Simulation Of Ductile Fracture Research Articles

A coupled microvoid elongation and dilation based ductile fracture model for structural steels

This paper presents the formulation, implementation, calibration and validation of microvoid elongation and dilation based continuum damage model (MED-CDM) for ductile fracture simulation. The damage evolution in this model is derived from micromechanical analyses that accounts for the microscopic damage due to microvoid elongation and dilation. The proposed model is implemented in Abaqus® finite element program, and is calibrated and validated using existing experimental data for a range of stress triaxialities. The proposed model is shown to accurately predict the load displacement behavior, fracture strains, fracture initiation locations and the underlying microscopic damage mechanisms that are responsible for the fracture initiation in ASTM A992 structural steels.

Numerical simulation and experimental investigation of ductile fracture in SPIF using modified GTN model

Incremental sheet forming (ISF) is a relatively new flexible forming process with excellent adaptability to CNC milling machines due to the fact that it does not require any high capacity presses or dies of a specific shape and this makes the process cost-effective and easy to automate for various applications. The purpose of this work is to develop a modified Gurson-Tvergaard-Needleman (GTN) model that can be used to predict ductile fracture in the ISF process. The GTN damage constitutive model was implemented in Abaqus/Explicit via a VUMAT user subroutine. Tensile tests and a scanning electron microscope (SEM) were utilized to determine the parameters for the GTN model experimentally. The deformation on the surface of the tensile specimen was measured and observed by using a digital image correlation (DIC) system to evaluate necking and instability in the tensile specimens. Based on the results obtained by the SEM in the affected zone of tensile specimens, a modified GTN model was employed to predict the fracture of a pure titanium hyperbolic cone using the ISF process. A comparative study was carried out by using experimental testing and numerical simulation results of the ISF process to validate the modified GTN model.

Determination of GTN Damage Parameters for Application to Pipe Ductile Fracture Simulation

A number of works have been reported in the literature up to present on finite element ductile failure analysis. Depending on the model employed for damage simulating, the Gurson-Tvergaard-Needleman model can be broadly used. That model is using a micro-mechanical model for ductile fracture, incorporating void nucleation, growth and coalescence.As many researchers have already published a number of papers using these methods, applicability and validity of these methods have been well discussed in the literature. In this paper, to simulate long, stable ductile tearing in large-scale pipes using finite element analysis by GTN model. The GTN damage model and associate parameters are determined from fracture toughness test.

Estimating the Ultimate Energy Dissipation Capacity of Steel Pipe Dampers

Steel pipe damper specimen tests are very important but expensive and time consuming. Therefore efficient tools are needed to minimize the expense and the time by avoiding unnecessary or unneeded tests. This paper elaborates a practical component integrity assessment using finite element ductile fracture simulation based on local approach, and estimates the ultimate energy dissipation capacity of steel pipe dampers using energy based damage model. Ductile fracture in steel components that happens in fewer than twenty constant amplitude loading cycles is known as Ultra Low Cycle Fatigue (ULCF). Under ULCFs load steel pipe dampers experienced large scale cyclic yielding. Accurate preliminary prediction of ductile fracture is critical to estimate the performance of steel pipe dampers. The hysteretic behavior and ultimate energy dissipation capacity are investigated via finite element simulation after the component integrity assessment has been done. A micromechanics-based model which provide accurate criteria for predicting ductile fracture and an energy-based damage model to quantify the ultimate energy dissipation capacity of steel pipe dampers are applied. Using these approaches, ultimate energy dissipation capacity of steel pipe dampers can be estimated under various patterns of loadings. The approaches described here can also be applied to other steel dampers subjected to randomly flexural/shear stress reversals.

Real‐time simulation of ductile fracture with oriented particles

ABSTRACTThis paper proposes a practical approach for real‐time simulation of large deformation and ductile fracture. We adopt the oriented particles approach for robust and efficient simulation of large deformation and develop a practical model for plastic flow and fracture. The proposed method finds the optimal rotation and the optimal stretch in shape matching. The newly introduced optimal stretch leads to a material strain that can be employed in the plastic flow and fracture criteria. We also propose a graphics processing unit skinning method for real‐time rendering of a deformed, fractured visual mesh. Experimental results show that the proposed method can robustly simulate large, elastoplastic deformation and ductile fracture of large visual meshes in real time. Copyright © 2014 John Wiley & Sons, Ltd.

Finite hyperelastic–plastic constitutive equations for atomistic simulation of dynamic ductile fracture

The atomistic-based consistent finite hyperelastic–plastic constitutive (LH) model for multiscale simulation is proposed. The system Helmholtz energy is constructed from an embedded-atom method (EAM) potential with uniformly deformation assumption, which consists two parts: the volumetric part and the deviatoric part. A lattice structure related upscaling strategy is introduced to split the total energy. The volumetric strain energy is used to determine the elastic responses, while the deviatoric strain energy governs the plastic evolution. Based on the maximum plastic dissipation principle and the deviatoric strain energy, we derive the general form of atomistic-based plastic flow rule, which lays the theoretical foundation to build LH model. The evolution equation can be explicitly expressed by the kinematic variables of multiplicative decomposition, which directly relates the flow rule to basic physical processes that induce plasticity such as dislocation multiplications. The isotropic hardening parameters in von Mises yield function are fitted by plastic flow stresses. The integration algorithm of standard finite strain plasticity is developed for LH model. In case of zero deviatoric strain energy, LH model reduces to the classical elastic Cauchy-born (CB) model. Full atomistic simulation of dynamic crack propagation is carried out to validate this model. Plasticity captured by LH model, instead of the elasticity obtained from CB model, is observed after lattice instability, which implies that the ductile fracture can be governed by the collective behaviors of dislocations.

Ductile Fracture Simulation of Full-scale Circumferential Cracked Pipes: (II) Stainless Steel

This paper reports ductile fracture simulation of full-scale circumferentially cracked pipes using finite element (FE) damage analysis. In the structural integrity, without experimental investigations or with few ones, it is not an easy task to properly evaluate the crack initiation and crack propagation of large-scale components with a crack-like defect. Unfortunately, from an economic perspective, performing experiments of large-scale components would be consequently unfavorable. For these reasons, ductile fracture simulation using FE damage analysis to predict crack behavior is one efficient way to replace the test procedures. In order to simulate ductile tearing of large-scale cracked pipes, element-size-dependent critical damage model based on the stress-modified fracture strain model is proposed. To evaluate fracture behavior of full-scale cracked pipes, tensile and C(T) specimens are calibrated by FE analysis technique. Tensile properties and fracture toughness of stainless steel at 288oC are taken from Battelle Pipe Fracture Encyclopedia. After calibrations, simulated results of the full-scale pipes with a circumferential crack are compared with test data to validate the proposed method.

Ductile Fracture Simulation of Full-scale Circumferential Cracked Pipes: (I) Carbon Steel

This paper reports ductile fracture simulation and comparison with carbon steel test data of full-scale cracked pipes via 3-D finite element damage analysis based on stress-modified fracture strain model. Although there are few test results and discussion of full-scale cracked pipes, it is not an easy task to properly evaluate fracture behaviour of large-scale components for various pipe sizes with different shape of a crack-like defect. In such a situation, finite element (FE) damage analysis can be a competitive alternative to characterize the fracture behaviour of cracked components such as crack initiation and maximum loads. In recent years, a simple FE method [1] to simulate ductile fracture has been developed based on the stress-modified fracture strain model [2,3]. The technique appropriately simulated ductile failure for miscellaneous cracked components [4,5]. However, for some cases, large-scale components using small element size, FE analysis couldn’t give reliable values because of numerical instability. Element-size-dependent critical damage model enhanced by taking the effect of element-size on the ductile fracture damage analysis is introduced to overcome this problem and to be applicable to the large-scale structures. In order to validate proposed method, two types of carbon steel (A106 Gr. B and SA333 Gr. 6) pipes with a circumferential crack taken from [6] are considered, subjected to four-point bending only and combined loading. It is shown that predicted crack initiation and maximum loads are compared with experimentally measured values, showing overall good agreements.

Numerical Simulation of Ductile Fracture in Modified Arcan Test

In this study, the experimental results from a modified Arcan test on dual-phase steel are revisited in order to assess a numerical modelling approach for ductile fracture. This experiment is chosen since it displays a complex crack-path that undergoes three phases: fracture initiation and crack propagation with transition from Mode I to Mode II. A significantly larger amount of plastic dissipation is observed in the material at the point of fracture initiation compared to the material points along the crack-path. A numerical approach that is capable of capturing this complex crack-path is believed also to give good predictions in other complex fracture problems. The numerical simulations of the modified Arcan test are run with the nonlinear Finite Element (FE) code IMPETUS Afea Solver. Elements with tri-linear shape functions are utilized in combination with explicit time integration. The plastic behaviour of the material is modelled by the elastic-viscoplastic J2 flow theory using the Voce work-hardening rule. Both element erosion and node-splitting techniques are tested in order to simulate the crack propagation. It is shown that the node-splitting technique with a crack-tip enhancement gave the best prediction of the three different phases of the crack propagation.

Effect of Critical Void Volume Fraction f<sub>F</sub> on Results of Ductile Fracture Simulation for S235JR Steel under Multi-Axial Stress States

The article describes an example of the GTN material model parameters determination and application. The main objective of the study was to determine experimentally the value of the critical volume fraction of voids fFfor S235JR steel and to assess the impact of this parameter on the numerical force-elongation curve under the multi-axial stress state. Value of fFwas obtained by the quantitative analysis of the material microstructure at fracture surfaces. For a sake of comparison, two other values of fF, described in the literature, were also used in numerical simulations.

Ductile Fracture Simulation of Structural Steels under Monotonic Tension

AbstractThis paper aims to predict ductile fracture of mild steel under monotonic loading only from the test results of notchless tensile coupons. A simple fracture model based on the concept of a ...

Effective Yield Criterion Accounting for Microvoid Coalescence

An effective yield function is derived for a porous ductile solid near a state of failure by microvoid coalescence. Homogenization theory combined with limit analysis are used to that end. A cylindrical cell is taken to contain a coaxial cylindrical void of finite height. Plastic flow in the intervoid matrix is described by J2 theory while regions above and below the void remain rigid. Velocity boundary conditions are employed which are compatible with an overall uniaxial straining for the cell, a postlocalization kinematics that is ubiquitous during the coalescence of neighboring microvoids in rate-independent solids. Such boundary conditions are not of the uniform strain rate kind, as is the case for Gursonlike models. A similar limit analysis problem for a square-prismatic cell containing a square-prismatic void was posed long ago (Thomason, P. F., 1985, “Three-Dimensional Models for the Plastic Limit–Loads at Incipient Failure of the Intervoid Matrix in Ductile Porous Solids,” Acta Metallurgica, 33, pp. 1079–1085). However, to date a closed-form solution to this problem has been lacking. Instead, an empirical expression of the yield function proposed therein has been widely used in the literature. The fully analytical expression derived here is intended to be used concurrently with a Gursonlike yield function in numerical simulations of ductile fracture.

Ductile fracture simulation of 304 stainless steel pipes with two circumferential surface cracks

ABSTRACTIn this paper, ductile fracture behaviours of 304 stainless steel pipes with two circumferential surface cracks under pure bending are simulated using finite element damage analyses. Simulations are based on the stress‐modified fracture strain model with the concept that the critical accumulated damage for progressive cracking is assumed to be dependent on an element size. The proposed method can predict not only maximum loads but also complex ductile fracture patterns observed in experiments.

Simulations of ductile fracture in an idealized ship grounding scenario using phenomenological damage and cohesive zone models

Two complementary simulation methodologies for ductile fracture in large sheet metal components are presented and evaluated in this paper. The first approach is based on the phenomenological dilatational plasticity-damage model developed by Woelke and Abboud [46], which accounts for pressure-dependent volumetric damage growth through a scalar damage variable. The damage function represents phenomenologically micromechanical changes the material undergoes during the process of necking. Secondly, the cohesive zone model with an opening mode traction–separation law is employed to simulate the same ductile fracture problems accounting for significant variation of the multiaxial stress state along the crack path. Both methods are examined as to their capabilities to reproduce and predict the outcome of large-scale experimental fracture tests of welded and un-welded ductile plates subjected to large-scale penetration, simulating an idealized ship grounding. The results of the current study indicate that, with appropriate calibration, both approaches can be successfully employed to simulate ductile fracture in structural components under multiaxial stress. The advantages and shortcomings of each approach is discussed from the point of view of post-test numerical investigation as well as its predictive capabilities as an engineering tool.

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

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.

Simulation of Ductile Fracture in an Aluminum Alloy Using Various Criteria

There are many models and failure criteria have been developed to predict the ductile fracture (DF) in metal plastic deformation. But usually, it is difficult to select a suitable model and the corresponding criterion from them. So, finding a way to identity their applicability and reliability is useful for selecting these DF criteria. In this paper, ductile fracture of aluminum alloy A5052P-H34 is studied by experiments and finite element simulations. In experiments, the mode I crack was obtained by uniaxial tension of plate with a circular hole in the center. The von Mises yield model and continuum damage mechanics based Rousselier model and modified Rousselier model are chosen to describe the material behavior. Three failure criteria, including the Cockcroft-Latham integral, maximum shear stress theory and critical void volume fraction criterion are investigated to determine their reliability in ductile failure prediction. These constitutive models and DF criteria are implemented by user material subroutine in ABAQUS/Explicit to predict the crack. And the crack initiation and propagation is implemented by element erosion method. By comparing the experiments and simulations, the modified Rousselier’s model with the corresponding criterion shows agreement with the experiments.

A hybrid, massively parallel implementation of a genetic algorithm for optimization of the impact performance of a metal/polymer composite plate

A hybrid parallelization method composed of a coarse-grained genetic algorithm (GA) and fine-grained objective function evaluations is implemented on a heterogeneous computational resource consisting of 16 IBM Blue Gene/P racks, a single x86 cluster node and a high-performance file system. The GA iterator is coupled with a finite-element (FE) analysis code developed in house to facilitate computational steering in order to calculate the optimal impact velocities of a projectile colliding with a polyurea/structural steel composite plate. The FE code is capable of capturing adiabatic shear bands and strain localization, which are typically observed in high-velocity impact applications, and it includes several constitutive models of plasticity, viscoelasticity and viscoplasticity for metals and soft materials, which allow simulation of ductile fracture by void growth. A strong scaling study of the FE code was conducted to determine the optimum number of processes run in parallel. The relative efficiency of the hybrid, multi-level parallelization method is studied in order to determine the parameters for the parallelization. Optimal impact velocities of the projectile calculated using the proposed approach, are reported.

Simulation of Ductile Fracture of Multiple Flaws

The effect of the interaction of multiple flaws on ductile fracture is studied numerically by Gurson’s constitutive equation. Based on experimental results, two parallel flaw and three parallel flaw problems are simulated. Flaw coalescence does not occur in some problems but does occur in other cases. In all cases, ductile fracture processes are obtained, and the results are compared with the experimental results. The fracture pattern and load-displacement curves agree well with the experimental results. The void growth term is found to be dominant for the coalescence of flaws. The slant flaw problem and the nonuniform length flaw problem are simulated and an evaluation method for the multiple flaws problem is discussed.

Modeling and simulation of large-scale ductile fracture in plates and shells

The work is concerned with the modeling and simulation of large scale ductile fracture in plate and shell structures. A meshfree method – the reproducing kernel particle method (RKPM) – is used in numerical computations in order to enact dynamic crack propagation without remeshing. There are several novelties in the present approach. First, we have developed a crack surface approximation and particle split algorithm for three-dimensional through-thickness cracks. Second, to represent evolving crack surface in 3D shell structures, a 3D parametric visibility condition algorithm is proposed, which re-constructs the local connectivity map for particles near the crack tip or crack surfaces, so that the meshfree interpolation field can represent physical material separation in the computational domain. Third, the constitutive update formulas in explicit time integration by different versions of Gurson models and the rate-dependent Johnson–Cook model are implemented for 3D computations. Finally, the performance of different Gurson-type models are investigated and compared with the experimental data of large scale in-plate tear process. Numerical simulations of crack propagation in stiffened plates and shells demonstrate that the proposed method provides an effective means to simulate ductile fracture in large scale plate/shell structures with engineering accuracy.

Levenberg–Marquardt vs Powell’s dogleg method for Gurson–Tvergaard–Needleman plasticity model

The GTN continuous damage model is very popular in academia and industry for structural integrity assessment and ductile fracture simulation. Following Aravas’ influential 1987 paper, Newton’s method has been used widely to solve the GTN equations. However, if the starting point is far from the solution, then Newton’s method can fail to converge. Hybrid methods are preferred in such cases. In this work we translate the GTN equations into a non-linear minimization problem and then apply the Levenberg–Marquardt and Powell’s ‘dogleg’ hybrid methods to solve it. The methods are tested for accuracy and robustness on two simple single finite element models and two 3D models with complex deformation paths. In total nearly 137,000 different GTN problems were solved. We show that the Levenberg–Marquardt method is more robust than Powell’s method. Our results are verified against the Abaqus’ own solver. The superior accuracy of the Levenberg–Marquardt method allows for larger time increments in implicit time integration schemes.