Stiffness Of Composites Research Articles

Multi-level intelligent design of variable angle tow laminates via image-driven method

Compared with the traditional design of constant stiffness composite laminates, the fiber variable angle-tow (VAT) laminates as variable stiffness composites have more flexible design space. In order to obtain continuous fiber paths and high-precision analysis model, the design of VAT requires more detail features, yet the large number of design variables presents a significant obstacle to conventional optimization methods. In this study, an image-driven intelligent optimization method is proposed. The VAT fiber angle vector field is accurately described by the B-spline surface with different control points, and is mapped into a gray image with fixed size. Through the isogeometric analysis (IGA) method, the buckling load is gained as the label of the images, and the dataset is obtained to train convolutional neural networks (CNNs) for deep learning models. Then, a multi-level refinement of the local key design space effectively optimizes the VAT laminates while taking into account manufacturing constraints. Due to the images providing as a single unit of input data, the CNNs model can be applied in variable design space without resampling. Three numerical examples demonstrate the effectiveness, and obtain higher buckling load of VAT laminates in different level design spaces.

Progressive failure prediction of three-dimensional woven composites using a generic multi-scale analytical model

Three-dimensional (3D) woven composites with higher interlaminar fracture toughness, better damage tolerance and improved mechanical stability are being increasingly used in aviation and automotive industries. To expedite their analysis and design efficiency, developing simple and accurate modeling tools capable of quickly evaluating their mechanical properties and progressive failure is necessary. In this paper, a generic multi-scale analytical model for mechanical response prediction and failure behavior characterization of 3D woven composites is presented and validated by using the existing experimental and numerical results. Subsequently, the analytical model is adopted to evaluate the predictive capability of different unit cell model schemes and to investigate the effects of geometric parameters on stiffness and strength properties of composites. The results indicate that the interior-surface integrated cell model scheme has the overall best performance in mechanical response prediction of composites. In addition, an exponential model and a linear model are proposed to quantify the relationships between the number of weft layer and yarn density and mechanical properties, respectively. The present results demonstrate the analytical model can be a useful tool to provide an insight for the analysis and design of other 3D woven composites.

Efficient beam element model for analysis of composite beam with partial shear connectivity

A two-node beam element formulation with eight degrees of freedom was developed and implemented into MATLAB to simulate the nonlinear behavior of the composite. The beam element includes the reinforced concrete slab, the concrete reinforcement, the steel beam, and the shear connectors. One dimension-softening model was used to simulate the concrete, the bilinear isotropic plastic model was used to simulate the steel, and a multi-linear spring was used to simulate the partial shear connectivity. Experimental results were used to validate the numerical model, which was able to closely predict the experimental response by a 2% difference. The model was able to simulate several material nonlinearities such as the plasticity of the steel beam and cracking and crushing of concrete slab. A parametric study was performed to investigate the effects of strength of shear connector, concrete slab thickness, steel beam depth, and the concrete compressive strength on the overall response of steel–concrete composite beams. It was found that the variation in shear connector strength significantly affects the response of the composite beams. In addition, the composite beam stiffness and strength increased with the increase of the steel beam depth, significantly.

Analysis of ballistic performance and penetration damage mechanisms of aramid woven fabric reinforced polycarbonate composites with different matrix content

High-performance lightweight ballistic composites have attracted much attention. The amount of matrix is always a principal factor influencing ballistic performance. However, the effect of matrix content on the ballistic properties of composites has not been studied deeply. In this work, polycarbonate, a potential matrix for ballistic composites, was introduced in the aramid fabric reinforced composites (AF/PCC) for ballistic protection because of its excellent impact resistance. AF/PCC with different matrix content were designed to investigate the ballistic performance and damage mechanism deeply by combining quasi-static tests, ballistic test and numerical simulation. The results revealed that neat fabrics exhibited optimal ballistic performance, with the main damage mechanism being yarns tensile breakage. The presence of matrix in composites reduced the ballistic performance of neat fabrics. The surplus matrix (more than 20 wt%) enhanced composite stiffness, thereby inhibiting the delamination and bending deformation of the composites, which was similar to thermosetting matrix composites, reducing energy absorption. As the matrix content decreased from 30 wt% to 10 wt%, the main ballistic damage mechanisms of AF/PCC changed from compression-shear failure to delamination and tensile failure. The analysis of matrix content on ballistic performance provides an efficient guideline for better design of ballistic composites.

Application and Performance Discussion of Fiber-Reinforced Composite Materials in Sports Equipment Based on Image Processing Technology

Fiber reinforced composites can meet the needs of lightweight, heavy load, long-span, high strength, and modern structures and work under strict conditions. Therefore, it is widely used in various fields. Sports equipment generally cannot meet the requirements of high-strength use, so there is an urgent need for new fiber materials to make these instruments. Aiming at improving the efficiency of measurement and the reliability of measurement results, this paper studies the quantitative characterization method of the microstructure of short fiber reinforced composites. The swept-frequency OCT system is a newly developed high-resolution biomedical imaging system. The method of this paper is to study the performance parameters of swept frequency OCT system, deduce the application of image processing technology in fiber material detection, and then study the detection of residual strength and residual stiffness of composites, so as to obtain a composite material detection method that can be popularized. On this basis, image processing and performance analysis of the composites were carried out, and the composites were tested. And the prospect of application in this field has been analyzed. The experimental results show that better fiber length can be obtained by this method, and the maximum relative error is only 4.5%, thus ensuring the accurate determination of fiber length. Calculations were carried out using the experimental data, and the results showed that the performance of the sports product increased from 15.4% to 48.6% with the graphite-rubber composite. Using this material can improve the comprehensive performance of sports equipment by 2.1%∼4.5%.

Curved Fiber-Reinforced Laminated Composite Panel and Variable Stiffness Influence on Eigenfrequency Responses: A Higher-Order FE Approach

Curved Fiber-Reinforced Laminated Composite Panel and Variable Stiffness Influence on Eigenfrequency Responses: A Higher-Order FE Approach

Carbon/Basalt Fibers Hybrid Composites: Hybrid Design and the Application in Automobile Engine Hood.

The low-velocity impact properties and the optimal hybrid ratio range for improving the property of hybrid composites are studied, and the application of hybrid composites in automobile engine hoods is discussed in this paper. The low-velocity impact properties of the hybrid composite material are simulated under different stacking sequences and hybrid ratios by finite element simulation, and the accuracy of the finite element model (FEM) is verified through experiments. Increasing the proportion of carbon fiber (CF) in the hybrid layer and placing the basalt fiber (BF) on the compression side can improve the energy absorption capacity under low-velocity impact loads. CF/BF hybrid composite hoods are optimized based on the steel hood and the low-velocity impact performance of the hybrid composite. The BCCC layer absorbs the most energy under low-velocity impact loads. Compared with CFRP, the energy absorbed under 10 J and 20 J impact energy is increased by 26.1% and 14.2%, respectively. Through the low-velocity impact properties of hybrid composites, we found that placing BF on the side of the load and keep the ratio below 50%, while increasing the proportion of CF in the hybrid laminate can significantly improve the property of the hybrid laminate. The results show that the stiffness and modal properties of the hybrid composite can meet the design index requirements, and the pedestrian protection capability of the hood will also increase with the increase in the proportion of BF.

Buckling analysis of variable stiffness composite plates with elliptical cutouts using an efficient RPIM based on naturally stabilized nodal integration scheme

Buckling resistance of variable stiffness composite (VSC) plates can be significantly improved by designing fiber orientation distributions, which affect structural stiffness and stress distribution. In this paper, a meshless model of VSC plates is developed using a radial basis point interpolation method (RPIM) based on the naturally stabilized nodal integration (NSNI) scheme, and the buckling behavior of the VSC plates with elliptical cutouts is investigated. In this model, a set of nodes is used to discretize the problem domain, which makes the loading response of the VSC plates can be calculated by its exact fiber angle. And the mechanical behavior of the VSC plates with various complex geometries and cutouts can be accurately simulated. This meshless method is verified by comparing with the finite element method (FEM) and semi-analytical solutions. And compared with the traditional RPIM based on the Gaussian integration (GI), NSNI-based RPIM achieves considerable enhancement in computational efficiency while maintaining high accuracy. Employing this efficient meshless method, the buckling behavior of the VSC plates with elliptical cutouts for different boundary conditions is studied. And the effects of size and location of the cutout on the buckling response of the VSC plates are analyzed. The research on the VSC plates with cutouts in this paper has practical implications for the design of cutouts for VSC plates and provides a new method for the numerical research of VSC plates.

Effect of Compatibilizer on the Properties of Areca-Fiber Reinforced Polypropylene Composites

ABSTRACT Areca fiber reinforced polypropylene (PP) composites were prepared at 20%, 30%, 40%, and 50% loading levels in the presence of compatibilizing agent, MAPP (maleic anhydride grafted polypropylene). The influence of compatibilizing agent and filler concentration on the mechanical properties of the resulting composites was studied. At 50% areca fiber loading, tensile, flexural strength, and modulus of the coupled samples were found to be superior when compared to the uncoupled samples. Coupled composites exhibited effective coupling/interfacial adhesion between the surface of areca fiber and polypropylene phase as evidenced by SEM microphotographs and FTIR spectroscopy. Increase in fiber percentage led to an increase in brittleness and stiffness of composites. A considerable increase in strength was noticed upon the addition of compatibilizing agent, due to proper incorporation of areca fiber into the polypropylene matrix and the efficient transfer of load from matrix to the fiber. Higher moisture absorption in case of composites with higher fiber content was found. Addition of MAPP reduces the percentage of water absorption as a result of encapsulation of areca fiber. Areca fiber can be effectively utilized to develop artificial wood with better properties to replace natural wood as a waste-to-wealth approach.

Fibrous anisotropy and mineral gradients within the radula stylus of chiton: Controlled stiffness and damage tolerance in a flexible biological composite

Over hundreds of millions of years, organisms have evolved architected structures via precise control over hierarchically assembled components, including the integration of dissimilar materials. One such example is found in the radula system of chitons, intertidal mollusks that feed on algae growing on the rock. Their radula consists of multiple rows of ultrahard teeth, each integrated with a foldable belt-like substrate via a stiff, yet flexible stylus, which is essential for efficient rasping during the feeding process. Here, we investigate the nano and micro-scale components and architectures as well as regional mechanical properties of the stylus, and their subsequent role during the rasping of Cryptochiton stelleri. Three important factors were determined to contribute to the regio-specific stiffness of the stylus: the presence of mineral components, highly oriented chitinous fibers, and a chemically cross-linked protein matrix. All these factors are varied throughout the stylus. There is a high mineral content on the trailing edge close to the tooth and a cross-linked matrix on the leading edge, both with orientational specific oriented chitin fibers that provide force transduction to the tooth. Conversely, there is a significant lack of mineral or cross-linked matrix in the proximal end as well as a low degree of fiber orientation, resulting in a flexible region that can accommodate torsion and flexure during rasping. Understanding the graded composite structure of the stylus and applying this unique design to various engineering fields such as soft robotics, biotechnology, and the medical industry, can inspire the production of high-performance materials.

Analytical study and design approach of the axial and flexural response of reinforced masonry columns confined with FRP jackets

Using fiber-reinforced polymer (FRP) wraps has proven to be an effective and optimized confinement technique to upgrade the axial and flexural capacity of reinforced concrete (RC) and reinforced masonry (RM) columns. The axial capacity of FRP-confined masonry depends on both the compressive strength of masonry before strengthening and the confinement pressure provided by the FRP jacket. This study proposes a simplified methodology to design and analyze prismatic fully grouted reinforced masonry columns (RMCs) strengthened with FRP Jackets. The proposed design methodology included an analytical confinement model to predict the compressive strength gain in RMCs due to the effective confinement pressure provided by FRP jackets. The proposed procedure was also designed to predict the nominal capacity of RMCs for practical design applications, with columns subjected to both axial loading and bending moment. The essential parameters to perform detailed section analysis were established, and suggested expressions were proposed to obtain the parameter values. Practical values for the equivalent rectangular stress block parameters were proposed. The theoretical axial force-moment interaction diagrams obtained by the proposed procedure were compared with available experimental data. The experimental test results were in good agreement with the analytical predictions by a reasonable marginal error. Furthermore, the effect of five design variables on the axial-flexural interaction of FRP-wrapped RMCs was investigated. The variables considered in the parametric study were the number of FRP layers, radius of the corners, stiffness of the FRP composite, cross-sectional aspect ratio, and masonry compressive strength. The results showed that increasing the FRP layers, corner radius, and FRP jacket stiffness significantly enhanced the axial capacity of the FRP-confined RMCs. However, the increase in the masonry compressive strength and cross-sectional aspect ratio negatively affected the axial capacity gain ratio of FRP-strengthened RMCs.

Nonlinear Matrix Ferroelectric Composites on Construction Quality and Safety Measures

With the continuous emergence of concrete construction projects such as long-span bridges, high-rise buildings, nuclear power plants, and offshore platforms, researchers at home and abroad have made great progress in construction quality and safety measures in recent years. Construction quality is the main factor affecting the appearance and function of the building. Compared with traditional building construction materials, it has indeed made great improvements in organization and performance. For example, for nonlinear matrix ferroelectric composites, this functional material can better solve the problem of poor compatibility between general composites and concrete structural materials. It is not sensitive to humidity and can work in a concrete environment for a long time. These materials are indeed popular in building construction, various materials complement each other in performance and produce a synergistic effect so that the comprehensive performance of composite materials is better than the original composition materials and meets various high-quality requirements. But some properties are still not very clear. Therefore, this study mainly studies the influence laws of viscoelasticity, elastic viscoplasticity, and macro mechanical behavior of nonlinear matrix ferroelectric composites. Through theoretical analysis and experiments, it is concluded that the change of effective stress relaxation stiffness of nonlinear matrix ferroelectric composites tends to be stable with the increase of time, and the effective stress relaxation stiffness of composites increases with the increase of age volume fraction at t = 0 s.

Variable Stiffness Conductive Composites by 4D Printing Dual Materials Alternately.

Materials that can be designed with programmable properties and which change in response to external stimuli are of great importance in numerous fields of soft actuators, involving robotics, drug delivery and aerospace applications. In order to improve the interaction of human and robots, materials with variable stiffness are introduced to develop their compliance. A variable stiffness composite has been investigated in this paper, which is composed of liquid metals (LMs) and silicone elastomers. The phase changing materials (LMs) have been encapsulated into silicone elastomer by printing the dual materials alternately with three-dimensional direct ink writing. Such composites enable the control over their own stiffness between soft and rigid states through LM effective phase transition. The tested splines demonstrated that the stiffness changes approximately exceeded 1900%, and the storage modulus is 4.75 MPa and 0.2 MPa when LM is rigid and soft, respectively. In the process of heating up, the stretching strain can be enlarged by at least three times, but the load capacity is weakened. At a high temperature, the resistance of the conductive composites changes with the deformation degree, which is expected to be applied in the field of soft sensing actuators.

Improved Voigt and Reuss Formulas with the Poisson Effect

The Poisson effect, measured by the Poisson’s ratio, plays an important role in the regulation of the elastic properties of composite materials, but it is not considered in the conventional Voigt (iso-strain) and Reuss (iso-stress) formulas, which explains why the formulas are inaccurate even if the iso-strain or the iso-stress conditions are satisfied. To consider the Poisson effect, we derived a set of new formulas based on the governing equations of elasticity. The obtained formulas show greater mathematical complexity. To further understand how the Poisson effect influences composite elastic properties, two types of finite element models (FEM) were constructed to simulate the situations where the Poisson effect does or does not have an influence. The results show that the conventional Voigt and Reuss formulas are special cases of the newly derived ones. The Poisson effect induces secondary strains and stresses into the phase materials, which demands more strain energy to achieve the same deformation in the primary (loading) direction, and thus increases composite stiffness; the magnitude of the increase is dependent on the contrast of phase properties. The findings may have significant impact on the study of the emerging nanocomposites and functionally graded materials, where the conventional Voigt and Reuss formulas have wide applications.

Investigation of the Mechanical Properties for Polymer Reinforced by Nanoparticles with Considering Viscoelasticity of the Matrix

Investigation of the Mechanical Properties for Polymer Reinforced by Nanoparticles with Considering Viscoelasticity of the Matrix

Non-orthogonal FE model with stress smoothing for progressive damage analysis of shear pre-deformed 3D angle-interlock woven composites under compression

3D angle interlock woven fabric usually needs to produce pre-deformation to meet the requirements of the complex shape of the target configuration. This cause the internal fabric angle to change, which makes it difficult to predict the mechanical properties and damage evolution. A novel non-orthogonal mesoscale finite element (FE) model considering local stress smoothing is proposed to predict the effective mechanical properties and reveal the damage evolution process of shear pre-deformed 3D angle-interlock woven composites under compressive load. The spurious stresses in the vicinity of the different material interface caused by the stepped block-like voxel boundaries are efficiently eliminated to improve the failure prediction accuracy. The predicted warp directional compressive stress-strain curves are good agreement with that of experiments, and the failure morphologies are verified by Scanning Electron Microscopy (SEM). The validated simulation model is further to investigate the effects of shear angle on the effective compressive stiffness and strength of 3D angle-interlock woven composites in different direction. And the effects of shear pre-deformation and stress smoothing on the damage and failure mechanisms are discussed. This study is great helpful of providing reliable guidance for analyzing and designing the pre-deformed 3D woven composite complex structures, such as aero-engine fan blades.

Three-dimensional printing of biomimetic variable stiffness composites with controlled orientations and volume fraction of fibers

In this paper, biomimetic continuous fiber reinforced composite structures were modeled and printed on the basis of variable distributions of fiber orientation and the fiber volume fraction, which are similar to the wood structure in the vicinity of a knot. The variable distributions of curved fibers in the variable stiffness composites (VSCs) without gaps and overlaps were realized during the 3D printing process by the control of positioning a nozzle and the adaptive feed of plastic. To demonstrate efficiency of the modeling method and the 3D printing process, a VSC plate with a hole under tensile loading was designed, printed and tested. The results of tensile tests showed that the main failure of the composite plates occurs far from the hole. Furthermore, numerical and experimental strain fields obtained by the finite element method and digital image correlation, respectively, are similar. The efficiency in fiber savings for the composite plates was improved by the change in reinforcement from unidirectional fibers to curved fibers adapted to heterogeneous stress fields.

Moderating Effect of the Lean Tissue Index on the Relationship between the Trabecular Bone Score and Augmentation Index in Dialysis Naïve Patients with Stage 5 Chronic Kidney Disease.

Osteopenia, sarcopenia, and increased vascular stiffness are common in patients with chronic kidney disease-mineral bone disorder (CKD-MBD) with protein energy wasting and can lead to worse clinical outcomes. We investigated the potential moderating role of the lean tissue index (LTI) in the relationship between bone microarchitecture and vascular stiffness in dialysis naïve patients with stage 5 CKD. Bioimpedance spectroscopy for evaluating LTI, lumbar spine dual energy X-ray absorptiometry for determining the trabecular bone score (TBS), and arterial applanation tonometry measurements for the central augmentation index, at a heart rate of 75 beats/minute (cAIx75), were simultaneously performed in 117 consecutive patients. A hierarchical regression analysis was conducted to assess the moderating effect of LTI on the relationship between TBS and cAIx75 after adjusting for age and sex. The effect of the interaction between LTI and TBS on cAIx75 was statistically significant (p = 0.030), demonstrating that the cAIx75 tends to decrease more, with the joint effect of LTI and TBS. In the separate analyses, the interaction effect was significant only in women (p = 0.048) and the group of diabetes (p = 0.042). Our study suggests that the evaluation of changes in body composition, bone health, and vascular stiffness needs to be performed simultaneously in patients with advanced-stage CKD. Further research in patients with different stages of CKD warranted to generalize and apply our results to patients in other stages.

Upper and lower bounds to the overall incremental stiffness of hyperelastic composites

Bounds to the overall stiffness of a composite are well-known within the classical theory of elasticity. They are based on the positive-definiteness of the local stiffness. A transfer to a prestressed state is not trivial. We may study the incremental stiffness that connects the nominal stress rate with the velocity gradient. But when there are mainly compressive stresses, then positive-definiteness can only be secured if this stiffness is replaced by a pseudo-stiffness. Its existence is equivalent to a strengthened form of uniform infinitesimal polyconvexity and is independent of the geometry. The same is the case with the crude Voigt and Reuss bounds. More refined kinematic or dynamic approximations do, of course, depend on the geometry. This is demonstrated with the unidirectional reinforcement of a matrix.

The Gut Microbiota and Vascular Aging: A State-of-the-Art and Systematic Review of the Literature.

The gut microbiota is a critical regulator of human physiology, deleterious changes to its composition and function (dysbiosis) have been linked to the development and progression of cardiovascular diseases. Vascular ageing (VA) is a process of progressive stiffening of the arterial tree associated with arterial wall remodeling, which can precede hypertension and organ damage, and is associated with cardiovascular risk. Arterial stiffness has become the preferred marker of VA. In our systematic review, we found an association between gut microbiota composition and arterial stiffness, with two patterns, in most animal and human studies: a direct correlation between arterial stiffness and abundances of bacteria associated with altered gut permeability and inflammation; an inverse relationship between arterial stiffness, microbiota diversity, and abundances of bacteria associated with most fit microbiota composition. Interventional studies were able to show a stable link between microbiota modification and arterial stiffness only in animals. None of the human interventional trials was able to demonstrate this relationship, and very few adjusted the analyses for determinants of arterial stiffness. We observed a lack of large randomized interventional trials in humans that test the role of gut microbiota modifications on arterial stiffness, and take into account BP and hemodynamic alterations.