Effective Simulation of Delamination in Aeronautical Structures Using Shells and Cohesive Elements

A cohesive element for shell analysis is presented. The element can be used to simulate the initiation and growth of delaminations between stacked, non-coincident layers of shell elements. The procedure to construct the element accounts for the thickness offset by applying the kinematic relations of shell deformation to transform the stiffness and internal force of a zero-thickness cohesive element such that interfacial continuity between the layers is enforced. The procedure is demonstrated by simulating the response and failure of the Mixed Mode Bending test and a skin-stiffener debond specimen. In addition, it is shown that stacks of shell elements can be used to create effective models to predict the inplane and delamination failure modes of thick components. The results indicate that simple shell models can retain many of the necessary predictive attributes of much more complex 3D models while providing the computational efficiency that is necessary for design

Cohesive Elements for Shells

A cohesive element for shell analysis is presented. The element can be used to simulate the initiation and growth of delaminations between stacked, non-coincident layers of shell elements. The procedure to construct the element accounts for the thickness offset by applying the kinematic relations of shell deformation to transform the stiffness and internal force of a zero-thickness cohesive element such that interfacial continuity between the layers is enforced. The procedure is demonstrated by simulating the response and failure of the Mixed Mode Bending test and a skin-stiffener debond specimen. In addition, it is shown that stacks of shell elements can be used to create effective models to predict the inplane and delamination failure modes of thick components. The results indicate that simple shell models can retain many of the necessary predictive attributes of much more complex 3D models while providing the computational efficiency that is necessary for design

A Micromechanics-Based Damage Model for the Strength Prediction of Composite Laminates

A new damage model based on a micromechanical analysis of cracked [+/-0deg/90deg(sub n)]s laminates subjected to multiaxial loads is proposed. The model predicts the onset and accumulation of transverse matrix cracks in uniformly stressed laminates, the effect of matrix cracks on the stiffness of the laminate, as well as the ultimate failure of the laminate. The model also accounts for the effect of the ply thickness on the ply strength. Predictions relating the elastic properties of several laminates and multiaxial loads are presented

Simulation of Delamination Under High Cycle Fatigue in Composite Materials Using Cohesive Models

A new thermodynamically consistent damage model is proposed for the simulation of high-cycle fatigue crack growth. The basis for the formulation is an interfacial degradation law that links Fracture Mechanics and Damage Mechanics to relate the evolution of the damage variable, d, with the crack growth rate da/dN. The damage state is a function of the loading conditions (R and (Delta)G) as well as the experimentally-determined crack growth rates for the material. The formulation ensures that the experimental results can be reproduced by the analysis without the need of additional adjustment parameters

Simulation of Delamination Propagation in Composites Under High-Cycle Fatigue by Means of Cohesive-Zone Models

A damage model for the simulation of delamination propagation under high-cycle fatigue loading is proposed. The basis for the formulation is a cohesive law that links fracture and damage mechanics to establish the evolution of the damage variable in terms of the crack growth rate dA/dN. The damage state is obtained as a function of the loading conditions as well as the experimentally-determined coefficients of the Paris Law crack propagation rates for the material. It is shown that by using the constitutive fatigue damage model in a structural analysis, experimental results can be reproduced without the need of additional model-specific curve-fitting parameters

An invariant based transversely-isotropic constitutive model for unidirectional fibre reinforced composites considering the matrix viscous effects

Fibres Reinforced Polymers (FRPs) are found in several applications in aeronautics, space and in the automotive industry. These applications are exposed to loading conditions, including impact, which results in a complex mechanical response that is vital to accurately predict. This is particularly important for a new generation of thermoplastic-based composites. The model proposed in this work is an invariant-based approach to represent viscous effects in polymer composites. The model developed only requires two viscous parameters to calibrate the viscoelastic behaviour. A good correlation between the simulations and experimental data obtained in off- axis tests in tension and compression is obtained.Postprint (author's final draft

A Micromechanics-Based Damage Model for [+/- Theta/90n]s Composite Laminates

A new damage model based on a micromechanical analysis of cracked [+/- Theta/90n]s laminates subjected to multiaxial loads is proposed. The model predicts the onset and accumulation of transverse matrix cracks in uniformly stressed laminates, the effect of matrix cracks on the stiffness of the laminate, as well as the ultimate failure of the laminate. The model also accounts for the effect of the ply thickness on the ply strength. Predictions relating the elastic properties of several laminates and multiaxial loads are presented

A Thermodynamically Consistent Damage Model for Advanced Composites

A continuum damage model for the prediction of damage onset and structural collapse of structures manufactured in fiber-reinforced plastic laminates is proposed. The principal damage mechanisms occurring in the longitudinal and transverse directions of a ply are represented by a damage tensor that is fixed in space. Crack closure under load reversal effects are taken into account using damage variables established as a function of the sign of the components of the stress tensor. Damage activation functions based on the LaRC04 failure criteria are used to predict the different damage mechanisms occurring at the ply level. The constitutive damage model is implemented in a finite element code. The objectivity of the numerical model is assured by regularizing the dissipated energy at a material point using Bazant's Crack Band Model. To verify the accuracy of the approach, analyses of coupon specimens were performed, and the numerical predictions were compared with experimental data

Effective simulation of the mechanics of longitudinal tensile failure of unidirectional polymer composites

An efficient computational model to simulate tensile failure of both hybrid and non-hybrid composite materials is proposed. This model is based on the spring element model, which is extended to a random 2D fibre packing. The proposed model is used to study the local stress fields around a broken fibre as well as the failure process in composite materials. The influence of fibre strength distributions and matrix properties on this process is also analysed. A detailed analysis of the fracture process and cluster development is performed and the results are compared with experimental results from the literature.Postprint (published version

Micro-mechanical analysis of composite materials using Phase-Field models of brittle fracture

Failure in fiber-reinforced composites is a complex phenomenon where different damage mechanisms interact and evolve through various scales. Micro-mechanical analysis using the finite element method has become an important alternative to study such failure phenomena and their interactions, by modeling explicitly the fiber, matrix, and fiber–matrix interface. In this work, the predictive capabilities of the finite element method together with the Phase-Field (PF) method for fracture has been assessed. The study compares different PF formulations, energy splits and numerical parameters, using Representative Volume Elements (RVEs) of different sizes, fiber distributions and with different Boundary Conditions (BCs). It is found that even though good approximations can be obtained and meso-scale failure envelopes for transverse loading generated, these are highly dependent on the modeling assumptions and PF parameters. The AT2 formulation combined with Amor’s energy split provides the best predictions when compared with an analytical failure surface. The best fit is found for transverse shear-dominated loading, while larger differences are found for compressive loading, whose strength predictions are strongly affected by the PF formulations and energy splits. It is demonstrated that meso-scale strength is conditioned by interface properties as interface damage is the dominant failure initiation mechanism under tensile-dominated loading. On the other hand, PF parameters have a stronger influence on compressive-dominated loading. Finally, it is shown that assuming a perfect fiber–matrix interface has a strong effect on the expected meso-scale strength, as failure is markedly delayed. Accordingly, based on the present results, especial care should be taken in properly assessing all the variables involved in the modeling methodology to draw conclusions from computational micro-mechanical analyses based on the PF approach.Postprint (published version