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Civil-Comp Proceedings
ISSN 1759-3433 CCP: 79
PROCEEDINGS OF THE SEVENTH INTERNATIONAL CONFERENCE ON COMPUTATIONAL STRUCTURES TECHNOLOGY Edited by: B.H.V. Topping and C.A. Mota Soares
Paper 56
Ductile Fracture under Dynamic Loading using a Strain-Rate Dependent Cohesive Model M. Anvari+, I. Scheider* and C. Thaulow+
+Department of Engineering Design and Materials, NTNU, Trondheim, Norway
Full Bibliographic Reference for this paper
M. Anvari, I. Scheider, C. Thaulow, "Ductile Fracture under Dynamic Loading using a Strain-Rate Dependent Cohesive Model", in B.H.V. Topping, C.A. Mota Soares, (Editors), "Proceedings of the Seventh International Conference on Computational Structures Technology", Civil-Comp Press, Stirlingshire, UK, Paper 56, 2004. doi:10.4203/ccp.79.56
Keywords: dynamic loading, ductile fracture, cohesive zone model, rate dependency, triaxiality, Gurson type model, centre cracked specimen, aluminium alloy, finite element method.
Summary
The rate of loading affects the mechanical response of ductile materials.
Depending on the loading speed and material properties, structural resistance to
dynamic loading is influenced by inertia, stress waves, rate sensitivity and adiabatic
heating. These parameters need to be considered more carefully when a structure
contains cracks. Different approaches can be applied for crack propagation and
failure analysis, one of which is a local approach named "cohesive zone modeling".
The approach is based on the models introduced by Dugdale [1] and Barenblatt [2]
called strip-yield models and later applied to finite element method by different
authors, e.g. [3,4]. In this approach, crack growth is modelled by introducing a
process zone ahead of a crack tip which accounts for traction and separation of
material in the damage-free surrounding. The application of the cohesive zone
model allows for dividing the dissipated energy into global plastic strain energy and
local separation energy. In the context of the finite element method, the surrounding
material is modelled using continuum elements and the crack growth is simulated by
introducing interface elements that are embedded between the continuum ones. The
cohesive properties, which are known as "traction separation law (TSL)" affect the
macroscopic stresses and strains and consequently the response of a structure.
Because ductile crack growth is the consequence of void nucleation, growth and
coalescence, it is wise to obtain the TSL based on the mechanical behavior of voided
cells, e.g. Gurson type model [5,6].
The effect of different parameters on the mechanical response of an In the paper, it is shown that cohesive elements have the potential of doing ductile crack growth simulation not only in static cases, but also in high speed dynamic loading. The cohesive model presented can be used for both small and large scale yielding and takes effects of different strain rates and triaxialities into account. Unloading at the crack tip, which happens because of stress waves, has been implemented by irreversible separation behaviour. The results of the analysis show that generally, ignoring constraint or local strain rate on TSL makes the analysis underestimate the toughness. The toughness of the structure increases under dynamic loading because of the inertia. Ignoring rate sensitivity of material in high speed loadings can lead to quite high energy absorption predictions. Depending on the load speed, material properties, the structure dimensions and the crack length, the effect of phenomena like elastic waves, strain rate, adiabatic heating and inertia forces might be different. References
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