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During the forming of aramid paper honeycomb sandwich component of aircraft composites in the autoclave, the structure bears thermo-mechanical coupling case, and it may exist the phenomenon of the honeycomb crack during the forming. The tensile, compressive, shear and thermal properties of aramid paper honeycomb core are obtained by testing. Meanwhile, the numerical simulation of thermo-mechanical sequential coupling of the whole honeycomb sandwich component and the local core area are carried out by combining the finite element analysis method and the inversion method. The results show that the thermal stress during the forming process will not cause the adhesive layer between honeycomb cells to crack, but gas pressure will. The difference value of pressure difference between honeycomb cells required for adhesive layer′s slight destruction and complete destruction is positively related to the initial normal fracture stress of adhesive layer. The pressure difference required for the adhesive layer′s complete destruction is decreased with the increase of the number of loaded cells.

Numerical analysis of growth of fractures from a wellbore under sleeve fracturing conditions demonstrates the impact of in situ stresses on initiation and propagation behavior of primary and secondary cracks at both field and lab scale. Crack initiation and propagation behavior are simulated in a Finite Element framework employing plane-strain cohesive element along the candidate crack paths. The model has been validated by benchmarking to Kirsch's solution as well as by comparing predictions to results of a laboratory block test. Indeed, the field scale simulation results show that the first fracture always orients with the opening in direction of maximum in situ stress while the opening direction of the second fracture can vary with different in situ stresses. Furthermore, the deviation of wellbore displacement from the Kirsch solution due to the fracture initiation can be captured by the simulation, which can be further applied to interpret the initiation pressure and location of cracks based on field test data. In the context of past contributions on sleeve fracturing, these numerical results demonstrate that a relatively large region near the crack tip undergoes inelastic deformation, which contrasts with the assumptions required to treat this problem using linear elastic fracture mechanics (LEFM). Notably, the finite tip plasticity appears to eliminate unstable jumping of the crack length(s) predicted by LEFM solution. The finite tip plasticity also leads to the prediction that the secondary fracture often occurs in orientations other than 90° to the primary fracture. In contrast, the LEFM solution predicts none of this growth because the stress intensity factor (SIF) never reaches the rock fracture toughness in spite of the fact that these alternate orientations have the largest SIFs over limited periods of time. Finally, the finite tip plasticity shows the strength of materials approach (based on stress analysis around the hole) to give a lower bound estimate of the pressure required for crack extension. [ABSTRACT FROM AUTHOR]

Modelling of thin joining zones is one application of Cohesive Zone Elements (CZE). It is shown that Traction Separation Laws (TSLs) used as constitutive models in most CZEs that can be found in literature cannot fully capture all damage causing deformation modes induced by forming of hybrid components. In this contribution we present an approach to overcome this deficiency by using bulk material models within a Cohesive Zone Element. Numerical examples demonstrate the excellent properties of the method by comparing the new formulation with volumetric modelling of a joining zone. • Membrane like forces and out of plane shear forces are relevant for bulk forming. • Most material models used in Cohesive Zone Elements cannot process these loads. • Internal Thickness Extrapolation combines Cohesive Zone Elements and bulk material. [ABSTRACT FROM AUTHOR]

Many high-strength composite materials have been developed for aircraft structures. GLAss fiber REinforced aluminum (GLARE) is one of the high-performance composites. The review of articles, however, yielded no study on the impact damage of heated GLARE laminates. This study, therefore, aimed at developing a numerical model that can delineate the continuum damage of GLARE 5A-3/2-0.3 laminates at elevated temperatures. In the first stage, the inter-laminar interface failure of heated GLARE laminate had been investigated at room temperature and 80 °C. The numerical analysis employed a three-dimensional GLARE 5A-3/2-0.3 model that accommodated volumetric cohesive interfaces between mating material layers. Lagrangian smoothed particles populated the projectile. The model considered the degradation of tensile and shear modulus of glass fiber reinforced epoxy (GF/EP) at 80 °C, while incorporated temperature-dependent critical strain energy release rate of cohesive interfaces. When coupled with the material particulars, an 82 m/s bird impact at room temperature exhibited delamination first in the GF/EP 90°/0° interface farthest from the impacted side. Keeping the impact velocity, interface failure propagated at a slower rate at 80 °C than that at room temperature, which was in agreement with the impact damage determined in the experiments. The outcomes of this study will help optimize a GLARE laminate based on the anti-icing temperature of aircraft.

The automotive industry is now extensively using multi-material structures to reduce vehicle weight. Joining these dissimilar materials effectively in a single structure demands the use of adhesive bonding, but the presence of stress concentrations leads to low mechanical performance. Mixed adhesive joints have been proposed as a solution for this issue, using two adhesives in the same joint, combining stiffness and flexibility on the same overlap, improving performance. This work focuses on the numerical modelling of the mechanical behaviour of these joints, using previously published experimental data for validation. The proposed models use cohesive laws with strain rate dependent data to simulate quasi-static and impact loads on several configurations of mixed adhesive joints, using four different adhesives. The models were able to predict the behaviour of most configurations under study, especially for the stiffer adhesives. The model was found to be less effective for modelling high ductility adhesives under impact. [ABSTRACT FROM AUTHOR]

The low velocity impact response of a novel foam-based composite structure inspired by microstructural features of dragonfly wings is investigated through the use of FE model and experiments. A nonlinear progressive damage model of the composite skins is incorporated into the FE code by VUMAT subroutine. Inter-laminar damage is reproduced using interface cohesive elements and the foam core is modeled as a crushable foam material. The numerical results are compared with experimental data acquired by impact testing on bio-inspired structures consisting of E-glass/epoxy skins filled by polyurethane foam, where a good agreement is achieved. To assess the contribution of the sandwich vein on the impact behavior of the structure, a comparison is made between different veiny structures and monolithic laminates with the same materials under low velocity impact. It is concluded that the sandwich vein can limit the damage propagation and makes the rest of the structure remain intact. [ABSTRACT FROM AUTHOR]

Dual-phase (DP) ferrite-martensite steels have a fine microstructure that consist of martensite islands, dispersed in a ferrite matrix. In the present work, a micromechanical-based finite element (FE) analysis is carried out in order to investigate the mechanical properties of this type of steels, such as yield stress and tensile strength. The step forward contribution of the present study is to insert the effect of the presence of the interface of the constituting phases in the FE model. In order to do so, a micromechanical FE model is developed in which the effect of the interface of the constituting phases is taken into account by means of cohesive elements. The required parameters of the cohesive elements are determined by a trial and error procedure which compares the numerical results to the experimental ones. Experimental data is used to verify the created FE model. This model is then used to investigate the effect of martensite volume fraction and small-to-large particle size ratio of martensite on the deformation behavior of the DP steel. This study shows that considering the interface of the constituting phases greatly affects the numerical results on stress and strain distributions and produces more realistic findings in agreement with experiments. The results show that applying cohesive elements between two phases have reduced tensile strength, yield stress, and the amount of uniform elongation. [ABSTRACT FROM AUTHOR]

The onset and growth of damage in fiber/matrix composites under transverse loads were modelled using cohesive elements and representative volume elements of randomly arranged fibers. Switching between iterative schemes, using an appropriate tolerance and load increment size, and using an extrapolated solution as an initial guess for load increments led to over an order of magnitude reduction in the solution time. The effect of several model parameters on the failure properties for the next larger scale was studied. The crack path did exhibit a dependence on the mesh, but the RVE strength and amount of dissipated energy in the representative volume element did not vary more than 4% for any of the mesh refinements considered. Periodic boundary conditions minimally interfered with the localization of damage when the localized band of damage did not extend across the entire RVE or when the damage naturally localized parallel to a boundary or diagonal of the representative volume element. A local method for quantifying the energy dissipated within the representative volume element was proposed, which provides an improved accuracy and flexibility. An approach to precisely define the dominant crack was given, which allowed the energy dissipate diffusely and along the dominant crack to be separated. It was shown that the predicted critical strain energy release rate for the representative volume element was sensitive to the representative volume element unless the diffusely dissipated energy was accounted for separately. The proposed technique for calculating failure properties within a multiscale framework has the potential to be applied to other damage models. [ABSTRACT FROM AUTHOR]

The wind turbine blades are complex structures in terms of their geometry and the materials used. They need to be modeled, on the one hand as accurately and precisely as possible, while on the other hand the models should be light enough to be run in a reasonable amount of time using reasonable computational resources. Sub-modeling is a technique used to reduce the domain size of a finite element model to a more manageable size. One of the motivations behind sub-modeling is the capacity to develop highly refined and detailed models, without using increased computational resources, as the refined model domain is small and hence has a smaller number of elements. There are different methods of sub dividing the problem domain into smaller simpler domains, of which the transfer of nodal displacement form one parent model to its child will be used in this study. Furthermore the use of surface to solid sub-models is also discussed. [ABSTRACT FROM AUTHOR]

This paper presents results for a new acoustic emission crack source model based on a finite element modelling approach which calculates the dynamic displacement field during crack formation. The specimen modelled is statically loaded until conditions for crack growth as defined by a failure criterion are fulfilled. Subsequently, crack growth is modelled by local degradation of the material stiffness utilizing a cohesive zone element approach. The displacements due to crack growth generate the acoustic emission signal and allow detailed examination of the principles of acoustic emission sources operation. Subsequent to crack growth signal propagation is modeled. The signal propagation is modeled superimposed on the static displacement field. The presented model comprises a multi-scale and multi-physics approach to consider the signal propagation from source to sensor, the piezoelectric conversion of the elastic wave to an electric signal and the interaction to the acquisition electronics. Validation of the modeling approach is done by investigating the acoustic emission signals of micromechanical experiments. Using a specifically developed load stage, carbon fiber filament failure and matrix cracking can be prepared as model sources. A comparison of the experimental signals to the modeled signals shows good quantitative agreement in signal amplitude and frequency content. A comparison between the present modeling work and analytical theories demonstrates the substantial differences not considered in previous modeling work of acoustic emission sources. [ABSTRACT FROM AUTHOR]

This paper presents a scheme for adaptive mesh refinement on unstructured polygonal meshes to better capture crack patterns in dynamic cohesive fracture simulations. A randomly seeded polygonal mesh leads to an isotropic discretization of the problem domain, which does not bias crack patterns, but restricts the number of paths that a crack may travel at each node. An adaptive refinement scheme is proposed and investigated through a detailed set of geometric studies. The refinement scheme is selectively chosen to optimize the number of paths that a crack may travel, while still maintaining a conforming domain discretization. The details of the refinement scheme are outlined, along with the criterion used to determine the region of refinement and the method of interpolating nodal attributes. Extrinsic cohesive elements are inserted when and where necessary, and follow the constitutive response of the Park-Paulino-Roesler cohesive model. The influence of bulk and cohesive material heterogeneity is investigated through the use of a statistical distribution of material properties. The adaptive mesh modifications are handled through a compact topological data structure. Numerical examples highlight the features of adaptive refinement in capturing physical fracture patterns while addressing computational cost. Thus, the present approach is a step towards obtaining accurate dynamic fracture patterns and fields with polygonal elements. [ABSTRACT FROM AUTHOR]

Abstract: In this work, stress- and fracture mechanics-based criteria are developed to predict initiation and evolution, respectively, of intra- and inter-laminar cracking developed in composite laminates subjected to a relatively low energy impact (⩽15J) with consideration of nonlinear shear behaviour. The damage model was implemented in the finite element (FE) code (Abaqus/Explicit) through a user-defined material subroutine (VUMAT). Delamination (or inter-laminar cracking) was modelled using interface cohesive elements while splitting and transverse matrix cracks (intralaminar cracking) that appeared within individual plies were also simulated by inserting cohesive elements along the fibre direction (at a crack spacing determined from experiments for computing efficiency). A good agreement is obtained when the numerically predicted results are compared to both experimentally obtained curves of impact force and absorbed energy versus time and X-ray radiography damage images, provided the interface element stiffness is carefully selected. This gives confidence to selected fracture criteria and assists to identify material fracture parameters that influence damage resistance of modern composite material systems. [Copyright &y& Elsevier]

Abstract: In this paper, the impact damage of composite laminates in the form of intra- and inter-laminar cracking was modelled using stress-based criteria for damage initiation, and fracture mechanics techniques to capture its evolution. The nonlinear shear behaviour of the composite was described by the Soutis shear stress–strain semi-empirical formula. The finite element (FE) method was employed to simulate the behaviour of the composite under low velocity impact. Interface cohesive elements were inserted between plies with appropriate mixed-mode damage laws to model delamination. The damage model was implemented in the FE code (Abaqus/Explicit) by a user-defined material subroutine (VUMAT). Numerical results in general gave a good agreement when compared to experimentally obtained curves of impact force and absorbed energy versus time. The various damage mechanisms introduced during the impact event were observed by non-destructive technique (NDT) X-ray radiography and were successfully captured numerically by the proposed damage evolution model. [Copyright &y& Elsevier]

Abstract: Fibre reinforced polymer composites (FRPCs) are being increasingly used in structural applications where high specific strength and stiffness are required. The mechanical performance of FRPCs is affected by multi-mechanism damage evolution under loading which in turn is affected by microstructural randomness in the material. Although the micro-scale fracture of a FRPC is a stochastic process, most analyses of these materials have treated them in a deterministic way. In this paper the effect of stochasticity in FRPCs is investigated through the application of cohesive zone elements in which random properties are introduced. These may be termed ‘stochastic cohesive zone elements’ and are used in this paper to investigate the effect of microstructural randomness on the fracture behaviour of composite double cantilever beams. It is seen from this investigation that microstructure can significantly affect the macroscopic response of FRPC’s, emphasising the need to account for microstructural randomness for accurate prediction of the performance of laminated composite structures. [Copyright &y& Elsevier]

Abstract: By its nature, metal fatigue has random characteristics, leading to extensive scatter in the results. Both initiation and propagation of a fatigue crack can be seen as random processes. This manuscript develops a numerical analysis using cohesive zone elements allowing the use of one single model in the finite element simulation of the complete fatigue life. The present formulation includes a damage evolution mechanism that reflects gradual degradation of the cohesive strength under cyclic loading. The uncertainties inherent to the fatigue process are assumed to be caused by the variability of the material properties, which are modeled using random fields. An extrapolation scheme is proposed to reduce the computational time. First, the accuracy of the proposed formulation is assessed considering a deterministic crack growth problem. Second, the effect of randomness in the material properties on the total fatigue life of a structure is then analyzed. [Copyright &y& Elsevier]

Hybrid laminates made of polymer matrix composite plies with a metal sheet are called fiber metal laminates (FMLs). This study presents a new numerical approach for examining the fatigue damage progress in FMLs. A layer-wise finite element along with a cohesive element are used to predict the fatigue damage progress for splitting, transverse cracking and delamination. Four-node cohesive elements are introduced to express 0° ply splitting and transverse cracking. Eight-node cohesive elements are inserted into the ply interfaces to represent delamination. The most important character of this model is that the proposed simulation introduces a damage-mechanics concept into the degradation process in cohesive elements in order to express the damage progress due to cyclic loading. This enables us to address the complicated fatigue damage process observed in a FML. We applied this model to titanium/glass fiber-reinforced plastic (Ti/GFRP) laminates and compared the simulated results with the experiment data reported in the references. We confirmed that this model can reproduce the fatigue damage process in a FML. The effect of the parameters of the cohesive element on Ti crack growth and the delamination profile were also investigated. The Ti crack-growth rate was found to be strongly associated with the delamination profile near the crack tip. [ABSTRACT FROM AUTHOR]

Laminated carbon fibre-reinforced polymer (CFRP) composites are already well established in structural applications where high specific strength and stiffness are required. Damage in these laminates is usually localised and may involve numerous mechanisms, such as matrix cracking, laminate delamination, fibre de-bonding or fibre breakage. Microstructures in CFRPs are non-uniform and irregular, resulting in an element of randomness in the localised damage. This may in turn affect the global properties and failure parameters of components made of CFRPs. This raises the question of whether the inherent stochasticity of localised damage is of significance in terms of the global properties and design methods for such materials. This paper presents a numerical modelling based analysis of the effect of material randomness on delamination damage in CFRP materials by the implementation of a stochastic cohesive-zone model (CZM) within the framework of the finite-element (FE) method. The initiation and propagation of delamination in a unidirectional CFRP double-cantilever beam (DCB) specimen loaded under mode-I was analyzed, accounting for the inherent microstructural stochasticity exhibited by such laminates via the stochastic CZM. Various statistical realizations for a half-scatter of 50 % of fracture energy were performed, with a probability distribution based on Weibull's two-parameter probability density function. The damaged area and the crack lengths in laminates were analyzed, and the results showed higher values of those parameters for random realizations compared to the uniform case for the same levels of applied displacement. This indicates that deterministic analysis of composites using average properties may be non-conservative and a method based on probability may be more appropriate. [ABSTRACT FROM AUTHOR]

The time-dependent fracture of fluid-infiltrated porous materials is known to be affected by fluid diffusion, which is strongly influenced by the initial pore pressure field attributed to loading and the permeability conditions on the boundaries. In this research, the concurrent solid deformation and fluid migration for a traction-free crack of finite length 2 a in a poroelastic medium are analyzed. The remote boundaries of the medium are subjected to constant stress or constant stain in the direction perpendicular to crack faces. Without confining the range of the fluid diffusion to the neighborhood of the crack, four types of permeability conditions are considered on the boundaries: (I) the remote boundaries and the crack faces are both impermeable, (II) the remote boundaries are permeable to constant pore pressure, but the crack faces are impermeable, (III) the remote boundaries are impermeable, but the crack faces are permeable to constant pore pressure, and (IV) the remote boundaries and the crack faces are both permeable to constant pore pressure. The singularities of the stress and the pore pressure gradient at small flow times are obtained from finite element simulations and compared to asymptotic predictions. The instantaneous fracture energy J is evaluated using a method that uses the J-integral around the cohesive zone embedded ahead of the crack tip in the simulations. The numerical results of J are validated with the asymptotes predicted from stress intensities at small flow times. When there is a difference between the imposed pore pressure and the initial pore pressure caused by applying loading on the boundaries, the variation in J (t) is affected by short and long ranges of fluid flow, and J (t) reaches its drained limit at a time on the order of W2/ c , where W is the dimensional size and c is the consolidation coefficient of porous materials. The drained limit may not be the maximum value of J , especially when the difference on the remote boundaries is positive under strain control. For stress-control cases, whether the maximum J occurs before the drained state depends not only on the difference in pore pressure but also on the location of the permeable boundaries. [ABSTRACT FROM AUTHOR]

Many high-strength composite materials have been developed for aircraft structures. GLAss fiber REinforced aluminum (GLARE) is one of the high-performance composites. The review of articles, however, yielded no study on the impact damage of heated GLARE laminates. This study, therefore, aimed at developing a numerical model that can delineate the continuum damage of GLARE 5A-3/2-0.3 laminates at elevated temperatures. In the first stage, the inter-laminar interface failure of heated GLARE laminate had been investigated at room temperature and 80 °C. The numerical analysis employed a three-dimensional GLARE 5A-3/2-0.3 model that accommodated volumetric cohesive interfaces between mating material layers. Lagrangian smoothed particles populated the projectile. The model considered the degradation of tensile and shear modulus of glass fiber reinforced epoxy (GF/EP) at 80 °C, while incorporated temperature-dependent critical strain energy release rate of cohesive interfaces. When coupled with the material particulars, an 82 m/s bird impact at room temperature exhibited delamination first in the GF/EP 90°/0° interface farthest from the impacted side. Keeping the impact velocity, interface failure propagated at a slower rate at 80 °C than that at room temperature, which was in agreement with the impact damage determined in the experiments. The outcomes of this study will help optimize a GLARE laminate based on the anti-icing temperature of aircraft. [ABSTRACT FROM AUTHOR]