A. Turon

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 noncoincident 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 skinstiffener debond specimen. In addition, it is shown that stacks of shell elements can be used to create effective models to predict the in-plane 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 three-dimensional models while providing the computational efficiency that is necessary for design.

Analytical and Numerical Investigation of the Length of the Cohesive Zone in Delaminated Composite Materials

An accurate prediction of the length of the cohesive zone ahead of a crack tip is fundamental for the correct simulation of delamination in composite materials under both quasi-static and fatigue loading. To ensure a correct dissipation of energy during delamination propagation, several cohesive finite elements have to span the cohesive zone. The length of the cohesive zone depends on the material properties, the geometry/size of the structure, and on the loading mode. This chapter presents new expressions to estimate the length of the cohesive zone under general mixed-mode loading conditions and for finite-sized geometries. The analytical model is validated by comparing its predictions with numerical results based on cohesive-zone models. The relevance of the proposed analytical solutions to the effective simulation of delamination is demonstrated by simulating delamination growth under mixed-mode loading using meshes with the length of the elements greater than the cohesive zone length.

Interface elements for fatigue-driven delaminations in advanced composite materials

Abstract This chapter presents the formulation for an extension of quasi-static cohesive zone models to simulate fatigue-driven delaminations. The basis for the fatigue formulation is to link the evolution of the damage variable to the crack growth rate da/dN that can be measured in experiments. The model relates damage accumulation to the number of load cycles while taking into account the loading conditions (load ratio, energy release rate, and mode ratio). When used in a structural analysis, the model can simulate the dependence of the crack growth rate on these parameters. Moreover, the capability of the model to predict fatigue delamination over specimens without initial crack is presented. We also show that the model can be used to reproduce the curves Gmax–N used to predict the onset of fatigue delamination.

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.

Experimental Study of Time-dependent Behaviour of Concrete Members Reinforced with GFRP Bars

This study presents the results and discussion of an experimental tests program in which 8 concrete beams reinforced with glass fibre reinforced polymer (GFRP) were maintained under a constant load for a period of 150 days. Two different ratios of reinforcement and two different levels of sustained load were used. The beams were instrumented and monitored to analyze the time-dependent behaviour due to concrete creep and shrinkage. The measured deflections are compared with those calculated using methodologies available for steel reinforced concrete structures as CEB procedure, as well as with ACI 440.1R-06 and CSA-S806-02 for FRP reinforced concrete structures. The comparisons of the theoretical and experimental long-term deflections indicate that CEB procedure gives reasonable predictions in all 8 beams. However, some differences can be found when ACI 440.1R-06 or CSA-S806-02 procedures are applied.

Delamination propagation under cyclic loading

Publisher Summary The increased use of advanced composite laminates in primary structures of commercial aircraft requires a thorough understanding of the inelastic response of composites under general loading conditions. One of the most relevant mechanisms that contribute to the loss of stiffness and to the structural collapse of composite structures is delamination. Besides degrading the structural integrity of composites, delamination is also difficult to detect using traditional non-destructive inspection methods. The majority of the analytical and experimental investigations of delamination have been focused on the study of delamination growth under quasi-static loads. However, under cyclic loading, delamination might grow up to a critical size for loads well below the critical load for quasi-static loading conditions. Currently, the design methodologies barely consider the possibility of interlaminar crack growth under fatigue loading, being more oriented to prevent fatigue damage by assuring that a stress or strain threshold for delamination onset is not exceeded. This approach is evolving towards more powerful physically based analyses implemented in advanced computational models. This is a result of the industrial interest to partially replace the experimental tests required to certify new composite components (qualification of materials, design allowables, sub-components and full components) by virtual (numerical) tools able to simulate the inelastic response of the composite materials up to structural failure.

Mode I fatigue behaviour and fracture of adhesively-bonded fibre-reinforced polymer (FRP) composite joints for structural repairs

This chapter discusses the methodologies used to analyse fatigue-induced damage of bonded joints between composite components under mode I cyclic loads. It first reviews the existing types of bonded joints used in manufacturing or repair procedures. Then, the chapter presents some general considerations related to mode I testing of adhesive joints, such as the stick-slip effects or the influence of the failure modes on the results. A description is given of the experimental test used at a coupon level to obtain onset and crack propagation curves. Finally, the chapter includes a concise review of the current numerical techniques used to simulate the behaviour of a bonded joint.

8.8 Analysis of Delamination Damage in Composite Structures Using Cohesive Elements

This chapter describes the main topics related to the use of cohesive elements in the simulation of delamination in polymer composite materials. The kinematic and constitutive idealizations used in cohesive zone models, as well as the numerical implementation of cohesive zone models in the context of the finite element methods is discussed. Guidelines for the use of cohesive elements are presented; this includes both the identification of material properties and modeling aspects. Finally, simulations of structural components at different time scales using cohesive elements are presented.

Experimental Study on the Tension Stiffening Effect of GFRP RC Elements

Direct tension test experiments are accepted to be adequate to study the tension stiffening effect of fibre reinforced polymer reinforced concrete (FRP RC) members. In this paper, an experimental program on direct tension tests on GFRP RC is presented. Four different reinforcing ratios were considered; the ties were instrumented to analyze the member behaviour (load-strain relationship and crack width). Measured member deformation and crack widths are compared with those calculated using the procedures of available codes for steel reinforced concrete, like EC-2 and ACI 224, as well as with ACI 440 for FRP reinforced concrete structures.