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Stephen R Hallett - One of the best experts on this subject based on the ideXlab platform.
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virtual characterization of nonlocal continuum damage model parameters using a high fidelity finite element model
Composite Structures, 2021Co-Authors: Johannes Reiner, Stephen R Hallett, Navid Zobeiry, Reza Vaziri, Michael R WisnomAbstract:Abstract A virtual finite element framework is presented to provide the transition from a high fidelity model to a more computationally efficient continuum model for simulating fibre-dominated damage behavior in composite laminates subjected to tensile loadings. The high fidelity method in LS-DYNA is based on the use of Cohesive Interface elements to simulate both intra-laminar matrix cracks and delamination. Applied to IM7/8552 carbon fibre reinforced plastic laminates in over-height compact tension (OCT) tests, this modeling strategy is used to determine the effective damage parameters for the nonlocal continuum damage model, CODAM2, implemented as MAT219 in LS-DYNA. The prediction of CODAM2 is thoroughly assessed against quantitative and qualitative results obtained from the high fidelity model. Furthermore, the characterized CODAM2 model is applied to simulations of large-scale OCT, open–hole tension (OHT) and center-notched tension (CNT) specimens and results are compared to corresponding experimental data. Due to its nonlocal feature to track the crack trajectory without the need to align the mesh with fibre orientation, the continuum damage model CODAM2 is able to accurately predict the structural response in all large-scale OCT, OHT and CNT test cases with significant computational efficiency.
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Finite element modelling of Dyneema® composites: From quasi-static rates to ballistic impact
Composites Part A-applied Science and Manufacturing, 2018Co-Authors: Mark K. Hazzard, Richard S. Trask, Ulrich Heisserer, Mirre Van Der Kamp, Stephen R HallettAbstract:Abstract A finite element methodology to predict the behaviour of Dyneema® HB26 fibre composites at quasi-static rates of deformation, under low velocity drop weight impact, and high velocity ballistic impact has been developed. A homogenised sub-laminate approach separated by Cohesive tied contacts was employed. The modelling approach uses readily available material models within LS-DYNA, and is validated against experimental observations in literature. Plane-strain beam models provide accurate mechanisms of deformation, largely controlled through Mode II Cohesive Interface properties and kink band formation. Low velocity drop weight impact models of HB26 give force-deflection within 10% of new experimental observations, with in-plane shear strain contour plots from models directly compared with experimental Digital Image Correlation (DIC). Ballistic impact models utilising rate effects and damage showed similar modes of deformation and failure to that observed in literature, and provide a good approximation for ballistic limit under 600 m/s impact speed.
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experimental determination of through thickness compression ttc enhancement factor for mode ii fracture energy
Composites Science and Technology, 2018Co-Authors: Michael R Wisnom, Xiaoyang Sun, Tamas Rev, Stephen R HallettAbstract:Abstract Mode II fracture energy, GIIC, is a critical parameter for determining the propagation of delamination in composite laminates. Its value can be affected by Through-Thickness Compression (TTC) stress acting on the crack tip and here this effect has been studied using IM7/8552 carbon/epoxy laminates with cut central plies. External TTC loads were applied through bi-axial testing. Unidirectional (UD) cut-ply specimens were used to determine the TTC enhancement factor, ηG, for GIIC. A similar enhancement effect was also found in Quasi-isotropic (QI) specimens with 2 extra cut central 0° plies inserted into the layup. The TTC enhancement factor was implemented in a Finite Element Analysis (FEA) framework using Cohesive Interface elements, showing that the determined ηG can be successfully used to model the effect of TTC on delamination.
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an improved delamination fatigue Cohesive Interface model for complex three dimensional multi Interface cases
Composites Part A-applied Science and Manufacturing, 2018Co-Authors: Chongcong Tao, Supratik Mukhopadhyay, Bing Zhang, Luiz F Kawashita, Jinhao Qiu, Stephen R HallettAbstract:Abstract This work presents a Cohesive Interface model for predicting interlaminar failure of composite laminates under tension-tension fatigue loading. The model features improvements on previous formulations and utilizes four-integration-point elements, which offer several new advantages, while maintaining the merits of the previous single-integration-point elements. An element-based crack tip tracking algorithm is incorporated to confine fatigue damage to crack-tip elements only. A new local rate approach is proposed to ensure accurate integration of strain energy release rate from local elements. Furthermore, a dynamic fatigue characteristic length is proposed to offer a more accurate estimation of fatigue characteristic length in complex three-dimensional cases. Fatigue initiation is incorporated by using a strength reduction method, without changing the propagation characteristics. The numerical approach has been verified and validated using multiple cases and was then applied to fatigue damage development in open-hole laminates, where a good agreement between numerical analysis and experimental results was obtained.
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modelling delamination with Cohesive Interface elements
Numerical Modelling of Failure in Advanced Composite Materials, 2015Co-Authors: Stephen R Hallett, Paul W HarperAbstract:Cohesive Interface elements are becoming a popular choice in the analysis of composite material failure, particularly for delamination. Use of such elements is not without pitfalls, which include the correct selection of material parameters and mesh size. This chapter gives an introduction to the use of Cohesive elements for analysis of composites delamination together with some examples of use, particularly where caution is needed in the interpretation of results.
Oded Rabinovitch - One of the best experts on this subject based on the ideXlab platform.
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stochastic Cohesive Interface analysis of layer debonding
International Journal of Solids and Structures, 2021Co-Authors: N Malkiel, Oded RabinovitchAbstract:Abstract Interfacial debonding in layered structures is recognized as a critical failure mode of such structural forms. The way uncertainty of input parameters affects the debonding mechanism is still an open question. In this paper, a methodology for the stochastic analysis of debonding in layered structures is developed. The layered structure is modeled using the Cohesive Interface approach while parameter uncertainty is represented by random fields of material and interfacial properties. To analyze the influence of uncertainty, the methodology integrates the Cohesive Interface approach into the perturbation based stochastic finite elements method. A representative model problem of a bar bonded through a Cohesive Interface to a rigid substrate and subjected to debonding is looked at. The proposed methodology is developed through the stochastic analysis of displacement profile, debonding length, and reaction force at the displaced end. The formulation is followed by numerical results demonstrating the capabilities of the method in describing the influence of input uncertainty on the structural response. The results are validated by comparison with a reference MCS analysis conducted for this purpose. Opposed to the MCS, which heavily draws upon computational power, the proposed method is expandable and can be applied to more general cases with a tolerable increase in complexity. The current investigation thus establishes a stochastic analysis tool for assessing uncertainty in more advanced models for debonding of layered structures.
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modeling of interfacial debonding propagation in sandwich panels
International Journal of Solids and Structures, 2017Co-Authors: Itay Odessa, Y Frostig, Oded RabinovitchAbstract:Abstract The paper presents a nonlinear model for the analysis of the process of debonding between a face sheet and the core in sandwich panels. The model incorporates the Extended High-Order Sandwich Panel Theory with a Cohesive Interface modeling of the crack nucleation and propagation at the Interface between a face sheet and the core. The derivation of the model combines the first order shear deformation kinematic assumptions for the face sheets with the high order small deformations kinematic assumptions that include out of plane compressibility for the core. The Cohesive Interfaces combine the components of the sandwich panel together and introduce the nonlinearity and the interfacial failure process into the model by means of nonlinear traction-separation laws. The properties of the Cohesive Interface are calibrated and the proposed model is validated through comparison with experimental results taken from the literature. Two cases that include the Double Cantilever Beam (DCB) and the Cracked Sandwich Beam (CSB) specimens are considered. In the DCB test, the panel is subjected to a global mode I loading, while in the CSB test is subjected to a loading scheme that yields a global mode II. On the local scale, a mixed mode is observed in the two tests. For the CSB case, contact constraints between the debonded face sheet and the core are also included. The comparison of the analytical results with the experimental ones focuses on the initial linear response, the nucleation of the interfacial debonding crack, and the propagation of the crack. The comparison validates the model and explores the deobnding failure process in sandwich panels.
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an extended high order Cohesive Interface approach to the debonding analysis of frp strengthened beams
International Journal of Mechanical Sciences, 2014Co-Authors: Oded RabinovitchAbstract:Abstract The paper studies the debonding mechanisms in concrete beams strengthened with externally bonded fiber reinforced polymer (FRP) composite strips. The paper develops a nonlinear analytical model that combines an extended high order multi-layer consideration of the strengthened beam with a Cohesive Interface modeling of its physical Interfaces. The handling of the adhesive layer uses 2D elasticity and considers the layer as an isotropic medium with shear, vertical-normal, and longitudinal-normal stiffness. This allows for capturing the variation of all stress components through the thickness of the adhesive layer and for implementing two distinct Cohesive Interfaces, one at the adhesive–concrete Interface and one at the adhesive–FRP Interface. By means of this combination, the model responds automatically with the nucleation and evolution of debonding mechanisms in each physical Interface. The study looks into the debonding mechanisms triggered by the localized effects near the edge of the bonded layer and near a flexural crack in the concrete beam. The numerical results, the quantitative comparison with reference results, and the qualitative comparison with experimental observations reported in the literature gain insight into the nucleation and evolution of the failure mechanism. This effort takes another step towards the understanding of the debonding phenomenon and towards the development of a high-resolution analytical platform for its handling.
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Debonding analysis of fiber-reinforced-polymer strengthened beams : Cohesive zone modeling versus a linear elastic fracture mechanics approach
Engineering Fracture Mechanics, 2008Co-Authors: Oded RabinovitchAbstract:A linear elastic fracture mechanics (LEFM) approach and a Cohesive Interface (Cohesive zone) modeling approach to the debonding analysis of concrete beams strengthened with externally bonded fiber-reinforced-polymer (FRP) strips are studied and compared. The analytical models that are based on the two approaches are presented and discussed. The Cohesive Interface model is formulated using a potential function and it takes into account the shear effects, the effect of the peeling stresses, and the coupling of the shear and the peeling effects. This model takes the form of a set on nonlinear differential equations. The LEFM model combines stress analysis using the high order theory and fracture analysis using the concepts of the energy release rate and the J-integral. In addition, an algorithm that converts the results of the LEFM model into the equilibrium path of the debonding process is developed. The main advantages and disadvantages of the two approaches are also discussed. The two approaches are compared in terms of their applicability to quantify and describe the debonding process in various cases that include a single shear test, an edge peeling test, and a beam specimen strengthened with FRP.
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Cohesive Interface modeling of debonding failure in frp strengthened beams
Journal of Engineering Mechanics-asce, 2008Co-Authors: Oded RabinovitchAbstract:A theoretical model that incorporates the concept of the Cohesive Interface approach for the debonding analysis of reinforced concrete beams strengthened with externally bonded fiber reinforced polymer (FRP) strips is presented. The Cohesive Interface concept is adopted for modeling of the debonding process near the critical adhesive-concrete Interface, whereas the adhesive layer itself is modeled as a two-dimensional elastic medium. Thus, the stress and deformation fields within the adhesive layer, the coupling between the shear and normal stresses and, especially, their influence on the tractions across the Cohesive Interface are taken into account. The nonlinear relations between the tractions and the displacement jumps across the Cohesive Interface are derived using a potential function and account for the peeling effects and for the coupling between the shear-slip and the peeling-separation laws. Numerical results that examine the capabilities of the model, provide insight into the stability characte...
Zhenjun Yang - One of the best experts on this subject based on the ideXlab platform.
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monte carlo simulations of mesoscale fracture of concrete with random aggregates and pores a size effect study
Construction and Building Materials, 2015Co-Authors: Xiaofeng Wang, Zhenjun Yang, Andrey P JivkovAbstract:Abstract Size effect in concrete under tension is studied by Monte Carlo simulations of mesoscale finite element models containing random inclusions (aggregates and pores) with prescribed volume fractions, shapes and size distributions (called meso-structure controls). For a given size and a set of controls, a number of realisations with different spatial distribution of inclusions are simulated to produce statistical data for macroscopic load/stress–strain curves. The complex meso-crack initiation and propagation is captured by pre-inserted Cohesive Interface elements. The effects of specimen size and meso-structure controls on macroscopic strength and toughness are analysed, and empirical size-effect laws for their dependences are proposed by data regression. It is also shown that the mesoscale porosity affects both strength and toughness and should not be ignored in size effect studies of concrete.
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monte carlo simulations of mesoscale fracture modelling of concrete with random aggregates and pores
Construction and Building Materials, 2015Co-Authors: Xiaofeng Wang, Zhenjun Yang, Andrey P Jivkov, J R Yates, Chuanzeng ZhangAbstract:Abstract A procedure for generating two-dimensional heterogeneous meso-scale concrete samples is developed, in which the multi-phasic features including the shape, size, volume fraction and spatial distribution of aggregates and pores are randomised. Zero-thickness Cohesive Interface elements with softening traction–separation relations are pre-inserted within solid element meshes to simulate complex crack initiation and propagation. Extensive Monte Carlo simulations (MCS) of uniaxial tension tests were carried out to investigate the effects of key multi-phasic features on the fracture patterns and load-carrying capacities. It is found that the fracture behaviour and stress-displacement responses of the numerical specimens are highly dependent on the random mesostructures, especially the post-peak softening responses. The specimens fail with either one or two macro-cracks, regardless of the shapes and volume fractions of aggregates and pores. Assuming that the aggregate–mortar Interface is weaker than the mortar, using polygonal rather than circular or elliptical aggregates, or increasing the aggregate volume fraction will reduce the tensile strength of specimens. The porosity is found to have severely adverse effects on the specimen strength and cannot be neglected in mesoscale fracture modelling of concrete.
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modelling multiple Cohesive crack propagation using a finite element scaled boundary finite element coupled method
Engineering Analysis With Boundary Elements, 2009Co-Authors: Zhenjun YangAbstract:Abstract This paper presents an extension of the recently-developed finite element–scaled boundary finite element (FEM–SBFEM) coupled method to model multiple crack propagation in concrete. The concrete bulk and fracture process zones are modelled using SBFEM and nonlinear Cohesive Interface finite elements (CIEs), respectively. The CIEs are automatically inserted into the SBFEM mesh as the cracks propagate. The algorithm previously devised for single crack propagation is augmented to model problems with multiple cracks and to allow cracks to initiate in an un-cracked SBFEM mesh. It also addresses crack propagation from one subdomain into another, as a result of partitioning a coarse SBFEM mesh, required for some mixed–mode problems. Each crack in the SBFEM mesh propagates when the sign of the Mode-I stress intensity factor at the crack tip turns positive from negative. Its propagation angle is determined using linear elastic fracture mechanics criteria. Three concrete beams involving multiple crack propagation are modelled. The predicted crack propagation patterns and load–displacement curves are in good agreement with data reported in literature.
Yanfei Gao - One of the best experts on this subject based on the ideXlab platform.
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flow coupled Cohesive Interface framework for simulating heterogeneous crack morphology and resolving numerical divergence in hydraulic fracture
Extreme Mechanics Letters, 2021Co-Authors: Wei Liu, Xin Cai, Xinpu Shen, Yanfei GaoAbstract:Abstract A user-friendly and generic finite element framework for simulating hydraulic fracture with a complex crack network in unconventional oil and gas exploitation is developed in this work with the following unique features. While the Cohesive zone model (CZM) removes crack singularities, its finite element simulations suffer numerical convergence problem during crack nucleation and growth, which can be regularized by a fictitious viscosity approach. The decoupling of Cohesive-cracked solid and fluid into separate free body diagrams allows the development of a weak-form finite element formulation for the former and a finite-difference approach for the transport analysis in the latter. Enforcing the Kirchhoff condition in polycrystalline geomaterials allows the study of the fluid-driven complex fracture propagation process. Our method has been verified by analytical solutions, and then employed to simulate the synergistic roles of confining pressure and grain boundary anisotropy on the fracking morphology. Numerical implementation into a user-defined element (UEL) subroutine in ABAQUS provides easy adaptation and further development for the research community.
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diffusion coupled Cohesive Interface simulations of stress corrosion intergranular cracking in polycrystalline materials
Acta Materialia, 2017Co-Authors: Yanfei Gao, Yanli Wang, T L ShamAbstract:Abstract To study the stress corrosion intergranular cracking mechanism, a diffusion-coupled Cohesive zone model (CZM) is proposed for the simulation of the stress-assisted diffusional process along grain boundaries and the mechanical response of grain boundary sliding and separation. This simulation methodology considers the synergistic effects of impurity diffusion driven by pressure gradient and degradation of grain boundary strength by impurity concentration. The diffusion-coupled CZM is combined with crystal plasticity finite element model (CPFEM) to simulate intergranular fracture of polycrystalline material under corrosive environment. Significant heterogeneity of the stress field and extensive impurity accumulation is observed at grain boundaries and junction points. Deformation mechanism maps are constructed with respect to the grain boundary degradation factor and applied strain rate, which dictate the transition from internal to near-surface intergranular fracture modes under various strain amplitudes and grain sizes.
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Cohesive Interface simulations of indentation cracking as a fracture toughness measurement method for brittle materials
Acta Materialia, 2012Co-Authors: J H Lee, Yanfei Gao, Kurt E Johanns, G M PharrAbstract:Abstract Cracks in brittle solids induced by pyramidal indenters are ideal for toughness evaluation since the indentation stress fields decay rapidly from the contact center and any cracks will be eventually arrested. Thus, if the applied energy release rate can be determined analytically, the material toughness can be deduced by measuring the crack length. However, such a driving force calculation is a nontrivial task because of the complex stress fields; only a number of limit cases can be solved, such as the long half-penny cracks (at least two times larger than the contact size) in the classic Lawn–Evans–Marshall (LEM) model. Important questions such as the evolution from short cracks to median/radial and then to half-penny cracks, the form of the scaling relationship that relates fracture toughness to material hardness and indenter angles, the threshold load for indentation cracking, etc., cannot easily be answered without a detailed knowledge of the co-evolution history of the stress fields and crack morphology. To this end, a finite element model of four-sided pyramidal indentation adopting Cohesive Interface elements is developed to study the effects of indenter geometry, load, Cohesive Interface parameters, and material properties on the initiation and propagation of the median/radial/half-penny crack systems. The validity and artifacts of the Cohesive Interface model are carefully examined, and the crack morphologies under various indentation and material parameters are systematically studied. Numerical predictions lead to a quantitative evaluation of the threshold load for indentation fracture, and an improved method for the evaluation of material toughness from the indentation load, crack size, hardness, elastic constants, and indenter geometry, which compare favorably to a large set of experiments in the literature. It is also found that the toughness evaluation method is very sensitive to Poisson’s ratio – an observation that has previously received little attentions. An approximate analysis for short cracks is developed based on the fracture mechanics of annular cracks and the embedded-center-of-dilatation model for indentation-induced residual stress fields.
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neutron and x ray diffraction studies and Cohesive Interface model of the fatigue crack deformation behavior
Philosophical Magazine Letters, 2008Co-Authors: Rozaliya Barabash, Yanfei Gao, Yinan Sun, Soo Yeol Lee, Hahn Choo, Peter K Liaw, Donald W Brown, Gene E IceAbstract:The crack-tip deformation behavior during a single overload, fatigue test of ferritic stainless steel, and Ni-based HAYNES 230 superalloy is studied at different structural levels using (1) neutron-diffraction, from which both the elastic-lattice strain and volume-averaged total dislocation densities are obtained, (2) polychromatic X-ray microdiffraction to probe the geometrically necessary dislocations and boundaries distribution, and (3) an irreversible and hysteretic Cohesive Interface model which has been implemented into a finite element framework to simulate the stress/strain evolution near the fatigue crack tip. Neutron strain measurements and finite element simulations are in qualitative agreement on the macroscopic length scale. Large plastic deformation induced by the overload and the resulting compressive residual strains are observed in front of the crack tip after the overload, and are the principal reason for the fatigue-crack-growth retardation. Strong strain gradients surrounding the crack...
G M Pharr - One of the best experts on this subject based on the ideXlab platform.
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Cohesive Interface simulations of indentation cracking as a fracture toughness measurement method for brittle materials
Acta Materialia, 2012Co-Authors: J H Lee, Yanfei Gao, Kurt E Johanns, G M PharrAbstract:Abstract Cracks in brittle solids induced by pyramidal indenters are ideal for toughness evaluation since the indentation stress fields decay rapidly from the contact center and any cracks will be eventually arrested. Thus, if the applied energy release rate can be determined analytically, the material toughness can be deduced by measuring the crack length. However, such a driving force calculation is a nontrivial task because of the complex stress fields; only a number of limit cases can be solved, such as the long half-penny cracks (at least two times larger than the contact size) in the classic Lawn–Evans–Marshall (LEM) model. Important questions such as the evolution from short cracks to median/radial and then to half-penny cracks, the form of the scaling relationship that relates fracture toughness to material hardness and indenter angles, the threshold load for indentation cracking, etc., cannot easily be answered without a detailed knowledge of the co-evolution history of the stress fields and crack morphology. To this end, a finite element model of four-sided pyramidal indentation adopting Cohesive Interface elements is developed to study the effects of indenter geometry, load, Cohesive Interface parameters, and material properties on the initiation and propagation of the median/radial/half-penny crack systems. The validity and artifacts of the Cohesive Interface model are carefully examined, and the crack morphologies under various indentation and material parameters are systematically studied. Numerical predictions lead to a quantitative evaluation of the threshold load for indentation fracture, and an improved method for the evaluation of material toughness from the indentation load, crack size, hardness, elastic constants, and indenter geometry, which compare favorably to a large set of experiments in the literature. It is also found that the toughness evaluation method is very sensitive to Poisson’s ratio – an observation that has previously received little attentions. An approximate analysis for short cracks is developed based on the fracture mechanics of annular cracks and the embedded-center-of-dilatation model for indentation-induced residual stress fields.
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nanoscale incipient asperity sliding and Interface micro slip assessed by the measurement of tangential contact stiffness
Scripta Materialia, 2006Co-Authors: B N Lucas, G M Pharr, W C OliverAbstract:Experiments with a multidimensional nano-contact system have shown that, prior to kinetic frictional sliding, there is a significant reduction of the tangential contact stiffness relative to the elastic prediction. The reduction occurs at contact sizes below about 50–200 nm for aluminum single crystals and several other materials. Using a Cohesive Interface model, we find that this reduction corresponds to a transition from a small-scale-slip to large-scale-slip condition of the Interface.