Stiffness Of Composites Research Articles

Mechanical and thermal properties of PVC and polyurethane foam hybrid composites

Abstract Sandwich foam composites are mostly used today in structural applications in aerospace and automobile industries due to light weight and high stiffness of the sandwich composites. In this research work, the composites were fabricated by using polyurethane and PVC foam and carbon fiber as skin. Sandwich composites were fabricated by the hand lay-up method. Mechanical tests such as tensile, flexural, impact, double shear, inter delamination and hardness tests were conducted. A thermo gravimetric analysis (TGA) test was also performed for the analysis of the temperature characteristics of the sandwich composites. The internal structures of tested samples were analyzed through a scanning electron microscope (SEM). The results show that the tensile, flexural, impact, thermal and inter delamination strengths of PVC foam composites are superior to those of other composites.

Finite Element Free Vibration Analysis of Marine Composite Laminates with Variable Stiffness

Ouyang, X., 2020. Finite element free vibration analysis of marine composite laminates with variable stiffness. In: Yang, D.F. and Wang, H. (eds.), Recent Advances in Marine Geology and Environmental Oceanography. Journal of Coastal Research, Special Issue No. 108, pp. 270–273. Coconut Creek (Florida), ISSN 0749-0208.Reinforced variable stiffness composites are commonly used in marine engineering and marine engineering structures. The free vibration behavior of composite laminates varies with varying stiffness. Classical layering theory and first-order shear deformation theory are used to model the structure. On the basis of the virtual work principle and the finite element method, the vibration equation is established. The curved fiber can improve the design performance of composite laminates, and the free vibration behavior of composite laminates with variable stiffness can be changed by changing the orientation of the fiber.

Reuse of Natural Waste to Improve the Thermal Stability, Stiffness, and Toughness of Postconsumer Polypropylene Composites

Reuse of Natural Waste to Improve the Thermal Stability, Stiffness, and Toughness of Postconsumer Polypropylene Composites

Enhanced interfacial adhesion in glass fiber fabric/epoxy composites employing fiber surface treatment with aminosilane-functionalized graphene oxide

An economical and effective method was developed to optimize the interface of glass fiber fabric (GFf)-reinforced epoxy composites (GFfE) by dispersing aminosilane-functionalized graphene oxide (GO) on the fiber surface. The effects of γ-aminopropyltrimethoxysilane (APS) or APS hydrolysis on the dispersion of GO and the interfacial properties of resultant composites were investigated in detail. The results indicated that the uniform dispersion of GO in composites and strong fiber/matrix adhesion could be achieved, based on grafting of APS hydrolysis onto GO. The interlaminar shear, flexural and tensile strengths of resultant composites were improved by 28%, 22% and 19%, respectively; the storage modulus and dynamic glass transition temperature (1 Hz) were significantly enhanced, compared with pure GFfE. In particular, the work of fracture received from interlaminar load–deflection curves increased by 97%, indicating the toughening effect of GO. This work demonstrates that it is possible to enhance the strength, stiffness and toughness of fiber-reinforced composites by incorporating GO into the interface between the fiber and the matrix.

Structure and Performance of Benzoxazine Composites for Space Radiation Shielding.

Innovative multifunctional materials that combine structural functionality with other spacecraft subsystem functions have been identified as a key enabling technology for future deep space missions. In this work, we report the structure and performance of multifunctional polymer matrix composites developed for aerospace applications that require both structural functionality and space radiation shielding. Composites comprised of ultra-high molecular weight polyethylene (UHMWPE) fiber reinforcement and a hydrogen-rich polybenzoxazine matrix are prepared using a low-pressure vacuum bagging process. The polybenzoxazine matrix is derived from a novel benzoxazine resin that possesses a unique combination of attributes: high hydrogen concentration for shielding against galactic cosmic rays (GCR), low polymerization temperature to prevent damage to UHMWPE fibers during composite fabrication, long shelf-life, and low viscosity to improve flow during molding. Dynamic mechanical analysis (DMA) is used to study rheological and thermomechanical properties. Composite mechanical properties, obtained using several standardized tests, are reported. Improvement in composite stiffness, through the addition of carbon fiber skin layers, is investigated. Radiation shielding performance is evaluated using computer-based simulations. The composites demonstrate clear advantages over benchmark materials in terms of combined structural and radiation shielding performance.

The edge regions in tergites of the desert isopod Hemilepistus reaumuri: the transition from hard cuticle to flexible arthrodial membrane

The arthrodial membrane is a thin and flexible type of cuticle that inserts at the edge regions of neighbouring rigid skeletal elements creating a flexible connection. In the present study, we analyzed the structure, mineral composition, calcite organization and local stiffness and hardness of edge regions that form transitions to the arthrodial membranes in the tergites of the desert isopod Hemilepistus reaumuri. For the transitions to the arthrodial membrane, the results show an increase in the thickness of the epicuticle at cost of the distal exocuticle and a calcite layer, an increase in the ratio of phosphorus to calcium and a decrease in the local mechanical properties. The posterior edge region contains an unusually large stack of unidirectionally oriented parallel fibrils projecting to the lateral sides. At the edge, it turns down into a long ventral cuticle overlapping an anterior part of the neighbouring tergite. It forms a thin arched gap between the tergites that can help reducing water loss through the arthrodial membrane and protects the arthrodial membrane upon predation. A thick ventral ridge near the transition to the arthrodial membrane carrying bristles can prevent sand grains from access to the arthrodial membrane. From the dorsal cuticle to the transition to the arthrodial membrane, calcite units become larger and single crystalline turning their c-axes orientation perpendicular to the sagittal section plane. Comparison with edge regions of the beach isopod Tylos europaeus reveal common characteristics of the edge region, but also specific adaptations to the desert habitat of H. reaumuri.

Optimization of curvilinear fiber orientation of composite plates and its experimental validation

Variable stiffness composite (VSC) is a viable design extension of carbon fiber reinforced plastic (CFRP) laminates to obtain unique mechanical properties by changing the fiber orientation in a curvilinear manner. We optimized a VSC for open-hole tension laminates considering the automatic processing by the tow prepreg curve placement and evaluated the strength improvement with experiments. Multi-objective optimization using fracture criterion and mean curvature as objective functions resulted in higher strength gains for solutions with higher mean curvature and lower processability. Besides, by giving the molding conditions as constraints for optimization, the strength was improved while satisfying the molding conditions. The fabrication of optimum VSCs involved using a tabletop automated fiber placement. Strength tests showed a 34.3% improvement in the curvilinear prepreg path.

A general isogeometric polar approach for the optimisation of variable stiffness composites: Application to eigenvalue buckling problems

This study presents a general approach for the multi-scale design of variable stiffness composites (VSCs). The first-level problem of the multi-scale two-level optimisation strategy (MS2LOS) is solved to determine the optimal distribution of the VSC stiffness properties at the macroscopic scale satisfying the requirements of the problem at hand. In this phase, the VSC laminate is modelled as an equivalent homogeneous anisotropic plate whose behaviour is described in terms of polar parameters (PPs), which vary locally over the structure. The First-order Shear Deformation Theory is used to take into account the influence of the transverse shear stiffness on the mechanical response of the VSC and Basis Spline (B-Spline) surfaces are employed to represent the PPs fields. In this background, the expression of the gradient of the buckling factor is determined analytically by exploiting the properties of the polar formalism and of the B-Spline surfaces. Moreover, the effect of the discrete variables, involved in the definition of the B-Spline surfaces, on the performances of the optimised solution is investigated. The effectiveness of the approach is proven on two benchmark problems dealing with the maximisation of the first buckling load of a VSC laminate, subject to feasibility and geometric requirements, taken from the literature. The results obtained by means of the MS2LOS based on the polar formalism outperform those reported in the literature, which are obtained through an optimisation strategy based on lamination parameters.

Tribo-mechanical characterization and optimization of green flexible composites

The use of natural fibers as a substitute for synthetic fibers as reinforcement in polymer-based composites is finding prominence nowadays. The present paper deals with the comparative study of biodegradable flexible composites fabricated using jute fabric and rubber with different configurations – namely, jute/rubber/jute (JRJ), jute/rubber/rubber/jute (JRRJ) and jute/rubber/jute/rubber/jute (JRJRJ) – using their tribo-mechanical properties as the attributes and making use of a multi-attribute decision making (MADM) approach – namely, technique for order of preference by similarity to ideal solution (Topsis). The tensile strength, tear strength, interlaminar shear strength and specific wear rate of the proposed composites are chosen as the attributes for comparison using Topsis. The results show that JRJRJ is the best configuration among the considered configurations of flexible composites, providing the overall best tribo-mechanical properties, and the fractography study shows that the proposed flexible composites undergo failure mainly due to matrix tearing in the case of tensile and tear tests as opposed to matrix cracking in the case of stiff composites. Also, the wear study reveals that rubber exhibits better resistance to wear. This study shows the efficient application of the statistically based MADM tool Topsis for material selection.

Comparison on mechanical properties, damage evolution and aging effects of multi-delaminated composites under three point bending

The study on natural aging of fiber reinforced composite, delamination damage behavior and damage pattern recognition plays a significant role in structural health monitoring of the material. In this study, acoustic emission is used to investigate damage evolution process and damage mechanisms in unidirectional glass and carbon-glass orthogonal woven fiber reinforced composites containing symmetric multiple delaminations under three point bending testing. Based on the principle of control variables, four kinds of specimens are manufactured to investigate the comparative study for the effects of delaminations, reinforced materials and fiber orientation on buckling failure behaviors and the effects of aging on mechanical properties. The results indicate that the degradation of strength and stiffness of composite can be influenced by the existence of delamination defects and natural aging condition. Both negative and positive effects can be generated by natural aging condition. Moreover, reinforced materials, porosity and fiber orientation are important factors influencing the reliability of composite. Clustering results of unidirectional fiber reinforced composites indicate interphase failure is the main damage component, moreover, the existence of symmetrical multiple delaminations will aggravate the damage of composite and make instability failure characterized by intermittency.

Mechanical and electrical properties of Cu-Steel bimetallic porous composite with a double-helix entangled structure

Cu-steel bimetallic porous composite structures can be used as lightweight, damping, and electro-conductive elements in critical environments where traditional single materials become ineffective. In the present work, a novel Cu-steel bimetallic porous composite with a double-helix entangled structure was developed. The mechanical properties (static stiffness, loss factor, and tangent modulus) of the as-synthesized composite were characterized by compressive tests, and its electrical sensitivity to compressive force was examined by mechanical–electrical coupling tests. The effect of the copper/steel weight ratio on mechanical and electrical performances was analyzed in detail. It is found that the Cu-steel bimetallic porous composite with a relatively high weight ratio has a larger average loss factor and lower stiffness than that with a low weight ratio. The structural characteristics and complex microstructural changes of the deformed specimen allow the tangent modulus and stiffness to exhibit a non-proportional relationship with the weight ratio over a range of displacement. The resistance-force curves exhibit an obvious nonlinearity and degradation due to the contact between internal bimetallic wires. The resistance-stiffness history reveals that the mitigation of electrical conductivity continued with the enlargement of stiffness and weight ratio.

Reinforcement of polypropylene with alkali-treated sugarcane bagasse fibers: Mechanism and consequences

Polypropylene composites were prepared from neat and alkali-treated sugarcane bagasse fibers. The results showed that alkali treatment leads to an increase in composite stiffness and strength. A maximum is achieved in these properties at around 5 wt% NaOH content of the treating solution. The increase in properties was assigned to the improvement in inherent fiber characteristics. Acoustic emission testing and electron microscopy showed that the two main local deformation processes related to the fibers are their fracture and debonding; the latter is accompanied by the shear yielding of the matrix. Increased inherent strength of the fibers results in an increase in the fracture initiation stress and fracture energy of the composites. Interfacial adhesion has a slight effect on stiffness, but more significant on strength and impact resistance. Changing adhesion modifies the relative importance of local deformation processes, the number of debonding events decreases, while fiber fracture increases with increasing adhesion. Increased interfacial adhesion improves stress transfer and the load bearing capacity of the fibers as well, but suppresses matrix yielding. Alkali treatment increases inherent fiber strength, which can be directly correlated with composite strength.

A Method of Stiffness Calculation and Optimization Design of Marine Composite Sandwich Plates Based on Ocean Environment

Fu, X.; Mei, Z.Y.; Zhang, F.; Chen, G.T.; Bai, X.F., and Lu, C., 2020. A method of stiffness calculation and optimization design of marine composite sandwich plates based on ocean environment. In: Bai, X. and Zhou, H. (eds.), Advances in Water Resources, Environmental Protection, and Sustainable Development. JournalofCoastalResearch, Special Issue No. 115, pp. 549-555. Coconut Creek (Florida), ISSN 0749-0208.In order to improve the performance of the ship's dome in the complex marine environment, this paper takes the optimization design of the shell plate of the sonar dome as the background, aiming to solve the problem that the structure of the ship's dome bears large flow shock load and occasional attack load in the negative pressure area, as well as the contradiction between the sound transmission and the rigidity design of the composite shell plate of the ship caused by the problem, therefore, a scheme of carbon fiber reinforced floating core sandwich shell is put forward Through the combination of theoretical analysis, numerical simulation and experimental verification, the research on the stiffness change law of acoustic sandwich plate is carried out. According to the research, it is found that the formula of composite laminate bending stiffness is also applicable to sandwich plate. Sandwich plate has the best efficiency in the stiffness optimization, and the accuracy of the stiffness optimization scheme can be verified by combining the material acoustic tube test data. The results show that the proposed optimization scheme can basically meet the needs of the marine environment, and can guide the optimization design of the stiffness of the acoustic sandwich plate in accordance with the best stiffness efficiency.

Stability and bifurcations in oscillations of composite laminates with curvilinear fibres under a supersonic airflow

Nonlinear flutter of variable stiffness composite plates—using a reduced-order model benefiting from a large-enough number of modes or degrees of freedom—is the subject of this research. Dynamic of such plates is enriched by different scenarios including period-n (un)stable limit cycle oscillations, quasi-periodic or chaotic solutions, along with Hopf, symmetry breaking and period doubling bifurcations. The fibre path orientation, in any individual ply of these rectangular plates, changes linearly from the left edge to the right one. The plates are subjected to a supersonic airflow. The plate’s displacement field is modelled using a third-order shear deformation theory; a p-version finite element is used to discretize the displacement components. Aerodynamic pressure due to the airflow is approximated using linear piston theory. The self-exciting vibrational full-order model (FOM) is formed by employing the principle of virtual work, and then, the ROM is extracted from the FOM using two reduction techniques, namely static condensation and modal summation method. The inclusion of a large-enough number of modes of vibration in vacuum in the latter technique is critical for the observation of, often overlooked, complex dynamic behaviour of these plates. The equations of motion representing the ROM are solved using the shooting method (to obtain stable and unstable LCOs) and using Newmark method (for stable or non-oscillatory solutions). The rich dynamics of these plates are explored with the help of vibration and flutter mode shapes, phase-plane plots, bifurcation diagrams, fast Fourier transform diagrams and Poincare sections.

New insights into the synergistic influence of voids and interphase characteristics on effective properties of unidirectional composites

Micro-voids are common imperfections generated during manufacturing or prolonged service of composites. The present paper emphasizes on the effect of not only matrix-pores, but also the interphase-defects on the effective stiffness of unidirectional fiber-reinforced composites (UFRCs). To this aim, a hierarchical modeling of porous UFRCs is established, in which an interphase model is incorporated to characterize the linkage among the fiber content, the content and properties of the interphase, as well as the trait of the interphase-pores. An effective and easy-to-implement two-step homogenization scheme, based on Mori-Tanaka method and Huang’s bridging method, is developed for the effective properties of such porous UFRCs. Comparison studies against available experimental data are conducted to demonstrate the validity of the proposed model. Afterwards, detailed parametric analyses are performed, and new insights into the effect of pores on UFRCs are provided, which may have important implications in the further manufacturing and defects speculation of composites.

Large deformation mechanical modeling with bilinear stiffness for Macro-Fiber Composite bimorph based on extending mixing rules

Macro-Fiber Composite bimorph is a kind of piezoelectric actuator that allow large bending deformation. However, macro-fiber composites exhibit strong stiffness nonlinearity in their operation range, so it is difficult to accurately estimate their large deformation behavior based on a linear constitutive model. In addition, the macro-fiber composites have active and inactive parts, that significantly differ in their material sizes and properties, so it is not reasonable to consider them as uniform material. Thus, it is necessary develop an accurate modeling and analysis method for the large deformation macro-fiber composite structures. First, the mixing rules are extended to derive the three-dimensional homogenized mechanical and electrical parameters of the macro-fiber composite active part; based on these parameters, the actuation results of linear finite element model is in good agreement with the official data. Then a finite element model of the axially compressed macro-fiber composite bimorph is established, the bilinear tensile stiffness of macro-fiber composite is realized by secondary development in ANSYS. Comparison with the experimental results reveals high accuracy of the established finite element model. Thus, the developed method can be effectively used for the performance evaluation and design of the macro-fiber composite devices with large deformation.

Physical, chemical and thermomechanical characterisation of glass textile-reinforced concretes (TRC): Effect of elevated temperature and of cementitious matrix nature on properties of TRC

In this paper, the thermomechanical behaviour of two glass textile-reinforced concretes (TRCs), with temperature levels ranging from 25 °C to 672 °C, was firstly characterised, compared and analysed. Each TRC had three layers of alkali-resistant glass grid but a different cementitious matrix (one with normal mortar and the other refractory mortar). The first novelty and achievement of this study, compared to existing studies in the literature, is that the size of particles within the cementitious matrix of TRC has a very important effect on the ultimate stress and the post-cracked composite stiffness of TRC under thermomechanical condition. The resistance of TRC with normal mortar is greater than that of the TRC with fire mortar because the size of particles within the cementitious matrix is finer for the normal mortar. The second novelty and achievement of this study is that the fire mortar, with refractory property, leads to greater thermal and mechanical stability of TRC when it is simultaneously subjected to a tensile pre-loading and exposed to increasing elevated temperature. The third novelty and achievement of this study is that the evolution of thermomechanical behaviour of the two TRCs before 300 °C has been experimentally justified and analyzed thanks to physical, chemical and thermal characterization that has been carried out on crushed hardened cementitious matrix or on small TRC specimens. Between 25 °C and 110 °C, the decrease of the ultimate stress and the post-cracked composite stiffness of TRC corresponds to dehydration of components of cementitious matrix. However, between 110 °C and 300 °C, with water molecules released, hydration of anhydrous particles of the cementitious matrix (mortar) occurs, a gain in the ultimate stress and the cracked-composite stiffness of glass TRC consequently occurs.

An element-free study of variable stiffness composite plates with cutouts for enhanced buckling and post-buckling performance

In this paper, an efficient element-free modeling framework for variable stiffness composite laminates with cutouts is developed. In comparison with other numerical methods or semi-analytical methods, element-free meshless methods offer many advantages in modeling variable stiffness materials and structures, e.g. complex geometries with cut-outs, providing accurate stress computations and stable nonlinear behavior modeling. In this study, an element-free Galerkin (EFG) method based modeling framework is developed to model variable stiffness composite plates containing circular holes, specifically for buckling and post-buckling analysis. Weighted orthogonal basis functions are used to further reduce the computational costs of the meshless method. Modeling accuracy for variable stiffness composite structures using node-based fiber angle definitions in this element-free framework are validated against FEM results obtained from ABAQUS. The critical buckling loads of linearly varying fiber composite laminates with different sizes of central circular holes are then computed. Based on the proposed element-free modeling scheme, nonlinear post-buckling equilibrium paths of variable stiffness composite plates are traced using an incremental loading step control method, which enables efficient post-buckling analysis. Changes in both critical buckling loads and nonlinear post-buckling performance with respect to hole size and the linear variation of fiber angles are investigated with the results presented in the form of parametric studies. The numerical results show that stress redistribution remains the major factor driving improvements in the buckling and post-buckling performance of variable stiffness composite plates with cut-outs. The research work presented in this paper for composite plates with linearly varying stiffness provides a basis for the analysis of general variable stiffness soft materials in the future.

Filters for manufacturability in design optimization of variable stiffness composites

In the design optimization of variable stiffness composites, manufacturing constraints imposed by the automated fiber placement technology must be considered. In the present paper, two filters are proposed to address this issue, and they are incorporated into the Shepard interpolation-based design optimization framework developed in our previous studies. The fiber angle arrangement of a composite is represented by a continuous function that interpolates fiber angles at scattered design points. Two filters are appointed for each design point to deal with two typical manufacturing constraints, i.e., fiber curvature and gap/overlap. At each design point, the sensitivity is first filtered in a rectangular region around this point, and by this means the fiber curvature is controlled; then in another rectangular region around this design point, the filtered sensitivities are averaged, and the result is used to update the corresponding design variable. Several numerical examples are investigated, and the results show that the proposed method is effective.

The Effect of Increasing Fracture Site Stiffness on Bone-Pin Interface Stress and Foot Contact Pressure within the Equine Distal Limb Transfixation Cast: A Finite Element Analysis.

The aim of this study was to determine how increasing stiffness of fracture site tissues distal to the pins in an equine distal limb transfixation cast influences stress at the bone-pin interface, within the bones distal to the transcortical pins, and contact pressure between the foot and the cast. A transfixation cast finite element model was used to compare the bone-pin interface stress, pin stress, bone stress distal to the pins and contact pressure between the foot and the cast, using six stiffness values for a composite tissue block representing progressive stages of fracture healing. Increasing stiffness of the composite tissue block resulted in a decrease in the maximum stresses at the bone-pin interface, an increase in stresses distal to the transcortical pins and a decrease in the maximum pin stresses. As the composite tissue block stiffness was increased, contact pressure between the bottom of the composite tissue block and the cast increased and the stress patterns surrounding the pin holes became less focal. The findings of this study illustrate that with good foot to cast contact within a transfixation cast, increases in tissue stiffness due to progressive fracture healing are expected to reduce bone-pin interface stresses, and increase fracture site loading and stress. Increasing the contact pressure between the foot and the cast could reduce transfixation casting complications such as pin loosening, pin hole fracture and poor fracture healing, if these results transfer to ex vivo and in vivo settings.