C. Betegón

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.

Strain gradient plasticity modeling of hydrogen diffusion to the crack tip

Abstract In this work hydrogen diffusion towards the fracture process zone is examined accounting for local hardening due to geometrically necessary dislocations (GNDs) by means of strain gradient plasticity (SGP). Finite element computations are performed within the finite deformation theory to characterize the gradient-enhanced stress elevation and subsequent diffusion of hydrogen towards the crack tip. Results reveal that GNDs, absent in conventional plasticity predictions, play a fundamental role on hydrogen transport ahead of a crack. SGP estimations provide a good agreement with experimental measurements of crack tip deformation and high levels of lattice hydrogen concentration are predicted within microns to the crack tip. The important implications of the results in the understanding of hydrogen embrittlement mechanisms are thoroughly discussed.

Comparison of the static and dynamic fracture behaviour of an AE-460 structural steel

Abstract The ductile-to-brittle transition curves of a structural steel under low strain rate (static conditions) and impact (dynamic conditions) are compared in this work, and the influence of the geometry of the initial defect that promotes fracture (notch or crack) is evaluated. The toughness of the precracked specimens was characterised by the J-integral parameters obtained at the initiation of the stable crack growth, Jc, and when the unstable fracture takes place, Ju. The different methods used in dynamic tests in order to calculate the dynamic J-integral parameter, Jd, were evaluated. The use of cracked specimens, instead of notched ones, modifies the transition curve to higher temperatures and, finally, the transition temperature shift between static and dynamic test results is also quantified

Non-local plasticity effects on notch fracture mechanics

The authors gratefully acknowledge financial support from the Ministry of Economy and Competitiveness of Spain through grant MAT2014-58738-C3. E. Martinez-Paneda also acknowledges financial support from the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme (FP7/2007-2013) under REA grant agreement n° 609405 (COFUNDPostdocDTU).

A cohesive zone framework for environmentally assisted fatigue

Abstract 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.

Implementation of an elastic-viscoplastic ductile model for the numerical simulation of the ductile crack growth in notched tensile and Charpy impact tests

A mathematical algorithm which integrates the constitutive equations for the ductile fracture process in viscoplastic materials is described. The algorithm has been implemented in the finite-element commercial code ABAQUS by means of a constitutive USER subroutine. Based on the computational cell methodology proposed by Xia and Shih, the R-curves for pre-cracked Charpy specimens under different dynamic load conditions are obtained. In all cases it is observed that the mathematical algorithm is able to reproduce the increase in the material resistance to ductile tearing as the impact speed increases.