Simulation Of Ductile Crack Growth Research Articles

Ductile Damage Model Based on Void Growth Analysis for Application to Ductile Crack Growth Simulation

AbstractThis study aims to propose the damage model based on of the mechanism for ductile fracture related to void growth and to confirm the applicability of the proposed model to ductile crack growth simulation for steel. To figure out void growth behavior, elastoplastic finite element analyses using a unit cell model with an initial void were methodically performed. From the results of those analyses, it was evident that the relationships between normalized void volume fraction and normalized strain by each critical value corresponding to crack initiation were independent of stress–strain relationship of material and stress triaxiality state. Based on this characteristic associated with void growth, damage evolution law was derived. Then, using the damage evolution law, simple and phenomenological ductile damage model reflecting void growth behavior and ductility of material was proposed. To confirm the validation and application of the proposed damage model, the damage model was implemented in finite element models and ductile crack growth resistance was simulated for cracked components were performed. Then, the simulated results were compared with experimental ones and it was found that the proposed damage model could accurately predict ductile crack growth resistance and applied to ductile crack growth simulation.

Application of damage model based on void growth analysis to ductile crack growth simulation

ABSTRACT The aim of this study is to confirm that ductile crack growth resistance of cracked components can be predicted by simulation model with the proposed damage model, which is derived by unit cell analyses with an initial void and reflects void growth behaviour until ductile crack initiation. Critical strain under constant stress triaxiality history, which is used in the damage model as ductility of the material, could be determined properly using tensile test results by inverse analyses. Tensile tests using notched round bar specimens, which simply represent stress triaxiality history similar to cracked component, were conducted for two different steels and the proposed damage model was found to be validated for evaluation of ductile crack initiation limit. Then, ductile crack growth tests of two types of bend specimens with a machined notch and a fatigue pre-crack were conducted for a mild steel. It was demonstrated that ductile crack growth resistance could be reproduced using simulation model with the proposed damage model with high accuracy and the influence of plastic constraint to ductile crack growth behaviour could be simulated.

Application of damage model based on void growth analysis to ductile crack growth simulation

The aim of this study is to confirm that ductile crack growth resistance of cracked components can be predicted by simulation model with the proposed damage model, which is derived by unit cell anal...

Simulation of ductile fracture toughness test under monotonic and reverse cyclic loading

This paper presents a ductile crack growth simulation method for monotonic and reverse cyclic loading with large-scale yielding using the damage model based on the multi-axial fracture strain energy. The damage model has two parameters which can be determined from tensile and fracture toughness data under the monotonic loading condition. The determined damage model then can be used to simulate ductile crack growth subjected to reverse cyclic loading with large-scale yielding. The proposed method is validated against test data for SA508 Gr.1a and TP316 under monotonic and reverse cyclic loading with three different reverse loading ratios.

Application of the GTN model to ductile crack growth simulation in through-wall cracked pipes

This paper presents ductile fracture simulation using the GTN (Gurson-Tvergaard-Needleman) model of through-wall cracked STPT410 carbon steel pipe test under pure bending. Three issues related to practical application are addressed; (i) how to determine the parameters in the GTN model from fracture toughness data, (ii) how to incorporate the element-size effect to the GTN model and (iii) the effect of the crack-tip mesh design on determination of the GTN model parameters. It is found that determined values of the parameters depend on the crack-tip mesh design and the finite element size used in simulation. The split of the crack tip element into two makes parameter determination easier and gives better simulation results, but is difficult to apply to three-dimensional surface crack problems.

Numerical simulation of ductile crack growth in medium wide plate specimens using 3-D computational cells

This work examines the applicability of a local damage method based on the computational cell methodology, incorporating the Gurson-Tvergaard (GT) yielding model, to predict ductile crack extension of circumferential crack-like defects in Medium Wide Plates (MWP) specimens, extracted from girth welds of pipelines, using standard single edge bend SE(B) specimen and recently standardized clamped single edge tension SE(T) specimens to obtain model parameters. Laboratory testing of SE(B) and SE(T) specimens at room temperature provides necessary and sufficient information to calibrate the parameters of the GT yielding law. After the model parameters have been defined using small specimens, predictions of crack growth behavior can be carried out for other geometries and loading modes. Therefore, parameter transferability between different specimens can be investigated and comparative studies of predicted crack driving forces can be performed. In this study, the simulated tearing resistance curves are compared to experimental data of MWP specimens, where the former is obtained by the cell model calibrated using either SE(T) or SE(B) specimens and the latter is obtained by large-scale experiments. Overall, a good agreement between numerical and experimental δ-Δa curves was observed demonstrating that the cell methodology can be considered as a valid engineering tool for studying the integrity of structural components.

Numerical simulation of ductile crack growth under cyclic and dynamic loading with a damage–viscoplasticity model

In this paper, the crack propagation in ductile materials is simulated under cyclic and dynamic loading. The adaptive finite element method is used to model the discontinuity due to crack propagation. The ductile fracture assumptions and continuum damage mechanics are utilized to model the material rupture behavior. Moreover both the rate-independent and rate-dependent constitutive equations are elaborated and the crack closure effect and combined hardening model are discussed in addition to some aspects of finite element implementation. Finally, a comparison is performed between the numerical simulation results and those of experiments to illustrate the robustness of proposed computational algorithm.

On critical assessment of the use of local and nonlocal damage models for prediction of ductile crack growth and crack path in various loading and boundary conditions

In this work, we have assessed the results of the local and nonlocal versions of Rousselier’s damage model, which have been used here for simulation of ductile crack growth. There are several issues regarding the accuracy of the results which has been addressed in this paper, e.g., accuracy in simulation of crack path, extent and width of the damaged region, fracture resistance behaviour in situations such as symmetric vs. non-symmetric boundary-value problems, mixed-mode loading vs. mode-I loading of the crack-tip, etc. It was also observed that the shape and orientation of the elements at the crack-tip, in addition to their size, influence the results of the local damage model. In this work, it was shown that the above issues can be resolved through the use of nonlocal damage models. The predictions of the nonlocal model are also consistent with the experimental observations unlike its local counterpart. Several examples were presented, where the results as obtained by both the local and nonlocal models were compared. From this experience, it is recommended that the local damage models should not be used blindly by the analysts for all kinds of mesh design, loading, boundary conditions, etc.

Simulation of ductile crack growth in thin panels using the crack tip opening angle

The aim of this work is the assessment of the efficiency of the crack tip opening angle (CTOA) with respect to the transferability from one geometry to another, in particular the transferability obtained from Kahn tear tests to M(T) panels. The load–displacement behaviour recorded during a Kahn tear test was reproduced by means of finite element analysis using a variable CTOA as a function of crack length. The CTOA extracted from Kahn tests has then been used to simulate the R-curve of M(T) panels with different widths. Experiments and simulations were run first on a 6013-T6 aluminium alloy and then also on butt, friction stir welded butt joints of the same material.

Assessment of Geometry Independent Fracture Resistance Characteristics Based on Local Approach

The influences of stress triaxiality on ductile fracture have been emphasized to explain the geometry independent fracture resistance characteristics of specimens and structures during past two decades. For the estimation of this material behavior, two-parameter global approach and local approach can be used as case by case manner. Recently, the interests for the local approach and micro-mechanical damage model are increased again due to progress of computational environments. In this paper, the applicability of the local approach has been assessed through a series of finite element analyses incorporating both modified GTN model and Rousselier model. The ductile crack growth behaviors are examined to guarantee the transferability on different sizes and geometries of C(T) specimens and SE(T) specimens. The material fitting constants are determined from calibration of tensile tests and numerical analyses results, and used to simulate the fracture behaviors of typical specimens. Then, a comparison is drawn between the numerically estimated crack resistance curves and experimentally determined ones. The comparison results show a good agreement and the two damage models are regarded as promising solutions for ductile crack growth simulation.

Simulation of ductile crack growth in thin aluminum panels using 3-D surface cohesive elements

This work describes the formulation and application of a 3-D, interface-cohesive finite element model to predict quasi-static, ductile crack extension in thin aluminum panels for mode I loading and growth. The fracture model comprises an initially zero thickness, interface element with constitutive response described by a nonlinear traction-separation relationship. Conventional volumetric finite elements model the nonlinear (elastic-plastic) response of background (bulk) material. The interface-cohesive elements undergo gradual decohesion between faces of the volumetric elements to create new traction free crack faces. The paper describes applications of the computational model to simulate crack extension in C(T) and M(T) panels made of a 2.3 mm thick, Al 2024-T3 alloy tested as part of the NASA-Langley Aging Aircraft program. Parameters of the cohesive fracture model (peak opening traction and local work of separation) are calibrated using measured load vs. outside surface crack extensions of high constraint (T-stress > 0) C(T) specimens. Analyses of low constraint M(T) specimens, having widths of 300 and 600 mm and various a/W ratios, demonstrate the capabilities of the calibrated model to predict measured loads and outside surface crack extensions. The models capture accurately the strong 3-D effects leading to various degrees of crack front tunneling in the C(T) and M(T) specimens. The predicted crack growth response shows rapid convergence with through-thickness mesh refinement. Adaptive load increment procedures to control the rate of decohesion in the interface elements leads to stable, rapidly converging iterations in the globally implicit solution procedures.

3d nonlocal simulation of ductile crack growth—a numerical realization

In this article we present a realization of a three dimensional ductile damage analysis. The Rousselier model is used in a finite strain formulation to represent the material behavior, so that a th...

Ductile crack growth simulation from near crack tip dissipated energy

A method to calculate ductile tearing in both small scale fracture mechanics specimens and cracked components is presented. This method is based on an estimation of the dissipated energy calculated near the crack tip. Firstly, the method is presented. It is shown that a characteristic parameter G fr can be obtained, relevant to the dissipated energy in the fracture process. The application of the method to the calculation of side grooved crack tip (CT) specimens of different sizes is examined. The value of G fr is identified by comparing the calculated and experimental load line displacement versus crack extension curve for the smallest CT specimen. With this identified value, it is possible to calculate the global behaviour of the largest specimen. The method is then applied to the calculation of a pipe containing a through-wall thickness crack subjected to a bending moment. This pipe is made of the same material as the CT specimens. It is shown that it is possible to simulate the global behaviour of the structure including the prediction of up to 90-mm crack extension. Local terms such as the equivalent stress or the crack tip opening angle are found to be constant during the crack extension process. This supports the view that G fr controls the fields in the vicinity near the crack tip.

Damage and strain localisation during crack propagation in thin-walled shells

Based on the GURSON damage model a simulation of ductile crack growth in thin-walled shell structures is presented. The proposed Finite Element model for shells is based on a 5-parameter theory taking bending effect into acount. The problem of mesh dependence in strain softening materials is treated by a non-local regularisation of the damage evolution equation. As an example a cylindrical tube under internal pressure is considered. Results for the equivalent stress, the equivalent strain and the damage parameter are shown.

The importance of the triaxiality modified J concept for the assessment of pressurized components with surface flaws

With the progress of stable crack growth of surface flaws observed in panels or pressure vessels a canoe-shaped crack front is formed. The crack propagation in the longitudinal direction is more pronounced that in the wall thickness direction. Therefore, the canoe effect is important with respect to a leak-before-break assessment because the actual through crack length is influenced by this effect. Based on the J integral concept crack initiation and crack propagation in ductile materials are described by J resistance curves which were found to be dependent on the constraint effect of the specimen geometry. Prediction of local crack growth by taking a conservative (flat) J R- curve into account results in a nonconservative estimate of the axial extension of the surface crack [W. Brocks, H. Veith and K. Wobst, in K. Kussmaul (ed.), Fracture Mechanics Verification by Large Scale Testing, Mech. Eng. Publication Limited, London, 1991]. This means that the influence of local constraint effects on crack resistance has to be considered. Ductile crack growth of semi-elliptical surface cracks in side-grooved specimens F(SCTsg) under tension made from German standard steel StE 460 will be reported on. The development of the canoe effect of an SCTsg specimen was also analysed by a finite element simulation of ductile crack growth which was modelled by using the node shift and node release technique and controlled by crack mouth opening displacement versus crack growth curves from the experiment. The simulation allows the determination of local J R- curves in dependence on the local multiaxility of the stress state to verify the constraint modified J concept. It is demonstrate that the slope of the J R- curves decreases with increasing multiaxiality of the stress state near by the crack front.