Imperial College London

DrEmilioMartinez-Paneda

Faculty of EngineeringDepartment of Civil and Environmental Engineering

Lecturer in Mechanics of Materials
 
 
 
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Contact

 

+44 (0)20 7594 8188e.martinez-paneda Website

 
 
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Location

 

249Skempton BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

40 results found

Kristensen PK, Niordson CF, Martínez-Pañeda E, 2020, A phase field model for elastic-gradient-plastic solids undergoing hydrogen embrittlement, Journal of the Mechanics and Physics of Solids, Vol: 143, Pages: 104093-104093, ISSN: 0022-5096

Journal article

Saeimi Sadigh MA, Paygozar B, da Silva LFM, Martínez-Pañeda Eet al., 2020, Creep behaviour and tensile response of adhesively bonded polyethylene joints: Single-Lap and Double-Strap, International Journal of Adhesion and Adhesives, Vol: 102, Pages: 1-10, ISSN: 0143-7496

The static and time-dependent behaviours of adhesively bonded polyethylene Double-Strap (DS) joints were investigated to assess the viability of this joint configuration relative to the Single-Lap (SL) joints. Both experiments and finite element simulations are conducted. First, we individually characterise the tensile and creep behaviour of the adhesive and adherent materials; an epoxy-based adhesive and polyethylene, respectively. This information is used to develop suitable constitutive models that are then implemented in the commercial finite element package ABAQUS by means of user material subroutines, UMATs. The numerical models are used to design the creep tests on the adhesive joints. Afterwards, an extensive experimental campaign is conducted where we characterise the static and creep behaviour of two joint configurations, SL and DS joints, and three selected values of the overlap length. In regard to the static case, results reveal an increase in the failure load with increasing overlap length, of up to 10% for an overlap length of 39 mm. Also, slightly better performance is observed for the SL joint configuration. For the creep experiments, we show that the DS adhesive joint configuration leads to much shorter elongations, relative to the SL joints. These differences diminish with increasing overlap length but remain substantial in all cases. In both joint configurations, the elongation increases with decreasing overlap length. For instance, increasing the overlap length to 39 mm led to a 50% and a 30% reduction in elongation for SL and DS joints, respectively. Moreover, the numerical predictions show a good agreement with the experiments. The stress redistribution is investigated and it is found that the shear stress is highly sensitive to the testing time, with differences being more noticeable for the DS joint system. The findings bring insight into the creep behaviour of polyethylene-based adhesive joints, a configuration of notable industrial interes

Journal article

García-Guzmán L, Reinoso J, Valverde A, Martínez-Pañeda E, Távara Let al., 2020, Numerical study of interface cracking in composite structures using a novel geometrically nonlinear Linear Elastic Brittle Interface Model: mixed-mode fracture conditions and application to structured interfaces, Composite Structures, Vol: 248, ISSN: 0263-8223

Interface cracking is one of the most prominent failure modes in fibrereinforced polymer (FRP) composites. Recent trends in high-tech applications of FRP composites exploit the limits of the load bearing capacity, generally encompassing the development of notable nonlinear effects from geometrical and material signatures. In this investigation, we present a comprehensive assessment of the new Linear Elastic Brittle Interface Model (LEBIM) in geometrically nonlinear applications undergoing mixed-mode fracture conditions. This interface model for triggering fracture events is formulated through the advocation of continuum-like assumptions (for initial non-zero interface thickness) and allows the incorporation of the potential role of the in-plane deformation effects. The performance of the present interface model is demonstrated through the simulation of specimens with mixed-mode delamination, with special attention for its application in samples equipped with structured interfaces. Current predictions exhibit an excellent agreement with respect to experimental data, validating the proposed methodology.

Journal article

Martinez-Paneda E, Wright L, Díaz A, Turnbull Aet al., 2020, Generalised boundary conditions for hydrogen transport at crack tips, Corrosion Science, Vol: 173, ISSN: 0010-938X

We present a generalised framework for resolving the electrochemistry-diffusion interface and modelling hydrogen transport near a crack tip. The adsorption and absorption kinetics are captured by means of Neumann-type generalised boundary conditions. The diffusion model includes the role of trapping, with a constant or evolving trap density, and the influence of the hydrostatic stress. Both conventional plasticity and strain gradient plasticity are used to model the mechanical behaviour of the solid. Notable differences are found in the estimated crack tip hydrogen concentrations when comparing with the common procedure of prescribing a constant hydrogen concentration at the crack surfaces.

Journal article

Díaz A, Cuesta II, Martínez-Pañeda E, Alegre JMet al., 2020, Influence of charging conditions on simulated temperature-programmed desorption for hydrogen in metals, International Journal of Hydrogen Energy, ISSN: 0360-3199

Journal article

Kristensen PK, Martínez-Pañeda E, 2020, Phase field fracture modelling using quasi-Newton methods and a new adaptive step scheme, Theoretical and Applied Fracture Mechanics, Vol: 107, Pages: 1-13, ISSN: 0167-8442

We investigate the potential of quasi-Newton methods in facilitating convergence of monolithic solution schemes for phase field fracture modelling. Several paradigmatic boundary value problems are addressed, spanning the fields of quasi-static fracture, fatigue damage and dynamic cracking. The finite element results obtained reveal the robustness of quasi-Newton monolithic schemes, with convergence readily attained under both stable and unstable cracking conditions. Moreover, since the solution method is unconditionally stable, very significant computational gains are observed relative to the widely used staggered solution schemes. In addition, a new adaptive time increment scheme is presented to further reduces the computational cost while allowing to accurately resolve sudden changes in material behavior, such as unstable crack growth. Computation times can be reduced by several orders of magnitude, with the number of load increments required by the corresponding staggered solution being up to 3000 times higher. Quasi-Newton monolithic solution schemes can be a key enabler for large scale phase field fracture simulations. Implications are particularly relevant for the emerging field of phase field fatigue, as results show that staggered cycle-by-cycle calculations are prohibitive in mid or high cycle fatigue. The finite element codes are available to download from www.empaneda.com/codes.

Journal article

Díaz A, Cuesta II, Martinez-Paneda E, Alegre JMet al., 2020, Analysis of hydrogen permeation tests considering two different modelling approaches for grain boundary trapping in iron, International Journal of Fracture, Vol: 223, Pages: 17-35, ISSN: 0376-9429

The electrochemical permeation test is one of the most used methods forcharacterising hydrogen diffusion in metals. The flux of hydrogen atomsregistered in the oxidation cell might be fitted to obtain apparentdiffusivities. The magnitude of this coefficient has a decisive influence onthe kinetics of fracture or fatigue phenomena assisted by hydrogen and dependslargely on hydrogen retention in microstructural traps. In order to improve thenumerical fitting of diffusion coefficients, a permeation test has beenreproduced using FEM simulations considering two approaches: a continuum 1Dmodel in which the trap density, binding energy and the input latticeconcentrations are critical variables and a polycrystalline model wheretrapping at grain boundaries is simulated explicitly including a segregationfactor and a diffusion coefficient different from that of the interior of thegrain. Results show that the continuum model captures trapping delay, but itshould be modified to model the trapping influence on the steady state flux.Permeation behaviour might be classified according to different regimesdepending on deviation from Fickian diffusion. Polycrystalline syntheticpermeation shows a strong influence of segregation on output flux magnitude.This approach is able to simulate also the short-circuit diffusion phenomenon.The comparison between different grain sizes and grain boundary thicknesses bymeans of the fitted apparent diffusivity shows the relationships between theregistered flux and the characteristic parameters of traps.

Journal article

Hirshikesh, Martínez-Pañeda E, Natarajan S, 2020, Adaptive phase field modelling of crack propagation in orthotropic functionally graded materials, Defence Technology, ISSN: 2214-9147

In this work, we extend the recently proposed adaptive phase field method tomodel fracture in orthotropic functionally graded materials (FGMs). A recoverytype error indicator combined with quadtree decomposition is employed foradaptive mesh refinement. The proposed approach is capable of capturing thefracture process with a localized mesh refinement that provides notable gainsin computational efficiency. The implementation is validated againstexperimental data and other numerical experiments on orthotropic materials withdifferent material orientations. The results reveal an increase in thestiffness and the maximum force with increasing material orientation angle. Thestudy is then extended to the analysis of orthotropic FGMs. It is observedthat, if the gradation in fracture properties is neglected, the materialgradient plays a secondary role, with the fracture behaviour being dominated bythe orthotropy of the material. However, when the toughness increases along thecrack propagation path, a substantial gain in fracture resistance is observed.

Journal article

Martinez-Paneda E, Ivan Cuesta I, Fleck NA, 2020, Mode II Fracture of an Elastic-Plastic Sandwich Layer, JOURNAL OF APPLIED MECHANICS-TRANSACTIONS OF THE ASME, Vol: 87, ISSN: 0021-8936

The shear strength of a pre-cracked sandwich layer is predicted, assuming that the layer is linear elastic or elastic-plastic, with yielding characterized by either J2 plasticity theory or by a strip-yield model. The substrates are elastic and of dissimilar modulus to that of the layer. Two geometries are analysed: (i) a semi-infinite crack in a sandwich layer, subjected to a remote mode II K-field and (ii) a centre-cracked sandwich plate of finite width under remote shear stress. For the semi-infinite crack, the near tip stress field is determined as a function of elastic mismatch, and crack tip plasticity is either prevented (the elastic case) or is duly accounted for (the elastic-plastic case). Analytical and numerical solutions are then obtained for the centre-cracked sandwich plate of finite width. First, a mode II K-calibration is obtained for a finite crack in the elastic sandwich layer. Second, the analysis is extended to account for crack tip plasticity via a mode II strip-yield model of finite strength and of finite toughness. The analytical predictions are verified by finite element simulations and a failure map is constructed in terms of specimen geometry and crack length.

Journal article

Martinez-Paneda E, Harris ZD, Fuentes-Alonso S, Scully JR, Burns JTet al., 2020, On the suitability of slow strain rate tensile testing for assessing hydrogen embrittlement susceptibility, Corrosion Science, Vol: 163, Pages: 1-17, ISSN: 0010-938X

The onset of sub-critical crack growth during slow strain rate tensile testing(SSRT) is assessed through a combined experimental and modeling approach.A systematic comparison of the extent of intergranular fracture and expectedhydrogen ingress suggests that hydrogen diffusion alone is insufficient to ex-plain the intergranular fracture depths observed after SSRT experiments ina Ni-Cu superalloy. Simulations of these experiments using a new phase fieldformulation indicate that crack initiation occurs as low as 40% of the timeto failure. The implications of such sub-critical crack growth on the validityand interpretation of SSRT metrics are then explored.

Journal article

Cuesta II, Martinez-Paneda E, Díaz A, Alegre JMet al., 2019, The essential work of fracture parameters for 3D printed polymer sheets, Materials and Design, Vol: 181, ISSN: 0264-1275

Additive manufacturing is becoming increasingly popular in academia and industry. Accordingly, there has been a growing interest in characterizing 3D printed samples to determine their structural integrity behaviour. We employ the Essential Work of Fracture (EWF) to investigate the mechanical response of polymer sheets obtained through additive manufacturing. Our goal is twofold; first, we aim at gaining insight into the role of fibre reinforcement on the fracture resistance of additively manufactured polymer sheets. Deeply double-edge notched tensile (DDEN-T) tests are conducted on four different polymers: Onyx, a crystalline, nylon-reinforced polymer, and three standard polymers used in additive manufacturing – PLA, PP and ABS. Results show that fibre-reinforcement translates into a notable increase in fracture resistance, with the fracture energy of Onyx being an order of magnitude higher than that reported for non-reinforced polymers. On the other hand, we propose the use of a miniature test specimen, the deeply double-edge notched small punch specimens (DDEN-SP), to characterize the mechanical response using a limited amount of material. The results obtained exhibit good alignment with the DDEN-T data, suggesting the suitability of the DDEN-SP test for measuring fracture properties of additively manufactured polymers in a cost-effective manner.

Journal article

Martinez-Paneda E, Fuentes-Alonso S, Betegon C, 2019, Gradient-enhanced statistical analysis of cleavage fracture, European Journal of Mechanics A: Solids, Vol: 77, ISSN: 0997-7538

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.

Journal article

Ivan Cuesta I, Martinez-Paneda E, Diaz A, Manuel Alegre Jet al., 2019, Cold isostatic pressing to improve the mechanical performance of additively manufactured metallic components, Materials, Vol: 12, ISSN: 1996-1944

Additive manufacturing is becoming a technique with great prospects for the production of components with new designs or shapes that are difficult to obtain by conventional manufacturing methods. One of the most promising techniques for printing metallic components is binder jetting, due to its time efficiency and its ability to generate complex parts. In this process, a liquid binding agent is selectively deposited to adhere the powder particles of the printing material. Once the metallic piece is generated, it undergoes a subsequent process of curing and sintering to increase its density (hot isostatic pressing). In this work, we propose subjecting the manufactured component to an additional post-processing treatment involving the application of a high hydrostatic pressure (5000 bar) at room temperature. This post-processing technique, so-called cold isostatic pressing (CIP), is shown to increase the yield load and the maximum carrying capacity of an additively manufactured AISI 316L stainless steel. The mechanical properties, with and without CIP processing, are estimated by means of the small punch test, a suitable experimental technique to assess the mechanical response of small samples. In addition, we investigate the porosity and microstructure of the material according to the orientations of layer deposition during the manufacturing process. Our observations reveal a homogeneous distribution independent of these orientations, evidencing thus an isotropic behaviour of the material.

Journal article

Hirshikesh, Natarajan S, Annabattula RK, Martinez-Paneda Eet al., 2019, Phase field modelling of crack propagation in functionally graded materials, Composites Part B: Engineering, Vol: 169, Pages: 239-248, ISSN: 0961-9526

We present a phase field formulation for fracture in functionally graded materials (FGMs). The model builds upon homogenization theory and accounts for the spatial variation of elastic and fracture properties. Several paradigmatic case studies are addressed to demonstrate the potential of the proposed modelling framework. Specifically, we (i) gain insight into the crack growth resistance of FGMs by conducting numerical experiments over a wide range of material gradation profiles and orientations, (ii) accurately reproduce the crack trajectories observed in graded photodegradable copolymers and glass-filled epoxy FGMs, (iii) benchmark our predictions with results from alternative numerical methodologies, and (iv) model complex crack paths and failure in three dimensional functionally graded solids. The suitability of phase field fracture methods in capturing the crack deflections intrinsic to crack tip mode-mixity due to material gradients is demonstrated. Material gradient profiles that prevent unstable fracture and enhance crack growth resistance are identified: this provides the foundation for the design of fracture resistant FGMs. The finite element code developed can be downloaded from www.empaneda.com/codes.

Journal article

Martinez-Paneda E, Deshpande VS, Niordson CF, Fleck NAet al., 2019, The role of plastic strain gradients in the crack growth resistance of metals, Journal of the Mechanics and Physics of Solids, Vol: 126, Pages: 136-150, ISSN: 0022-5096

Crack advance from short or long pre-cracks is predicted by the progressive failure of a cohesive zone in a strain gradient, elasto-plastic solid. The presence of strain gradients leads to the existence of an elastic zone at the tip of a stationary crack, for both the long crack and the short crack cases. This is in sharp contrast with previous asymptotic analyses of gradient solids, where elastic strains were neglected. The presence of an elastic singularity at the crack tip generates stresses which are sufficiently high to activate quasi-cleavage. For the long crack case, crack growth resistance curves are predicted for a wide range of ratios of cohesive zone strength to yield strength. Remarkably, this feature of an elastic singularity is preserved for short cracks, leading to a severe reduction in tensile ductility. In qualitative terms, these predictions resemble those of discrete dislocation calculations, including the concept of a dislocation-free zone at the crack tip.

Journal article

Martinez-Paneda E, Fleck NA, 2019, Mode I crack tip fields: Strain gradient plasticity theory versus J2 flow theory, European Journal of Mechanics A: Solids, Vol: 75, Pages: 381-388, ISSN: 0997-7538

The mode I crack tip asymptotic response of a solid characterised by strain gradient plasticity is investigated. It is found that elastic strains dominate plastic strains near the crack tip, and thus the Cauchy stress and the strain state are given asymptotically by the elastic K-field. This crack tip elastic zone is embedded within an annular elasto-plastic zone. This feature is predicted by both a crack tip asymptotic analysis and a finite element computation. When small scale yielding applies, three distinct regimes exist: an outer elastic K field, an intermediate elasto-plastic field, and an inner elastic K field. The inner elastic core significantly influences the crack opening profile. Crack tip plasticity is suppressed when the material length scale of the gradient theory is on the order of the plastic zone size estimation, as dictated by the remote stress intensity factor. A generalized J-integral for strain gradient plasticity is stated and used to characterise the asymptotic response ahead of a short crack. Finite element analysis of a cracked three point bend specimen reveals that the crack tip elastic zone persists in the presence of bulk plasticity and an outer J-field.

Journal article

Juul KJ, Martinez-Paneda E, Nielsen KL, Niordson CFet al., 2019, Steady-state fracture toughness of elastic-plastic solids: Isotropic versus kinematic hardening, Engineering Fracture Mechanics, Vol: 207, Pages: 254-268, ISSN: 0013-7944

The fracture toughness for a mode I/II crack propagating in a ductile material has been subject to numerous investigations. However, the influence of the material hardening law has received very limited attention, with isotropic hardening being the default choice if cyclic loads are absent. The present work extends the existing studies of monotonic mode I/II steady-state crack propagation with the goal to compare the predictions from an isotropic hardening model with that of a kinematic hardening model. The work is conducted through a purpose-built steady-state framework that directly delivers the steady-state solution. In order to provide a fracture criterion, a cohesive zone model is adopted and embedded at the crack tip in the steady-state framework, while a control algorithm for the far-field, that significantly reduces the number of equilibrium iterations is employed to couple the far-field loading to the correct crack tip opening. Results show that the steady-state fracture toughness (shielding ratio) obtained for a kinematic hardening material is larger than for the corresponding isotropic hardening case. The difference between the isotropic and kinematic model is tied to the non-proportional loading conditions and reverse plasticity. This also explains the vanishing difference in the shielding ratio when considering mode II crack propagation as the non-proportional loading is less pronounced and the reverse plasticity is absent.

Journal article

Cuesta II, Willig A, Diaz A, Martinez-Paneda E, Alegre JMet al., 2019, Pre-notched dog bone small punch specimens for the estimation of fracture properties, Engineering Failure Analysis, Vol: 96, Pages: 236-240, ISSN: 1350-6307

In recent years, the pre-notched or pre-cracked small punch test (P-SPT) has been successfully used to estimate the fracture properties of metallic materials for cases in which there is not sufficient material to identify these properties from standard tests, such as CT or SENB specimens. The P-SPT basically consists of deforming a pre-notched miniature specimen, whose edges are firmly gripped by a die, using a high strength punch. The novelty of this paper lies in the estimation of fracture properties using dog-bone-shaped specimens with different confinement levels. With these specimens, three confinement variations have been studied. The results obtained enable the establishment of a variation of fracture properties depending on the level of confinement of each miniature specimen and selection of the most appropriate confinement for this goal.

Journal article

Martinez-Paneda E, 2019, On the finite element implementation of functionally graded materials, Materials, Vol: 12, ISSN: 1996-1944

We investigate the numerical implementation of functionally graded properties in the context of the finite element method. The macroscopic variation of elastic properties inherent to functionally graded materials (FGMs) is introduced at the element level by means of the two most commonly used schemes: (i) nodal based gradation, often via an auxiliary (non-physical) temperature-dependence, and (ii) Gauss integration point based gradation. These formulations are extensively compared by solving a number of paradigmatic boundary value problems for which analytical solutions can be obtained. The nature of the notable differences revealed by the results is investigated in detail. We provide a user subroutine for the finite element package ABAQUS to overcome the limitations of the most popular approach for implementing FGMs in commercial software. The use of reliable, element-based formulations to define the material property variation could be key in fracture assessment of FGMs and other non-homogeneous materials.

Journal article

Martinez-Paneda E, Golahmar A, Niordson CF, 2018, A phase field formulation for hydrogen assisted cracking, Computer Methods in Applied Mechanics and Engineering, Vol: 342, Pages: 742-761, ISSN: 0045-7825

We present a phase field modeling framework for hydrogen assisted cracking. The model builds upon a coupled mechanical and hydrogen diffusion response, driven by chemical potential gradients, and a hydrogen-dependent fracture energy degradation law grounded on first principles calculations. The coupled problem is solved in an implicit time integration scheme, where displacements, phase field order parameter and hydrogen concentration are the primary variables. We show that phase field formulations for fracture are particularly suitable to capture material degradation due to hydrogen. Specifically, we model (i) unstable crack growth in the presence of hydrogen, (ii) failure stress sensitivity to hydrogen content in notched specimens, (iii) cracking thresholds under constant load, (iv) internal hydrogen assisted fracture in cracked specimens, and (v) complex crack paths arising from corrosion pits. Computations reveal a good agreement with experiments, highlighting the predictive capabilities of the present scheme. The work could have important implications for the prediction and prevention of catastrophic failures in corrosive environments. The finite element code developed can be downloaded from www.empaneda.com/codes.

Journal article

Mathew T, Natarajan S, Martinez-Paneda E, 2018, Size effects in elastic-plastic functionally graded materials, Composite Structures, Vol: 204, Pages: 43-51, ISSN: 0263-8223

We develop a strain gradient plasticity formulation for composite materials with spatially varying volume fractions to characterize size effects in functionally graded materials (FGMs). The model is grounded on the mechanism-based strain gradient plasticity theory and effective properties are determined by means of a linear homogenization scheme. Several paradigmatic boundary value problems are numerically investigated to gain insight into the strengthening effects associated with plastic strain gradients and geometrically necessary dislocations (GNDs). The analysis of bending in micro-size functionally graded foils shows a notably stiffer response with diminishing thickness. Micro-hardness measurements from indentation reveal a significant increase with decreasing indenter size. And large dislocation densities in the vicinity of a crack substantially elevate stresses in cracked FGM components. We comprehensively assess the influence of the length scale parameter and material gradation profile to accurately characterize the micro-scale response and identify regimes of GNDs relevance in FGMs.

Journal article

Martinez-Paneda E, Fleck NA, 2018, Crack growth resistance in metallic alloys: the role of isotropic versus kinematic hardening, Journal of Applied Mechanics-Transactions of the ASME, Vol: 85, ISSN: 0021-8936

The sensitivity of crack growth resistance to the choice of isotropic or kinematic hardening is investigated. Monotonic mode I crack advance under small scale yielding conditions is modeled via a cohesive zone formulation endowed with a traction–separation law. R-curves are computed for materials that exhibit linear or power law hardening. Kinematic hardening leads to an enhanced crack growth resistance relative to isotropic hardening. Moreover, kinematic hardening requires greater crack extension to achieve the steady-state. These differences are traced to the nonproportional loading of material elements near the crack tip as the crack advances. The sensitivity of the R-curve to the cohesive zone properties and to the level of material strain hardening is explored for both isotropic and kinematic hardening.

Journal article

Martínez Pañeda E, 2018, Strain gradient plasticity-based modeling of damage and fracture, Publisher: Springer International Publishing, ISBN: 9783319633831

Book

Martinez-Paneda E, del Busto S, Betegon C, 2017, Non-local plasticity effects on notch fracture mechanics, Theoretical and Applied Fracture Mechanics, Vol: 92, Pages: 276-287, ISSN: 0167-8442

We investigate the influence of gradient-enhanced dislocation hardening on the mechanics of notch-induced failure. The role of geometrically necessary dislocations (GNDs) in enhancing cracking is assessed by means of a mechanism-based strain gradient plasticity theory. Both stationary and propagating cracks from notch-like defects are investigated through the finite element method. A cohesive zone formulation incorporating monotonic and cyclic damage contributions is employed to address both loading conditions. Computations are performed for a very wide range of length scale parameters and numerous geometries are addressed, covering the main types of notches. Results reveal a strong influence of the plastic strain gradients in all the scenarios considered. Transitional combinations of notch angle, radius and length scale parameter are identified that establish the regimes of GNDs-relevance, laying the foundations for the rational application of gradient plasticity models in damage assessment of notched components.

Journal article

del Busto S, Betegon C, Martinez-Paneda E, 2017, A cohesive zone framework for environmentally assisted fatigue, Engineering Fracture Mechanics, Vol: 185, Pages: 210-226, ISSN: 0013-7944

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.

Journal article

Martinez-Paneda E, Natarajan S, Bordas S, 2017, Gradient plasticity crack tip characterization by means of the extended finite element method, Computational Mechanics, Vol: 59, Pages: 831-842, ISSN: 0178-7675

Strain gradient plasticity theories are being widely used for fracture assessment, as they provide a richer description of crack tip fields by incorporating the influence of geometrically necessary dislocations. Characterizing the behavior at the small scales involved in crack tip deformation requires, however, the use of a very refined mesh within microns to the crack. In this work a novel and efficient gradient-enhanced numerical framework is developed by means of the extended finite element method (X-FEM). A mechanism-based gradient plasticity model is employed and the approximation of the displacement field is enriched with the stress singularity of the gradient-dominated solution. Results reveal that the proposed numerical methodology largely outperforms the standard finite element approach. The present work could have important implications on the use of microstructurally-motivated models in large scale applications. The non-linear X-FEM code developed in MATLAB can be downloaded from www.empaneda.com/codes.

Journal article

Papazafeiropoulos G, Muniz-Calvente M, Martinez-Paneda E, 2017, Abaqus2Matlab: a suitable tool for finite element post-processing, Advances in Engineering Software, Vol: 105, Pages: 9-16, ISSN: 0965-9978

A suitable piece of software is presented to connect Abaqus, a sophisticated finite element package, with Matlab, the most comprehensive program for mathematical analysis. This interface between these well-known codes not only benefits from the image processing and the integrated graph-plotting features of Matlab but also opens up new opportunities in results post-processing, statistical analysis and mathematical optimization, among many other possibilities. The software architecture and usage are appropriately described and two problems of particular engineering significance are addressed to demonstrate its capabilities. Firstly, the software is employed to assess cleavage fracture through a novel 3-parameter Weibull probabilistic framework. Then, its potential to create and train neural networks is used to identify damage parameters through a hybrid experimental–numerical scheme, and model crack propagation in structural materials by means of a cohesive zone approach. The source code, detailed documentation and a large number of tutorials can be freely downloaded from www.abaqus2matlab.com.

Journal article

Martínez-Pañeda E, Niordson CF, Deshpande VS, Fleck NAet al., 2017, Gradient effects on crack tip mechanics, Pages: 757-758

© 2017 ICF 2017 - 14th International Conference on Fracture. All rights reserved. In this work a general framework for damage and fracture assessment accounting for the role of geometrically necessary dislocations (GNDs) is provided. Crack tip fields are characterized, within both infinitesimal and finite deformation theories, by means of different strain gradient plasticity (SGP) formulations and their finite element implementation. Moreover, the impact of SGP theories on fracture and damage problems is further assessed by modeling crack growth initiation and subsequent resistance. Results show a strong influence of GNDs, revealing the need to incorporate its influence in continuum models in order to adequately characterize behavior at the small scales involved in crack tip deformation.

Conference paper

Martinez-Paneda E, Papazafeiropoulos G, Muniz-Calvente M, 2017, ABAQUS2MATLAB: A NOVEL TOOL FOR FINITE ELEMENT POST-PROCESSING, 7th International Conference on Mechanics and Materials in Design (M2D), Publisher: INEGI-FEUP, Pages: 185-186

Conference paper

Fuentes-Alonso S, Martinez-Paneda E, 2017, CRACK TIP MECHANICS IN DISTORTION GRADIENT PLASTICITY, 7th International Conference on Mechanics and Materials in Design (M2D), Publisher: INEGI-FEUP, Pages: 509-510

Conference paper

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