Mechanical Behavior Of Composite Structures Research Articles

A refined model for functionally graded graphene nanoplatelets reinforced composite beams under thermal loads

Graphene is highly regarded as a promising material for various engineering purposes due to its remarkable physical characteristics. However, existing higher-order shear deformation theories may struggle to accurately capture the interlaminar mechanical response of functionally graded graphene nanoplatelets (FG-GNP) reinforced composite beams subjected to thermal loads. This arises from the pronounced discrepancies in material properties and thermal expansion coefficients among the various layers, making the interlaminar mechanical behavior of composite structures exceedingly complex under thermal loading. As a result, a meticulous analysis of interlaminar stresses becomes necessary for the structural assessment of GNP-reinforced composite beams under temperature variations. To overcome this limitation, we propose an advanced model incorporating transverse normal thermal deformation, tailored specifically for FG-GNP reinforced composite beams subjected to thermal loads. The developed model primarily boasts two advantages. Firstly, although transverse normal deformation is introduced into the displacement field, no additional displacement variables are increased, thereby maintaining model simplicity and efficiency. Secondly, the model achieves a high-precision interlaminar shear stress field taking into account the interlaminar continuity conditions and temperature effects. Furthermore, the elimination of second-order derivatives of in-plane displacement parameters from the transverse shear stress components leads to a simplification in the finite element implementation. Based on the proposed model, a simple three-node beam element is constructed. The 3D elasticity solutions and the results from alternative modeling frameworks are used to assess the performance of the developed model. Additionally, a comprehensive investigation is conducted to explore the effects of various parameters on the deformations and stresses of GNP-reinforced composite beams.

A unified phase-field method-based framework for modeling quasi-brittle fracture in composites with interfacial debonding

Interfacial debonding affects the mechanical behaviors of composite structures. In the present work, we developed a new phase-field-based cohesive zone model for modeling debonding at interfaces. The traction-separation law and evolution of phase-field considering the mixed-mode scheme of fracture toughness are given in a variational form. Besides, the present interfacial model can work with the phase-field cohesive zone model for the bulk region, in which a common phase-field value ϕ is shared for both regions. The interaction between the bulk region and interfacial cracking in both displacement, as well as phase-field, are taken into account directly to avoid the over-estimation of the free surface energy. The framework is implemented with the finite element method and validated with several numerical examples. The present work provides a unified approach for modeling quasi-brittle fracture in the bulk region and interfaces and shows its advantage in describing interactions between bulk and interfacial cracking.

Assessment of a unique reinforcement construction on mechanical behaviour of composite structures

The construction type of reinforcement material is a critical factor playing an essential role in the mechanical performance of composite materials. Recently, much attention has been attracted to discovering the mechanical behaviour of glass woven fabric, such as plain and different variations of twill and satin reinforced composites. In this paper, it is aimed to test the effect of a combination of unique characteristics of woven and knitted fabrics in the same matrix on the mechanical behaviour of composites. In this direction, carbon tows are fabricated into plain woven, 2/2 twill woven and interlock knitted fabrics. The effect of reinforcement construction on bending and tensile properties like strength, modulus and strain of epoxy composites manufactured in three variations as woven, knitted and woven/knitted reinforced was investigated by conducting 3-point bending and tensile tests. Experimental results indicated that hybrid composites displayed lower bending strength than woven fabric composites. However, a combination of 2/2 twill woven and knitted fabric in hybrid composites, including five fabric layers, had a higher bending modulus than 2/2 twill woven composites, including seven fabric layers. Also, it was observed that the mechanical performance of hybrid composites under tensile load was poor compared to the woven fabric composites.

Dispersion of Elastic Waves in Functionally Graded CNTs-Reinforced Composite Beams

This work deals with the wave propagation analysis in functionally graded carbon nanotubes (CNTs)-reinforced composite beams lying on an elastic medium. Despite the large amount of experimental and theoretical studies in the literature on the mechanical behavior of composite structures strengthened with CNTs, limited attention has been paid to the effect of an axial graduation of the reinforcing phase on the mechanical response of CNTs-reinforced composite beams. In this paper, CNT fibers are graded across the beam length, according to a power-law function, which expresses a general variation from a linear to parabolic pattern. An Euler-Bernoulli beam theory is considered herein to model the CNTs-reinforced composite structure resting on a Winkler–Pasternak foundation, whose governing equations are derived from the Hamiltonian principle. The theoretical solution of the problem checks for the sensitivity of the mechanical response to different parameters, i.e., the wave number, power index, Winkler and Pasternak coefficients, that could serve for further computational/experimental studies on the same problem, even from a design standpoint.

Effect of nanoparticles on the improving mechanical behavior of GFRP composites in a corrosive environment

Corrosive environment can significantly effect on the mechanical behavior of composite structures which can improve by using hydrophobic nanoparticles. The aim of this study is to investigate the improvement of the mechanical properties of incorporated glass fiber reinforced polymer (GFRP) composite specimens with 3 wt. % of nanoclay and nanosilica in a corrosive environment. Filled GFRP composites were fabricated using the mechanical stirrer following by sonication method, and the hand layup method. After preparing the neat and incorporated GFRP according to ASTM standards, all the samples were immersed in 5% sulfuric acid solution for 0, 1, and 3 months. As the immersion time increased, the samples containing nanosilica absorbed more water than the other samples. The tensile and compressive tests were performed at different immersion times to obtain the ultimate tensile and compressive strength and tensile modulus. The results showed that by adding nanoparticles, the mechanical properties were increased, which GFRP containing nanoclay showed a better behavior in the corrosive environment. By adding 3 wt. % of nanoclay, the ultimate tensile and compressive strength and the tensile modulus decreased after one month of immersion by only 0.34%, 1.81% and 2.95%, respectively, and after three months of immersion only decreased by 0.43%, 10.88% and 6.95%, respectively. Finally, SEM images of all specimens were examined to investigate the fracture mechanisms and the corrosion behavior of fabricated nanocomposites.

Higher-gradient and micro-inertia contributions on the mechanical response of composite beam structures

In the current work, we study the role of higher-order and micro-inertia contributions on the mechanical behavior of composite structures. To that scope, we compute the complete set of the effective static and dynamic properties of composite beam structures using a higher-order dynamic homogenization method which incorporates micro-inertia effects. We consider different inner composite element designs, with material constituents that are of relevance for current engineering practice. Thereupon, we compute the effective static longitudinal higher-gradient response, quantifying the relative difference with respect to the commonly employed, Cauchy-mechanics formulation. We observe that within the static analysis range, higher-order effects require high internal length values and highly non-linear strain profile distributions for non-negligible higher-order effects to appear. We subsequently analyze the longitudinal, higher-gradient eigenfrequency properties of composite structural members, accounting for the role of micro-inertia contributions. Thereupon, we derive analytical expressions that relate the composite material's effective constitutive parameters with its macroscale vibration characteristics. We provide for the first-time evidence that micro-inertia contributions can counteract the effect of second-gradient properties on the eigenfrequencies of the structure, with their relative significance to depend on the mode of interest. What is more, we show that the internal length plays a crucial role in the significance of micro-inertia contributions, with their effect to be substantial for low, rather than for high internal length values, thus for a wide range of materials used in engineering practice.

Mechanical behaviour of composite structures made of topologically interlocking concrete bricks with soft interfaces

In this study, the mechanical responses of an assembly plate made of topologically interlocking concrete bricks with rubber as soft interfaces are investigated. The principle of topological interlocking is combined with the concept of hybrid material design. A series of quasi-static tests are conducted to compare the mechanical behaviour of the hybrid interlocking assembly plate with those obtained from the monolithic plate and the interlocking assembly plate without soft interfaces. The results show that the hybrid assembly plate has a remarkable flexural compliance with less damage in the bricks compared to the other two. The effect of lateral confining load on the structural behaviour of the hybrid interlocking assembly plate is also investigated. In addition, a 3D numerical model is established to predict the overall behaviour, failure mode and the damage distributions of the hybrid assembly plate. Then the hybrid assembly plate under various lateral confining loads and with different friction factors between rubber and bricks is studied numerically. It is found that, although the maximum transverse force is influenced by the initial confining load and the friction factor, the mechanical behaviour of the hybrid structure is consistent in large deformation.

Prediction of the mechanical behavior of fiber-reinforced composite structure considering its shear angle distribution generated during thermo-compression molding process

In this study, a combined forming-structural analysis of composite structures was performed. Fiber orientation and shear angle changes of the composite materials during its thermo-compression molding processes were predicted with commercial software PAM-FORM. In addition, the material properties, such as elastic modulus and strength, from the shearing of woven fabrics, were measured. The predicted shear angle changes were transformed and reflected to the structural analysis by a self-developed vector-mapping program. The load-displacement curves of the composite structures from the combined forming and structural simulation were compared with the experimental compression and bending test results of hemispherical and U-shaped components, respectively. Finally, it was found that the mechanical behaviors of composite structures can be accurately predicted by the developed combined thermo-forming structural analysis method.

Improving Composite Tensile Properties during Resin Infusion Based on a Computer Vision Flow-Control Approach.

Liquid composite manufacturing techniques, mainly applied in the transport industry, have been studied and optimized for decades while defect analysis and its minimization have been a goal to increase reliability and mechanical performance. Researchers have found that many process parameters have a strong influence on the mechanical behavior of composite structures where the flow front velocity, closely related to voids, plays a considerable role. In this work, the optimal flow front velocity was evaluated and controlled using a computer vision system for different laminates improving the mechanical tensile properties and void content. Enhanced mechanical tensile properties were found using a feedback flow-controller vision system which was able to keep the optimal flow front velocity constant to reduce the air traps among tows and fibers. Tensile strength was enhanced up to 18% for fiber orientation at 0° and 3.3% at 90°, whereas tensile modulus was increased up to 18.4% for fibers at 0° and 8.7% at 90°. A novel methodology is presented through this work, aiming to improve the robustness of resin film infusion (RFI) processes while ensuring the quality of the composite material.

Mechanical behavior of composite structures subjected to constant slamming impact velocity: An experimental and numerical investigation

The interaction between deformable structures and free water surfaces can modify the fluid flow and change the estimated hydrodynamic loads in relation to rigid bodies, due to the appearance of hydroelastic effects. The flexibility and damage failure modes in composite materials introduce additional complexity for predicting hydrodynamic loads when interactive with water. This is considered to be a key challenge when using these materials in marine applications. Therefore, particular attention should be paid to this fact in the design phase and over their period of use. The aim of this work is to study the structural behavior and the effect of the flexibility of composite panels on hydrodynamic loads and the dynamic deformation response experimentally and numerically. To study these effects, composite panels with two different rigidities were subjected to various impact velocities and investigated. It should be noted that all the panels tested at a10° deadrise angle. A high velocity shock machine was used to maintain constant velocity during water entry at impact velocities of 4 m/s, 6 m/s, 8 m/s and 10 m/s. The general analysis of experimental results indicated that compared to the higher stiffness panels, the more flexible panel has a higher peak force as velocity increases. This has been attributed to the change in local velocity and local deadrise angle along the water-panel interface. The numerical model was implemented based on the Coupled Eulerian–Lagrangian Model (CEL) built-in Abaqus/Explicit finite element software. The numerical results showed a good agreement compared with experimental data for both the hydrodynamic force and the deformation response. These quantitative structural-loading data can provide a clear guide for maritime ship design.

Fire Condition Effects on the Mechanical Behaviour of Composite Structures

The fire behaviour of Polymeric composite structures is one of the most critical aerospace research topics. Indeed, the exposure of Polymeric composite structures to high temperatures leads to material decomposition, associated to thermal and mechanical properties degradation. This degradation causes a reduction of the mechanical performances, which can be of main concern for safety reasons. In this paper, the tensile behaviour of Carbon Fibre Composite Polymer specimens, subjected to fire, has been experimentally and numerically investigated. The material properties degradation has been estimated according to an Arrhenius shape function, which relates the mechanical properties of the composite to the temperature. At first, static structural analyses have been carried out to assess the mechanical behaviour of the investigated specimen without fire effects. Then, a coupled thermo-structural analysis allowed evaluating the fire effect on the specimens’ mechanical and the thermal behaviour. In order to preliminary validate the proposed degradation model, the numerical results, in terms of Load versus Displacements curves, have been compared against data obtained from an ad-hoc experimental campaign where fire condition have been suitably replicated during the mechanical tests.

Effect of Embedded Strain Gage on the Mechanical Behavior of Composite Structures

Fiber reinforced composites are increasingly used in several fields such as aeronautics and civil engineering due to their increased strength, durability, corrosion resistance, resistance to fatigue and damage tolerance characteristics. The embedding of sensor networks into such composite structures can be achieved. In the present study, glass fiber reinforced Epoxy composite with integrated strain gage was analysed. Firstly, the mechanical behaviour of this material with embedded strain gage is investigated. The as-prepared samples have been tested under tensile and flexural loading in order to study the effects of the strain gage embedding on the structural stiffness and strength of the composite. It was found that the tensile stiffness decreases by 5.8% and the tensile strength decrease by 1.5% when the strain gage embedded in the material. On the other hand, the flexural strength and stiffness is increased, respectively, by 1.5% and 5.5% with an embedded strain gage. The experiments showed that embedded strain gage is functional and demonstrated the successful integration of sensor networks into composite parts. The obtained results confirm that integrated strain gage can be used for the Structural Health Monitoring (SHM) of glass fiber reinforced Epoxy composite.

Considerations in Low-Temperature Mechanical Behavior of Polymer Composite Materials

In this paper it has been sought to highlight the mechanical behavior of composite structures at low temperatures compared to mechanical behavior at room temperature. For researches an experimental program has been conceived and built using factorial method. In this method, as dependent variable was taken the tensile strength (σr), while as independent variables were taken: the fiber orientation angles (θ), the degree of reinforcement (Mf) of the composite structure and the temperature (t) at which the tests were carried out (+25°C,-25°C and-50°C respectively). It has been used a complex experimental installation, specific to tests at low temperatures.

Numerical Simulation of Mechanical Behavior of Composite Structures by Supercomputing Technology

This paper will examine the possibilities of the virtual tests of composite structures by simulating mechanical behaviors by using supercomputing technologies, which have now become easily available and powerful but relatively inexpensive. We will describe mainly the applications of large scale finite element analysis using the direct numerical simulation (DNS), which describes composite material properties considering individual constituent properties. DNS approach is based on the full microscopic concepts, which can provide detailed information about the local interaction between the constituents and micro-failure mechanisms by separate modeling of each constituent. Various composite materials such as metal matrix composites (MMCs), active fiber composites (AFCs), boron/epoxy cross-ply laminates and 3-D orthogonal woven composites are selected as verification examples of DNS. The effective elastic moduli and impact structural characteristics of the composites are determined using the DNS models. These DNS models can also give the global and local information about deformations and influences of high local in-plane and interlaminar stresses induced by transverse impact loading at a microscopic level inside the materials. Furthermore, the multi-scale models based on DNS concepts considering microscopic and macroscopic structures simultaneously are also developed and a numerical low-velocity impact simulation is performed using these multi-scale DNS models. Through these various applications of DNS models, it can be shown that the DNS approach can provide insights of various structural behaviors of composite structures.

Feasibility of integrated optical fibre sensors for condition monitoring of composite structures. Part II: Combination of Bragg-sensors and acoustic emission detection

This paper discusses the possibility of continuously monitoring the mechanical behaviour of composite structures by means of optical fibre sensors. In the first paper* some basic principles of optical fibre sensors in general and Bragg-sensors in particular have been briefly reported. Experimental results showed that optical fibre Bragg-sensors are much more reliable compared to traditionally used electrical-resistance strain gauges. In the introduction to this second paper, the discussion is extended to the broader context of condition monitoring. Filament wound pressure vessels have again been used as a representative composite structure. Instrumentation of the vessel consisted of an embedded Bragg-sensor, an acoustic emission detector and a pressure transducer. The results of a burst pressure experiment are discussed in this second paper. In this 'fully monitored' experiment, Bragg-sensors are used for strain monitoring and damage is monitored through acoustic emission detection. With additional research effort, and provided that adequate tools for the simulation of a degrading fibre-reinforced structure become available, permanent condition monitoring of critical structures can become realistic in the near future. The Bragg-sensor type can be considered a key instrument for monitoring applications of composite structures, provided that they become available at economically interesting prices.

Feasibility of integrated optical fibre sensors for condition monitoring of composite structures. Part I: Comparison of Bragg-sensors and strain gauges

This first paper discusses the possibility of continuously monitoring the mechanical behaviour of composite structures by means of optical fibre sensors. In the introduction, some inherent advantages of optical fibre sensors, which make them ideally suited for this purpose, are discussed. Optical fibres with an intracore Bragg-grating were chosen as an alternative for classical electrical-resistance strain gauges. The main principles of Bragg-sensors are briefly discussed and some previously obtained results are summarized. Filament-wound pressure vessels have been used as a representative composite structural element. These vessels have been equipped with surface-mounted Bragg-sensors and classical strain gauges. Comparative measurements between these two sensor types are discussed in this first of two papers. The results of these experiments lead to the conclusion that optical fibre Bragg-sensors are more reliable than classical strain gauges. In the second paper 'fully monitored' experiments will be discussed: strain is monitored by means of Bragg-sensors and damage is monitored through acoustic emission detection. With additional research effort, and provided that adequate tools for the simulation of a degrading fibre-reinforced structure become available, permanent condition monitoring of critical structures could become realistic in the near future. The Bragg-sensor type optical fibre sensor can be considered a key instrument for monitoring applications of composite structures providing that they become available at economically interesting prices.

柔軟性界面相を有する一方向繊維強化複合材料の数値モデル化手法

The interphase has an important role, that is, conducts the stress transmission between fiber and resin. Authors introduced the concept of flexible interphase and established the fabrication method of prepregs. In this study, a new numerical model considering the fiber/resin interphase was proposed, in order to design the flexible interphase. For the mechanical behaviors of composite structures were predicted by the finite element method, the consideration of the heterogeneity is an important key. Authors have established a numerical modeling method, namely, a quasi-threedimensional model which can consider the heterogeneity. Based on the quasi-three-dimensional modeling concept, a new numerical model which can independently consider the fiber/matrix interphase was proposed. The mechanical behaviors of unidirectional CFRP laminates with various flexural interphase thickness under the three-point bending load were simulated. By comparing the prediction with experimental results, the validity of the proposed model was examined. It was concluded that the model was effective for the fiber/resin interphase designing in the CFRP composite structures.

Basic material data and structural analysis of fibre composite components for space application

Fibre composites are widely used for space applications such as antennas, solar arrays and support structures for cryogenic tanks. For calculation of the mechanical behaviour of composite structures, basic material data are presented. The main mechanical properties of importance are thermal conductivity, thermal expansion, stiffness and strength. For the unidirectional data, special fibre orientations and lay ups are used to optimize the laminates, taking special requirements into account. In this paper the optimized designs of some composite parts for different space applications are shown, with special reference to aspects of the structural analysis and the tests performed to check the calculations.