50 research outputs found
Modeling damage and fracture within strain-gradient plasticity
In this work, the influence of the plastic size effect on the fracture
process of metallic materials is numerically analyzed using the strain-gradient
plasticity (SGP) theory established from the Taylor dislocation model. Since
large deformations generally occur in the vicinity of a crack, the numerical
framework of the chosen SGP theory is developed for allowing large strains and
rotations. The material model is implemented in a commercial finite element
(FE) code by a user subroutine, and crack-tip fields are evaluated thoroughly
for both infinitesimal and finite deformation theories by a boundary-layer
formulation. An extensive parametric study is conducted and differences in the
stress distributions ahead of the crack tip, as compared with conventional
plasticity, are quantified. As a consequence of the strain-gradient
contribution to the work hardening of the material, FE results show a
significant increase in the magnitude and the extent of the differences between
the stress fields of SGP and conventional plasticity theories when finite
strains are considered. Since the distance from the crack tip at which the
strain gradient significantly alters the stress field could be one order of
magnitude higher when large strains are considered, results reveal that the
plastic size effect could have important implications in the modelization of
several damage mechanisms where its influence has not yet been considered in
the literature
Gradient-enhanced statistical analysis of cleavage fracture
We present a probabilistic framework for brittle fracture that builds upon
Weibull statistics and strain gradient plasticity. The constitutive response is
given by the mechanism-based strain gradient plasticity theory, aiming to
accurately characterize crack tip stresses by accounting for the role of
plastic strain gradients in elevating local strengthening ahead of cracks. It
is shown that gradients of plastic strain elevate the Weibull stress and the
probability of failure for a given choice of the threshold stress and the
Weibull parameters. The statistical framework presented is used to estimate
failure probabilities across temperatures in ferritic steels. The framework has
the capability to estimate the three statistical parameters present in the
Weibull-type model without any prior assumptions. The calibration against
experimental data shows important differences in the values obtained for strain
gradient plasticity and conventional J2 plasticity. Moreover, local probability
maps show that potential damage initiation sites are much closer to the crack
tip in the case of gradient-enhanced plasticity. Finally, the fracture response
across the ductile-to-brittle regime is investigated by computing the cleavage
resistance curves with increasing temperature. Gradient plasticity predictions
appear to show a better agreement with the experiments
Analysis of the influence of microstructural traps on hydrogen assisted fatigue
We investigate the influence of microstructural traps on hydrogen diffusion
and embrittlement in the presence of cyclic loads. A mechanistic, multi-trap
model for hydrogen transport is developed, implemented into a finite element
framework, and used to capture the variation of crack tip lattice and trapped
hydrogen concentrations as a function of the loading frequency, the trap
binding energies and the trap densities. We show that the maximum value
attained by the lattice hydrogen concentration during the cyclic analysis
exhibits a notable sensitivity to the ratio between the loading frequency and
the effective diffusion coefficient. This is observed for both hydrogen
pre-charged samples (closed-systems) and samples exposed to a permanent source
of hydrogen (open-systems). Experiments are used to determine the critical
concentration for embrittlement, by mapping the range of frequencies where the
output is the same as testing in inert environments. We then quantitatively
investigate and discuss the implications of developing materials with higher
trap densities in mitigating embrittlement in the presence of cyclic loads. It
is shown that, unlike the static case, increasing the density of "beneficial
traps" is a viable strategy in designing alloys resistant to hydrogen assisted
fatigue for both closed- and open-systems
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Gradient-enhanced statistical analysis of cleavage fracture
We present a probabilistic framework for brittle fracture that builds upon Weibull statistics and strain gradient plasticity. The constitutive response is given by the mechanism-based strain gradient plasticity theory, aiming to accurately characterize crack tip stresses by accounting for the role of plastic strain gradients in elevating local strengthening ahead of cracks. It is shown that gradients of plastic strain elevate the Weibull stress and the probability of failure for a given choice of the threshold stress and the Weibull parameters. The statistical framework presented is used to estimate failure probabilities across temperatures in ferritic steels. The framework has the capability to estimate the three statistical parameters present in the Weibull-type model without any prior assumptions. The calibration against experimental data shows important differences in the values obtained for strain gradient plasticity and conventional J2 plasticity. Moreover, local probability maps show that potential damage initiation sites are much closer to the crack tip in the case of gradient-enhanced plasticity. Finally, the fracture response across the ductile-to-brittle regime is investigated by computing the cleavage resistance curves with increasing temperature. Gradient plasticity predictions appear to show a better agreement with the experiments
A general framework for decomposing the phase field fracture driving force, particularised to a Drucker–Prager failure surface
Due to its computational robustness and versatility, the phase field fracture model has become the preferred tool for predicting a wide range of cracking phenomena. However, in its conventional form, its intrinsic tension–compression symmetry in damage evolution prevents its application to the modelling of compressive failures in brittle and quasi-brittle solids, such as concrete or rock materials. In this work, we present a general methodology for decomposing the phase field fracture driving force, the strain energy density, so as to reproduce asymmetrical tension–compression fracture behaviour. The generalised approach presented is particularised to the case of linear elastic solids and the Drucker–Prager failure criterion. The ability of the presented model to capture the compressive failure of brittle materials is showcased by numerically implementing the resulting strain energy split formulation and addressing four case studies of particular interest. Firstly, insight is gained into the capabilities of the model in predicting friction and dilatancy effects under shear loading. Secondly, virtual direct shear tests are conducted to assess fracture predictions under different pressure levels. Thirdly, a concrete cylinder is subjected to uniaxial and triaxial compression to investigate the influence of confinement. Finally, the localised failure of a soil slope is predicted and the results are compared with other formulations for the strain energy decomposition proposed in the literature. The results provide a good qualitative agreement with experimental observations and demonstrate the capabilities of phase field fracture methods to predict crack nucleation and growth under multi-axial loading in materials exhibiting asymmetric tension–compression fracture behaviour
A cohesive zone framework for environmentally assisted fatigue
We present a compelling finite element framework to model hydrogen assisted
fatigue by means of a hydrogen- and cycle-dependent cohesive zone formulation.
The model builds upon: (i) appropriate environmental boundary conditions, (ii)
a coupled mechanical and hydrogen diffusion response, driven by chemical
potential gradients, (iii) a mechanical behavior characterized by finite
deformation J2 plasticity, (iv) a phenomenological trapping model, (v) an
irreversible cohesive zone formulation for fatigue, grounded on continuum
damage mechanics, and (vi) a traction-separation law dependent on hydrogen
coverage calculated from first principles. The computations show that the
present scheme appropriately captures the main experimental trends; namely, the
sensitivity of fatigue crack growth rates to the loading frequency and the
environment. The role of yield strength, work hardening, and constraint
conditions in enhancing crack growth rates as a function of the frequency is
thoroughly investigated. The results reveal the need to incorporate additional
sources of stress elevation, such as gradient-enhanced dislocation hardening,
to attain a quantitative agreement with the experiments
Biomass dust explosions: CFD simulations and venting experiments in a 1Â m3 silo
This study presents CFD simulations of biomass dust explosions in a newly developed experimental 1 m3 silo apparatus with variable venting, designed and fabricated to operate similarly to the explosivity test standards. The aim of the study is to validate a CFD model under development and investigate its capability to capture the transient effects of a vented explosion. The model is based on OpenFOAM and solves the multiphase (gas-particle) flow using an Eulerian-Lagrangian approach in a two-way regime. It considers the detailed thermochemical conversion of biomass, including moisture evaporation, devolatilization, and char oxidation, along with the homogeneous combustion of gases, turbulence, and radiative heat transfer. The explosion is analyzed in all stages, i.e., dust cloud dispersion, ignition, closed explosion, and vented explosion. The results indicate excellent agreement between the CFD model and experimental tests throughout the sequence. Our findings highlight the critical role of particle size in dust cloud distribution and pre-ignition turbulence, which significantly influences flame dynamics and the explosion itself. This model shows great promise and encourages its application for future investigations of biomass dust explosions in larger-scale geometries, especially in venting situations that fall out of the scope of the NFPA 68 or EN 14491 standards, and to help design effective safety measures to prevent such incidents