A gradient-enhanced damage model motivated by engineering approaches to ductile failure of steels

2019 ◽  
Vol 28 (8) ◽  
pp. 1261-1296 ◽  
Author(s):  
Andreas Seupel ◽  
Meinhard Kuna
Keyword(s):  
Damage Evolution ◽  
Damage Model ◽  
High Stress ◽  
Stress Triaxiality ◽  
Material Behavior ◽  
Pressure Vessel Steel ◽  
Vessel Steel ◽  
Damage Initiation ◽  
Gradient Enhancement ◽  
Von Mises Plasticity

Material models for ductile damage, crack initiation, and crack growth are of high interest, e.g. for metal forming simulations. Empirical engineering approaches are often applied, but the numerical results are sensitive to the discretization if no method is utilized to prevent ill-posedness of the underlying boundary value problem due to strain softening. In order to face this issue, an empirical damage model is equipped with a gradient-enhancement which introduces an additional length scale parameter. Until the initiation of damage, the material is modeled with standard von Mises plasticity. Damage initiation is taken into account by an uncoupled failure indicator. After damage initiation, material degradation is assumed to be driven by a non-local quantity, which depends on plastic deformation and stress triaxiality. During damage evolution, the macroscopic material behavior becomes dependent on hydrostatic stress, which is motivated by well known void growth and coalescence mechanisms. A calibration strategy is developed to determine the parameters of strain hardening, damage initiation, and damage evolution as well as the internal length step-by-step. The proposed model is calibrated to experimental data of a pressure vessel steel. Reasonable predictions of smooth and notched tensile tests as well as a small punch test show the validity of the model for loadings from moderate to high stress triaxialities.

Failure Limit State Investigation of Misaligned Welded Plates by Using a Damage Model-Based Upon Triaxiality and Lode Parameters

2020 ◽  
Author(s):  
Iago S. Santos ◽  
Diego F. B. Sarzosa
Keyword(s):  
Numerical Study ◽  
Limit State ◽  
Damage Model ◽  
Stress Triaxiality ◽  
Failure Behavior ◽  
Pressure Vessel Steel ◽  
Vessel Steel ◽  
Strain Capacity ◽  
Combined Loads ◽  
Welded Structure

Abstract This paper presents a numerical study using the finite element method to assess the structural integrity of welded plates. Different levels of weld misalignment were introduced on the FEM models to investigate the influence of this welding imperfection parameter on the limit state of the structure. The models were loaded under displacement-controlled condition to introduce traction and torsion loads seeking to understand the effects of combined loads on the strain capacity of the misaligned welded structure. Surface elliptical cracks having different crack-size ratios were modeled to study the crack growth behavior by taking into account the misalignment of the weld and combined loads. The damage model is based on a failure surface and post-initiation behavior to model the ductile crack initiation and propagation steps, respectively. The models provide useful information to track the evolution of damage on the hot spot point of the welded structure. The model used is dependent on stress triaxiality and a Lode-based parameter and the damage level is driven by the plastic strain. The evolution of stress triaxiality and Lode parameter with loading are presented, and the influence of misalignment on them are shown. An exponential softening law was adopted to predict post-initiation failure behavior. The calibration steps of the parameters required for damage model application are shown for a A285 pressure vessel steel. Overall, the numerical models reveal the deleterious effects of weld misalignment and combined torsional and tensile loads on the strain capacity of the weld.

Prediction of Ductile Fracture as a Process Controlled by Cavitation Instability

2009 ◽  
Author(s):  
Jiri Novak
Keyword(s):  
Ductile Fracture ◽  
Deformation Theory ◽  
Fracture Strain ◽  
High Stress ◽  
Stress Triaxiality ◽  
Plastic Behavior ◽  
Pressure Vessel Steel ◽  
Vessel Steel ◽  
Theory Of Plasticity ◽  
Cavitation Instability

Recently, we applied criterion of initiation of deformation bands based on bifurcation analysis as a criterion of ductile fracture. Experience shows that this procedure yields realistic results if plastic behavior is described by deformation theory of plasticity, with corresponding stress-strain dependence — especially with transition between strain hardening stages III and IV. But it is generally known that under high stress triaxilities, fracture strain depends strongly on stress triaxiality. If deformation theory of plasticity is suitable for modeling of constitutive properties of polycrystalline metals, it should lead to good results in prediction of cavitation instability as a criterion of ductile fracture under high triaxialities as well. We present prediction of fracture strains for reactor pressure vessel steel, in comparison with experimental results. Criterion of cavitation instability based on deformation theory of plasticity predicts similar dependence of fracture strain on stress triaxiality as the classical Rice-Tracey void growth model does, but, moreover, in contrast to the Rice-Tracey model, it predicts absolute values of critical strains. Finally, important role of deformation theory of plasticity in other areas of material engineering and structural integrity analysis is shortly remembered.

scholarly journals A Modified Phase-Field Damage Model for Metal Plasticity at Finite Strains: Numerical Development and Experimental Validation

Metals ◽  
2020 ◽  
Vol 11 (1) ◽  
pp. 47
Author(s):  
Jelena Živković ◽  
Vladimir Dunić ◽  
Vladimir Milovanović ◽  
Ana Pavlović ◽  
Miroslav Živković
Keyword(s):  
Phase Field ◽  
Damage Evolution ◽  
Deformation Gradient ◽  
Damage Model ◽  
Steel Structures ◽  
Plasticity Model ◽  
Metal Plasticity ◽  
Damage And Fracture ◽  
Von Mises ◽  
Von Mises Plasticity

Steel structures are designed to operate in an elastic domain, but sometimes plastic strains induce damage and fracture. Besides experimental investigation, a phase-field damage model (PFDM) emerged as a cutting-edge simulation technique for predicting damage evolution. In this paper, a von Mises metal plasticity model is modified and a coupling with PFDM is improved to simulate ductile behavior of metallic materials with or without constant stress plateau after yielding occurs. The proposed improvements are: (1) new coupling variable activated after the critical equivalent plastic strain is reached; (2) two-stage yield function consisting of perfect plasticity and extended Simo-type hardening functions. The uniaxial tension tests are conducted for verification purposes and identifying the material parameters. The staggered iterative scheme, multiplicative decomposition of the deformation gradient, and logarithmic natural strain measure are employed for the implementation into finite element method (FEM) software. The coupling is verified by the ‘one element’ example. The excellent qualitative and quantitative overlapping of the force-displacement response of experimental and simulation results is recorded. The practical significances of the proposed PFDM are a better insight into the simulation of damage evolution in steel structures, and an easy extension of existing the von Mises plasticity model coupled to damage phase-field.

Probabilistic Fatigue Assessment of a Notched Detail Taking Into Account Mean Stress Effects

2012 ◽  
Vol 134 (2) ◽  
Author(s):  
Abílio M. P. De Jesus ◽  
M. Luisa Ruiz Ripoll ◽  
Alfonso Fernández-Canteli ◽  
Enrique Castillo ◽  
Hélder F. S. G. Pereira
Keyword(s):  
Probabilistic Model ◽  
Fatigue Behavior ◽  
Material Behavior ◽  
Test Series ◽  
Model Parameters ◽  
Pressure Vessel Steel ◽  
Mean Stress ◽  
Vessel Steel ◽  
Life Data ◽  
R Ratio

Probabilistic fatigue models are required to account conveniently for the several sources of uncertainty arising in the prediction procedures, such as the scatter in material behavior. In this paper, a recently proposed stress-based probabilistic model is assessed using fatigue data available for the P355NL1 steel (a pressure vessel steel). The referred probabilistic model is a log-Gumbel regression model, able to predict the probabilistic Wöhler field (P–S–N field), taking into account the mean stress (or stress R-ratio) effects. The parameters of the probabilistic model are identified using stress-life data derived for the P355NL1 steel, from smooth specimens, for three distinct stress R-ratios, namely R = −1, R = −0.5, and R = 0. The model requires a minimum of two test series with distinct stress R-ratios. Since data from three test series is available, extrapolations are performed to test the adequacy of the model to make extrapolations for stress R-ratios other than those used in the model parameters assessment. Finally, the probabilistic model is used to model the fatigue behavior of a notched plate made of P355NL1 steel. In particular, the P–S–N field of the plate is modeled and compared with available experimental data. Cyclic elastoplastic analysis of the plate is performed since plasticity at the notch root is developed. The probabilistic model correlated appropriately the stress-life data available for the P355NL1 steel and was able to perform extrapolations for stress ratios other than those used in the model identification. The P–S–N field identified using data from smooth specimens led to consistent predictions of the P–S–N field for a notched plate, demonstrating the adequacy of the probabilistic model also to predict the probabilistic Wöhler field for notched components.

scholarly journals Development of a Microscopic Damage Model for Low Stress Triaxiality

2011 ◽  
Vol 473 ◽  
pp. 460-467 ◽  
Author(s):  
Mohamed Achouri ◽  
Guenael Germain ◽  
Phillippe dal Santo ◽  
Serge Boude ◽  
Jean Lou Lebrun ◽  
...  
Keyword(s):  
Hsla Steel ◽  
Damage Model ◽  
Stress Triaxiality ◽  
High Strength ◽  
Material Behavior ◽  
Shear Tests ◽  
Damage Process ◽  
Microscopic Damage ◽  
Evolution Of Microstructure

This work dealsa contribution to ductile damageof High-Strength Low-Alloy (HSLA) steel under low stress triaxiality. This work is based on micrographics observations and in-situ shear tests to examine the evolution of microstructure in this kind of loading and to identify the damage process associated. Numerical simulations by finites elements has been performed to simulate the material behavior of nucleation mechanism and the interaction between cavities during the coalescence phase, as well as the effect of the relative position of the inclusions in the shear plane.The model used as a reference in this work is the Gurson-Tvergaard- Needleman (GTN) model. It has been recently improved in order to take into account the effects of low triaxiality during shearing. A new modelisunderdevelopmentto takeintoaccounttheeffects oflowtriaxiality stresses (or loading) during shearing.

Elasto-Plastic Behavior of Polycrystalline Steel at Mesoscopic and Macroscopic Levels

2003 ◽  
Author(s):  
Marko Kovacˇ ◽  
Igor Simonovski ◽  
Leon Cizelj
Keyword(s):  
Voronoi Tessellation ◽  
Anisotropic Elasticity ◽  
Model Material ◽  
Material Behavior ◽  
Grain Structure ◽  
Stress Fields ◽  
Plastic Behavior ◽  
Pressure Vessel Steel ◽  
Vessel Steel ◽  
Homogeneity Assumption

An important drawback of the classical continuum mechanics is idealization of inhomogenous microstructure of materials. Approaches, which model material behavior on mesosocopic level and can take inhomogenous microstructure of materials into the account, typically appeared over the last decade. Nevertheless, entirely anisotropic approach towards material behavior of a single grain is still not widely used. The proposed approach divides the polycrystalline aggregate into a set of grains by utilizing Voronoi tessellation (random grain structure). Each grain is assumed to be a monocrystal with random orientation of crystal lattice. Mesoscopic response of grains is modeled with anisotropic elasticity and crystal plasticity. Strain and stress fields are calculated using finite element method. Material parameters for pressure vessel steel 22 NiMoCr 3 7 are used in analysis. The analysis is limited to 2D models. Applications of the proposed approach include (a) the estimation of the minimum component/specimen size needed for the homogeneity assumption to become valid and (b) the estimation of the correlation lengths in the resulting mesoscopical stress fields, which may be used in well-established macroscopical material models. Both applications are supported with numerical examples and discussion of numerical results.

Behavior of polyethylene under different triaxial stress states: An experimental and numerical study

2020 ◽  
pp. 146442072097058
Author(s):  
Limei Han ◽  
Yi Zhang ◽  
Shifeng Xue ◽  
Bo Zhou ◽  
Cuiwei Liu
Keyword(s):  
Finite Element ◽  
Finite Element Simulation ◽  
Damage Evolution ◽  
Fracture Strain ◽  
Damage Model ◽  
Stress Triaxiality ◽  
True Stress ◽  
Triaxial Stress ◽  
Element Simulation ◽  
Stress States

The behavior of a semi-crystalline polymer under different triaxial stress states is studied through the combination of experimental testing and finite element simulation. Polyethylene round bar specimens with four different notch radii were stretched at crosshead speed of 1 mm/min until fracture. The continuum damage mechanics damage model and Gurson–Tvergaard–Needleman damage model were proposed and applied to the finite element simulation. The results of engineering stress–displacement curves determined from finite element simulation match experimental results. Finite element simulation without considering damage and with the consideration of damage was conducted to determine the damaged and undamaged true stress–strain relationship of polyethylene materials, respectively. Damage evolution model was established based on the degradation of true stress. The finite element model was further applied to study the distribution of stress triaxiality for specimens with different notch radii and the effect of stress triaxiality on damage evolution, critical damage parameters, and fracture strain. The results show that the distribution of the stress triaxiality on the cross section of the specimen is not uniform, and as the stress triaxiality increases, the position where the maximum stress triaxiality occurs moves from the center point to two-third the radius from the center. Furthermore, the damaged true stress and the undamaged true stress increases with the decrease of the stress triaxiality when the strain is below 0.3, but decreases with the increase of stress triaxiality when the strain is larger than 0.3. In addition, it was found that the greater the stress triaxiality, the earlier the onset of damage and the faster the evolution, but the smaller the fracture strain.

Prediction of Cleavage Fracture in the Ductile-to-Brittle Transition Region of Pressure Vessel Steels: A Probabilistic Model

2006 ◽  
Vol 324-325 ◽  
pp. 283-286
Author(s):  
Xiao Sheng Gao ◽  
Gui Hua Zhang ◽  
T.S. Srivatsan
Keyword(s):  
Fracture Toughness ◽  
Cleavage Fracture ◽  
Pressure Vessel ◽  
Stress Triaxiality ◽  
Weibull Model ◽  
Pressure Vessel Steel ◽  
Vessel Steel ◽  
Proposed Model ◽  
Modified Weibull ◽  
Weibull Stress

This paper presents a modified Weibull stress model, which accounts for the effects of plastic strain and stress triaxiality at the crack tip region. The proposed model is applied to predict cleavage fracture in a modified A508 pressure vessel steel. It is demonstrated that the Weibull modulus (m) remains constant in the temperature range considered, while the threshold Weibull stress (σw-min) decreases with an increase in temperature due to reduction of the yield stress and the scale parameter of the Weibull model (σu) increases with temperature reflecting the influences of temperature on both material flow properties and toughness. The proposed model accurately predicts the scatter of the measured fracture toughness data and the strong effects of constraint and temperature on cleavage fracture toughness.

Probabilistic Fatigue Assessment of a Notched Detail Taking Into Account Mean Stress Effects

2010 ◽  
Author(s):  
Abi´lio M. P. De Jesus ◽  
M. Luisa Ruiz Ripoll ◽  
Norberto J. Gonc¸alves ◽  
He´lder F. S. G. Pereira
Keyword(s):  
Probabilistic Model ◽  
Notch Root ◽  
Fatigue Behavior ◽  
Material Behavior ◽  
Test Series ◽  
Model Parameters ◽  
Pressure Vessel Steel ◽  
Mean Stress ◽  
Vessel Steel ◽  
R Ratio

Probabilistic fatigue models are required to account conveniently for the several sources of uncertainty arising in the prediction procedures, such as the scatter in material behavior. In this paper, a recently proposed stress-based probabilistic model is assessed using fatigue data available for the P355NL1 steel (a pressure vessel steel). The referred probabilistic model is a log-Gumbel regression model, able to predict the probabilistic Wo¨hler field (P-S-N field), taking into account the mean stress (or stress R-ratio) effects. The parameters of the probabilistic model are identified using stress-life data derived for the P355NL1 steel, from smooth specimens, for three distinct stress R-ratios, namely R = −1, R = −0.5 and R = 0. The model requires a minimum of two test series with distinct stress R-ratios. Since data from three test series is available, extrapolations are performed to test the adequacy of the model to make extrapolations for stress R-ratios other than those used in the model parameters assessment. Finally, the probabilistic model is used to model the fatigue behavior of a notched plate made of P355NL1 steel. In particular, the P-S-N field of the plate is modeled and compared with available experimental data. Cyclic elastoplastic analysis of the plate is performed since plasticity at the notch root is developed.

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