalo Pincheira from Universidad de Talca, Professor Laurent Duchêne and the Vice Dean of Research Anne-Marie Habraken from the Engineering School of University of Liège studied the mechanical behavior of Ti6A14V alloys until fracture based on three plasticity models. These models were assessed by comparing their respective finite element predictions with the obtained experimental results i.e. loads and displacement of field subjected to different stress triaxiality values. The main objective was to determine the most suitable plasticity model for designing bulk Ti6A14V parts. Their research work is currently published in the journal, Meccanica.

Briefly, their experiment comprised of both tensile and compression tests. The former was performed on round bars with V-notches, through-holes or radial notches while the later was performed on elliptical cross-section samples. The three models included the advanced orthotropic yield criterion CPB06 with distortional hardening, anisotropic Hill’48 yield criterion with distortional hardening and the classical Hill’48 yield locus with Voce isotropic hardening.

Results show satisfactory predicted plastic behavior of the CPB06 model attributed to the minimized the global. Even though the Hill model provided lower load errors in tensile tests, it was not effective in the global evaluation due to significant error observed in the compression state. Furthermore, the predicted plastic behavior of the isotropic Hill’48 model has shown to be only satisfactory for positive stress triaxilities. This poor performance was attributed to its inability to describe the strength differential effects of the alloy.

The authors demonstrated the significant impact of distortion hardening on the quality of global model predictions. For instance, the numerical simulations results show clearly that none of the models could be used for perfect prediction of both the measured loads and sample shapes. It was worth noting that the microscopic observations like the texture and crystal plasticity have a considerable influence on the evolution of yield locus of Ti6A14V alloys and thus should be taken into consideration during modeling. Additionally, accurate Voce law based on true stress-strain identification should be used in modeling of the post-necking behavior especially in cases involving large strains.

In a nutshell, the study insights highlight the impact of mechanical features such as hardening, plastic anisotropy and tension-compression asymmetry on the prediction of the post-necking deformation behavior in Ti6A14V alloys and why they should be taken into account during modeling for more authentic results. Based on the findings of the research, Professor Víctor Tuninetti in a statement to Advances in Engineering noted that his research will advance the prediction of damage and fractures of high performance titanium alloys.

About the author

Dr. Tuninetti is associate professor at the Department of Mechanical Engineering (http://www.dim.ufro.cl/), Universidad de La Frontera (Chile). He received his BS and MS degrees from University of Concepcion and PhD in engineering from the University of Liège, Belgium in 2014. He is currently the Director of internationalization and former Director of R&D in the Macrofacultad Consortium of Regional State Universities in the Central South of Chile (http://www.macrofacultad.cl/perfil/39).

He leads a research group at the Universidad de La Frontera focused on material characterization, finite element modeling, experimental mechanics, manufacturing processes and additive manufacturing.

About the author

Anne Marie Habraken was graduated in civil Engineering at the University of Liège in 1984. Since 1989, she holds a PhD from the same University. She got a permanent position as Research Associate of F.N.R.S. (Belgian National Funds for Scientific Research) in 1991 and became Research Director in 2006. To get this current position a second thesis was necessary, entitled Contributions to Constitutive laws of metals: micro-macro and damage models. President of ESAFORM European Scientific Association for material FORMing from 2004 to 2008, she is Vice Dean of research of the Engineering School of the university of Liège since October 2015.

She leads a numerical team which has a strong experimental involvement. Mainly focused on MATERIAL BEHAVIOUR (steel, Ti, Al, Ni..), her activities consist in the development and the identification of constitutive thermo-mechanical-metallurgical laws by multi-scale approaches. Her rheological models are implemented within different FE codes to predict microstructures, damage and rupture during forming process, or under specific static loading or in presence of fatigue, creep and corrosion phenomena. In collaboration with Uliege soil mechanic group, her team develops the Lagamine finite element code.

She was the thesis advisor of more than 15 PhD theses covering processes such as continuous casting, additive manufacturing, single point incremental forming, straightening cold roll forming and microforming.

Reference

Tuninetti, V., Gilles, G., Flores, P., Pincheira, G., Duchêne, L., & Habraken, A. (2019). Impact of distortional hardening and the strength differential effect on the prediction of large deformation behavior of the Ti6Al4V alloy. Meccanica, 54(11-12), 1823-1840.

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