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Understanding the formation of impact craters on planetary surfaces has fundamental importance in geology, impact dynamics, and planetary sciences. The finite element-smoothed particle hydrodynamics (FE-SPH) adaptive method combines the advantages of smoothed particle hydrodynamics (SPH) and finite element method (FEM) in solving hypervelocity impact (HVI) problems. Employing FE-SPH adaptive method is a new idea for the analysis of planetary impact cratering which is different from impact-simplified arbitrary Lagrangian Eulerian (iSALE). In this paper, we establish a complete numerical model of planetary impact cratering through the FE-SPH method, including material failure criterion, thermodynamic analysis, gravity preloading, etc. We reproduce the numerical results by Collins et al. (2012) and highly consistent results are obtained through comparative analysis. This study simulates the formation process of simple and complex impact craters, and the temperature, pressure, density, and geometric shape of the impact zone can be quantitatively obtained. We further analyze the distribution of polymorphs and rock fragmentations in the impact zone based on the theories of shock metamorphism. The proposed analysis method may assist geologists in conducting geological studies. • We establish a complete numerical model of planetary impact cratering different from iSALE. • We analyze the distribution of high-pressure polymorphs based on shock metamorphism. • The FE-SPH model has higher numerical precision for planetary cratering impacts. [ABSTRACT FROM AUTHOR]

Abstract In this paper, compressive material tests on unconfined/confined resin and steel reinforced resin were experimentally evaluated in order to validate the numerical results. The uniaxial model which combines damage mechanics and Ramberg-Osgood relationship is proposed in this paper to describe the uniaxial compressive behaviour of resin and steel reinforced resin. Numerical homogenization is conducted to predict the tensile and shear behaviour of steel reinforced resin after validated by compressive material test results. The friction angle β , the ratio of the yield stress in triaxial tension to the yield stress in triaxial compression K, and the dilation angle ψ of the linear drucker-prager plastic model are obtained based on experiments and numerical homogenization simulation. The confinement effects on resin and steel reinforced resin could be effectively simulated by combing above parameters and uniaxial compressive model. Finite element simulations on unconfined/ confined resin and steel reinforced resin material tests were conducted to validate the material parameters proposed in this paper. A good agreement is observed, indicating the model and parameters proposed in this paper could be effectively used in the finite element simulation of injected bolts. [ABSTRACT FROM AUTHOR]

Continuum models of fracture should be equipped with a localization limiter to prevent pathological discretization sensitivity of finite element simulations. Moreover, stiffness recovery, called also crack closing effect, should be reproduced in the modelling of quasi-brittle materials when reversed loading is applied. In this paper the efficiency of two different established damage-plasticity models for concrete is assessed from the viewpoint of the two above-mentioned aspects. The first description is the so-called concrete damaged plasticity model, available in the ABAQUS package. In this model the plasticity theory is not only augmented with stiffness degradation and recovery, but also with the crack band approach and viscoplastic regularization. The gradient-enhanced damage description, which is the second one considered, can also be coupled to plasticity. In this model additional averaging equation prevents the pathological discretization sensitivity of fracture simulations. Two fields, displacements and averaged strain measure, are interpolated and suitable finite elements are programmed by the authors in the FEAP package. Basic concepts of the numerical analysis of the crack closing phenomenon are reviewed. Both models are capable of avoiding mesh-sensitivity, but it is achieved in a different manner and it is shown in the paper that the users of ABAQUS should employ the crack band options with particular care. Representative examples in the context of the examined issues are demonstrated: uniaxial tension of a bar and a cantilever beam. The results obtained using both the models are compared and the effects of regularization and crack closing are illustrated. [ABSTRACT FROM AUTHOR]

Highlights • An intrinsic cohesive zone approach is proposed for the impact failure of automotive windshield laminated glass. • Gradual energy dissipation process of glass-PVB debonding can be modeled by means of the cohesive zone model. • A windshield finite element model is developed to account for the two failure patterns, including glass fracture and glass-PVB debonding. • The effectiveness of the proposed model is validated by comparing simulation results under center and corner impact with experimental ones. • Parametric studies are conducted to investigate the effects of adhesion parameters, PVB material parameters, and impact velocities and angles on the impact fracture behavior and energy absorption performance of the windshield. Abstract Though windshield laminated glass is a simple sandwiched composite structure, it is of vital importance to investigate its damage and energy absorption behavior for the purpose of pedestrian safety protection. This paper proposes an intrinsic cohesive zone approach to achieve this end. A finite element windshield glazing model is established by inserting cohesive elements into the common surfaces between glass elements, and the interfaces between glass and PVB prior to simulations. The appeal of this model is that both of the two main failure patterns and their energy absorptions, i.e. glass fracture and glass-PVB debonding, of the windshield can be captured by means of the intrinsic cohesive zone approach. The effectiveness of the proposed model is validated by comparing the simulation results under center and corner impact with the corresponding experimental outcomes. Finally, the effects of adhesion parameters, PVB material parameters, and impact velocities and angles on the impact fracture behavior and energy absorption performance of the windshield are thoroughly investigated. [ABSTRACT FROM AUTHOR]

On the basis of an experimental investigation [1], numerical investigation is conducted in this paper on damaged seamless metallic pipelines with metal loss (diameter-to-thickness ratio D / t around 21) through nonlinear finite element method (FEM). Numerical models are developed and validated through test results by using the measured material properties and specimen geometry, capable of predicting the residual ultimate strength of pipes in terms of bending capacity ( M c r ) and critical curvature ( κ c r ). By changing the metal loss parameters, i.e. length ( l m ), width ( w m ) and depth ( d m ), a series of numerical simulations are carried out. Results show that the larger the d m or l m is, the less the bending capacity will be. The increase of notch width slightly reduces the pipe strength, presenting a linear tendency. Based on the FEM results, empirical formulas are proposed to predict the residual ultimate strength of metallic pipes with metal loss under pure bending moment. The prediction results match well with the results from the tests, the numerical simulations as well as the theoretical derivation. Such formulas can be therefore used for practice purposes and facilitate the decision-making of pipe maintenance after mechanical interference. [ABSTRACT FROM AUTHOR]

In this paper an efficient numerical tool is proposed to investigate delamination type failure in multi-layered composite shells. In the current contribution the extended finite element method (XFEM), the mixed-mode cohesive zone model, the contact formulation, and the damage criterion are incorporated into a new algorithm to study the interfacial delamination initiation and growth with less computational effort. A flat-shell formulation is developed in the geometrically non-linear regime to study the response of shells in small strains and moderate rotations. In addition, the equivalent single layer theory (ESLT) is applied to simulate the multi-layered laminates. This formulation is enhanced through the XFEM topology to be able to model discontinuous domains and a mixed-mode bilinear cohesive formulation to track the delamination growth. In the current study, the simulation can be initiated in an intact laminate. Thus, unlike formulations in existing finite element models, incorporating cohesive zone model at all available interfaces is not necessary. The interlaminar stresses are calculated during post-processing and they are being used in the delamination onset criterion. As soon as the criterion is satisfied at a specific layer and location, the formulation of that corresponding element is locally changed to XFEM and the cohesive behaviour. Consequently, the possibility to track delamination growth is locally provided; and hence, the computational cost is reduced. [ABSTRACT FROM AUTHOR]

This paper investigates the effect of size and complexity of composite structures on the formation of low-velocity impact damage via experimental tests and numerical modelling. The ASTM standard low-velocity impact test and a scaled-up version of the test were conducted. A novel numerical technique is presented that combines 3D solid and thin 2D shell elements for modelling different domains to achieve a high level of fidelity locally under the impact location, whilst achieving good computational efficiency for large structures. Together with the experimental studies at the different scales, the predictive capability of the numerical models was systematically validated. This modelling method demonstrated an advanced computational efficiency without compromising predictive accuracy. The models are applied to a case study of low-velocity impact of a large-scale stringer-stiffened panel, showing this modelling approach to be suitable for predicating low-velocity impact damage and structural response of laminated composites over a range of sizes and complexities. [ABSTRACT FROM AUTHOR]

Abstract: The numerical simulation of fatigue and the prediction of the life time of structures is more than 50years after the introduction of the Finite-Element-Method (FEM) still a problem. Another numerical method, older than the Finite Element idea, is the Discrete-Element-Method (DEM), with which a material can be represented as a continuum as well as a discontinuum. Over the last few years engineers have become very fascinated by this method, since it enables a very realistic fracture modelling. Furthermore it is also possible to make life time predictions with the DEM. This paper shows a new way of obtaining S–N curves for metallic materials, affected by mechanical loads. For the numerical simulation of life time a virtual specimen is represented by Discrete Elements. When using this method, no initial cracks or initial crack lengths are needed. Materials always show a variation in their properties. Since the discrete variation of the material parameters has an influence on the mechanical behaviour of the structure and its life time, statistical aspects are also discussed briefly in this paper. These stochastically distributed microscopic effects are included in the DEM model. [Copyright &y& Elsevier]

This paper aims at developing a pore-scale numerical model for non-Darcy fluid flow through rough-walled fractures. A simple general relationship between the local hydraulic conductivity and the flow velocity is proposed. A new governing equation for non-Darcy fluid flow through rough-walled fractures is then derived by introducing this relationship into the Reynolds equation. Based on the non-linear finite element method, a self-developed code is used to simulate the non-Darcy fluid flow through fractures. It is found that the macroscopic results obtained by the numerical simulation agree well with the experimental results. Furthermore, some interesting experimental observations can be reproduced. [ABSTRACT FROM AUTHOR]

In this paper, the constraint effect caused by a side-groove on creep crack growth behavior in P92 steel was studied using finite element analysis. Creep crack growth simulations were conducted on compact tension specimens for side-grooves with different depths and angles. The simulated outcome indicated that the creep crack growth rate increased as the depth and the angles of the side-grooves in the specimen increased. Next, the profile of the crack front for the different specimens was discussed. The distribution of the average stress triaxiality and the average equivalent stress on the crack front were studied to investigate the mechanical mechanism describing the constraint effect on creep crack growth behavior. Finally, the creep crack growth rate determined from numerical simulation results was determined to be consistent with existing experimental data. [ABSTRACT FROM AUTHOR]

This paper introduces a numerical simulation tool using the Finite Element Method (FEM) for near-surface mounted (NSM) strengthening technique using fibre reinforced polymers (FRP) applied to concrete elements. In order to properly simulate the structural behaviour of NSM FRP systems there are three materials (concrete, FRP and the adhesive that binds them) and two interfaces (FRP/adhesive and adhesive/concrete) that shall be considered. This work presents the major details of a discontinuous-based constitutive model which aims at simulating NSM FRP interfaces implemented in the FEMIX FEM software. This constitutive model was adapted from one available in the literature, originally employed for fracture simulation in meso-scale analyses of quasi-brittle materials, which is based on the classical Flow Theory of Plasticity combined with fracture mechanics concepts. The most important features of the implemented constitutive model are the consideration of both fracture modes I and II and the possibility of performing 2D and 3D analysis. In the end, results based on FEM simulations are presented with the aim of investigating the soundness and accuracy of the constitutive model to simulate NSM FRP systems’ interfaces. [ABSTRACT FROM AUTHOR]

Among the different energy absorption mechanisms, axial splitting/curling is known as an efficient method due to its large stroke to length ratio. This paper presents a finite element model for numerical simulation of splitting of circular brazen tubes. Surface-based cohesive technique is employed for modeling fracture and cracks growth. The required material parameters are extracted from stress-strain diagram of simple tensile test. Some experimental tests were performed to validate the numerical results. For implementation of experimental procedure, the specimens were prepared by cutting the commercial brazen tube and some edge notches were created on one end side of tubes. The specimens were axially compressed between a rigid plate on top and a conical die at the bottom using Zwick universal testing machine. Good agreements were found between numerical simulations and experimental tests. The effects of several parameters such as number and length of initial notches, diameters and thickness of tubes and vertex angle of dies on energy absorption capacity of specimens are investigated, numerically. [ABSTRACT FROM AUTHOR]

This paper compares the stress and strain state simulated in a twin-disc test and a full scale wheel/rail experiment. The simulation is done by two-dimensional finite element models. The simulations regard the scaling factor between these experiments. This factor includes the contact patch size and the deformation depth. Same stresses and strains are calculated for both experiments considering this scale factor. For the investigation of cracks in these experiments, initiated short cracks or manually produced initial cracks are needed. The transferability of the crack growth predictions from twin-disc tests to the full scale wheel/rail experiments affords a scaling of the initial crack size to the size of the contact patches. Out of practical reasons usually the initial crack tips are located in both experiments in another depth as the scaling would afford and therefore, the tips are located in another zone of the plastic deformation. Very short cracks in the twin-disc tests are removed by wear or are influenced by the always existing surface roughness. It is shown that stresses and strains scale with the experiments but this will be not the case for wear and surface roughness. [ABSTRACT FROM AUTHOR]

In this paper, we present an extension of the Stable Generalized FEM (SGFEM) for 3-D fracture mechanics problems. Numerical experiments show that the use of available singular enrichment bases derived from the 2-D asymptotic crack tip solution leads to a severe loss of accuracy in the SGFEM. An enrichment scheme based on singular bases and linear polynomials is shown to recover the optimal convergence of the SGFEM. The proposed SGFEM for 3-D fractures delivers the same rate of growth in condition number as the standard FEM for proper choice of singular enrichment functions. The accuracy and conditioning obtained with the SGFEM is compared with the Generalized FEM (GFEM) for different types of singular enrichment bases. A fully 3-D fracture mechanics problem is considered to highlight issues that arise only in 3-D problems. The convergence of Stress Intensity Factors (SIFs) extracted from GFEM and SGFEM solutions and the effect of the size of enrichment sub-domains on the accuracy of extracted SIFs are also studied. It is shown that the accuracy of SIFs may deteriorate with mesh refinement when the topological enrichment strategy is adopted. The proposed SGFEM delivers much better accuracy than the GFEM for both geometrical and topological enrichment strategies. [ABSTRACT FROM AUTHOR]

Hypervelocity impact dynamics is the theoretical support of spacecraft shielding against space debris. The numerical simulation has become an important approach for obtaining the ballistic limits of the spacecraft shields. Currently, the most widely used algorithm for hypervelocity impact is the smoothed particle hydrodynamics (SPH). Although the finite element method (FEM) is widely used in fracture mechanics and low-velocity impacts, the standard FEM can hardly simulate the debris cloud generated by hypervelocity impact. This paper presents a successful application of the node-separation technique for hypervelocity impact debris cloud simulation. The node-separation technique assigns individual/coincident nodes for the adjacent elements, and it applies constraints to the coincident node sets in the modeling step. In the explicit iteration, the cracks are generated by releasing the constrained node sets that meet the fracture criterion. Additionally, the distorted elements are identified from two aspects – self-piercing and phase change – and are deleted so that the constitutive computation can continue. FEM with the node-separation technique is used for thin-wall hypervelocity impact simulations. The internal structures of the debris cloud in the simulation output are compared with that in the test X-ray graphs under different material fracture criteria. It shows that the pressure criterion is more appropriate for hypervelocity impact. The internal structures of the debris cloud are also simulated and compared under different thickness-to-diameter ratios ( t / D ). The simulation outputs show the same spall pattern with the tests. Finally, the triple-plate impact case is simulated with node-separation FEM. [ABSTRACT FROM AUTHOR]

The concept of ratchetting strain as a crack driving force in controlling crack growth has previously been explored at Portsmouth using numerical approaches for nickel-based superalloys. In this paper, we report the first quantitative experimental evidence of near-tip strain ratchetting with cycles, as captured in situ by digital image correlation (DIC) technique on a compact tension specimen of stainless steel 316L, using both Stereo and SEM systems. The evolution of the near-tip strains with loading cycles was monitored whilst the crack tip was kept stationary. The strains normal to the crack plane were examined over selected distances from 6 to 57 μm to the crack tip for a number of cycles. The results show that strain ratchetting occurs with loading cycles, and is particularly evident close to the crack tip and under higher loads. 3D finite element models have also been developed to simulate the experiments and the results from the simulation are compared with those from the DIC measurements. This is the first time that near-tip strain ratchetting has been captured in situ at the peak loads during cyclic loading. [ABSTRACT FROM AUTHOR]

This paper presents the results of numerical studies carried out on concrete columns confined by Carbon Fiber Reinforced Polymer (CFRP) and loaded in uni-axial compression. A numerical simulation of confined columns experimentally tested by [1] is performed for relatively small concrete specimens of constant size but with different cross-section corner radius. The experimental results, reported in terms of axial stress–strain relationships and failure modes, constitute a useful database for the calibration of numerical models. These results clearly demonstrate that CFRP confinement is much less effective in square than in circular cross-sections. Therefore, the influence of the corner radius on the non-uniform stress distribution due to confinement, is numerically investigated using the proposed microplane-based model that takes into account the effect of multi-axial stress states in concrete. The experimental results are used to calibrate and verify the prediction capability of a three-dimensional finite element code (3D FE) that is based on the microplane constitutive law for concrete [2] . In the finite element model carbon fibers are modelled using nonlinear truss elements, while epoxy resin as well as concrete are modelled using microplane-based constitutive law and 3D finite elements. Given that experimental results for unconfined and confined configuration are available for each specimen, the 3D FE concrete model has been preliminarily calibrated on unconfined concrete specimens and then used in the analysis of a confined specimen. It is demonstrated that numerical models can predict behavior of confined concrete columns from the experimental investigations, confirming the predictability of the numerical microplane-based approach used. [ABSTRACT FROM AUTHOR]

Computation cost is a key issue for finite element methods to simulate hydraulic fracturing. In this paper a coupled finite element method combined with a condensation technique is proposed to address this issue. Removing the node displacements that have no contribution to fracture widths from the coupled equations, the condensation technique reduces the size of the coupled equations in the proposed method. The numerical method with the condensation technique is verified. Simulations show that the condensation technique can reduce the computation cost effectively, in particular when the fracture propagation regime is viscosity-dominated or the simulation is on the early stage. The effects of the condensation technique on the simulation accuracy, stability, and convergence of the numerical method are discussed. The condensation technique is applicable to other finite element methods that are based on linear elastic fracture mechanics. [ABSTRACT FROM AUTHOR]

Abstract: The ductile fracture of the clinched joint is studied by the experiments and simulations using a modified Rousselier model. Although tension failure has been successfully predicted by the classical Rousselier damage model, its predictive capability on shear failure is seldom discussed. The microstructure of the clinched joint reveals that the ductile fracture occurs in the neck thickness under the shear and tensile force. So this paper is mainly concerned with application of a modified Rousselier damage model developed by the Guo on the description of the failure behavior of the clinched joint. The three experimental specimens were designed in order to calibrate the damage coefficients k ω and the initial void volume fraction f 0. The material parameters of the modified Rousselier model were identified by an inverse method comparing the experimental results with the numerical results. The modified Rousselier model could capture both tension and shear failure behavior of the clinched joint. The numerical results and the experimental results indicate the Lode parameters in the new damage evolution model can capture the failure behavior and the evolution process during the clinching process. [Copyright &y& Elsevier]

Abstract: This paper presents the advanced object-oriented design of finite element representations in a complex multi-physics finite element environment OOFEM [1–3]. The focus is on reuse of existing single-physics capabilities when implementing elements for coupled simulations. This has been achieved by decoupling the description of element geometry, element problem specific capabilities, element interpolation, and integration schemes. The individual problem specific capabilities, represented by a hierarchy of classes derived form ElementEvaluator, can be assembled together to define an evaluator for coupled analysis. The presented design leads to an extremely flexible implementation, with clean modular design. The application is demonstrated on an implicit gradient formulation of a damage-plastic model and on coupled heat and mass transfer. [Copyright &y& Elsevier]

Abstract: This paper investigates the deformation and failure mechanisms of wood at the ultrastructural scale. At this level, wood is composed of a periodic alternation of amorphous and crystalline cellulose fractions, embedded in a soft hemicellulose-lignin matrix. The mechanical response of wood is calculated under tensile loading conditions by means of the computational homogenisation of a representative volume element (RVE) of material. Three potential mechanisms of failure are suggested: axial straining of the crystalline fraction of cellulose, accumulation of plastic strain in the amorphous portion of cellulose and tensile rupture in the hemicellulose-lignin matrix due to cellulose fibres separation. In order to validate the present multi-scale framework, we compare our numerical predictions for the reorientation of cellulose fibres with experimental data, finding a good agreement for a wide range of strains. Furthermore, we assess successfully our numerical predictions for ultimate strains at the instant of failure when compared to experimental values. Numerical simulations show that our model is able to provide new clues into the understanding of how trees and plants optimise their microstructure in order to develop larger strains without apparent damage. A remarkable prediction by our model suggests that the extensibility of the material is maximised for initial microfibril angles (MFA) between 50° and 55°, a range of values found typically in branches of trees, in which the extensibility is an essential requirement. These findings are likely to shed more light into the dissipative mechanisms of wood and natural materials, which are still not well-understood at present. [Copyright &y& Elsevier]

Abstract: This paper reports the analysis and simulation of the catastrophic failure of a compressed natural gas (CNG) fuel tank. The initial analyses of the deformation and cracking patterns, along with the observed fractographic features, were indicative of an internal gaseous combustion. Accordingly, a set of transient-dynamic elasto-plastic finite element (FE) analyses was carried out to simulate the structural response of the tank to a special type of combustion-induced dynamic pressure. The FE model was composed of 3D brick elements equipped with interface cohesive elements for crack growth analysis. Excellent agreements were found between the final simulation results and the observed deformation and fracture patterns. The simulation results clearly revealed that the observed failure characteristics, like the overall asymmetric deformation and fracture patterns, initiation and partial growth of parallel cracks at the same section, multiple cracking at the neck, and the self-similar growth of the main axial crack were all caused by traveling of a deflagration-induced sonic pressure wave from the neck towards the bottom of the tank. Finally, a comparison was made between the characteristics of deflagration-induced and detonation-induced deformation and fracture behaviors of closed-end cylinders. [Copyright &y& Elsevier]

Abstract: A systematic numerical and analytical approach, to predict the various global failure modes such as flexural, punching shear and mixed flexure-shear failures along with the local failure modes such as crushing, cracking, spalling and scabbing, under impact load is investigated in this study. Numerical simulation on reinforced concrete (RC) slab impacted with cylindrical drop hammer has been carried out using ABAQUS finite element code to obtain the failure modes. Also a comparison has been made to validate the results of numerical simulation with the Zineddin and Krauthammer experimental results. The simulation results are in excellent agreement with the experimental failure modes reported by Zineddin and Krauthammer. The behavior of RC slab under impact load is simulated with the conventional limiting strain criteria as well as fracture energy softening models within the framework of damage plasticity model. It is found that the limiting strain criterion suffers from the limitation of mesh sensitivity and results from the fracture energy approach are close to the actual experimental results. Therefore the fracture energy softening models have been recommended as robust method for these types of studies. Besides the complexities due to concrete heterogeneity, the inelastic concrete and rebar behavior, strain rate effect and mesh sensitivity issues are addressed in the present paper to evolve a robust nonlinear transient dynamic finite element simulation. It is illustrated that the global and local failure modes can be numerically predicted through identifiable and measurable parameters such as the point of inflection, bi-axial failure criteria, strain based failure criteria, shear failure criteria, rebar strain and tensile damage profiles. [Copyright &y& Elsevier]

Abstract: This paper presents a comprehensive finite element model for the numerical simulation of Hydrogen Induced Cracking (HIC) in steel pipelines exposed to sulphurous compounds, such as hydrogen sulphide (H2S). The model is able to mimic the pressure build-up mechanism related to the recombination of atomic hydrogen into hydrogen gas within the crack cavity. In addition, the strong couplings between non-Fickian hydrogen diffusion, pressure build-up and crack extension are accounted for. In order to enhance the predictive capabilities of the proposed model, problem boundary conditions are based on actual in-field operating parameters, such as pH and partial pressure of H2S. The computational results reported herein show that, during the extension phase, the propagating crack behaves like a trap attracting more hydrogen, and that the hydrostatic stress field at the crack tip speed-up HIC related crack initiation and growth. In addition, HIC is reduced when the pH increases and the partial pressure of H2S decreases. Furthermore, the relation between the crack growth rate and (i) the initial crack radius and position, (ii) the pipe wall thickness and (iii) the fracture toughness, is also evaluated. Numerical results agree well with experimental data retrieved from the literature. [Copyright &y& Elsevier]

Abstract: In a companion paper (Part I), the processing of two-layer parts by co-sintering of two powder blends has been investigated. As a complement to this experimental study, a finite element simulation of this operation has been achieved. This simulation was based on constitutive equations identified from specific experiments performed with each blend. The numerical results provided deformation and internal stress information that have been found to be fully consistent with the experimental observation. In particular they confirm that cracking occurs in the course of heating, when one material starts densifying and the other one is still brittle, whereas a large densification mismatch at high temperature is not harmful because the low viscosity of both materials leads to the relaxation of internal stresses. [Copyright &y& Elsevier]

Abstract: Series of ballistic experiments and numerical simulation were adopt to analyze the anti-penetration performance of explosively welded steel/aluminum, steel/aluminum/steel and aluminum/steel/aluminum plates impacted by spherical projectile vertically in this paper. A 14.5mm slip chamber gun was used to launch the spherical steel projectile with diameter of 6mm and the finite element code LS-DYNA 3D was used to simulate the penetration process. The effects of layer number, thickness, combination state of the interface and strength of the plates on the anti-penetration performance and damage mechanism were analyzed based on the results of experiment and numerical simulation. The results show that the damage mechanism of the front plate and middle plate is shearing and plugging, respectively, the steel rear plate is petalling deformation and the aluminum rear plate is ductile deformation impacted by projectile vertically. The anti-penetration performance of three-layered plates is better than that of two-layered plates with the same total thickness, especially the anti-penetration performance of three-layered steel/aluminum/steel welded plates is the best. As to two-layered steel/aluminum plates, the best anti-penetration performance can be attained when the thickness ratio is about 3:1. The good accordance of numerical and experimental results indicates that the finite element method (FEM) can predict the anti-penetration performance of multi-layered explosively welded plates well. [Copyright &y& Elsevier]

Abstract: A finite element model for 3D random fiber networks was constructed to simulate deformation and failure behavior of networks with dynamic bonding/debonding properties. Such fiber networks are ubiquitous among many living systems, soft matters, bio-materials, and engineering materials (papers and non-woven). A key feature of this new network model is the fiber–fiber interaction model that is based on AFM measurements from our earlier study. A series of simulations have been performed to investigate strain localization behavior, strength statistics, in particular, the variations of strength, strain-to-failure and elastic modulus, and their size dependence. Other variables investigated are fiber geometries. The result showed that, in spite of its disordered structure, strength and elastic modulus of a fiber network varied very little statistically, as long as the average number of fibers in the simulated specimen and the degree of fiber orientation are kept constant. However, strain-to-failure showed very significant statistical variations, and thus more sensitivity to the disordered structures. [Copyright &y& Elsevier]

Abstract: In this paper, the recently developed singular edge-based smoothed finite element method (sES-FEM) is further developed for dynamic crack analysis in two-dimensional elastic solids. The objective of this work is to provide an efficient and accurate numerical simulation tool for the dynamic fracture behaviors of linear elastic solids in the framework of the strain smoothing approaches. Following this approach, the strains are smoothed and the system stiffness matrix is thus performed using the strain smoothing technique over the smoothing domains associated with the element edges. In order to accurately capture the singular fields at the crack-tip, a two-layer singular 5-node crack-tip element is employed. The governing dynamic equations are transformed into a weakened weak (W2) form, which is then discretized into a sES-FEM system of time-dependent equations to be solved by the unconditionally stable implicit Newmark time integration method. To analyze the fracture behaviors of linear elastic solids, mixed-mode dynamic stress intensity factors (DSIFs) are evaluated using the domain forms of the interaction integrals in terms of the smoothing technique. Four test examples including pure mode-I and mixed-modes are studied to validate the accuracy of the proposed method. The computed results for the normalized DSIFs are compared with analytical and other numerical reference solutions in a wide range of benchmark dynamic crack problems which shows high accuracy of the sES-FEM. [Copyright &y& Elsevier]

Abstract: The simulation of delamination using the Finite Element Method (FEM) is a useful tool to analyse fracture mechanics. In this paper, simulations are performed by means of two different fracture mechanics models: Two Step Extension (TSEM) and Cohesive Zone (CZM) methods, using implicit and explicit solvers, respectively. TSEM is an efficient method to determine the energy release rate components GIc , GIIc and GIIIc using the experimental critical load (Pc ) as input, while CZM is the most widely used method to predict crack propagation (Pc ) using the critical energy release rate as input. The two methods were compared in terms of convergence performance and accuracy to represent the material behaviour and in order to investigate their validity to predict mode I interlaminar fracture failure in unidirectional AS4/8552 carbon fibre composite laminates. The influence of increasing the loading speed and using mass scaling was studied in order to decrease computing time in CZ models. Finally, numerical simulations were compared with experimental results performed by means of Double Cantilever Beam specimens (DCB). Results showed a good agreement between both FEM models and experimental results. [Copyright &y& Elsevier]

Abstract: The crack in a double cantilever beam is the most fundamental one-dimensional fracture problem. It has caused considerable confusion due to its in-depth subtleness and complex entanglement with different theories and numerical simulations. The present paper presents completely analytical theories based on Euler and Timoshenko beam theories using a brand new approach which reveals the hidden mechanics of the problem. Orthogonal pairs of pure modes are found and used to partition mixed modes. The developed theories are extensively validated against numerical simulations using finite element methods. Moreover, the fracture mode partition space is thoroughly investigated and crack tip running contact is found which results in a region of pure mode II. The theories are finally applied to general one-dimensional fracture in beams and axisymmetric plates. [Copyright &y& Elsevier]

Abstract: In this paper, we established a strain-gradient damage model based on microcrack analysis for brittle materials. In order to construct a damage-evolution law including the strain-gradient effect, we proposed a resistance curve for microcrack growth before damage localization. By introducing this resistance curve into the strain-gradient constitutive law established in the first part of this work (), we obtained an energy potential that is capable to describe the evolution of damage during the loading. This damage model was furthermore implemented into a finite element code. By using this numerical tool, we carried out detailed numerical simulations on different specimens in order to assess the fracture process in brittle materials. The numerical results were compared with previous experimental results. From these studies, we can conclude that the strain gradient plays an important role in predicting fractures due to singular or non-singular stress concentrations and in assessing the size effect observed in experimental studies. Moreover, the self-regularization characteristic of the present damage model makes the numerical simulations insensitive to finite-element meshing. We believe that it can be utilized in fracture predictions for brittle or quasi-brittle materials in engineering applications. [Copyright &y& Elsevier]

Abstract: This paper shows how experimental test results from a split-Hopkinson tension bar (SHTB) and numerical simulations of the test set-up can be used for mutual verification. Firstly, a SHTB where the tension stress wave is generated by pre-stretching a part of the incident bar is briefly presented. This SHTB is used to carry out tensile tests of four aluminium alloys at high rates of strain, while tests at low to medium strain rates were performed in a servo-hydraulic tensile test machine. Using the test results, the parameters of an anisotropic thermoelastic-thermoviscoplastic constitutive relation and a one-parameter fracture criterion are identified for the materials at hand. Subsequently, the material model is used in explicit finite element analyses of the SHTB tests, including the entire experimental set-up and the stress wave propagation during the test. The numerical predictions were found to represent the observed behaviour in the experimental tests fairly well. [Copyright &y& Elsevier]

Abstract: In this paper, as a typical example, a valve in a coal-fired thermal power plant with a semi-elliptic crack in its body is presented to illustrate an effective method to predict the surface crack shape under thermo-mechanical loading. The 3D finite element analysis (FEA) is employed to estimate the stress intensity factor (SIF) based on the 3D constraint effect at a set of points along the crack front, and then the evolution of the crack shape is studied according to the SIF. Some numerical techniques were well applied to simulate the real charge condition during the valve operation. It has been assumed that each point in the front of the crack advances in direction perpendicular to such a front, according to the Paris–Erdogan Law. Numerical results of the present method agree fairly well with detective results. [Copyright &y& Elsevier]

Abstract: The load response of a rock-socketed steel H-pile can be strongly influenced by the nonlinear interfacial behavior between the grout and the steel H-pile, and between the pile and the rock mass. This paper focuses on the load-transfer mechanism of the former interface through experimental push-out tests and numerical simulation of the tests. The study is divided into two parts. In the first part, a series of push-out tests have been carried out on four types of specimens (with studs+confinement (Case I); no stud+confinement (Case II); no stud+no confinement (Case III); and studs+no confinement (Case IV)). The second part is a numerical study based on three-dimensional finite element technique, which takes into account possible damage and cracking in grout, as well as bond-slip along the interface. It is shown that the numerical predictions of the four cases compare favorably with the corresponding test results, including the load–displacement response and the development of cracks. Furthermore, parametric study has been carried out to investigate the influence of various factors, including the studs, the casing confinement, the grout fracture energy, and the dilation property of steel-grout interface. Lastly, some implications, based on the test and numerical results, on the design of socketed steel H-piles are discussed. [Copyright &y& Elsevier]

Abstract: The computational modeling of failure mechanisms in solids due to fracture based on sharp crack discontinuities suffers in situations with complex crack topologies. This can be overcome by a diffusive crack modeling based on the introduction of a crack phase field. Following our recent work [C. Miehe, F. Welschinger, M. Hofacker, Thermodynamically-consistent phase field models of fracture: Variational principles and multi-field fe implementations, International Journal for Numerical Methods in Engineering DOI:10.1002/nme.2861] on phase-field-type fracture, we propose in this paper a new variational framework for rate-independent diffusive fracture that bases on the introduction of a local history field. It contains a maximum reference energy obtained in the deformation history, which may be considered as a measure for the maximum tensile strain obtained in history. It is shown that this local variable drives the evolution of the crack phase field. The introduction of the history field provides a very transparent representation of the balance equation that governs the diffusive crack topology. In particular, it allows for the construction of a new algorithmic treatment of diffusive fracture. Here, we propose an extremely robust operator split scheme that successively updates in a typical time step the history field, the crack phase field and finally the displacement field. A regularization based on a viscous crack resistance that even enhances the robustness of the algorithm may easily be added. The proposed algorithm is considered to be the canonically simple scheme for the treatment of diffusive fracture in elastic solids. We demonstrate the performance of the phase field formulation of fracture by means of representative numerical examples. [ABSTRACT FROM AUTHOR]

Abstract: Ore body formation and mineralization are the consequence of transport, focusing and mixing processes of geofluids from the deep earth into the upper crust of the earth. These processes largely depend on geological structures, particularly heterogeneity such as cracks and faults. Both geological stress and geofluid-induced hydraulic pressure can create and alter the heterogeneity, thus changing the transport, focusing and mixing patterns. As a preliminary study, this paper investigates the intrusion and focusing of geofluids through simulation of the hydraulic fracturing process. First, the deformation of porous media and geofluid flow are coupled through the Biot''s consolidation theory to consider both the elastoplasticity of porous media and the deformation induced evolution of permeability. Second, the crack induced anisotropy of stiffness and permeability is described, on the basis of a smeared theory and an equivalent anisotropic medium, to establish a criterion for tensile crack initiation and propagation at an element level. Finally, the resulting nonlinear equations are solved by a coupled algorithm of the meshless method and the finite element method (FEM). Numerical examples indicate that the proposed method is able to simulate the stress-induced crack propagation, geofluid intrusion and focusing in fluid-saturated porous media. It thus provides a potential tool for simulating the detailed mineralization process associated with the geological stress and geofluid intrusion. [Copyright &y& Elsevier]

Abstract: This paper deals with the numerical simulation of a stable crack propagation experiment at in a 16MND5 steel. At this temperature, the material is viscoplastic. A cohesive zone model is formulated in order to simulate the rupture of a CT specimen. A large displacement 3D cohesive element with eight nodes is implemented in the finite element code ABAQUS. The associated traction–separation law is of Tvergaard and Hutchinson type, in which an hardening slope has been added. This hardening simulates the material strengthening associated to the increasing strain rate in front of the crack tip when crack tip starts to propagate. We show that in this case the form of the cohesive law has great impact on the simulated propagation velocity. [Copyright &y& Elsevier]

Abstract: The crack repair technique by “crack grinding”, due to its efficiency, has been widely used to extend the service life of the cracked structural components. The “crack grinding” technique is carried out by removing the materials which contains the crack. So it is of great importance to study the effects of this repair technique. In this paper, the finite element method is used to analyse the behavior of repaired cracks in mode I by computing the stress-intensity factors at the crack tip. 3D numerical simulation was carried out to model the steam governor valve in a coal-fired thermal power plant with a semi-elliptic crack. Since no extra auxiliary solutions are required for the computation, the proposed method appears to be visual and can be used for evaluation of the associated stress-intensity factors. The plot of the stress-intensity factors according to the crack length in mode I, shows that the stress-intensity factor exhibits an asymptotic behavior as the crack length increases. It’s to be noted that a transition zone allows connecting the refined specific crack tip mesh to the global structural discretization. So this technique can be used for complex industrial structures. [Copyright &y& Elsevier]

Abstract: This paper deals with a finite element algorithm for the creep crack growth process in a viscoelastic medium. The main developments focus on the coupling between the M-integral and an incremental formulation for the viscoelastic behavior. In this context, mixed mode configurations are simulated for orthotropic symmetries. An algorithm uncoupling viscoelastic incremental formulation and the fracture procedure is resolved with finite element software. The global approach is validated in terms of the evolution of energy release rate versus time and the advance of cracks. Numerical simulations are based on a Constant Tension Shear model. The insensitivity of the M-integral to the integration domain is shown from creep crack growth simulations for mixed mode configurations. [Copyright &y& Elsevier]

Abstract: In the present paper an extended finite element method (XFEM) containing strong discontinuity within elements is introduced and implemented in the commercial general purpose software ABAQUS. The algorithm allows introducing a new crack surface at arbitrary locations and directions in elements. To consider fatigue crack nucleation and propagation in quasi-brittle materials the XFEM is combined with a cyclic cohesive model. Accumulative material damage is described by separate evolution equations. The crack path is completely independent of the mesh structure but determined by the mixed-mode loading cases. Numerical simulations illustrate the ability of this method to simulate fracture with unstructured meshes. The computational results agree with known fracture experiment data. Known fatigue observations can be predicted using the present model. [Copyright &y& Elsevier]

• Fatigue analysis of valve bodies DN100 and DN50. • Crack initiation on a valve body according to Basquin-Coffin-Manson equation. • Stress intensity factor estimation with eXtended Finite Element Method (XFEM). • Cycles for crack growth across the valve body thickness. • The engineering and the true stress–strain diagrams and cycle fatigue behaviour of GP240GH steel. Global industry depends of the different type of valves, such as globe valves which are used for precise flow regulation. Since most of the power plants rely on performance of globe valves, it is necessary that valves function without interruption. In this paper, the main part of the globe valve, valve body, is investigated. Simulation of crack initiation and its growth in a wall of valve body is carried out. Required number of cycles for crack initiation is estimated according to Basquin-Coffin-Manson equation. Afterwards, stress intensity factor is estimated through eXtended Finite Element Method (XFEM) and required number of cycles for crack growth across the valve body thickness is calculated according to Paris' law. The aforementioned method is validated on the results from the available literature by employing relatively simple shapes such as steel pipe with integrated flaw. Subsequently, the method is used on a relatively complex geometry, as is the valve body. The obtained results indicate lack of brittle fracture in a valve body. In other words, the results imply occurrence of Leakage Before Brake (LBB). This is utterly important for power plant industries, since it ensures the proper reaction of the user and possibly avoids possible catastrophic consequences for personnel and the equipment located in the proximity of the valve. [ABSTRACT FROM AUTHOR]