The stress triaxiality effect on the strain required for void nucleation by particle‐matrix debonding has been investigated by means of micromechanical modelling. A unit‐cell model considering an ...elastic spherical particle embedded in an elastic‐plastic matrix was developed to the purpose. Particle‐matrix decohesion was simulated through the progressive failure of a cohesive interface. It has been shown that the parameters of matrix‐particle cohesive interface are correlated with macroscopic material properties. Here, a simple relationship for the maximum cohesive opening at interface failure as a function of material fracture toughness and yield stress has been derived. Results seem to confirm that, increasing stress triaxiality, the strain at which void nucleation is predicted to occur decreases exponentially in a similar way as for fracture strain. This result has substantial implications in modelling of ductile damage because it indicates that if the stress triaxiality is high enough, ductile fracture can occur at plastic strain lower than that necessary to nucleate damage for moderate or low stress triaxiality regime.
A physical-based model for the flow stress of bcc metals is presented. Here, thermally activated and viscous drag regimes are considered. For the thermally activated component of the flow stress, the ...diffusion-controlled regime at elevated temperature is also taken into account assuming the non-linear dependence of the activation volume on temperature. The model was applied to A508 (16MND5) steel showing the possibility to accurately describe the variation of the flow stress over the entire temperature range (from 0 K to Tm) and over a wide strain-rate range.
To test the hypothesis that blunt trauma shockwave propagation may cause macular and peripheral retinal lesions, regardless of the presence of vitreous. The study was prompted by the observation of ...macular hole after an inadvertent BB shot in a previously vitrectomized eye.
The computational model was generated from generic eye geometry. Numeric simulations were performed with explicit finite element code. Simple constitutive modeling for soft tissues was used, and model parameters were calibrated on available experimental data by means of a reverse-engineering approach. Pressure, strain, and strain rates were calculated in vitreous- and aqueous-filled eyes. The paired t-test was used for statistical analysis with a 0.05 significance level.
Pressure at the retinal surface ranged between -1 and +1.8 MPa at the macula. Vitreous-filled eyes showed significantly lower pressures at the macula during the compression phase (P < 0.0001) and at the vitreous base during the rebound phase (P = 0.04). Multiaxial strain reached 20% and 25% at the macula and vitreous base, whereas the strain rate reached 40,000 and 50,000 seconds(-1), respectively. Both strain and strain rates at the macula, vitreous base, and equator reached lower values in the vitreous- compared with the aqueous-filled eyes (P < 0.001). Calculated pressures, strain, and strain rate levels were several orders of magnitude higher than the retina tensile strength and load-carrying capability reported in the literature.
Vitreous traction may not be responsible for blunt trauma-associated retinal lesions and can actually damp shockwaves significantly. Negative pressures associated with multiaxial strain and high strain rates can tear and detach the retina. Differential retinal elasticity may explain the higher tendency toward tearing the macula and vitreous base.
•A plasticity-damage model consistent with fracture micromechanisms is presented.•Specific dissipation potential for void growth, sheeting, and shearing is proposed.•Damage is given by as sum ...plasticity, triaxiality, and Lode angle contributed terms.•Stress triaxiality effect on void nucleation is considered.•The existence of a cut-off for negative stress triaxiality is questioned.
A model for plasticity and damage, developed in the context of continuum damage mechanics and consistent with micromechanisms of void nucleation, growth, and coalescence, is presented. The damage dissipation potential has been formulated specifically to account for different damage contributions due to intervoid necking, shearing, and sheeting under arbitrary stress states. The existence of a cut-off for the negative stress triaxiality was questioned and unilateral conditions for ductile damage have been reformulated accordingly, considering the combination of stress triaxiality and Lode parameter. The model allows the derivation, although in implicit form, of the fracture locus for the whole range of stress triaxiality. An example application to Al2024-T351 and Al6061-T6 is presented. The proposed formulation represents an attempt to reconcile CDM theoretical framework with the ductile fracture specific mechanisms and their sequential progression under general loading conditions, making advantages of micromechanical modeling to establish correlations between void evolution and continuum scale physical quantities. Finite element model implementation and plasticity model formulation will be discussed in a forthcoming companion paper.
•The plasticity damage self-consistent (PDSC) model is formulated for implementation in finite element code.•The PDSC model is used to predict ductile fracture initiation and propagation in Al6061-T6 ...under different stress states.•In notched flat specimen with different thickness the transition from flat to slant fracture is accurately predicted.•In flat specimens under combined loading, computed crack propagation ductile tearing marks agree with experiments.•In upsetting cylinder, in agreement with experimental results, the model predict surface crack initiation and propagation.
The plasticity damage self-consistent (PDSC) model is a damage model developed in the context of continuum damage mechanics in which the damage dissipation potential is formulated to account for damage contributions due to intervoid necking, shearing, and sheeting for an arbitrary stress state. The synergic action of different micromechanisms contributing to ductile fracture is accounted for by their dependence on the plastic deformation, stress triaxiality, third invariant of the deviatoric of the stress tensor, and unilateral conditions. In this work, the PDSC has been implemented in the commercial finite element code MSC MARC via user subroutines and used to predict ductile failure response under different stress state conditions. Firstly, the model implementation has been verified by comparing calculated damage evolution as a function of plastic strain, for prescribed combinations of stress triaxiality and Lode parameter under proportional loading, with an analytical solution. Successively, the model has been validated in predicting ductile fracture for Al6061-T6 in different specimen geometries and comparing with experiment results. Model capability to anticipate fracture initiation conditions, together with specimen global response, and ductile crack propagation also for complex load paths, is demonstrated.
The Bonora damage model was extended to account for shear-controlled damage. To this purpose, a new definition for the damage dissipation potential in which an explicit dependence on the third ...invariant of deviatoric stress was proposed. This expression leads to damage rate equation in which two contributions, the first for void nucleation and growth damage process the latter for shear fracture, can be recognized. For the JIII controlled damage contribution, only two additional material parameters are necessary of easy experimental identification The extended model formulation was verified predicting the failure locus for AL 2024-T351 alloy. Finally, the failure locus for stress state combinations, where the minimum material ductility is expected, was determined.
Discontinuous dynamic recrystallization can occur during dynamic tensile extrusion of copper, which is subjected to uniaxial tensile strains of ~5 and strain rates up to 10
6
s
−1
in the extruded ...section. Through high-resolution transmission Kikuchi diffraction, we show that nucleation occurs through subgrain rotation and grain boundary bulging at boundaries between 〈001〉 and 〈111〉 oriented grains. The observed nuclei consist of subgrains with a size of approximately 200 to 400 nm.
The extended Bonora damage model was used to investigate joinability of materials in self-piercing riveting process. This updated model formulation accounts for void nucleation and growth process and ...shear-controlled damage which is critical for shear fracture sensitive materials. Potential joint configurations with dissimilar materials have been investigated computationally. In particular the possible combination of DP600 steel, which is widely used in the automotive industry, with AL2024-T351, which is known to show shear fracture sensitivity, and oxygen-free pure copper, which is known to fail by void nucleation and growth, have been investigated. Preliminary numerical simulation results indicate that the damage modelling is capable to discriminate potential criticalities occurring in the SPR joining process opening the possibility for process parameters optimization and screening of candidate materials for optimum joint.
Strain-based design for offshore pipeline requires a considerable experimental work aimed to determine the material fracture toughness and the effective strain capacity of pipe and welds. Continuum ...damage mechanics can be used to limit the experimental effort and to perform most of the assessment analysis and evaluation in a simulation environment. In this work, the possibility to predict accurately fracture resistance of X65 steel using a CDM model proposed by the authors, is shown. The procedure for material and damage model parameters identification is presented. Damage model predictive capability was demonstrated predicting ductile crack growth in SENB and SENT fracture specimens.
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