Effect of an interphase layer on the dynamic stress concentration in a Mg-matrix surrounding a SiC-particle

The potential of using an interphase layer to reduce stress concentrations under a dynamic loading in a Mg-matrix surrounding a SiC-particle is investigated in this study. An interphase layer was applied between the particle and the matrix and the contact between them was assumed to be perfect. Both constant-property materials and functionally graded materials were considered for the interphase. A constant-property interphase was modelled as a single layer while a functionally graded interphase was divided into a number of sublayers and each sublayer was treated as having constant material properties. Numerical results reveal that the interphase layer made of a constant-property material shows better stress concentration reduction than that made of functionally graded materials. An interphase layer with low values of both shear modulus and Poisson’s ratio is necessary for a significant stress concentration reduction. Studies were focused on determining the maximum stress concentration that occurs over a range of frequencies. This investigation has revealed that a stress concentration reduction of up to 44% could be realized.     

Effect of an interphase region on debonding of a CNT reinforced polymer composite

The effect on stiffness and debonding of an interphase zone of altered polymer properties surrounding each carbon nanotube (CNT) in a CNT reinforced polymer composite is investigated. The interphase zone has position dependent material properties that merge with those of the polymer at a sufficiently large distance from the inclusion. There is evidence that such an interphase zone must be included in models in order to represent the overall composite properties. The analyses are based on an axisymmetric unit cell model of the composite. An elastic–viscoplastic conventional continuum constitutive relation (a size-independent relation between stress, strain and strain rate) is taken to characterize the bulk polymer material and the interphase, with the material properties being position dependent in the interphase. The interface between the polymer and the CNT is modeled by a phenomenological cohesive relation that allows for complete separation and the creation of new free surface. The effect of varying interface strength on the composite stress–strain response and on debonding is analyzed both with and without an interphase. The presence of an interphase increases the composite stiffness but promotes debonding which ultimately reduces composite stress carrying capacity. The compliance of the interface also affects the stress–strain response prior to debonding and leads to stress redistributions within both the fiber and the matrix (and/or interphase) which can affect the fracture mode that occurs.     

On the relation between the mode I fracture toughness of a composite laminate and that of a 0° ply: Analytical model and experimental validation

This paper proposes a simple model to predict the fracture toughness of multidirectional carbon–epoxy composite laminates using the fracture toughness of the 0° ply. The model is based on a combination of Linear-Elastic Fracture Mechanics and lamination theory, and uses as material properties the ply elastic properties and the fracture toughness of the 0° ply measured in compact tension test specimens. A good correlation is obtained by comparing the model predictions and experimental data obtained in center-cracked specimens manufactured using different lay-ups and materials.     

On the fracture toughness of ceramic matrix composites

Based on the resistance curve (R-curve) behaviour of ceramic matrix composites (CMCs) determined under either quasi-static or cyclic loading, the crack-face fibre bridging stress field is determined for the compact tension (CT) test specimen geometry. Two different methods have been used for the analysis of the bridging stresses. The first considers a compliance approach. Using the difference in compliance calibration curves with and without bridging and assuming a power-law relation between bridging stress and crack opening displacement, the bridging stress field was calculated. The second approach uses the existence of an invariant stress reversal point in the CT geometry and assuming that the material exhibits linear elastic fracture behaviour, yields a recurrence relation for the bridging stresses resulting in a piece-wise constant stress function. Both models are applied to the experimentally determined fracture behaviour of a 2D carbon/carbon (C/C) composite, and the resulting bridging stress distributions are discussed.     

Evaluation of FRP and hybrid FRP cables for super long-span cable-stayed bridges

This paper evaluates the safety factors, the applicable lengths, and relative cost of FRP (fiber reinforced polymer) and hybrid FRP cables that are potentially suitable for cable-stayed bridges with a super long-span of between 1000 m and 10,000 m. Following previous studies on 1000-m scale cable-stayed bridges with FRP cables, two kinds of hybrid FRP cables – the previously discussed hybrid basalt and carbon FRP (B/CFRP) cable and the newly-developed basalt and steel-wire FRP (B/SFRP) cable – as well as conventional steel cable, CFRP cable, and BFRP cable are further investigated focusing on their promise in meeting potential requirements for super long-span bridges. Some major results are as follows: (1) a three-stage model for determining safety factors of cables with different kinds and lengths is proposed; (2) a threshold of λ² is suggested to achieve both high material and stiffness utilization efficiency, based on which the applicable lengths for different kinds of cables were evaluated; and (3) hybrid B/SFRP cables and BFRP cables are comparable in cost to steel cables within a 3000 m span, while hybrid B/CFRP cables and CFRP cables demonstrate a superior performance/cost ratio over a longer span.     

Effects of interphase material properties in unidirectional fibre reinforced composites

A three-dimensional (3D) micromechanical study has been performed in order to investigate local damage in unidirectional (UD) composite materials with epoxy resin under transverse tensile loading. In particular the effect of different mechanical properties of a 3D interphase within the hexagonal array RVE have been considered and effects of thermal residual stress arising during the curing process have been accounted for in this study. To examine the effect of interphase properties and residual stress on failure, a study based on the temperature-dependent properties of matrix and interphase and a stiffness degradation technique has been used for damage analysis of the unit cell subjected to mechanical loading. Results indicate a strong dependence of damage onset and its evolution from the different interphase properties within the RVE (representative volume element). Moreover, predicted mechanical properties, damage initiation and evolution are also clearly influenced by the presence of residual stress. Numerical results and experimental data (in the literature) have also shown an interesting agreement.     

Gradient of nanomechanical properties in the interphase of cellulose nanocrystal composites

The nanoscale transitional zone between a nanofiber and surrounding matrix (interphase) defines the ultimate mechanical characteristics in nanocomposite systems. In spite of this importance, one can hardly find quantitative data on the mechanical properties of this transitional zone in the cellulose–nanofiber composites. In addition, most of the theoretical models to predict the mechanical properties of interphase are developed with the assumption that this transitional zone is independent of the nanofiber size. In the current study, we show that the mechanical properties of interphase in cellulose nanocrystal (CNC) composites can be quantitatively characterized and the correlation with the size of CNCs can be mapped. The peak force tapping mode in atomic force microscope (AFM) was used to characterize deformation, adhesion, and modulus gradient of the interphase region in poly(vinyl alcohol) (PVA)–poly(acrylic acid) (PAA)–cellulose nanocrystal (CNC) composites. In comparison to the polymer matrix, the adhesion force of CNC was lower. The average elastic modulus in the interphase varied from 12.8 GPa at the interface of CNC to 9.9 GPa in PVA–PAA matrix. It was observed that the existence of PAA increased the gradient of mechanical and adhesion properties of the interphase zone. This occurs due to the variation in the ester linkage density from the CNC interface to the polymer matrix. Finally, it is shown that interphase thickness is higher for CNCs with larger diameter.     

A review on the driving performance of FRP composite piles

Fibre composites have been a viable option in replacing traditional pile materials such as concrete, steel and timber in harsh environmental conditions. However, driving composite piles require more careful consideration due to their relatively low stiffness. Currently, there are no specific guidelines on the installation of composite piles which limits their acceptance in load-bearing applications. There is a need therefore to understand their behaviour during driving in order for composite piles to be safely and economically driven into the ground. This paper presents an overview on composite pile technologies and an examination on the different factors that affects their driving performance. Emphasis on the potential use of hollow fibre reinforced polymer (FRP) piles and the need for further study on their impact behaviour is highlighted. It is expected that the information provided in this paper will help researchers and engineers to develop efficient techniques and guidelines in driving composites piles.     

Effects of interphase properties in unidirectional fiber reinforced composite materials

A micromechanical study has been performed to investigate the mechanical properties of unidirectional fiber reinforced composite materials under transverse tensile loading. In particular, the effects of different properties of interphase within the representative volume element (RVE) on both the transverse effective properties and damage behavior of the composites have been studied. In order to evaluate the effects of interphase properties on the mechanical behaviors of unidirectional fiber reinforced composites considering random distribution of fibers, the interphase is represented by pre-inserted cohesive element layer between matrix and fiber with tension and shear softening constitutive laws. Results indicate a strong dependence of the RVE transverse effective properties on the interphase properties. Furthermore, both the damage initiation and its evolution are also clearly influenced by the interphase properties.     

Thermal expansivities in fibrous composites incorporating hybrid interphase regions

The aim of this paper is the prediction of coefficients of thermal expansion in unidirectional fiber-reinforced composites. The representative volume element is a three phase composite structure subjected to a uniform temperature change. The advanced hybrid interphase concept is introduced, in which the interphase thickness depends on the property under consideration. This model involves also imperfect adhesion by immediate softening of material properties. Equations for the prediction of coefficients of thermal expansion are presented. Results are illustrated and discussed in terms of fiber volume fraction and adhesion coefficient. To validate the accuracy of these results finite element analysis has also been utilized. Predictions of coefficients of thermal expansion are in good agreement with experimental, finite element analysis and previous published results. The coefficients of thermal expansion of the considered polymer matrix composites are affected significantly by the parameters characterizing interphase.     

Effect of fibre content on the interlaminar fracture toughness of unidirectional glass-fibre/polyamide composite

In this paper, the effects of fibre content on the interlaminar fracture in continuous glass-fibre/polyamide 12 composite have been investigated under model I (DCB) loading condition. The specimens were fabricated with different fibre volume contents (21%, 26%, 34% and 39%) by using a powder impregnation method. It was observed that the values of G_IC(NL) and G_IC(PROP) of this material have a dropping tendency with increasing fibre volume content in the range of 21%–39%, while no general trends in G_IC(5%) and G_IC(VIS). Results show that the glass-fibre/polyamide 12 composites possess high mode I fracture toughness, which is mainly attributed to the high ductility of the polyamide 12 matrix, and the increased fibre bridging caused by the increasing of the fibre volume content can not change the decrease tendency of G_IC(PROP). The fracture surfaces of the specimens were observed by scanning electron microscopy, and the fracture mechanism was analysed.     

Strengthening RC beams with composite fiber cement plate reinforced by prestressed FRP rods: Experimental and numerical analysis

This paper deals with the development of a new strengthening system for reinforced concrete beams with externally-bonded plate made of composite fiber cement reinforced by rebars made of fiber-reinforced plastic (FRP) [1]. The proposed strengthening material involves the preloading of FRP rod before mortar casting. The paper presents experimental and numerical analysis carried out on many large-scale beams strengthened by well-known reinforcement techniques, such as externally bonded Carbon Fiber-Reinforced Plastic (CFRP) plate and the Near Surface Mounted (NSM) technique, which are compared to the proposed new strengthening material through four-point bending tests. Results are analyzed with regard to the load-displacement curve, bending stiffness, cracking load, yield strength and failure load. The developed numerical model is in agreement with the experimental results. It clearly shows the effects of prestressed FRP rod on cracking mechanisms and internal strength distribution in the analyzed beams.     

GBT formulation to analyse the buckling behaviour of FRP composite open-section thin-walled columns

This paper presents a Generalised Beam Theory (GBT) formulation to analyse the local and global buckling behaviour of FRP (fibre-reinforced polymer) composite thin-walled columns with arbitrary open cross-sections, which takes into account both shear deformation and cross-section deformation effects. After describing the steps and procedures involved in performing the GBT cross-section analysis of an arbitrarily branched composite (laminate plate) thin-walled member, the paper addresses the numerical implementation of the proposed GBT formulation, carried out by means of the finite element method (GBT-based beam element) – particular attention is devoted to the derivation of the element linear and geometric stiffness matrices, which incorporate all the material coupling effects. In order to illustrate the application and capabilities of the proposed formulation and implementation, several numerical results are presented and discussed, dealing with the local and global buckling behaviour of FRP composite I-section columns with different ply orientations and stacking sequences. Taking advantage of the GBT modal features, deep insight is acquired on the complex composite member buckling mechanics, namely those involving bending–torsion or global–local coupling effects. In particular, one investigates the influence of (i) the constitutive assumption regarding the transverse extension occurring in the cross-section composite walls and (ii) the distribution of pre-buckling normal stresses (due to axial compression) on the buckling behaviour of I-section columns. For validation purposes, the above results are compared with values recently reported in the literature and estimates obtained from shell finite element analyses.     

拉挤成型复合材料板桩抗弯刚度简化计算

将折线型截面复合材料板桩整体刚度表示成翼缘和腹板刚度之和,基于层合板理论分层计算翼缘和腹板的局部刚度,然后提出整体结构抗弯刚度和抗剪刚度的简化计算公式,因此结构在弯曲荷载作用下的位移理论值可采用Timoshenko方程获得。开展拉挤复合材料板桩四点弯试验,将测量的荷载-位移曲线与理论值进行对比,表明本文推导的刚度计算公式在弹性范围内具有较好的精度,其中跨高比更大的试件理论与试验值更接近。该理论公式也可用于计算Z型、槽型、工字型等截面复合材料结构的刚度和位移。     

Mode I and mode II fracture toughness of E-glass non-crimp fabric/carbon nanotube (CNT) modified polymer based composites

In this study, mode I and mode II interlaminar fracture toughness, and interlaminar shear strength of E-glass non-crimp fabric/carbon nanotube modified polymer matrix composites were investigated. The matrix resin containing 0.1 wt.% of amino functionalized multi walled carbon nanotubes were prepared, utilizing the 3-roll milling technique. Composite laminates were manufactured via vacuum assisted resin transfer molding process. Carbon nanotube modified laminates were found to exhibit 8% and 11% higher mode II interlaminar fracture toughness and interlaminar shear strength values, respectively, as compared to the base laminates. However, no significant improvement was observed for mode I interlaminar fracture toughness values. Furthermore, Optical microscopy and scanning electron microscopy were utilized to monitor the distribution of carbon nanotubes within the composite microstructure and to examine the fracture surfaces of the failed specimens, respectively.     

Light intensity effect on UV cured FRP coupled composite pipe joints

In this study, 36 composite pipes were joined with an eight-layer chopped mat and woven fabric that was adhered with UV curing vinylester resin. The joined composite pipes were cured vertically with UV lamps at three different light intensities: 80, 35, and 15 m W/cm². The mechanical properties of the cured pipes were evaluated by conducting internal pressure testing and simply supported 4-point bending testing. The effect of UV light intensity on the internal pressure rating, the ultimate bending load, and stiffness was evaluated based on the test results. A finite element analysis, which considered the under-curing along the axial direction and radial direction, was conducted to validate the test results. There was a direct correlation observed between increased light intensity and increased residual mechanical properties such as internal pressure rating, stiffness and peak failure load. The mechanisms for variation in the system properties were found to be under-curing and non-uniform curing in the FRP joint resulting in a loss of ability to effectively transfer load from the pipe to the joint. Additional observed phenomenon was gravity leaching due to vertical curing. All mechanisms were simulated by the introduction of a sliding modulus technique in the conducted finite element analysis. FEA results agreed well with the observed failure modes.     

Effect of flax fibres individualisation on tensile failure of flax/epoxy unidirectional composite

The study of plant fibres composites is a widespread research topic; nevertheless, the reinforcement mechanism understanding of these materials must be still improved. The paper presents a study of the effect of the mechanical properties, the dispersion and the fibre/matrix interface property of elementary fibres on the tensile properties of unidirectional composites. Our work shows that the mechanical performances of unidirectional composites could be linked to those of the elementary fibres as well as to the composites microstructure. Flax fibres individualisation, linked to the homogeneity of the microstructure, is highly dependent on the fibre extraction process. The importance of the composites homogeneity has been confirmed by the Rosen model, which could be used thanks to interfacial shear strength measurements.     

Seismic strengthening of shear critical post-heated circular concrete columns wrapped with FRP composite jackets

An experimental study was carried out to investigate the seismic performance of post-heated circular reinforced concrete columns wrapped with glass or carbon fibre reinforced polymer jackets. Eight shear critical reinforced circular columns with a shear span-to-depth ratio of 2.5 were tested under a combined constant axial and cyclic lateral displacement history, simulating earthquake loading. The columns were tested in three groups, unheated, post-heated and post-heated repaired with either glass fibre reinforced polymer (GFRP) or carbon fibre reinforced polymer (CFRP). In terms of seismic performance the test results indicated that using GFRP or CFRP jackets significantly increased the shear capacity, ductility and energy dissipation of the post-heated damaged columns. However, the GFRP or CFRP did not increase the stiffness of the post-heated damaged columns. It was found that the unheated and post-heated damaged columns failed in a brittle shear mode while the mode of failure of posted-heated columns repaired with GFRP or CFRP was successfully shifted from a shear to a ductile flexural failure.     

Micromechanics of stress transfer through the interphase in fiber-reinforced composites

A micromechanical model to predict the interphasial/interfacial stress transfer in a three-phase fiber-reinforced composite is presented. The axisymmetric system consists of a fiber embedded in a compliant matrix having an interphase between them. Each constituent of the composite is regarded as a linear elastic continuum. The matrix is treated as an isotropic material while the fiber and interphase are considered as a transversely isotropic material. Traction-free boundary conditions are strictly enforced. It is assumed that the interfaces are perfect and strong. A pair of uncoupled governing partial differential equations is obtained in terms of unknown displacements. Furthermore, assuming that the Eigenvalues exist for this system of equations, Eigenfunction expansion method is employed to derive an exact solution in terms of the Bessel functions. Analytical solutions are obtained for free boundary conditions at the external surface of the matrix cylinder to model a single fiber pull-out problem, and for fixed boundary conditions to approximately model a hexagonal array of fibers in the matrix material. This formulation provides an analytical framework for the analysis of interphasial and interfacial stresses as well as displacements in the entire 3D axisymmetric system. Finite element (FE) analysis was also performed to simulate stress transfer from the fiber to the matrix through the interphase. Analytically obtained stress fields are verified with FE results. Shear and radial interphasial stresses provide insight into the design of engineered interfaces/interphases.     

Polymeric composite materials incorporating an elastomeric interphase: A mathematical assessment

A mathematical model based upon the method of cells is extended in order to describe three-phase composite materials containing an interphase. A parametric study is performed wherein the effective properties of the composites are determined as a function of interphase material properties, interphase thickness and fiber volume fraction. The simulation is designed in particular towards describing the behavior of model composites which incorporate elastomeric polymers as an interphase to bond carbon fibers chemically to a polymeric matrix. Two hypothetical interphases are considered: one whose properties are representative of elastomers (Young's modulus less than fiber and matrix) and one whose properties are intermediate with respect to the fiber and matrix. The two cases provide a broad assessment of how the interphase properties influence the effective composite properties.