Publications

@article{Goudarzi2019,

title = {Fiber neutrality in fiber-reinforced composites: Evidence from a computational study},

author = {M. Goudarzi and A. Simone},

doi = {10.1016/j.ijsolstr.2018.07.023},

year  = {2019},

date = {2019-01-01},

journal = {INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES},

volume = {156-157},

pages = {14–28},

publisher = {Elsevier Ltd},

abstract = {We report numerical evidence for neutrality of thin fibers to a prescribed uniform stress field in a fiber-reinforced composite. Elastic finite element analyses of fiber-reinforced composites are carried out with a conventional fully-resolved model and a novel dimensionally-reduced fiber model.The two modeling approaches are compared in the analysis of mechanical properties and matrix-fiber slip profiles. An analysis of the effectiveness of various fiber orientations with respect to the loading direction shows that the notion of inclusion neutrality, originally formulated for rigid line inclusions by Wang et al. [Journal of Applied Mechanics, 52(4), 814–822, 1985], holds also for linear elastic thin fibers with imperfect interface.},

keywords = {Embedded reinforcement, Fiber neutrality, Fiber-reinforced composites},

pubstate = {published},

tppubtype = {article}

}

@article{Verners2019,

title = {Characterization of the structural response of a lithiated SiO2 / Si interface: A reactive molecular dynamics study},

author = {O. Verners and A. Simone},

doi = {10.1016/j.mechmat.2019.04.001},

year  = {2019},

date = {2019-01-01},

journal = {MECHANICS OF MATERIALS},

volume = {136},

publisher = {Elsevier B.V.},

abstract = {We report the results of a computational study regarding the mechanical properties of a lithiated Si/SiO2 interface using reactive molecular dynamics. The study is motivated by an intended application of SiO2-coated Si nanotubes as fibers in structural batteries with a fiber-reinforced composite architecture while serving as anodes. According to the results, main failure properties due to partly irreversible bond breakage during mechanical deformation are identified, indicating agreement with bond energy/bond order based estimates. Microscopic failure properties are also identified and interpreted in view of the observed processes of bonding degradation. In particular, the effect of Li distribution on the shear deformation response is evaluated as significant.},

keywords = {Composite cathode, Molecular dynamics, Silicon, Silicon oxide, Structural battery},

pubstate = {published},

tppubtype = {article}

}

@article{Goudarzi2019a,

title = {Discrete inclusion models for reinforced composites: Comparative performance analysis and modeling challenges},

author = {M. Goudarzi and A. Simone},

doi = {10.1016/j.cma.2019.06.026},

year  = {2019},

date = {2019-01-01},

journal = {COMPUTER METHODS IN APPLIED MECHANICS AND ENGINEERING},

volume = {355},

pages = {535–557},

publisher = {Elsevier B.V.},

abstract = {We report the results of a comparative analysis of mesh independent discrete inclusion models and point out some shortcomings of classical approaches in the approximation of the strain field across an inclusion (artificial continuity) and the slip profile along an inclusion (oscillatory behavior). We also present novel embedded reinforcement models based on partition of unity enrichment strategies, adaptive h-refinement, and order/regularity extensions. These novel models are assessed by means of mesh convergence studies and it is shown that they improve the quality of the solution by significantly decreasing local spurious oscillations in the slip profile along an inclusion.},

keywords = {Embedded reinforcement, Fiber-reinforced composite, Non-smooth slip profile, Platelet inclusion},

pubstate = {published},

tppubtype = {article}

}

@article{Ghavamian2019,

title = {Accelerating multiscale finite element simulations of history-dependent materials using a recurrent neural network},

author = {F. Ghavamian and A. Simone},

doi = {10.1016/j.cma.2019.112594},

year  = {2019},

date = {2019-01-01},

journal = {COMPUTER METHODS IN APPLIED MECHANICS AND ENGINEERING},

volume = {357},

publisher = {Elsevier B.V.},

abstract = {FE2 multiscale simulations of history-dependent materials are accelerated by means of a recurrent neural network (RNN) surrogate for the history-dependent micro level response. We propose a simple strategy to efficiently collect stress-strain data from the micro model, and we modify the RNN model such that it resembles a nonlinear finite element analysis procedure during training. We then implement the trained RNN model in the FE scheme and employ automatic differentiation to compute the consistent tangent. The exceptional performance of the proposed model is demonstrated through a number of academic examples using strain-softening Perzyna viscoplasticity as the nonlinear material model at the micro level.},

keywords = {Deep learning, Machine learning, Multiscale modeling, Recurrent neural network, Strain softening, Viscoplasticity},

pubstate = {published},

tppubtype = {article}

}

@article{Shabir2019,

title = {On the applicability of linear elastic fracture mechanics scaling relations in the analysis of intergranular fracture of brittle polycrystals},

author = {Z. Shabir and E. Van der Giessen and C. A. Duarte and A. Simone},

doi = {10.1007/s10704-019-00381-x},

year  = {2019},

date = {2019-01-01},

journal = {INTERNATIONAL JOURNAL OF FRACTURE},

volume = {220},

number = {2},

pages = {205–219},

publisher = {Springer},

abstract = {Crack propagation in polycrystalline specimens is studied by means of a generalized finite element method with linear elastic isotropic grains and cohesive grain boundaries. The corresponding mode-I intergranular cracks are characterized using a grain boundary brittleness criterion that depends on cohesive law parameters and average grain boundary length. It is shown that load–displacement curves for specimens with the same microstructure and for various cohesive law parameters can be obtained from a master load–displacement curve by means of simple linear elastic fracture mechanics scaling relations. This property is a consequence of the independence of intergranular crack paths from cohesive law parameters. Perfect scaling is obtained for cases characterized by the same grain boundary brittleness number, irrespective of its value, whereas scaling is approximated for cases with different but relatively large values of the grain boundary brittleness number. The former case corresponds to grain boundary traction profiles that are identical apart from a scale factor; in the latter case, a large grain boundary brittleness number implies similar, apart from a scale factor, traction profiles. By exploiting this property, it is demonstrated that computationally expensive simulations can be avoided above a certain grain boundary brittleness threshold value.},

keywords = {Brittle fracture, generalized finite element method, Linear elastic fracture mechanics, polycrystals, Scaling},

pubstate = {published},

tppubtype = {article}

}

@article{Venugopalan2019,

title = {Indentation of a plastically deforming metal crystal with a self-affine rigid surface: A dislocation dynamics study},

author = {S. P. Venugopalan and L. Nicola},

doi = {10.1016/j.actamat.2018.10.020},

year  = {2019},

date = {2019-01-01},

journal = {ACTA MATERIALIA},

volume = {165},

pages = {709–721},

publisher = {Acta Materialia Inc},

abstract = {Although indentation of elastic bodies by self-affine rough indenters has been studied extensively, little attention has so far been devoted to plasticity. This is mostly because modeling plasticity as well as contact with a self-affine rough surface is computationally quite challenging. Here, we succeed in achieving this goal by using Green's function dislocation dynamics, which allows to describe the self-affine rough surface using wavelengths spanning from 5 nm to 100 micron. The aim of this work is to gain understanding in how plastic deformation affects the contact area, contact pressure and hardness, gap profile and subsurface stresses, while the roughness of the indenter is changed. Plastic deformation is found to be more pronounced for indenters with larger root-mean-square height and/or Hurst exponent, and to be size dependent. The latter means that it is not possible to scale observables, as typically done in elastic contact problems. Also, at a given indentation depth (interference) the contact area is smaller than for the corresponding elastic contact problem, but gap closure is more pronounced. Contact hardness is found to be much larger than what reported by classical plasticity studies. Primarily, this is caused by limited dislocation availability, for which the stiffness of the deforming crystal is in between that of a linear elastic and an elastic-perfectly plastic material. When calculating hardness and nominal contact pressure, including very small wavelength in the description of the surface is not necessary, because below a given wavelength the subsurface stresses become invariant to a further decrease in true contact area. This is true for both elastic and plastic materials. Considering small wavelengths is instead required to capture accurately roughening and contact stress distribution.},

keywords = {contact mechanics, dislocation dynamics, Plasticity, Self-affine surfaces},

pubstate = {published},

tppubtype = {article}

}

@article{VanDokkum2019,

title = {Green's function molecular dynamics including viscoelasticity},

author = {J. S. Van Dokkum and L. Nicola},

doi = {10.1088/1361-651X/ab3031},

year  = {2019},

date = {2019-01-01},

journal = {MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING},

volume = {27},

number = {7},

publisher = {Institute of Physics Publishing},

abstract = {The contact mechanical response of various polymers is controlled by the viscoelastic behavior of their bulk and the adhesive properties of their interface. Due to the interplay between viscoelasticity and adhesion it is difficult to predict the contact response, even more when surfaces are rough. Numerical modeling could be of assistance in this task, but has so far mostly dealt with either adhesion or viscoelasticity and focused on simple geometries. Ideally, one would need a model that can concurrently describe viscoelasticity, surface roughness, and interfacial interactions. The numerical technique named Green's function molecular dynamics (GFMD) has the potential to serve this purpose. To date, it has been used to model contact between adhesive elastic bodies with self-affine surfaces. Here, as a first step, we extend the GFMD technique to include the transient contact response of frictionless viscoelastic bodies. To this end, we derive the constitutive equation for a viscoelastic semi-infinite body in reciprocal space, then integrate it using the semi-analytical method, and find the quasi-static solution through damped dynamics of the individual modes. The new model is then applied to study indentation as well as rolling of a rigid cylinder on a frictionless isotropic half-plane that follows the Zener model when loaded in shear. Extension of the method to a generalized viscoelastic model is straightforward, but the computational effort increases with the number of time-scales required to describe the material. The steady-state response of the rolling cylinder was provided analytically by Hunter in the sixties. Here, we use his analytical solution to validate the steady-state response of our model and provide additionally the transient response for bodies with various shear moduli.},

keywords = {contact mechanics, Green's function, viscoelasticity},

pubstate = {published},

tppubtype = {article}

}

@article{KhajehSalehani2019,

title = {Modeling adhesive contacts under mixed-mode loading},

author = {M. Khajeh Salehani and N. Irani and L. Nicola},

doi = {10.1016/j.jmps.2019.06.010},

year  = {2019},

date = {2019-01-01},

journal = {JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS},

volume = {130},

pages = {320–329},

publisher = {Elsevier Ltd},

abstract = {Experiments show that when an adhesive contact is subjected to a tangential load the contact area reduces, symmetrically or asymmetrically, depending on whether the contact is under tension or compression. What happens after the onset of sliding is more difficult to be assessed because conducting experiments is rather complicated, especially under tensile loading. Here, we provide through numerical simulations, a complete picture of how the contact area and tractions of an adhesive circular smooth punch evolve under mixed-mode loading, before and after sliding. First, the Green’s function molecular dynamics method is extended to include the description of the interfacial interactions between contacting bodies by means of traction–separation constitutive laws that enforce coupling between tension (or compression) and shear. Next, simulations are performed to model sliding of a circular smooth punch against a flat rigid substrate, under tension and compression. In line with the experimental observations, the reduction in the contact area during shear loading is found to be symmetric under tension and asymmetric under compression. In addition, under tensile loading, full detachment is observed at the onset of sliding with a non-zero value of the tangential force. After the onset of sliding and the occurrence of slip instability, the contact area abruptly increases (reattachment), under both tension and compression. For interfaces with high friction, the reattachment occurs only partially. However, a full reattachment is attainable when friction is low.},

keywords = {Adhesive contacts, Contact area, Green's function molecular dynamics, Onset of sliding, Reattachment},

pubstate = {published},

tppubtype = {article}

}

@article{Irani2019,

title = {Modelling surface roughening during plastic deformation of metal crystals under contact shear loading},

author = {N. Irani and L. Nicola},

doi = {10.1016/j.mechmat.2019.02.007},

year  = {2019},

date = {2019-01-01},

journal = {MECHANICS OF MATERIALS},

volume = {132},

pages = {66–76},

publisher = {Elsevier B.V.},

abstract = {During plastic deformation, metal surfaces roughen and this has a deleterious impact on their tribological performance. It is therefore desirable to be able to predict and control the amount of roughening caused by subsurface plasticity. As a first step, we focus on modelling plastic deformation during contact shearing of an FCC metallic single crystal, employing a finite strain Discrete Dislocation Plasticity (DDP) formulation. This formulation allows us to capture the finite lattice rotations induced in the material by shearing and the corresponding local rotation of the crystallographic slip planes. The simulations predict a pronounced material pile-up in front of the contact and a sink-in at its rear, which are strongly crystal-orientation dependent. By comparing finite and small strain DDP, we can assess the effect of slip plane rotation on surface roughening and on metal plasticity in general. Results of the simulations are also compared with crystal plasticity, which is also capable of predicting a pile-up and sink-in, but not the crystal-orientation dependency of roughening.},

keywords = {Contact shearing, Dislocations, Finite strains, size effects, Surface roughening},

pubstate = {published},

tppubtype = {article}

}

@article{Venugopalan2019a,

title = {Plastic contact of self-affine surfaces: Persson's theory versus discrete dislocation plasticity},

author = {S. P. Venugopalan and N. Irani and L. Nicola},

doi = {10.1016/j.jmps.2019.07.019},

year  = {2019},

date = {2019-01-01},

journal = {JOURNAL OF THE MECHANICS AND PHYSICS OF SOLIDS},

volume = {132},

publisher = {Elsevier Ltd},

abstract = {Persson's theory allows for a fast and effective estimate of contact area and contact stress distributions when a flat and a self-affine rough surface are pressed into contact. For elastic bodies, the results of the theory have been shown to be in very good agreement with rather costly simulations. The theory has also been extended to plastic bodies. In this work, the results of Persson's theory for plastic bodies are compared with those of discrete dislocation plasticity. The area-load curves obtained by theory and simulations are found to be in good agreement when the rough surface has a very small root-mean-square (rms) height. For larger rms heights, which are more realistic for metal surfaces, the agreement is no longer good unless in the theory, instead of a size-independent material strength, one uses a rms height- and resolution-dependent yield strength. A modification of this type, i.e., the use of a yield strength dependent on size, does however not lead to agreement between the probability distributions of the contact stress, which is much broader in the simulations than in the theory. The most likely reason for this discrepancy is that the theory, apart from neglecting plasticity size dependence, only applies to elastic-perfectly plastic bodies and therefore, neglects strain hardening.},

keywords = {contact mechanics, dislocation dynamics, Persson's theory, Plasticity, Self-affine surfaces},

pubstate = {published},

tppubtype = {article}

}

@article{Srinivasan2019,

title = {The effectiveness of reference-free modified embedded atom method potentials demonstrated for NiTi and NbMoTaW},

author = {P. Srinivasan and A. I. Duff and T. A. Mellan and M. H. F. Sluiter and L. Nicola and A. Simone},

doi = {10.1088/1361-651X/ab2604},

year  = {2019},

date = {2019-01-01},

journal = {MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING},

volume = {27},

number = {6},

publisher = {Institute of Physics Publishing},

abstract = {One of the effective potentials that has proven to be very versatile and useful for describing metals is the modified embedded atom method (MEAM) potential. The reference-free version of the MEAM (RF-MEAM) potential provides more flexibility for fitting than the 2NN-MEAM because it also describes the pair potential as an explicit function. In this work, we present a methodology to fit RF-MEAM potentials to DFT data. We then evaluate the performance of the fitted potential by comparing MD simulations with experimental and DFT data. As an example, the methodology is applied to a binary and a quaternary alloy, namely NiTi and NbMoTaW. In the case of the equi-atomic NiTi shape memory alloy, our attention focuses on designing a potential that properly captures its mechanical behavior, given that the existing potentials fail to predict elastic constants in agreement with experiments. To reach our aim, we included the stress tensors of different high temperature NiTi configurations in the fitting database. The obtained RF-MEAM potential outperforms existing EAM and MEAM potentials in predicting the lattice and elastic constants of austenitic and martensitic phases as well as the corresponding transformation temperatures. To demonstrate the suitability of this methodology also for more complex systems, a RF-MEAM potential is fitted to model the multi-component NbMoTaW high-entropy alloy. Validation is achieved through comparison between observables obtained through the MD output and ab initio data. The article also reports key improvements to the optimization code MEAMfit v2 and the freely-available LAMMPS implementation of the RF-MEAM formalism. Most notably, resorting to analytic derivatives of the objective function with respect to the potential parameters rather than derivatives through finite differences, the time necessary for fitting has decreased by an order of magnitude.},

keywords = {interatomic potential fitting, Molecular dynamics, multi-component alloy, nickel titanium, Phase transformation, reference-free MEAM},

pubstate = {published},

tppubtype = {article}

}

@article{Verners2018,

title = {Salt concentration dependence of the mechanical properties of LiPF6/poly(propylene glycol) acrylate electrolyte at a graphitic carbon interface: A reactive molecular dynamics study},

author = {Osvalds Verners and Alexey V. Lyulin and A. Simone},

doi = {10.1002/polb.24584},

year  = {2018},

date = {2018-01-01},

journal = {JOURNAL OF POLYMER SCIENCE. PART B, POLYMER PHYSICS},

volume = {56},

number = {9},

pages = {718–730},

publisher = {John Wiley and Sons Inc.},

abstract = {This reactive molecular dynamics study explores the salt concentration dependence of the viscoelastic and mechanical failure properties of a poly(propylene glycol)/LiPF6-based solid polymer electrolyte (SPE) at a graphitic carbon electrode interface. To account for the finite-size effect of interface-confined SPE films, the properties of two distinct film thicknesses are compared with the respective bulk properties. Additionally, the effect of uniaxial compression in the interface-normal direction on free energy profiles of Li-ion SPE-desolvation is studied.},

keywords = {Failure properties, Molecular dynamics, Solid polymer electrolyte, Viscoelastic properties},

pubstate = {published},

tppubtype = {article}

}

@article{Vandoren2018,

title = {Modeling and simulation of quasi-brittle failure with continuous anisotropic stress-based gradient-enhanced damage models},

author = {B. Vandoren and A. Simone},

doi = {10.1016/j.cma.2017.12.027},

year  = {2018},

date = {2018-01-01},

journal = {COMPUTER METHODS IN APPLIED MECHANICS AND ENGINEERING},

volume = {332},

pages = {644–685},

publisher = {Elsevier B.V.},

abstract = {Two anisotropic stress-based gradient-enhanced damage models are proposed to address the issue of spurious damage growth typical of continuous standard gradient-enhanced damage models. Both models are based on a decreasing interaction length upon decreasing stresses and do not require additional model parameters or extra degrees of freedom when compared to standard gradient-enhanced models. It is observed that with the proposed models damage spreading is significantly reduced due to the occurrence of non-physical oscillations in the nonlocal strain field near the strain localization band. Model improvements to eliminate these strain oscillations upon vanishing length scale values are proposed. The capability of the models and their patched versions to correctly simulate damage initiation and propagation is investigated by means of mode-I failure, shear band and four point bending tests.},

keywords = {Anisotropic damage, Gradient-enhanced damage, Quasi-brittle failure, Transient length scale},

pubstate = {published},

tppubtype = {article}

}

@article{Dokkum2018,

title = {On the Proportionality Between Area and Load in Line Contacts},

author = {J. S. Dokkum and M. Khajeh Salehani and N. Irani and L. Nicola},

doi = {10.1007/s11249-018-1061-7},

year  = {2018},

date = {2018-01-01},

journal = {TRIBOLOGY LETTERS},

volume = {66},

number = {3},

publisher = {Springer New York LLC},

abstract = {The relative contact area of rough surface contacts is known to increase linearly with reduced pressure, with proportionality factor . In its common definition, the reduced pressure contains the root-mean-square gradient (RMSG) of the surface. Although easy to measure, the RMSG of the entire surface does not coincide, at small loads, with the RMSG over the actual contact area , which gives a better description of the contact between rough surfaces. It was recently shown that, for Hertzian contacts, linearity between area and load is indeed obtained only if the RMSG is determined over the actual contact area. Similar to surface contacts, in line contacts, numerical data are often studied using theories that predict linearity by design. In this work, we revisit line contact problems and examine whether or not the assumption of linearity for line contacts holds true. We demonstrate, using Green's function molecular dynamics simulations, that for line contacts is not a constant: It depends on both the reduced pressure and the Hurst exponent. However, linearity holds when the RMSG is measured over the actual contact area. In that case, we could compare for line and surface contacts and found that their ratio is approximately 0.9. Finally, by analytically deriving the proportionality factor using in the original model of Greenwood and Williamson, a value is obtained that is surprisingly in good agreement with our numerical results for rough surface contacts.},

keywords = {Contact area, Greenwood and Williamson, Random rough surface, Reduced pressure, Root-mean-square gradient},

pubstate = {published},

tppubtype = {article}

}

@article{Vakis2018,

title = {Modeling and simulation in tribology across scales: An overview},

author = {A. I. Vakis and V. A. Yastrebov and J. Scheibert and L. Nicola and D. Dini and C. Minfray and A. Almqvist and M. Paggi and S. Lee and G. Limbert and J. F. Molinari and G. Anciaux and R. Aghababaei and S. Echeverri Restrepo and A. Papangelo and A. Cammarata and P. Nicolini and C. Putignano and G. Carbone and S. Stupkiewicz and J. Lengiewicz and G. Costagliola and F. Bosia and R. Guarino and N. M. Pugno and M. H. Müser and M. Ciavarella},

doi = {10.1016/j.triboint.2018.02.005},

year  = {2018},

date = {2018-01-01},

journal = {TRIBOLOGY INTERNATIONAL},

volume = {125},

pages = {169–199},

publisher = {Elsevier Ltd},

abstract = {This review summarizes recent advances in the area of tribology based on the outcome of a Lorentz Center workshop surveying various physical, chemical and mechanical phenomena across scales. Among the main themes discussed were those of rough surface representations, the breakdown of continuum theories at the nano- and microscales, as well as multiscale and multiphysics aspects for analytical and computational models relevant to applications spanning a variety of sectors, from automotive to biotribology and nanotechnology. Significant effort is still required to account for complementary nonlinear effects of plasticity, adhesion, friction, wear, lubrication and surface chemistry in tribological models. For each topic, we propose some research directions.},

keywords = {Adhesion, Contact, friction, Lubrication, Multiphysics modeling, Multiscale modeling, roughness, Tribochemistry, tribology, Wear},

pubstate = {published},

tppubtype = {article}

}

@article{KhajehSalehani2018,

title = {Modelling coupled normal and tangential tractions in adhesive contacts},

author = {M. Khajeh Salehani and N. Irani and M. H. Müser and L. Nicola},

doi = {10.1016/j.triboint.2018.03.022},

year  = {2018},

date = {2018-01-01},

journal = {TRIBOLOGY INTERNATIONAL},

volume = {124},

pages = {93–101},

publisher = {Elsevier Ltd},

abstract = {This paper presents a nanoscale-inspired continuum model to capture the coupling of adhesion and friction in contact-mechanics problems. The method relies on Green's function molecular dynamics to calculate the elastic body fields and on a phenomenological mixed-mode coupled cohesive-zone model to describe the interplay between normal and tangential tractions, i.e. adhesion and friction. While the presented formulation is applicable to linearly elastic solids with generic surface roughness, the focus of our analysis is on the indentation of an array of circular rigid punches into a flat, deformable solid. Our results show that the coupling between adhesion and friction leads to an increase in the contact size and a decrease in the pull-off load.},

keywords = {Adhesion and friction, Contact area, Pull-off load},

pubstate = {published},

tppubtype = {article}

}

@article{Srinivasan2018,

title = {Atomistic modeling of the orientation-dependent pseudoelasticity in NiTi: Tension, compression, and bending},

author = {Prashanth Srinivasan and L. Nicola and A. Simone},

doi = {10.1016/j.commatsci.2018.07.028},

year  = {2018},

date = {2018-01-01},

journal = {COMPUTATIONAL MATERIALS SCIENCE},

volume = {154},

pages = {25–36},

publisher = {Elsevier B.V.},

abstract = {Pseudoelasticity in NiTi shape memory alloy single crystals depends on the loading direction. Here, we present a comprehensive study in which molecular dynamics simulations of austenitic bulk single crystals under strain-controlled tensile and compressive loading along the <110>, <111> and <100> directions are performed, and the mechanical response of the crystals are contrasted. All simulations are performed using the MEAM interatomic potential proposed by Ko et al. (2015). The transformation strains and the Young’s modulus of the initial austenitic and the final martensitic phases are compared with values obtained from the lattice deformation model and experimental results from the literature. Results show that depending on orientation the transformation occurs either through the formation of martensitic Lüders bands or through the transient formation of a multivariant martensite which, upon reorientation, becomes a dominant final single variant. 

 Simulations are also performed to assess the orientation-dependent behavior of nano-wires subjected to bending, since the flexibility of the wires is orientation dependent.},

keywords = {Molecular dynamics, Phase transformation, Pseudoelasticity, Shape-memory alloy},

pubstate = {published},

tppubtype = {article}

}

@article{Malagu2017,

title = {Diameter-dependent elastic properties of carbon nanotube-polymer composites: Emergence of size effects from atomistic-scale simulations},

author = {M. Malagù and M. Goudarzi and A. Lyulin and E. Benvenuti and A. Simone},

doi = {10.1016/j.compositesb.2017.07.029},

year  = {2017},

date = {2017-01-01},

journal = {COMPOSITES. PART B, ENGINEERING},

volume = {131},

pages = {260–281},

publisher = {Elsevier Ltd},

abstract = {We propose a computational procedure to assess size effects in nonfunctionalized single-walled carbon nanotube (CNT)-polymer composites. The procedure upscales results obtained with atomistic simulations on a composite unit cell with one CNT to an equivalent continuum composite model with a large number of CNTs. Molecular dynamics simulations demonstrate the formation of an ordered layer of polymer matrix surrounding the nanotube. This layer, known as the interphase, plays a central role in the overall mechanical response of the composite. Due to poor load transfer from the matrix to the CNT, the reinforcement effect attributed to the CNT is negligible; hence the interphase is regarded as the only reinforcement phase in the composite. Consequently, the mechanical properties of the interface and the CNT are not derived since their contribution to the elastic response of the composite is negligible. To derive the elastic properties of the interphase, we employ an intermediate continuum micromechanical model consisting of only the polymer matrix and a three-dimensional fiber representing the interphase. The Young's modulus and Poisson's ratio of the equivalent fiber, and therefore of the interphase, are identified through an optimization procedure based on the comparison between results from atomistic simulations and those obtained from an isogeometric analysis of the intermediate micromechanical model. Finally, the embedded reinforcement method is employed to determine the macroscopic elastic properties of a representative volume element of a composite with various fiber volume fractions and distributions. We then investigate the role of the CNT diameter on the elastic response of a CNT-polymer composite; our simulations predict a size effect on the composite elastic properties, clearly related to the interphase volume fraction.},

keywords = {Atomistic simulations, Finite element analysis, Interface/interphase, Polymer-matrix composites},

pubstate = {published},

tppubtype = {article}

}

@article{Ghavamian2017,

title = {POD-DEIM model order reduction for strain softening viscoplasticity},

author = {F. Ghavamian and P. Tiso and A. Simone},

doi = {10.1016/j.cma.2016.11.025},

year  = {2017},

date = {2017-01-01},

journal = {COMPUTER METHODS IN APPLIED MECHANICS AND ENGINEERING},

volume = {317},

pages = {458–479},

publisher = {Elsevier B.V.},

abstract = {We demonstrate a Model Order Reduction technique for a system of nonlinear equations arising from the Finite Element Method (FEM) discretization of the three-dimensional quasistatic equilibrium equation equipped with a Perzyna viscoplasticity constitutive model. The procedure employs the Proper Orthogonal Decomposition-Galerkin (POD-G) in conjunction with the Discrete Empirical Interpolation Method (DEIM). For this purpose, we collect samples from a standard full order FEM analysis in the offline phase and cluster them using a novel -means clustering algorithm. The POD and the DEIM algorithms are then employed to construct a corresponding reduced order model. In the online phase, a sample from the current state of the system is passed, at each time step, to a nearest neighbor classifier in which the cluster that best describes it is identified. The force vector and its derivative with respect to the displacement vector are approximated using DEIM, and the system of nonlinear equations is projected onto a lower dimensional subspace using the POD-G. The constructed reduced order model is applied to two typical solid mechanics problems showing strain-localization (a tensile bar and a wall under compression) and a three-dimensional square-footing problem.},

keywords = {Discrete empirical interpolation method, k-means clustering algorithm, Machine learning, Model order reduction, Perzyna viscoplasticity, Proper orthogonal decomposition, Strain softening},

pubstate = {published},

tppubtype = {article}

}

@article{Kim2017,

title = {Mesh refinement strategies without mapping of nonlinear solutions for the generalized and standard FEM analysis of 3-D cohesive fractures},

author = {J. Kim and A. Simone and C. A. Duarte},

doi = {10.1002/nme.5286},

year  = {2017},

date = {2017-01-01},

journal = {INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING},

volume = {109},

number = {2},

pages = {235–258},

publisher = {John Wiley and Sons Ltd},

abstract = {A robust and efficient strategy is proposed to simulate mechanical problems involving cohesive fractures. This class of problems is characterized by a global structural behavior that is strongly affected by localized nonlinearities at relatively small-sized critical regions. The proposed approach is based on the division of a simulation into a suitable number of sub-simulations where adaptive mesh refinement is performed only once based on refinement window(s) around crack front process zone(s). The initialization of Newton-Raphson nonlinear iterations at the start of each sub-simulation is accomplished by solving a linear problem based on a secant stiffness, rather than a volume mapping of nonlinear solutions between meshes. The secant stiffness is evaluated using material state information stored/read on crack surface facets which are employed to explicitly represent the geometry of the discontinuity surface independently of the volume mesh within the generalized finite element method framework. Moreover, a simplified version of the algorithm is proposed for its straightforward implementation into existing commercial software. Data transfer between sub-simulations is not required in the simplified strategy. The computational efficiency, accuracy, and robustness of the proposed strategies are demonstrated by an application to cohesive fracture simulations in 3-D.},

keywords = {adaptive mesh refinement, cohesive fracture, generalized finite element method, Newton-Raphson method},

pubstate = {published},

tppubtype = {article}

}

@article{Aragon2017,

title = {The Discontinuity-Enriched Finite Element Method},

author = {Alejandro M. Aragón and A. Simone},

doi = {10.1002/nme.5570},

year  = {2017},

date = {2017-01-01},

journal = {INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING},

volume = {112},

number = {11},

pages = {1589–1613},

publisher = {John Wiley and Sons Ltd},

abstract = {We introduce a new methodology for modeling problems with both weak and strong discontinuities independently of the finite element discretization. At variance with the eXtended/Generalized Finite Element Method (X/GFEM), the new method, named the Discontinuity-Enriched Finite Element Method (DE-FEM), adds enriched degrees of freedom only to nodes created at the intersection between a discontinuity and edges of elements in the mesh. Although general, the method is demonstrated in the context of fracture mechanics, and its versatility is illustrated with a set of traction-free and cohesive crack examples. We show that DE-FEM recovers the same rate of convergence as the standard FEM with matching meshes, and we also compare the new approach to X/GFEM.},

keywords = {cohesive cracks, fracture mechanics, GFEM, IGFEM, strong discontinuities, XFEM},

pubstate = {published},

tppubtype = {article}

}

@article{Arash2017,

title = {Effect of water content on the thermal degradation of amorphous polyamide 6,6: A collective variable-driven hyperdynamics study},

author = {Behrouz Arash and Barend J. Thijsse and Alessandro Pecenko and A. Simone},

doi = {10.1016/j.polymdegradstab.2017.10.019},

year  = {2017},

date = {2017-01-01},

journal = {POLYMER DEGRADATION AND STABILITY},

volume = {146},

pages = {260–266},

publisher = {Elsevier Ltd},

abstract = {Thermal degradation under wet conditions is considered as an important aging mechanism in polyamide 6,6 (PA 6,6). The effect of water on thermal degradation of amorphous PA 6,6 is investigated at relatively low temperatures, varying from 1000 to 2000 K, using reactive force field molecular dynamics (MD) and collective variable-driven hyperdynamics simulations. The simulation of the related long-term chemical reactions is made possible thanks to the self-learning accelerated MD concept of hyperdynamics in combination with the corresponding accurate reproduction of the correct dynamics, consistent with unbiased MD simulations. The kinetics of cleavage reactions of the amide bonds in the backbone of the polymer chains, responsible for the thermal degradation of the polymer, is studied, and the influence of water content on the activation energy and pre-exponential factor of the cleavage reactions is explored. The results show that activation energy and pre-exponential factor are in agreement with experimental data. The proposed simulation framework not only estimates kinetic properties of thermal degradation that are consistent with experimental observations but also provides a predictive tool for studying long-term thermal degradation of PA 6,6.},

keywords = {6, Hyperdynamics, Molecular dynamics, Polyamide 6, Reactive molecular simulations, Thermal degradation},

pubstate = {published},

tppubtype = {article}

}

@article{Venugopalan2017,

title = {Green's function molecular dynamics meets discrete dislocation plasticity},

author = {Syam P. Venugopalan and Martin H. Müser and L. Nicola},

doi = {10.1088/1361-651X/aa7e0e},

year  = {2017},

date = {2017-01-01},

journal = {MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING},

volume = {25},

number = {6},

publisher = {Institute of Physics Publishing},

abstract = {Metals deform plastically at the asperity level when brought in contact with a counter body even when the nominal contact pressure is small. Modeling the plasticity of solids with rough surfaces is challenging due to the multi-scale nature of surface roughness and the length-scale dependence of plasticity. While discrete-dislocation plasticity (DDP) simulations capture size-dependent plasticity by keeping track of the motion of individual dislocations, only simple two-dimensional surface geometries have so far been studied with DDP. The main computational bottleneck in contact problems modeled by DDP is the calculation of the dislocation image fields. We address this issue by combining two-dimensional DDP with Green's function molecular dynamics. The resulting method allows for an efficient boundary-value-method based treatment of elasticity in the presence of dislocations. We demonstrate that our method captures plasticity quantitatively from single to many dislocations and that it scales more favorably with system size than conventional methods. We also derive the relevant Green's functions for elastic slabs of finite width allowing arbitrary boundary conditions on top and bottom surface to be simulated.},

keywords = {contact mechanics, dislocation dynamics, Green's functions},

pubstate = {published},

tppubtype = {article}

}

@article{NgWeiSiang2017,

title = {Static friction of sinusoidal surfaces: a discrete dislocation plasticity analysis},

author = {Kelvin Ng Wei Siang and L. Nicola},

doi = {10.1080/14786435.2017.1344785},

year  = {2017},

date = {2017-01-01},

journal = {PHILOSOPHICAL MAGAZINE},

volume = {97},

number = {29},

pages = {2597–2614},

publisher = {Taylor and Francis Ltd.},

abstract = {Discrete dislocation plasticity simulations are carried out to investigate the static frictional response of sinusoidal asperities with (sub)-microscale wavelength. The surfaces are first flattened and then sheared by a perfectly adhesive platen. Both bodies are explicitly modelled, and the external loading is applied on the top surface of the platen. Plastic deformation by dislocation glide is the only dissipation mechanism active. The tangential force obtained at the contact when displacing the platen horizontally first increases with applied displacement, then reaches a constant value. This constant is here taken to be the friction force. In agreement with several experiments and continuum simulation studies, the friction coefficient is found to decrease with the applied normal load. However, at odds with continuum simulations, the friction force is also found to decrease with the normal load. The decrease is caused by an increased availability of dislocations to initiate and sustain plastic flow during shearing. Again in contrast to continuum studies, the friction coefficient is found to vary stochastically across the contact surface, and to reach locally values up to several times the average friction coefficient. Moreover, the friction force and the friction coefficient are found to be size-dependent.},

keywords = {Contact, discrete dislocation plasticity, friction, Size effect},

pubstate = {published},

tppubtype = {article}

}

@article{Venugopalan2017a,

title = {Green's function molecular dynamics: Including finite heights, shear, and body fields},

author = {Syam P. Venugopalan and L. Nicola and Martin H. Müser},

doi = {10.1088/1361-651X/aa606b},

year  = {2017},

date = {2017-01-01},

journal = {MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING},

volume = {25},

number = {3},

publisher = {Institute of Physics Publishing},

abstract = {The Green's function molecular dynamics (GFMD) method for the simulation of incompressible solids under normal loading is extended in several ways: shear is added to the GFMD continuum formulation and Poisson numbers as well as the heights of the deformed body can now be chosen at will. In addition, we give the full stress tensor inside the deformed body. We validate our generalizations by comparing our analytical and GFMD results to calculations based on the finite-element method (FEM) and full molecular dynamics simulations. For the investigated systems we observe a significant speed-up of GFMD compared to FEM. While calculation and proof of concept were conducted in two-dimensions only, the methodology can be extended to the three-dimensional case in a straightforward fashion.},

keywords = {contact mechanics, Green's function, tribology},

pubstate = {published},

tppubtype = {article}

}