THEORETICAL ANALYSIS ON THE LOCAL CRITICAL STRESS AND SIZE EFFECT FOR INTERFACIAL DEBONDING IN PARTICLE REINFORCED RHEOLOGICAL MATERIALS

The mechanisms of interfacial debonding of particle reinforcedrheological materials are studied. Based on an energy criterion, asimple formula of local critical stress for interfacial debonding isderived and expressed in terms of the interfacial energy. Theparticle size effect on interface debond- ing can then be analyzedeasily owing to the fact that critical stress is inverselyproportional to the square root of particle radius. By takingPP/CaCO_3 system as an example, the present energy criterion iscompared with the mechanical debonding criterion, and it is foundthat under the condition that bond strength is equal to matrixstrength and particle radius not over 0.2μm, the mechanicaldebonding cri- terion can be automatically satisfied if the energycirterion is satisfied.

Study on mechanical properties of composite materials by in-situ tensile test

The mechanical properties of the SiC fiber-reinforced Mg-Al metal matrix composite materials have been studied on internal microstructure by (scanning electron microscopy) SEM in-situ tensile test. The emergence and propagation of the crack, and the fracture behavior in materials have been observed and studied. It is found that in the case of the tensile test, the crack emerged in SiC fiber initially. In the case of the strong cohesion of the fiber-metal interface, the crack propagated in the fiber, meanwhile the fibers in the neighborhood of the cracked fiber began to crack and the Mg-Al metal deformed plastically, and at last the material fractured. Otherwise the toughness of the materials grows in the case of the lower cohesion of the fiber-metal matrix interface.

Ballistic impact simulation of Kevlar-129 fiber reinforced composite material

The penetration resistance of Kevlar-129 fiber reinforced composite materials was investigated with AUTODYN software.The ballistic limits of the fragment that pierced 6kinds of target plates were obtained by finite element simulation when the 10 g fragment simulation projectile（FSP）impacting to the target plates of different thickness values of 8,10,12,14,16 and 18mm with appropriate velocity,respectively,and the influences of thickness on the ballistic limits and the specific energy absorption were analyzed.The results show that the ballistic limit of Kevlar-129 fiber reinforced composite plates presents linear growth with the increase of the target thickness in the range from 8to 18 mm.The specific energy absorption of plates presents approximately linear growth,but there is slightly slow growth in the range from 10 to 16mm of the target thickness.It also can be found that the influences of plate thickness and surface density on the varying pattern of specific energy absorption are almost the same.Therefore,both of them can be used to characterize the variation of specific energy absorption under the impact of the FSP fragment.

AN EXPERIMENTAL STUDY ON POST-BUCKLING BEHAVIOR OF SLENDER COLUMN WITH FIBER REINFORCED COMPOSITE MATERIAL

Equilibrium paths of post-buckling are measured for large slenderness column specimens made of the fiber reinforced composite material. The influence of the initial curvature is investigated experimentally and compared with the result of the initial post-buckling theory. Both the theoretical and experimental results reveal that the column with the initial curvature has stable post-buckling behaviors and is not sensitive to the imperfection in the form of initial curvature. The experimental results show that when the lateral buckling displacement is less than 20 percent of the column length, the experimental results agree with the results from the theory of initial post-buckling quite well, while they agree with the results from the large deflection theory in a quite large range.

Effect of Loading Frequency on the High Cycle Fatigue Strength of Flax Fiber Reinforced Polymer Matrix Composites

Among natural fibers,flax fiber reinforced polymer matrix composites show excellent dynamic/fatigue properties due to its excellent damping properties.Knowledge about fatigue limit and effect of loading frequency on fatigue limit is very crucial to know before being used a member as a structural component.Fatigue limit of fiber reinforced composite is measured through high cycle fatigue strength(HCFS).The effect of loading frequency on the HCFS of flax fiber reinforced polymer matrix composites was investigated using stabilized specimen surface temperature based thermographic and dissipated energy per cycle-based approaches.Specimens of unidirectional flax fiber reinforced thermoset composites were tested under cyclic loading at different percentages of applied stresses for the loading frequencies of 5,7,10,and 15 Hz in order to determine the stabilized surface temperature of the specimen and dissipated energy per fatigue cycle.Both approaches predicted similar fatigue limits(HCFS)which showed a good agreement with experimental results from Literature.HCFS of flax fiber reinforced composites decrease little with increasing loading frequency.Furthermore,effect of loading frequency on stabilized specimen temperature and dissipated energy per fatigue cycle was also investigated.Although specimen surface temperature increases with loading frequency,dissipated energy per-cycle does not change with loading frequency.Thermal degradation at higher loading frequencies may play a significant role in decreasing HCFS with increasing loading frequency.

Study on Permeability of 2.5D Woven Reinforcement Materials with Fractal Theory

Permeability is one of the key issues in the design of molds and in the molding process for composite manufacture. As a disordered fibrous assembly, 2.5- dimension （2.5 D） woven reinforcement materials have complex structure. It poses a challenge to the study of pore structure and the establishment of the theoretical permeability model. Toward addressing this problem, a powerful tool called fractal theory emerged. According to the analysis of 2.5 D woven reinforcement material stmcture using fractal theory, it is found that the structure has an obvious fractal character. Therefore, a permeability fractal model of 2.5D woven reinforcement material was established by cormbining the Hagen-Poiseulle equation with Darcy law according to the capillary vessel fractal model in this paper. The permeability was expressed as a function of the fractal dimension and microstructure parameter of the porous media in this model. The theoretical model is verified by experimental tests and the measurement data are in good agreement with the results obtained from the fractal medel .

Thermal Characteristics of Earth Blocks Stabilized by Rice Husks

The objective of this study is to determine the thermal characteristics of bricks produced from clay soils in Chad using the asymmetric plane method. Indeed, in Sahelian countries like Chad, temperature variations are excessive. The study of the thermal behavior of a recyclable local material with low environmental impact could not only improve thermal comfort in homes, but also help mitigate the effects of climate change. It is in this context that this study is envisaged. Before carrying out these measurements, we first produced different formulations of soil blocks 0%, 1% 1.5%, 2% and 2.5% by mass of rice husks (1.25 mm sieve refusal). Brick specimens of dimensions 10 cm × 10 cm × 1 cm were developed at 0 day, 7 days and 14 days of maturation of the formulated pastes. After, those bricks were submitted after drying to the measurements of various thermal parameters: in particular the conductivity, the effusivity, the volumetric capacity and the diffusivity. The obtained results show that the addition of rice husks to clay soils improves conductivity by 13% to 49%, effusivity by 19% to 24%, volumetric capacity by 23% to 27%, and diffusivity by 47% to 58% for the Moundou soils, depending on the maturation period. For the N’Djamena soil, these thermal characteristics are improved from 11% to 38%, from 11% to 13%, from 40% to 47% and from 39% to 40% respectively.

Mechanism Analysis of Helical and Twisted Reinforcement

Although helical and twisted reinforcement has been used to reinforce concrete for more than two decades, its rationale still remains unclear. With a brief review of current researches on the helical and twisted reinforcement properties, this paper describes some new phenomenon of the helical and twisted reinforcement in concrete and other matrix by experimental studies, and then discusses on mechanism of helical effect of strengthening. This paper also discusses the mechanism of accessional helical effect of strengthening and its significance in industrial practice. Extensive tests indicate that twisting is the most effective way to improve reinforcement mechanical properties. The main results are： （1） They can greatly enhance bond anchorage in base material. In some pull-out tests, the pull-out resistance increases with reinforcement slip within the specimens, which results not only in a higher pull-out load but also a larger slip up to 70%-80% of reinforcement embedded length. （2） Concrete reinforced by twisted bars demonstrates certain ductility at failure. （3） The bond strength depends on the pitch space directly. （4） The twisted effect on material strengthening is from a three-dimensional interlocking force which is formed from material untwisting when they were pulled out from base specimens.

Research on the Crack Detection of Conductive Components Using Pulsed Eddy Current Thermography

Crack of conductive component is one of the biggest threats to daily production. In order to detect the crack on conductive component,the pulsed eddy current thermography models were built according to different materials with the cracks based on finite element method(FEM) simulation. The influence of the induction heating temperature distribution with the different defect depths were simulated for the carbon fiber reinforced plastic(CFRP) materials and general metal materials. The grey value of image sequence was extracted to analyze its relationship with the depth of crack. Simulative and experimental results show that in the carbon fiber reinforced composite materials,the bigger depth of the crack is,the larger temperature rise of the crack during the heating phase is; and the bigger depth of the crack is,the faster the cooling rate of the crack during the cooling phase is. In general metal materials,the smaller depth of the crack is,the lager temperature rise of the crack during the heating phase is; and the smaller depth of the crack is,the faster the cooling rate of crack during the cooling phase is.

Damage detection method in complicated beams with varying flexural stiffness

A damage detection method for complicated beam-like structures is proposed based on the subsection strain energy method （SSEM）, and its applicability condition is introduced. For a beam with the continuously varying fiexural stiffness and an edge crack, the SSEM is used to detect the crack location effectively by numerical modal shapes. As a complicated beam, the glass fiber-reinforced composite model of a wind turbine blade is studied based on an experimental modal analysis. The SSEM is used to calculate the damage index from the measured modal parameters and locate the damage position in the blade model successfully. The results indicate that the SSEM based on the modal shapes can be used to detect the damages in complicated beams or beam-like structures for engineering applications.

The impact of the new composite material technology on the performance of the tennis rackets

With the continues improving of people＇s living standards, more and more people work out in all kinds of sports fields beyond the busy work. On the other hand, the development of the modem competitive sports also requires that the sports experts should not only strive for the scientific training, but should also pay much attention on the improvement and development of the sports equipment at the same time, which makes the sports equipment market have achieved unprecedented prosperity. This paper introduces the application of the fiber reinforced composite materials in the field of sports equipment, which is described mainly from the advantages of the fiber reinforced composite materials used in sports equipment areas, and from the aspects of the principles of material selection, the product varieties, the application examples and the status.

Estimation of Transverse Thermal Conductivity of Doubly-periodic Fiber Reinforced Composites

For steady-state heat conduction,a new variational functional for a unit cell of composites with periodic microstructures is constructed by considering the quasi-periodicity of the temperature field and in the periodicity of the heat flux fields. Then by combining with the eigenfunction expansion of complex potential which satisfies the fiber-matrix interface conditions, an eigenfunction expansion-variational method （EEVM）based on a unit cell is developed. The effective transverse thermal conductivities of doubly-periodic fiber reinforced composites are calculated, and the first-order approximation formula for the square and hexagonal arrays is presented, which is convenient for engineering application. The numerical results show a good convergency of the presented method,even though the fiber volume fraction is relatively high. Comparisons with the existing analytical and experimental results are made to demonstrate the accuracy and validity of the first-order approximation formula for the hexagonal array.

Direct joining of oxygen-free copper and carbon-fiber-reinforced plastic by friction lap joining

Oxygen-flee copper （Cu） was successfully joined to carbon-fiber-reinforced thermoplastic （CFRTP, polyamide 6 with 20wt% carbon fiber addition） by friction lap joining （FLJ） at joining speeds of 200-1600 mm/min with a constant rotation rate of 1500 rpm and a nominal plunge depth of 0.9 ram. It is the first time to report the joining of CFRTP to Cu by FLJ. As the joining speed increased, the tensile shear force （TSF） of joints increased first, and decreased thereafter. The maximum TSF could reach 2.3 kN （ 15 mm in width）. Hydrogen bonding formed between the amide group of CFRTP and the thin Cu20 layer on the Cu surface, which mainly contributed to the joint bonding. The influence factors of the TSF of the joints at different joining speeds were discussed. The TSF was mainly affected by the joining area, the degradation of the plastic matrix and the number and the size of bubbles. As the joining speed increased, the influence factors varied as follows： the joining area increased first and then decreased; the degra- dation of the plastic matrix and the number and the size of bubbles decreased. The maximum TSF was the comprehensive result of the relatively large joining area, small degradation of the plastic matrix and small number and sizes of bubbles.

Actuation performances of catkin fibers reinforced thiol-acrylate main-chain liquid crystalline elastomer

Liquid crystalline elastomers(LCEs)have been utilized as an important class of smart actuator materials.However,the modest actuation mechanical and robustness performances remain a challenge.Inspired by the specific structures,well mechanical properties and physico-chemical characteristics of some natural plant fibers,a composite of thiol-acrylate main-chain LCE matrix incorporated with catkin fibers is designed and developed.The catkin fibers build a network as reinforcement phase,and demon-strate effective compatibility and integration property with the matrix,their high flexibility can be adapted to the large deforma-tional performance of LCE matrix.The prepared LCE composite demonstrates strong mechanical actuation properties.The mod-ulus and driving force triggered by the stimuli are obviously increased.The tensile strength and fatigue failure resistant prop-erty under high loadings and repeated cycles of thermal actua-tion or photothermal actuation are greatly enhanced.While the stimulus response deformation rate,phase transition temperature and liquid crystal phase structure of the LCE matrix,and so on,do not weaken or change.This work promotes the LCE materi-als’application potential and broadens the application value of natural plant fibers.

COUPLING EFFECTS OF VOID SHAPE AND VOID SIZE ON THE GROWTH OF AN ELLIPTIC VOID IN A FIBER-REINFORCED HYPER-ELASTIC THIN PLATE

The growth of a prolate or oblate elliptic micro-void in a fiber reinforced anisotropic incompressible hyper-elastic rectangular thin plate subjected to uniaxial extensions is studied within the framework of finite elasticity. Coupling effects of void shape and void size on the growth of the void are paid special attention to. The deformation function of the plate with an isolated elliptic void is given, which is expressed by two parameters to solve the differential equation. The solution is approximately obtained from the minimum potential energy principle. Deformation curves for the void with a wide range of void aspect ratios and the stress distributions on the surface of the void have been obtained by numerical computation. The growth behavior of the void and the characteristics of stress distributions on the surface of the void are captured. The combined effects of void size and void shape on the growth of the void in the thin plate are discussed. The maximum stresses for the void with different sizes and different void aspect ratios are compared.