Insight into the role of triaxiality in mode-I, plane strain resistance curves of a representative ductile metal has been gained. Growth of a macroscopic crack is simulated as per modified boundary layer formulation for a range of constraint parameter with the fracture process represented by a triaxiality dependent cohesive model. In contrast to the predictions by a fixed cohesive law, the study shows that by including the effect of triaxiality on the work of separation, the stick-slip nature or the non-uniformity in the rate of the crack growth and its manifestations on the plastic wake and fracture surface can be predicted that are closer to trends observed in experimental literature. © 2014 Springer Science+Business Media Dordrecht.

Anuradha Banerjee

Department of Applied Mechanics

Cracks

Ductile fracture

Slip forming

Cohesive zone model

FRACTURE PROCESS

Fracture surfaces

MACROSCOPIC CRACKS

Modified boundary layers

RESISTANCE CURVES

TRIAXIALITY

WORK OF SEPARATION

Computer simulation

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

International Journal of Fracture

In the simulation of the ductile fracture process in a low ductility aluminum alloy, the limitations of the current implementation of a stress-state dependent cohesive model are identified. Ductile fracture data was generated at moderate triaxiality with experiments on a range of notched bars while at high triaxiality in growth of a pre-existing mode-I crack in compact test specimens. In the corresponding finite element analysis, cohesive elements obeying a stress-state dependent cohesive law were introduced in the plane where material separation was expected to occur. By recognizing that the effect of model parameters is decoupled in fracture at moderate triaxiality, a procedure is outlined to determine the unique combination of model parameters that is shown to reproduce the experimental data for the entire range of triaxiality well. It is argued that the necessity of a plane strain core and its thickness is largely driven by the extent to which plastic deformation spreads during the growth of crack. © 2017 Elsevier Ltd

Engineering Fracture Mechanics

Simulation of fracture in a low ductility aluminum alloy using a triaxiality dependent cohesive model

A recently formulated triaxiality dependent cohesive model for plane strain is implemented and its versatility is tested in simulation of ductile fracture of mild steel at different states of stress. The triaxiality dependent model was implemented as linear displacement formulation based elements whose constitutive behaviour was dependent on the stress-state of the neighbouring continuum element. By comparing the experimental data and predictions of corresponding plane strain simulations, the model parameters are estimated. The model is shown to be effective in reproducing characteristic features of the macroscopic response of both pre-cracked as well as geometries without a preexisting nominal defect. Since the model parameters are held constant for simulations at different stress-states, they are effectively material constants. © 2013 Springer Science+Business Media Dordrecht.

Implementation and validation of a triaxiality dependent cohesive model: Experiments and simulations

A stress-state dependent traction-separation law is combined with an irreversible damage parameter to formulate a cohesive model relevant for life assessment under complex stress-states. Evolution of damage is based on continuum damage laws requiring two cohesive fatigue parameters. For two representative metals, model is able to reproduce typical stress-life response, mean stress effects and sequencing effects under variable amplitude loads. Control of cohesive fatigue parameters on characteristics of the stress-life curve is established, followed by predictions for proportional, cyclic stress-states. With increasing constraint, initiation and growth of damage is more rapid such that higher constraint results in lower life expectancy. © 2011 Published by Elsevier Ltd.

International Journal of Fatigue

A cohesive model for fatigue failure in complex stress-states

Two different approaches that explicitly incorporate the stress triaxiality into cohesive zone models applicable to thin-walled structures are compared to identify the relative merits and limitation of these models. The number of model parameters involved, the ease of parameter determination and the predictive capabilities of the models over a wide range of thin-walled geometries are investigated. The first model, proposed recently by the authors, uses basic elastic-plastic constitutive equations combined with a model parameter depending on the average triaxiality in plane stress conditions. The second model incorporates stress-state through exponential dependence of cohesive strength on triaxiality, similar to plane strain studies earlier. The respective parameters for both models are identified and subsequently applied to several notched and precracked specimens. It is shown that in contrast to stress-state independent models, both constraint dependent models are able to predict well failure of a wide range of structures. While the model incorporating triaxiality dependent cohesive parameters has more parameters to be determined, it is not restricted to any specific stress condition and therefore can be extended to arbitrary three-dimensional stress-states. © 2010 Elsevier Ltd.

Fulltext

Comparison of different stress-state dependent cohesive zone models applied to thin-walled structures

A versatile cohesive zone model to predict ductile fracture at different states of stresses is proposed. The formulation developed for mode-I plane strain accounts for triaxiality of the stress-state explicitly by using basic elastic-plastic constitutive relations combined with two stress-state independent new model parameters. Comparison with available predictions of cohesive models based on porous plasticity damage models is used to demonstrate the efficacy of the proposed model. The effect of triaxiality on conventional cohesive parameters is well predicted as peak stresses are shown to increase while the cohesive energy decreases with triaxiality. © 2009 Elsevier Ltd. All rights reserved.

Journal	International Journal of Fracture
Publisher	Kluwer Academic Publishers
ISSN	03769429
Open Access	No