Crack Propagation in Mixed-Mode Ductile Fracture with Continuum Damage Mechanics

1989 ◽  
Vol 203 (3) ◽  
pp. 189-199 ◽  
Author(s):  
C L Chow ◽  
J Wang
Keyword(s):  
Crack Initiation ◽  
Crack Propagation ◽  
Ductile Fracture ◽  
Crack Growth ◽  
Mixed Mode ◽  
Damage Mechanics ◽  
Continuum Damage Mechanics ◽  
Propagation Model ◽  
Element Formulation ◽  
Continuum Damage

This paper presents a crack propagation model based on an anisotropic theory of continuum damage mechanics proposed earlier by the authors capable of characterizing sub-critical crack growth in mixed-mode ductile fracture. The embedded β angles of mixed-mode specimens include 75, 60, 45 and 30 degrees. The crack growth criteria chosen are essentially those developed for crack initiation and are postulated as: 1. A crack propagates in the direction of maximum effective damage equivalent stress αd. 2. The threshold condition of crack initiation is satisfied when the overall damage w in an element ahead of the crack tip at the prospective crack growth direction reaches its critical value, wc. The crack growth behaviours of the mixed-mode specimens are analysed using a finite element formulation of the non-proportional loading based on the anisotropic model, and the predicted and measured results are found to be satisfactory. The proposed model is also adapted to predict the critical loads of unstable crack propagation and the results are compared favourably with those determined experimentally. Hence the application of the above two fracture criteria can be made to address the entire history of crack development from initiation to rapid growth.

Parameter Calibration for Continuum Damage Mechanics Models to Simulate Ductile Fracture of High Strength Pipeline Steels

2019 ◽  
Author(s):  
Filip Van den Abeele
Keyword(s):  
Crack Propagation ◽  
Ductile Fracture ◽  
Damage Mechanics ◽  
Void Nucleation ◽  
High Strength ◽  
Continuum Damage Mechanics ◽  
Continuum Damage ◽  
Pipeline Steels ◽  
Gtn Model ◽  
Input Parameters

Abstract The ability to arrest a running crack is one of the key features in the safe design of pipeline systems. In the industry design codes, the crack arrest properties of a pipeline should meet two requirements: crack propagation has to occur in a ductile fashion, and enough energy should be dissipated during propagation. While the first criterion is assessed by the Battelle Drop Weight Tear Test (BDWTT) at low temperatures, the latter requirement is converted into a lower bound for the impact energy absorbed during a Charpy V-notch (CVN) impact test. However, the introduction of high strength pipelines steels (X70 and beyond) has revealed that the commonly used relations based on BDWTT and CVN no longer hold. For such scenarios, Continuum Damage Mechanics (CDM) models provide promising potential to obtain a more profound understanding of the mechanisms that govern ductile crack propagation in high strength pipeline steels. In recent years, different types of CDM models have been used to simulate ductile fracture of pipeline steels. This paper focuses on two of these models, i.e. the Gurson-Tvergaard-Needleman (GTN) model and the Modified Bai-Wierzbicki (MBW) model. The GTN model is based on the computation of void growth according to Rice and Tracey, and account for the local softening of the material due to void nucleation, growth and subsequent coalescence. The MBW model is a fully coupled damage model, where the yield surface depends on both the stress triaxiality and the Lode angle. Although both models can predict ductile fracture propagation, their widespread application in pipeline design is hampered by the large number of input parameters to be calibrated. The GTN model requires 10 input parameters, i.e. 3 Tvergaard damage parameters, 4 porosity parameters and 3 parameters to describe void nucleation. Whereas the Modified Mohr-Coulomb model originally proposed by Bai and Wierzbicki uses merely 2 parameters, the extended MBW model requires no less than 18 parameters to be calibrated: 11 plasticity parameters (5 stress + 3 strain rate + 3 temperature) and 7 damage parameters (4 initiation + 1 propagation + 2 failure). In this paper, different numerical/experimental strategies to calibrate these parameter sets are reviewed and compared. Sensitivity analyses are performed to assess the influence of the different input parameters on the model predictions. For both GTN and MBW models, the robustness and uniqueness of the calibrated parameter sets is investigated. Recommendations on optimum parameter values are derived, with special emphasis on high strength pipeline steels.

DUCTILE FAILURE SIMULATION IN SPHERODIZED STEEL USING A CONTINUUM DAMAGE MECHANICS COUPLED FINITE ELEMENT FORMULATION

2010 ◽  
Vol 07 (02) ◽  
pp. 319-348 ◽  
Author(s):  
SACHIN S. GAUTAM ◽  
P. M. DIXIT
Keyword(s):  
Ductile Fracture ◽  
Damage Mechanics ◽  
Ductile Failure ◽  
Fracture Initiation ◽  
Continuum Damage Mechanics ◽  
Critical Value ◽  
Experimental Results ◽  
Element Formulation ◽  
Continuum Damage ◽  
Mechanics Model

Ductile fracture occurs due to microvoid nucleation, growth and, finally, coalescence into microcracks. These microcracks grow in the presence of stresses leading to fracture. In this work, a criterion based on this phenomenon is used to simulate ductile fracture in a class of steel specimens. A critical value of the damage variable, estimated from experimental results, is used as an indicator of fracture initiation. A continuum damage mechanics model is employed to incorporate the damage in the constitutive relation of the material. A damage growth law based on experimental results is used. It is observed that the damage reaches the critical value first at the center in both the cylindrical and prenotched specimens. Thus, the failure begins at the center and then grows radially outward toward the free surface. The analysis is carried out till the damage reaches the critical value across the whole cross-section, at which point the specimen is considered to have failed.

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