On ductile fracture initiation toughness: Effects of void volume fraction, void shape and void distribution

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.

Download Full-text

The Effects of Non-Proportional Biaxial Loading Paths on Ductile Fracture Initiation: A Void Growth Analysis

Volume 6B: Materials and Fabrication ◽

10.1115/pvp2019-93312 ◽

2019 ◽

Author(s):

Zhaoyu Jin ◽

Xin Wang

Keyword(s):

Ductile Fracture ◽

Void Growth ◽

Biaxial Loading ◽

Crack Tip ◽

Fracture Initiation ◽

Initiation Toughness ◽

Proportional Loading ◽

Loading Paths ◽

Ductile Fracture Initiation ◽

Crack Tip Blunting

Abstract The effects of non-proportional biaxial loading paths on ductile fracture initiation toughness are studied in this paper. To this end, the growth of a cylindrical void (hole) located in front of a mode I plane strain crack has been studied using large deformation finite element analysis (FEA). A specific microstructural feature of a steel alloy was thoroughly studied by having a single void positioned at a fixed distance from the crack tip and void that was equal to 10 times the diameter of the void. In particular, the non-proportional biaxial loading path effects on the crack tip blunting, void-growth, ligament reduction and near-tip stress fields are investigated computationally. Under small-scale yielding conditions, one proportional loading and two non-proportional loading paths are applied to the modified boundary layer (MBL) model. It is observed that the non-proportional load paths have a marked effect on the void growth, crack tip blunting and their interaction. By applying the criteria for the coalescence of the crack tip and void, the ductile fracture initiation toughness is estimated. It is shown that the ductile fracture toughness is dependent on loading paths, and the T-stress ratios. Results from this study are of relevance to ductile fracture assessment of components or pressure vessels that operate under non-proportional biaxial loading conditions.

Download Full-text

Prediction of Central Bursting in Drawing and Extrusion of Metals

Journal of Manufacturing Science and Engineering ◽

10.1115/1.1961982 ◽

2004 ◽

Vol 127 (3) ◽

pp. 698-702 ◽

Cited By ~ 2

Author(s):

A. R. Ragab ◽

S. N. Samy ◽

Ch. A. R. Saleh

Keyword(s):

Volume Fraction ◽

Yield Function ◽

Void Volume Fraction ◽

Void Volume ◽

Process Conditions ◽

Flow Rule ◽

Mean Stress ◽

Band Formation ◽

Void Shape ◽

Associated Flow Rule

In this work central bursting in drawing and extrusion of metals is investigated. The analysis is based on a modified stress distribution within the die zone due to Shield (Shield, R. T., 1955, J. Mech. Phys. Solids, 3, pp. 246–258) together with Gurson–Tvergaard’s yield function (Tvergaard, V., 1981, Int. J. Fract., 17, pp. 389–407) and its associated flow rule for voided solids. The effects of hardening and evolution of void shape on void growth are considered. Various fracture criteria are employed to predict the process conditions at which central bursting occurs. The first criterion is due to Avitzur (Avitzur, B., 1968, ASME J. Eng. Ind., 90, pp. 79–91 and Avitzur, B., and Choi, J. C., 1986, ASME J. Eng. Ind., 108, pp. 317–321), the second and simplest criterion is based on vanishing mean stress while a suggested third criterion depends on the current value of the void volume fraction. Two other criteria which are basically due to Thomason’s internal necking condition (Thomason, P. F., 1990, Ductile Fracture of Metals, Pergamon, Oxford) as well as McClintock’s shear band formation criterion are applied (McClintock, F. A., Kaplan, S. M., and Berg, C. S., 1966, Int. J. Fract. Mech., 2, p. 614, and McClintock, F. A., 1968, in Ductility, ASM, Metals, Park, OH). The critical process conditions are predicted and compared with the available experimental data. Comparison showed that predictions based on the vanishing mean stress and the current void volume fraction criteria are closer to experiments than those based on Thomason’s internal necking and McClintock criteria.

Download Full-text

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

Volume 11A: Honoring Symposium for Professor Carlos Guedes Soares on Marine Technology and Ocean Engineering ◽

10.1115/omae2018-77182 ◽

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.

Download Full-text

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

Volume 6A: Materials and Fabrication ◽

10.1115/pvp2013-97924 ◽

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.

Download Full-text

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

International Journal of Modern Physics Conference Series ◽

10.1142/s2010194512003273 ◽

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.

Download Full-text

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

Journal of Marine Science and Application ◽

10.1007/s11804-019-00117-2 ◽

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

Download Full-text

Application of 3D X-Ray Tomography to Enhance the Calibration of Ductile Fracture Models

Volume 5: High-Pressure Technology; ASME NDE Division; 22nd Scavuzzo Student Paper Symposium and Competition ◽

10.1115/pvp2014-28318 ◽

2014 ◽

Cited By ~ 1

Author(s):

Michael Daly ◽

Fabien Leonard ◽

Andrew H. Sherry

Keyword(s):

Fracture Surface ◽

Ductile Fracture ◽

Volume Fraction ◽

Pressure Vessels ◽

Void Volume Fraction ◽

Void Volume ◽

Normal Loading ◽

X Ray ◽

Fracture Models ◽

Ductile Fracture Model

Reactor Pressure Vessels (RPV) are manufactured from medium strength low allow ferritic steel specifically selected for its high toughness and weldability. The normal operating temperature of RPV steels is sufficiently high to ensure that the material remains ductile throughout its service life with an extremely low probability of cleavage under normal and off-normal loading conditions. Understanding and having the ability to predict ductile fracture behaviour is consequently important. The ductile fracture mechanism is characterised by the nucleation, growth and coalescence of voids at initiating particles within the volume of high triaxial stress and plastic strain ahead of a crack-tip or stress concentrator. The fracture properties of the steels are conventionally determined using standard pre-cracked compact test specimens. Mechanistically based models of fracture can be calibrated against those data. This paper describes the use of 3D laboratory X-ray tomography to characterise the void distribution associated with the ductile fracture in test specimens and use the data to calibrate the Gurson-Tvergaard-Needleman ductile fracture model. The tomography successfully captures voids ≥ 6um in diameter and has been used to define the average distribution of void volume fraction as a function of distance below the fracture surface. The tomography results also allow an estimate of the critical and final void volume fractions to be made as well as capture secondary void peaks well below the fracture surface. This distribution of voids was used to calibrate the Gurson-Tvergaard-Needleman model in order to correlate experimental observations with the finite element models. The models have been able to replicate the observed trends of the void volume fraction distributions away from the fracture surface including the secondary peaks observed by tomography and to reproduce similar J-R curve behaviour as that observed in the test specimens.

Download Full-text

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

Materials ◽

10.3390/ma12111783 ◽

2019 ◽

Vol 12 (11) ◽

pp. 1783 ◽

Cited By ~ 4

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.

Download Full-text