C.S. Lopes

Structural composites for multifunctional applications: Current challenges and future trends

Abstract This review paper summarizes the current state-of-art and challenges for the future developments of fiber-reinforced composites for structural applications with multifunctional capabilities. After a brief analysis of the reasons of the successful incorporation of fiber-reinforced composites in many different industrial sectors, the review analyzes three critical factors that will define the future of composites. The first one is the application of novel fiber-deposition and preforming techniques together with innovative liquid moulding strategies. The second is the combination of these techniques by optimization tools based on novel multiscale modeling approaches, so fiber-reinforced composites with optimized properties can be designed and manufactured for each application. In addition, the third is the enhancement of composite applications by the incorporation of multifunctional capabilities. Among them, electrical conductivity, energy storage (structural supercapacitors and batteries) and energy harvesting (piezoelectric and solar energy) seem to be the most promising ones.

Strength and toughness of structural fibres for composite material reinforcement

The characterization of the strength and fracture toughness of three common structural fibres, E-glass, AS4 carbon and Kevlar KM2, is presented in this work. The notched specimens were prepared by means of selective carving of individual fibres by means of the focused ion beam. A straight-fronted edge notch was introduced in a plane perpendicular to the fibre axis, with the relative notch depth being a0/D≈0.1 and the notch radius at the tip approximately 50u2009nm. The selection of the appropriate beam current during milling operations was performed to avoid to as much as possible any microstructural changes owing to ion impingement. Both notched and un-notched fibres were submitted to uniaxial tensile tests up to failure. The strength of the un-notched fibres was characterized in terms of the Weibull statistics, whereas the residual strength of the notched fibres was used to determine their apparent toughness. To this end, the stress intensity factor of a fronted edge crack was computed by means of the finite-element method for different crack lengths. The experimental results agreed with those reported in the literature for polyacrylonitrile-based carbon fibres obtained by using similar techniques. After mechanical testing, the fracture surface of the fibres was analysed to ascertain the failure mechanisms. It was found that AS4 carbon and E-glass fibres presented the lower toughness with fracture surfaces perpendicular to the fibre axis, emanating from the notch tip. The fractured region of Kevlar KM2 fibres extended along the fibre and showed large permanent deformation, which explains their higher degree of toughness when compared with carbon and glass fibres. This article is part of the themed issue ‘Multiscale modelling of the structural integrity of composite materials’.

A Multi Material Shell Model for the Mechanical Analysis of Triaxial Braided Composites

An efficient numerical methodology based on a multi material shell (MMS) approximation is proposed in this paper for the analysis of the mechanical behavior of triaxial braided composites subjected to tensile loads. The model is based on a geometrical description of the textile architecture of the material at the Gauss point level of a standard shell including the corresponding yarn geometrical parameters. The mechanical properties at the yarn level were determined from values reported in the literature or by means of micromechanical homogenization of unidirectional fiber reinforced composites. Simulations were carried out on single representative unit cell subjected to periodic boundary conditions and on multiple cell representative volume elements corresponding to the size of the standard width of a tensile specimen. The numerical results were compared with the stress-strain curves obtained experimentally as well as the damage mechanisms progression during deformation captured using radiographs performed on interrupted tests.

Computational micromechanics strategies for the analysis of failure in unidirectional composites

This chapter summarises the current state of the art on the application of computational micromechanics to study the mechanical behaviour of unidirectional plies. We outline the simulation strategies to determine the strength and toughness of unidirectional plies together with the experimental techniques to determine the properties of the constituents (matrix, fibres and interfaces). Several examples in unidirectional plies under matrix-dominated failure conditions are presented and the future developments in computational micromechanics of composites are briefly summarised.

Surrogate models of the influence of the microstructure on the mechanical properties of closed- and open-cell foams

The mechanical behavior of closed- and open-cell foams was analyzed using a computational homogenization strategy based on the finite element simulation of a representative volume element of the foam microstructure. The representation of the foam took into account the main microstructural features, namely density, fraction of solid material in cell walls and struts, cell anisotropy, cell size and distribution and strut shape. A parametric study was carried out to ascertain the influence of these parameters on the elastic modulus and the plateau stress under compression. It was found that these properties mainly depended on the density, fraction of solid material in cell walls and struts and cell anisotropy. Building upon the results of the parametric study, surrogate models were developed to relate the influence of the microstructure on the mechanical properties of the foams. It is believed that these models can be used for the design of foams with optimized properties for particular applications.

Microscale Characterization Techniques of Fibre-Reinforced Polymers

Polymer matrices reinforced with structural fibres as carbon, glass or aramid (fibre-reinforced polymers or FRPs) possess excellent specific mechanical properties as strength and stiffness. As a result, structural composites are commonly used in applications driven by weight reduction in aerospace, although they are continuously expanding to other industrial sectors, such as automotive, energy, sports or civil engineering. Excellent examples of carbon composite applications in aerospace are found in the last two civil airplanes developed by Airbus and Boeing, the A350 and 787 Dreamliner, respectively, in which composites made up to 50 % in weight of structural parts ranging from fuselage barrels or wings to stabilizers. However, despite the increasing number of engineering applications of structural composites, the accurate prediction of their mechanical behaviour still remains an arduous task because of the complexity of the failure mechanisms involved, specially at the microscopic level.

Virtual testing of impact in fiber reinforced laminates

This chapter proposes a systematic simulation strategy to determine the mechanical behaviour of composite laminates under impact loading using a computational mesomechanics approach. The methodology includes modelization of the physical mechanisms of damage observed in laminates: intralaminar (ply failure) and interlaminar (delamination failure). The methodology was applied to analyse the impact behaviour of carbon–epoxy laminates subjected to low- and high-velocity impacts, and the results were compared with experimental data in terms of energy absorption capacity and failure mechanisms.

Development of a Mesoscale Finite Element Constitutive Model for Fiber Kinking

A mesoscale finite element material model is proposed to analyze structures that fail by the fiber kinking damage mode. To evaluate the assumptions of the mesoscale model, the results were compared with those of a high-fidelity micromechanical model. A direct comparison between the two models shows remarkable correlation, indicating that the key features of the fiber kinking phenomenon are appropriately accounted for in the mesoscale model. The mesoscale model is applied to structural analysis cases to demonstrate the capabilities of the model. A verification study is conducted with an unnotched compression specimen and preliminary validation is demonstrated with a notched compression specimen. The results show that the model is successful at representing the kinematics of fiber kinking while at the same time highlighting the need for further verification and validation.

Interlaminar and Intralaminar Fracture Behavior of Carbon Fiber Reinforced Polymer Composites

Interlaminar and intralaminar fracture behavior of carbon fiber reinforced composites have been experimentally studied. Unidirectional, woven reinforcement and thermoplastic and thermoset polymer matrix laminates have been characterized using double cantilever beam (DCB) and end notch flexure (ENF) specimens for Mode-I and Mode-II fracture toughness, respectively and compact tension (CT) specimens for intralaminar fracture. AS4/PEEK, AS4/8552 and AGP193PW/8552 laminates have been characterized in this study. The fracture toughness determined from the experimental data could be related to the constituents and reinforcements. It has been observed between the two UD laminates, AS4/PEEK exhibit higher fracture resistance under both interlaminar and intralaminar fracture. Woven reinforcement is found to show higher mode-II interlaminar fracture toughness.

8.12 Multiscale FE Modelling and Design of Composite Laminates Under Impact

Virtual testing is emerging as a powerful tool for the analysis and design of composite materials laminates subjected to a variety of loading conditions, static, dynamic, and impact events. The representative physical deformation and damage mechanisms, as intraply failure and interply delamination, were taken into account explicitly in the finite element discretization by means of a continuum damage mechanic model based on the LaRC04 failure criterion and a cohesive zone approach, respectively. The models were used to evaluate the residual velocity in monolithic composite and fiber metal laminates panels subjected to impact loading of metallic projectiles. Finally, future trends in relation with the use of the numerical tools for the design of impact shields made of multimaterial laminates are showed and discussed.