A finite element ductile failure simulation method using stress-modified fracture strain model

Existing and emerging methods in computational mechanics are rarely validated against problems with an unknown outcome. For this reason, Sandia National Laboratories, in partnership with US National Science Foundation and Naval Surface Warfare Center Carderock Division, launched a computational challenge in mid-summer, 2012. Researchers and engineers were invited to predict crack initiation and propagation in a simple but novel geometry fabricated from a common off-the-shelf commercial engineering alloy. The goal of this international Sandia Fracture Challenge was to benchmark the capabilities for the prediction of deformation and damage evolution associated with ductile tearing in structural metals, including physics models, computational methods, and numerical implementations currently available in the computational fracture community. Thirteen teams participated, reporting blind predictions for the outcome of the Challenge. The simulations and experiments were performed independently and kept confidential. The methods for fracture prediction taken by the thirteen teams ranged from very simple engineering calculations to complicated multiscale simulations. The wide variation in modeling results showed a striking lack of consistency across research groups in addressing problems of ductile fracture. While some methods were more successful than others, it is clear that the problem of ductile fracture prediction continues to be challenging. Specific areas of deficiency have been identified through this effort. Also, the effort has underscored the need for additional blind prediction-based assessments.

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The Sandia Fracture Challenge: blind round robin predictions of ductile tearing

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Computational aspects of nonlinear fracture mechanics

Ductile failure in polycrystalline aluminum alloys is explored through uniaxial tension and notched tension experiments. Specimens obtained through tests interrupted at various stages of deformation and failure evolution are examined through microscopy to discern the mechanisms of failure and to evaluate the local strain evolution quantitatively. Fractographic observations are used to identify the onset and evolution of damage processes during deformation and failure of these aluminum alloys. Local strain levels are estimated from measurements of the change in grain size with deformation and used to indicate that the local values of failure strains are likely to be much larger than that estimated from strains averaged over characteristic specimen dimensions such as the gage length or the specimen diameter. Lower bound estimates of the failure strain at moderate triaxiliaties are obtained from the experiments.

Ductile failure behavior of polycrystalline Al 6061-T6

CDM modeling of ductile failure in ferritic steels: Assessment of the geometry transferability of model parameters

Results of a European numerical round robin on the application of constitutive equations for ductile damage to simulate tearing of a ferritic steel are presented. The analysis of deformation and failure of a standard smooth tensile specimen is used to characterize the material and identify critical damage parameters for ductile tearing. Ductile crack growth in a C(T) specimen is then numerically simulated by applying the constitutive models of Gurson or Rousselier in order to predict a J R -curve. Although the tensile tests were satisfactorily described, two main problems arose. The first one is that the participants' results differ considerably from each other with respect to the critical damage parameter obtained in the first task and the predicted J R -curves in the second. The reasons were partially found in the strategies for parameter identification and the non-unique definitions for crack initiation and crack growth used in the simulations. Further reasons are expected in the individual realization of the damage model in the FE programmes or user subroutines. The second problem is a general discrepancy between the simulations and the experiment: most of the predicted J R -curves are too flat compared with the experimental one. Manifold conclusions can be drawn for future work aiming at improving the reliability of the numerical models and promoting their application in engineering practice.

G Bernauer

Papers

Micro‐mechanical modelling of ductile damage and tearing – results of a European numerical round robin