Micromechanical modelling of ductile fracture – local approach

Micromechanical modelling of ductile fracture by using the local approach has been presented as the review of previously obtained results for welded joints made of low alloyed high strength steel. Experimental work was performed on 3PB specimens and tensile panels, which were then modelled by finite element method, using two- and three-dimensional meshes, respectively. The local approach was used to simulate both for crack initiation and growth during ductile fracture process.

Deformation and fracture behavior of new strain-transformable titanium alloys: a multi-scale investigation

Titanium alloys possessing Twinning and Transformation Induced Plasticity effects show promising mechanical properties, particularly high ductility, hardenability, impact and fracture toughness. This work focuses on a strain-transformable, coarse-grained β-Ti-Cr-Sn alloy displaying TWIP effect. To account for the enhanced properties of this alloy, compared to more conventional β-Ti alloys, fracture and deformation features were correlated at different scales. Examinations evidenced a major role of twinning and, more generally, of plasticity-induced phenomena in the ductile fracture process. The resistance of this alloy to plastic deformation (work-hardening), and to crack initiation and propagation is interpreted in view of the progressive, multiscale twinning mechanisms that occur up to the very final stages of fracture.

A New Method in Identifying Critical Void Volume Fraction of GTN Model for NVA Mild Steel in Ship Collision and Grounding Scenario

The NVA mild steel is a commonly used material in shipbuilding, which possesses good ductility character. However, the description of ductile fracture process for NVA steel in numerical simulation is still a challenging task. A new method to predict the critical void volume fraction fc of Gurson-Tvergaard-Needleman (GTN) model is introduced in this paper. GTN-model is one of the well-known micromechanical models for ductile fracture. The traditional plasticity theory assumes that the plastic volume is incompressible and that the yield of the material is independent of the hydrostatic stress, whereas the yield surface of the GTN-model takes the effect of the macroscopic hydrostatic stress into account. The yield surface is reduced with the increase of the void volume fraction, which can reflect the deterioration characteristics of the material with development of damage during the deformation process. Therefore, GTN-model is a promising mathematical model for describing the ductile fracture process of the ship structures during accidental scenarios of collision and grounding. The traditional way to determine fc of GTN-model is using the inverse method directly, which has a high degree of uncertainty. A new method based on Hill, and Bressan & Williams’s assumptions proposed in this paper solve this problem effectively. Besides, the combined of Voce and Swift constitutive model is used to describe the mechanical property of the NVA material. Furthermore, numerical simulations were also conducted with code LS_DYNA by developing the user-defined subroutine. It is found that the model can predict the structural damage quite accurately, which proves its feasibility of being applied in the research of structural responses in ship collision and grounding accidents.

Damage Characteristics of Dynamically Induced Incipient Spall in Ultrapure Aluminum

A series of plate-impact experiments were performed to study the damage characteristics of incipient spall in ultrapure aluminum (99.999%). Surface profile measurement system based on the axial chromatic aberration was used to measure the cross section of “soft-recovered” samples and reconstruct the metallographic and 3D surface topography. Statistical analyses show that the size distribution, coalescence condition and connectivity factor of the voids are regularly affected by loading conditions. Based on this finding, the mechanisms that control the damage properties and evolution process of dynamic fracture are analyzed in detail. The results of this research offer an experimental quantitative microstructure description of the ductile fracture process.

A Reseach of Fracture Mechanism in Micro Cutting of 45 Steel Based on Material Properties

Though the observation in micro cutting of 45 steel, the deformation zone of fracture characteristics was studied, and the corresponding cutting model was established; on dynamic tensile and situ observation of 45 steel by transmission electron microscopy , micro cutting ductile fracture process is simulated by the experiment on the steel tensile of 45 film sample, and the mechanism on initiation and propagation of crack in micro cutting fracture was studied with dislocation theory.

Interaction Effect Evaluation of Multiple Through Cracks in Ductile Fracture Process

Study on the interaction effect of multiple cracks on the collapse load during ductile crack growth processes is important for the integrity evaluation of nuclear structure. The ductile fracture processes are well studied numerically by using Gurson’s constitutive equation and taking into account of void growth during plastic deformation. In this study, two through cracks are assumed to exist in a flat plate in tension and the interaction between two cracks is studied. The initial crack length, horizontal distance and vertical distance between two cracks are changed systematically. The interaction behavior between two cracks is studied precisely. Criteria to evaluate maximum load in the ductile fracture processes are discussed. The present study found H/l criteria can estimate the maximum load reasonably, and also the consideration of the effect of the horizontal distance between two cracks in H/l criteria may improve its estimation more reasonable. The effects on maximum load by changing the horizontal distance between two cracks are studied.

Interaction Effect Evaluation of Plural Surface Cracks in Ductile Fracture Process

Effect of surface flaw on ductile fracture behavior of non-aligned multiple flaws in plate is studied numerically using Gurson’s constitutive equation. Based on experiments, two parallel non-aligned crack problems are simulated. From experimental and simulation data, crack coalescence, interaction between two cracks and crack penetration was observed. In all cases, ductile fracture processes are obtained and results are compared with experimental ones. Fracture patterns agree well with experimental results. Close qualitative match was obtained by comparing each load displacement curves that are normalized by maximum load. It shows that the mechanism of ductile fracture process is properly captured. By selecting one normalized experimental data as a base setup, maximum load from related simulation results can be predicted. Aligned rules are checked based on these simulations, and H / a criteria verified it’s availability when average crack depth is used as crack depth.