A New Method in Identifying Critical Void Volume Fraction of GTN Model for NVA Mild Steel in Ship Collision and Grounding Scenario

2018 ◽  
Author(s):  
Zijie Song ◽  
Zhiqiang Hu
Keyword(s):  
Ductile Fracture ◽  
Volume Fraction ◽  
Hydrostatic Stress ◽  
Void Volume Fraction ◽  
New Method ◽  
Void Volume ◽  
Gtn Model ◽  
Ship Collision ◽  
Ductile Fracture Process ◽  
Critical Void

The NVA mild steel is a commonly used material in shipbuilding, which possesses good ductility character. However, the description of ductile fracture process for NVA steel in numerical simulation is still a challenging task. A new method to predict the critical void volume fraction fc of Gurson-Tvergaard-Needleman (GTN) model is introduced in this paper. GTN-model is one of the well-known micromechanical models for ductile fracture. The traditional plasticity theory assumes that the plastic volume is incompressible and that the yield of the material is independent of the hydrostatic stress, whereas the yield surface of the GTN-model takes the effect of the macroscopic hydrostatic stress into account. The yield surface is reduced with the increase of the void volume fraction, which can reflect the deterioration characteristics of the material with development of damage during the deformation process. Therefore, GTN-model is a promising mathematical model for describing the ductile fracture process of the ship structures during accidental scenarios of collision and grounding. The traditional way to determine fc of GTN-model is using the inverse method directly, which has a high degree of uncertainty. A new method based on Hill, and Bressan & Williams’s assumptions proposed in this paper solve this problem effectively. Besides, the combined of Voce and Swift constitutive model is used to describe the mechanical property of the NVA material. Furthermore, numerical simulations were also conducted with code LS_DYNA by developing the user-defined subroutine. It is found that the model can predict the structural damage quite accurately, which proves its feasibility of being applied in the research of structural responses in ship collision and grounding accidents.

scholarly journals Critical Void Volume Fraction Identification Based on Mesoscopic Damage Model for NVA Shipbuilding Steel

2019 ◽  
Vol 18 (4) ◽  
pp. 444-456
Author(s):  
Zijie Song ◽  
Zhiqiang Hu ◽  
Jonas W. Ringsberg
Keyword(s):  
Numerical Simulation ◽  
Mild Steel ◽  
Ductile Fracture ◽  
Volume Fraction ◽  
Fracture Behaviour ◽  
Void Volume Fraction ◽  
Void Volume ◽  
Simulation Results ◽  
Ship Collision ◽  
Critical Void

Abstract NVA mild steel is a commonly used material in the shipbuilding industry. An accurate model for description of this material’s ductile fracture behaviour in numerical simulation is still a challenging task. In this paper, a new method for predicting the critical void volume fraction fc in the Guson-Tvergaard-Needleman (GTN) model is introduced to describe the ductile fracture behaviour of NVA shipbuilding mild steel during ship collision and grounding scenarios. Most of the previous methods for determination of the parameter fc use a converse method, which determines the values of the parameters through comparisons between experimental results and numerical simulation results but with high uncertainty. A new method is proposed based on the Hill, Bressan, and Williams hypothesis, which reduces the uncertainty to a satisfying extent. To accurately describe the stress-strain relationship of materials before and after necking, a combination of the Voce and Swift models is used to describe the material properties of NVA mild steel. A user-defined material subroutine has been developed to enable the application of the new parameter determination method and its implementation in the finite element software LS-DYNA. It is observed that the model can accurately describe structural damage by comparing the numerical simulation results with those of experiments; thus, the results demonstrate the model’s capacity for structural response prediction in ship collision and grounding scenario simulations

Assessment of Gurson-Tveergard-Needleman Model Under High Stress Triaxiality

2016 ◽  
Author(s):  
Jiru Zhong ◽  
Tong Xu ◽  
Kaishu Guan
Keyword(s):  
Ductile Fracture ◽  
Volume Fraction ◽  
Stress Triaxiality ◽  
Fracture Initiation ◽  
Tensile Specimen ◽  
Void Volume Fraction ◽  
Model Parameters ◽  
Gtn Model ◽  
Notched Specimens ◽  
Critical Void

The Gurson-Tveergard-Needleman (GTN) model has been widely used to describe ductile fracture. In this paper, a series of tensile tests were carried out on notched specimens to assess the GTN model. The GTN model parameters were calibrated from a smooth tensile specimen by a hybrid particle swarm optimization, and the reliability of the calibrated parameters was verified by the profile of the smooth tensile specimen. The calibrated parameters were used to predict the ductile fracture of notched specimens. A comparison of fracture initiation sites between simulations and experiments indicates that the GTN model has a good performance on predicting fracture initiation site but fails at predicting fracture moment. The assessment of the transformability of the GTN model parameters was performed by comparing the load-displacement curves between simulations and experiments. It is observed that the GTN model parameters are material constant, except the critical void volume fraction fc. The influence of stress triaxiality on the critical void volume fraction fc is also discussed.

A STUDY ON EXPERIMENTS AND SIMULATIONS FOR DUCTILE FRACTURE OF ISOTROPIC MATERIAL USING ROUSSELIER'S DAMAGE MODEL

2012 ◽  
Vol 06 ◽  
pp. 257-262
Author(s):  
Junhang Guo ◽  
Ri-ichi Murakami ◽  
Shengdun Zhao
Keyword(s):  
Ductile Fracture ◽  
Damage Mechanics ◽  
Volume Fraction ◽  
Damage Model ◽  
Void Volume Fraction ◽  
Void Volume ◽  
Integration Algorithm ◽  
Stress Integration ◽  
Simulation Results ◽  
Critical Void

Ductile fracture has been a hot topic for a long time for its importance to mechanical design in evaluating the risk of failure. In this paper, the A5052BD-H14's ductile fracture is studied using a new constitutive equation based on the continuum damage mechanics. A novel full-implicit stress integration algorithm is developed based on Rousselier's damage model and implemented into finite element analysis (FEA) models by the ABAQUS/Explicit using the user material subroutine. The tensile tests of A5052BD-H14 with notch were taken and the load-displacement curves were recorded. By simulations, the evolutions of the void volume fraction are obtained and can be used as calibration for the critical void volume fraction. The validity of the damage model and the proposed stress integration algorithm are verified by comparing the experimental results and the simulation results. Further, by using the critical void volume fraction and element deletion, the simulation results show that this method is reliable, and can be used to predict the fracture of metals.

scholarly journals Forming Limit Stress Diagram Prediction of Pure Titanium Sheet Based on GTN Model

Materials ◽  
2019 ◽  
Vol 12 (11) ◽  
pp. 1783 ◽  
Author(s):  
Tao Huang ◽  
Mei Zhan ◽  
Kun Wang ◽  
Fuxiao Chen ◽  
Junqing Guo ◽  
...  
Keyword(s):  
Volume Fraction ◽  
Pure Titanium ◽  
Void Volume Fraction ◽  
Void Volume ◽  
Forming Limit ◽  
Stress And Strain ◽  
Gtn Model ◽  
Stress Diagram ◽  
Limit Stress ◽  
Hemispherical Punch

In this paper, the initial values of damage parameters in the Gurson–Tvergaard–Needleman (GTN) model are determined by a microscopic test combined with empirical formulas, and the final accurate values are determined by finite element reverse calibration. The original void volume fraction (f0), the volume fraction of potential nucleated voids (fN), the critical void volume fraction (fc), the void volume fraction at the final failure (fF) of material are assigned as 0.006, 0.001, 0.03, 0.06 according to the simulation results, respectively. The hemispherical punch stretching test of commercially pure titanium (TA1) sheet is simulated by a plastic constitutive formula derived from the GTN model. The stress and strain are obtained at the last loading step before crack. The forming limit diagram (FLD) and the forming limit stress diagram (FLSD) of the TA1 sheet under plastic forming conditions are plotted, which are in good agreement with the FLD obtained by the hemispherical punch stretching test and the FLSD obtained by the conversion between stress and strain during the sheet forming process. The results show that the GTN model determined by the finite element reverse calibration method can be used to predict the forming limit of the TA1 sheet metal.

scholarly journals Experimental determination of the void volume fraction for S235JR steel at failure in the range of high stress triaxialities

2017 ◽  
Vol 62 (1) ◽  
pp. 167-172 ◽  
Author(s):  
P. G. Kossakowski
Keyword(s):  
Volume Fraction ◽  
High Stress ◽  
Material Model ◽  
Tensile Tests ◽  
Void Volume Fraction ◽  
Void Volume ◽  
Advanced Technique ◽  
Quantitative Image ◽  
Critical Void

Abstract This paper is concerned with the critical void volume fraction fF representing the size of microdefects in a material at the time of failure. The parameter is one of the constants of the Gurson-Tvergaard-Needleman (GTN) material model that need to be determined while modelling material failure processes. In this paper, an original experimental method is proposed to determine the values of fF. The material studied was S235JR steel. After tensile tests, the void volume fraction was measured at the fracture surface using an advanced technique of quantitative image analysis The material was subjected to high initial stress triaxialities T0 ranging from 0.556 to 1.345. The failure processes in S235JR steel were analysed taking into account the influence of the state of stress.

A Stress-State Related Void Coalescence Criterion and its Validation

2010 ◽  
Vol 44-47 ◽  
pp. 2656-2660
Author(s):  
Zeng Tao Chen ◽  
Rahul Datta
Keyword(s):  
Stress State ◽  
Volume Fraction ◽  
Cell Model ◽  
Stress Triaxiality ◽  
High Strength Steels ◽  
Void Volume Fraction ◽  
Void Volume ◽  
Void Coalescence ◽  
Element Analysis ◽  
Critical Void

We propose a new critical void volume fraction (fc) criterion that identifies the onset of void coalescence based on the stress state of the material as compared to the definition of the phenomenological criterion by Tvergaard and Needleman [1], where void coalescence is predicted based merely on a constant value for critical void volume fraction. The new fc criterion is obtained using the finite element analysis of the unit cell model of clustered voids. Validation of this new criterion is done by implementing the new coalescence criterion into the Gurson-Tvergaard-Needleman (GTN) [1-3] model and simulating the ductile fracture experiment of a series of angularly notched sheet samples of dual phase (DP), advanced high strength steels (AHSS). A methodology has been devised to construct a stress triaxiality-based void coalescence criterion. Validation of the methodology has been performed using tensile tests of angularly notched samples of DP490 AHSS. Experimental data is compared with FE simulations in order to verify the dependency of void coalescence on stress triaxiality.

Advanced Assessment of the Ductile Fracture Mechanism in A508 Class 3 Reactor Pressure Vessel Steel Using Laboratory X-Ray Tomography

2013 ◽  
Author(s):  
Michael Daly ◽  
Fabien Leonard ◽  
John K. Sharples ◽  
Andrew H. Sherry
Keyword(s):  
Fracture Surface ◽  
Plastic Strain ◽  
Ductile Fracture ◽  
Volume Fraction ◽  
Void Volume Fraction ◽  
Void Volume ◽  
Ductile Damage ◽  
Pressure Vessel Steel ◽  
Vessel Steel ◽  
X Ray

Ductile damage is characterised by the nucleation, growth and coalescence of voids at initiating particles within the volume of high triaxial stresses and plastic strain ahead of a crack-tip or stress concentrator. To establish a more detailed understanding of the mechanism of ductile fracture in the A508 Class 3 ferritic RPV steels and to improve fracture models, the ductile damage was quantified below the fracture surface of tested compact test specimens using laboratory X-ray tomography imaging with sufficient resolution to image voids of approximately 10μm in diameter. The average distribution of void volume fraction as a function of distance below the fracture surface was quantified, and the initiating and coalescence mechanisms were characterised. The highest void volume fraction was observed at the fracture surface and this tends to decrease as a function of distance below the fracture surface. This decrease is periodically perturbed by large voids associated with inclusions which are distributed throughout the microstructure and act as further nucleating sites at low strains. This distribution of voids was correlated with the local variations in stress triaxiality and plastic strain derived from finite element analyses to provide a relationship between experimental observations and the Rice and Tracey model. These correlations aim to provide new data and understanding with which to calibrate mechanistically based models such as the Gurson-Tvergaard-Needleman (GTN) model.

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