The critical crack-tip-opening angle or displacement (CTOA/CTOD) fracture criterion is one of the oldest fracture criteria applied to metallic materials. Improved computer-aided photographic methods have been developed to measure CTOA during the fracture process; and elastic-plastic, finite-element analyses (ZIP2D) with a constant CTOA and a plane-strain core have been used to simulate fracture of laboratory specimens. The fracture criterion has been able to link the fracture of laboratory specimens to structural applications. This paper analyzes fracture of cracked thin-sheet 2219 aluminum alloy over an extremely wide range in width, crack-length-to-width ratio, and applied loading. The results from the critical CTOA fracture analyses on the thin-sheet material showed that the stress-intensity factor at failure (KIe) was linearly related to the net-section stress (Sn), as predicted by the Two-Parameter Fracture Criterion (TPFC).
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Themes: Ductile Fracture Under Complex Loading
CHARACTERIZATION AND NUMERICAL SIMULATION OF DUCTILE CRACK INITIATION AND PROPAGATION IN CT SPECIMENS OF DIFFERENT SIZES MACHINED FROM A 316L THICK PLATE
Measuring fracture toughness for ductile materials requires the specimen size to be large enough for the tests to be valid. The higher the toughness is, the larger the specimen must be. This paper uses experimental and numerical approaches to study the fracture behavior of as-received and aged 316L(N) steel and the effect of the size and thickness of the specimens on the evaluated toughness.
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APPLICATION OF A NOVEL UNIFIED PARAMETER ON CHARACTERIZING IN-PLANE AND OUT-OF-PLANE CRACK-TIP CONSTRAINTS FOR AL7075 T651 SEN(B) SPECIMENS
Crack-tip constraint can have a significant effect on fracture toughness. A loss of crack-tip constraint can cause an increase in fracture toughness. In this paper, a novel unified constraint parameter λ based on the plastic strain energy was proposed to quantify the crack-tip constraint level. The application of this parameter for assessing the in-plane and out-of-plane constraints of Al7075 T651 alloy SEN(B) specimens was investigated with a series of fracture bending experiments and numerical modelling.
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FRACTURE MODELLING AND ANALYSIS OF MULTIPLE SITE CRACKS IN PLATES UNDER LATERAL PRESSURE
Results of experimental and finite element study on fracture behavior of damaged thin plate specimens subjected to lateral pressure are presented. Plate specimens with a single crack or an array of collinear cracks were tested applying lateral pressure load by using a specially designed experimental setup. The elastic plastic fracture mechanics concept (EPFM) was employed in FE analyses, as large scale yielding occurred in ligaments of fractured specimens. The critical J-integral and crack tip opening displacement (CTOD) values associated with fracture onset were inferred from finite element simulation results. Assessed critical pressure loads for considered plate specimens were compared with experimentally obtained results and a good agreement was ob-served.
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A GURSON-TYPE LAYER MODEL FOR DUCTILE POROUS SOLIDS CONTAINING ARBITRARY ELLIPSOIDAL VOIDS WITH ISOTROPIC AND KINEMATIC HARDENING
Extensions of Gurson’s model for porous ductile materials have been done by Madou and Leblond (2012) for general ellipsoidal cavities made of rigid-plastic materials, and Morin et al. (2017), for spherical voids with rigid-hardenable matrices. The aim of this work is to provide a homogenized criterion for porous ductile materials incorporating both void shape effects and isotropic and kinematic hardening. A sequential limit-analysis is performed on an ellipsoidal representative volume made of some rigid-hardenable material, containing a confocal ellipsoidal cavity. The overall plastic dissipation is obtained by using the velocity field proposed by Leblond and Gologanu (2008) and that satisfies conditions of homogeneous strain rate on an arbitrary family of confocal ellipsoids. The heterogeneity of hardening is accounted for by discretizing the cell into a finite number of ellipsoids between each of which the quantities characterizing hardening are considered as homogeneous. The model is finally assessed through comparison of its predictions with the results of micromechanical finite element simulations. The numerical and theoretical overall yield loci are compared for various distributions of isotropic and kinematic pre-hardening with a very good agreement.
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A NON-LOCAL GURSON MODEL WITH TWO FRACTURE-MECHANISM ASSOCIATED LENGTH SCALES: SUPPORTED BY NUMERICAL ANALYSES AND EXPERIMENTS
An extension of Gurson’s porous plasticity model capable of preventing pathological strain localization, and describing crack initiation and propagation under both shearing and tension is investigated. This paper separates the progression of shear failure and flat dimple rupture based on the assumption that these two failure mechanisms are governed by different characteristic length scales, a deviatoric and a dilatational length scale, respectively. A set of numerical analyses is presented which brings out the effects of these length scales on the development of e.g. cup-cone and slant fracture. Guided by the outcome of the numerical study, a set of tests has been designed and carried out for calibration of these length scales.
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THE FOURTH SANDIA FRACTURE CHALLENGE – PREDICTING PUNCTURE IN A METAL STRUCTURE [Keynote]
The fourth Sandia Fracture Challenge (SFC4) investigated the puncture of aluminum structures through comparing various computational predictions to physical experiments. Five teams, internal to Sandia National Laboratories, submitted predictions with mixed success. Qualitatively, many teams were able to predict the deformation and failure modes at the critical velocity for puncture, but the extent of damage was underpredicted by all. Quantitatively, predictions for critical velocity varied widely, though were in the correct order of magnitude. The SFC4 highlighted difficulties in modeling damage and fracture in shear-dominated loading cases.
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PREDICTING DUCTILE FRACTURE FOR MIXED MODE OF LOADING USING THE MODIFIED MOHR-COULOMB CRITERION
Reliable and robust fracture prediction tools are necessary for designing and analyzing critical engineering structures. This paper uses a phenomenological damage model to study the fracture response of a pressure vessel steel under complex loading conditions. Details of the experiments and numerical procedures are provided for calibrating and validating the proposed framework for predicting ductile fracture.
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COUPLED CRYSTAL PLASTICITY PHASE-FIELD MODEL FOR DUCTILE FRACTURE IN POLYCRYSTALLINE MICROSTRUCTURES
A wavelet-enriched adaptive hierarchical, coupled crystal plasticity – phase-field finite element model is developed in this work to simulate crack propagation in complex polycrystalline microstructures. The model accommodates initial material anisotropy and crack tension-compression asymmetry through orthogonal decomposition of stored elastic strain energy into tensile and compressive counterparts. The crack evolution is driven by stored elastic and defect energies, resulting from slip and hardening of crystallographic slips systems. A FE model is used to simulate the fracture process in a statistically equivalent representative volume element reconstructed from electron backscattered diffraction scans of experimental microstructures. Multiple numerical simulations with the model exhibits microstructurally sensitive crack propagation characteristics.
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A UNIFIED NONLINEAR XFEM-CZM BASED METHODOLOGY TO DEAL WITH DUCTILE FRACTURE
The numerical treatment of the whole process of ductile fracture remains a challenging task, particularly when FEM is employed. The main issue regards pathologically mesh dependence of the numerical results, not only in the softening regime but also in the stages of strain localization and further crack propagation. In the literature, non-local approaches are adopted to mitigate these effects but they require a calibrated length scale and mesh refinement, thus being time consuming. This work focuses on the numerical treatment of ductile fracture in metal materials via a three-dimensional unified methodology that combines (i) the GTN model to describe diffuse damage using the standard FEM, the (ii) XFEM to represent the crack and (iii) the coupling of the XFEM with a cohesive zone model to account for the intermediate localization phase. We rely upon the Updated Lagrangian formulation to include large strains and rotations. The methodology, implemented in Abaqus commercial code as a user finite element (UEL), is capable of reproducing numerically the overall response of structures until rupture.
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