Ductile fracture at low stress triaxialities – LOTERIE

- tenson-torsion testing of bulk materials
- development of constitutive models based on non-linear homogenization
- tomography and in-situ testing

- new type of fracture specimen for tension-torsion testing

- comprehensive experimental program demonstrating the effect of the Lode angle on the strain to fracture at low stress triaxialities

• Papasidero, J., Doquet, V. and Mohr, D. (2012). Modified Lindholm Specimen to Characterize the Ductile Fracture of Bulk Metals at Low Stress Triaxialities, Experimental Mechanics, submitted for publication.

Understanding and predicting ductile fracture is critically important for the engineering of lightweight and safe primary load carrying structures or metal forming. Ductile fracture of metals at high stress triaxialities has been extensively studied over the past 50 years. However, recent findings suggest that the underlying mechanisms and micromechanics of ductile fracture change at low stress triaxialities (distortion and rotation of cavities rather than growth, early strain localization) so that classical models fail to predict ductility in that range. Furthermore, the importance of the normalized third stress invariant, the so-called Lode angle, has been recognized.

It is the objective of this joint research effort between the LMS (Ecole Polytechnique) and the CdM (Ecole des Mines de Paris) to design a new multi-axial testing technique to study the onset (and propagation) of ductile fracture over a wide range of low stress triaxialities and Lode angles. The experimental work will make use of a triaxial testing machine (tension/compression-torsion-internal pressure) along with a stereo digital image correlation system. In addition, biaxial fracture experiments will be designed for in-situ testing in a SEM and a tomograph for real-time 3D monitoring of damage. A new micromechanics-based constitutive model will be developed for general ellipsoidal voids microstructures subject to large deformations. In view of practical applications, an effort will be made to make use of a homogenization technique that provides a computationally efficient and stable numerical framework. In addition, a non-local theory will be developed to eliminate the well-known spurious mesh dependency caused by damage accumulation. All models will be implemented into a finite element software and validated by simulation of the above experiments.