Long-term moisture effects on the interfacial shear strength between surface treated carbon fiber and epoxy matrix

In recent years, carbon fiber reinforced polymer (CFRP) composites have found increasing applications in marine and offshore area, where the CFRP components are subjected to a persistent attack of moisture. The performance degradation of composites under those critical service conditions becomes a key issue. In this work, silane coating and multiwalled carbon nanotubes were applied on carbon fibers to enhance the fiber/matrix interfacial bonding strength. The long-term effects of moisture on the interfacial shear strength (IFSS) of the composites in underwater environments, such as de-ionized water and simulated seawater, have been studied using single fiber microbond method. The silane coating and carbon nanotube-modified silane coating are found to contribute 14.5% and 26.3% increase in IFSS of the CFRP in dry air, and well maintain this improvement during a 120-day immersion test in de-ionized water and simulated seawater.     

Effect of moisture on elastic and viscoelastic properties of epoxy and epoxy-based carbon fibre reinforced plastic filled with multiwall carbon nanotubes

In this study, we investigated the peculiarities of moisture absorption and moisture-induced effects on the elastic and viscoelastic flexural properties of epoxy resin and carbon fibre reinforced plastic (CFRP) filled with multiwall carbon nanotubes (MWCNTs). Short-term cyclic creep-recovery tests of moistened epoxy and CFRP filled with MWCNTs revealed improvements in creep resistance for both materials. The addition of MWCNTs to the epoxy resin suppressed the moisture absorption by the material, causing a reduction in the diffusion coefficient by 31% and equilibrium moisture content by 15%. The addition of MWCNTs reduced the flexural strength of moistened epoxy and CFRP samples by approximately half, and also lowered the flexural modulus by ∼1.4 and ∼3 times, elastic strain by 1.25 and 1.04 times, viscoelastic strain by 1.39 and 1.03 times, and plastic strain by 2.68 and 1.60 times, respectively.     

Linear and nonlinear torsional behavior of unidirectional CFRP and GFRP

The helicopter bearingless rotor flexbeam is usually made of glass-fiber reinforced plastic composite (GFRP). Carbon-fiber composites (CFRP) are candidate for future flexbeam materials due to their superior tensile fatigue strength. This research examines the feasibility of CFRP as a future flexbeam material. The torsion behaviors of unidirectional CFRP and GFRP with the same matrix resin were investigated. As a result, it was confirmed that the behavior of both CFRP and GFRP is comprised of linear/nonlinear domains. The initial torsional rigidity of CFRP was almost the same as that of GFRP. The torsional rigidities calculated from Lekhnitskii’s equations agreed with the experimental results, and they are mainly determined by the shear stiffness of the materials. The nonlinear torsional behavior was observed above 0.5% of the shear strain, and it is due to plastic deformation of the matrix resin. A 3D plasticity model proposed by Sun et al. was applied to the plasticity parameters obtained from off-axis tensile tests. The numerical curves agree with the experimental data below 1.5% of the shear strain. The experimental result suggests that GFRP can be replaced by CFRP as torsional elements of a helicopter flex beam without an increase in torsional rigidity.     

Investigation of the long term effects of moisture on carbon fibre and epoxy matrix composites

The aim of this work is to investigate the long term effects of moisture on the interface between a carbon fibre and an epoxy matrix. High modulus carbon fibres were used to prepare single fibre model composites based on an epoxy resin. The samples were immersed in the seawater and demineralised water and their moisture uptake behaviour was monitored. The equilibrium moisture content and diffusion coefficients for the samples were determined. DSC has been used to analyse the moisture effects on glass transition temperature and thermal stability of the pure epoxy specimens. These results showed a reduction in the glass transition temperature (T_g) after moisture absorption. Tensile tests were also carried out for the epoxy specimens and a general decrease in the mechanical properties of the epoxy matrix was observed. Raman spectroscopy was used to observe the effects of moisture on the axial strain of the carbon fibre within the composite and stress transfer at the interface as a function of exposure time. The results show that the decrease in the mechanical and interfacial properties of the model composites under the seawater immersion is more significant than under demineralised water immersion.     

Modification of polyester resin based composites induced by seawater absorption

This research is a study on seawater absorption ability and on mechanical performance (before and after immersion in seawater) of two composites that basically differ for what concerns the polyester resin (isophthalic or orthophthalic) employed in boats manufacture. Experimental tests, carried out on the two resins, evidenced that they differ for what concerns their structural organization, water diffusion coefficient, thermal stability, wet ability toward glass fibre beside commercial cost. Isophthalic resin resulted well bonded to glass fibres so that seawater absorption resistance was higher compared with that of orthophthalic one. As a consequence, the composite containing isophthalic resin showed higher flexural stiffness and strength, as well as higher shear strength.     

不同湿热条件下碳纤维/环氧复合材料湿热性能实验研究

对比研究了环氧5228A树脂及碳纤维/环氧5228A树脂复合材料层合板在3种湿热环境(水煮、70℃水浸,70℃85%相对湿度)下的湿热性能,考察了湿热条件对复合材料层间剪切性能的影响规律,并从吸湿特性、物理化学特性、树脂力学性能、湿应力等方面分析了不同湿热环境下复合材料性能衰减的机制。研究表明,碳纤维/高温固化环氧树脂复合材料层间剪切性能主要是由吸湿率决定,相同吸湿率不同湿热条件下性能的下降幅度基本相同;3种湿热条件下该树脂及其复合材料未发生化学反应、微裂纹等不可逆变化,复合材料层合板湿热老化机制主要是吸入水分后基体增塑和树脂、纤维湿应变不一致导致的湿应力对复合材料性能的负面作用。     

Flexural properties of z-pinned composite laminates in seawater environment

Unidirectional carbon-fibre reinforced composite laminates with and without z-pins were immersed in artificial seawater and exposed to two different temperature levels (?1.75 and 50 °C), as well as thaw–freeze cycles (+20/?20 °C). The investigation described is focused on the question to which degree seawater absorption, as well as bending properties are influenced by z-pin reinforcement. The results indicate an increasing influence of the z-pin reinforcement on the water sorption rate, while the sorption rate of unpinned laminates is lower. This is a result of the additional diffusion pathways of the moisture ingress into the laminate caused by the inserted z-pins which in turn change the micro-structure. Furthermore, the sorption rate depends on the immersion temperature. Laminates immersed into seawater with higher temperatures (50 °C) display a significantly higher diffusion rate than those immersed in colder seawater (?1.75 °C) or those immersed under thaw–freeze conditions (+20/?20 °C). Z-pin reinforced laminates with a unidirectional fibre orientation show a reduced bending strength by about 31 %, as well as a reduced flexural modulus by about 11 % in comparison to unpinned samples. Unpinned and z-pinned samples that were exposed to a seawater environment for 1344 h show a reduced flexural modulus depending on the immersion temperatures. As opposed to flexural modulus, flexural strength is not affected by immersion time or temperature. The overall bending strain energy that is necessary for a complete fracture of the unpinned samples under 4-point bending loads can be described with the value of the elastic bending strain energy. In contrast to this the overall bending strain energy of the z-pinned laminates is composed of two different components –the elastic bending strain energy and the post-fracture strain energy. The post-fracture strain energy occurs after exceeding the flexural strength. The overall bending strain energy of z-pinned and unpinned samples without immersion into seawater is around 7.2 J, while the percentage of the post-fracture energy of the pinned samples is 40 % with respect to the overall bending strain energy. The duration of the immersion into water and higher water temperatures increases the overall bending strain energy for both unpinned and pinned samples. The increase is higher for z-pinned samples and is mainly caused by the increase of the post-fracture energy.     

Transfer zone of prestressed CFRP reinforcement applied according to NSM technique for strengthening of RC structures

This study presents an experimental program to assess the tensile strain distribution along prestressed carbon fiber reinforced polymer (CFRP) reinforcement flexurally applied on the tensile surface of RC beams according to near surface mounted (NSM) technique. Moreover, the current study aims to propose an analytical formulation, with a design framework, for the prediction of distribution of CFRP tensile strain and bond shear stress and, additionally, the prestress transfer length. After demonstration the good predictive performance of the proposed analytical approach, parametric studies were carried out to analytically evaluate the influence of the main material properties, and CFRP and groove cross section on the distribution of the CFRP tensile strain and bond shear stress, and on the prestress transfer length. The proposed analytical approach can also predict the evolution of the prestress transfer length during the curing time of the adhesive by considering the variation of its elasticity modulus during this period.     

Edge Delamination and Residual Properties of Drilled Carbon Fiber Composites with and without Short-Aramid-Fiber Interleaf

Edge delamination is frequently observed in carbon fiber reinforced plastic (CFRP) laminates after machining, due to the low fracture toughness of the resin interfaces between carbon fiber plies. In this study, the effects of incorporating tough aramid fibers into the brittle CFRP system are quantified by measuring the residual properties of bolted CFRP. By adding short-aramid-fiber interleaves in CFRP laminates, the residual tensile strength have been substantially increased by 14 % for twill-weave laminates and 45 % for unidirectional laminates respectively. Moreover, tensile failure was observed as the major mode of toughened laminates, in contrast to shear failure of plain laminates. The qualitative FEM results agreed well with the experimental results that edge delamination would cause relatively higher shear stress and therefore alter the failure mode from tensile failure to shear failure.     

湿热环境下碳纤维增强乙烯基树脂复合材料长期力学性能

碳纤维增强聚合物基复合材料(CFRP)因其耐腐蚀、轻质高强等特点被广泛应用于海洋环境,进而长期遭受湿热环境的考验。为了解湿热环境和极端温度对碳纤维增强乙烯基树脂复合材料的影响,测试了湿热老化前后和不同温度下CFRP的压缩性能、面内剪切性能和层间剪切强度变化。FTIR和SEM结果表明：纯树脂试样在湿热环境中发生了水解,使试样表面的微裂纹和孔隙不断扩展并向试样内部渗透;碳纤维的埋入抑制了水的扩散和水解,因而CFRP的吸湿曲线与Fickian模型高度吻合;纯树脂由于水解反应影响了吸湿通道使吸湿曲线偏离Fickian模型。力学性能表明：湿热老化90天后压缩强度和层间剪切强度分别降低7.6%、12.3%;试样在高温(70℃)下的压缩强度、面内剪切强度、层间剪切强度分别急剧降低36.2%、26.9%、37.4%,且高温对试样力学性能的影响具有部分可逆性。     

Effect of Seawater and Warm Environment on Glass/Epoxy and Glass/Polyurethane Composites

A study of the durability of fiber reinforced polymer (FRP) materials in seawater and warm environment is presented in this paper. The major objective of the study is to evaluate the effects of seawater and temperature on the structural properties of glass/epoxy and glass/polyurethane composite materials. These effects were studied in terms of seawater absorption, permeation of salt and contaminants, chemical and physical bonds at the interface, degradation in mechanical properties, and failure mechanisms. Test parameters included immersion time, ranging from 3 months to 1 year, and temperature including room temperature and 65°C. Seawater absorption increased with immersion time and with temperature. The matrix in both composites was efficient in protecting the fibers from corrosive elements in seawater; however moisture creates a dual mechanism of stress relaxation—swelling—mechanical adhesion, and breakdown of chemical bonds between fiber and matrix at the interface. It is observed that high temperature accelerates the degradation mechanism in the glass/polyurethane composite. No significant changes were observed in tensile strength of glass/epoxy and in the modulus of both glass/epoxy and glass/polyurethane composites. However, the tensile strength of the glass/polyurethane composite decreased by 19% after 1 year of exposure to seawater at room temperature and by 31% after 1 year of exposure at 65°C. Plasticization due to moisture absorption leads to ductile failure in the matrix, but this can be reversed in glass/polyurethane composites after extended exposure to seawater at high temperature where brittle failure of matrix and fiber were observed.     

应用桥联模型预测复合材料吸湿老化剩余强度

采用试验与理论相结合的方法, 研究了玻璃纤维/环氧树脂复合材料的吸湿老化剩余强度, 测试了基体的吸湿性能, 对老化前后基体、 纤维及其复合材料的拉伸性能进行了试验研究。根据纤维和基体性能测试数据, 应用桥联模型计算了复合材料的吸湿老化剩余强度, 计算结果与试验结果一致。结果显示: 在小吸湿率条件下, 利用桥联模型能够准确预测复合材料的破坏演化规律及其最大剩余强度。当主承力层破坏由纤维控制时, 由湿老化引起的纤维强度的下降直接导致复合材料最大拉伸强度的下降; 当破坏由基体控制时, 影响复合材料吸湿剩余强度的主要因素是基体硬化及强度的下降。较小吸湿率对纤维与基体的界面粘结强度影响有限, 界面破坏不是层合板破坏的主要因素。      

Moisture absorption of unidirectional hybrid composites

Unidirectional hybrid composite rods were conditioned in humid air to investigate the sorption kinetics and the effects of moisture on mechanical and physical properties. Sorption curves were obtained for both hybrid and non-hybrid composite rods to determine characteristic parameters, including the diffusion coefficient (D) and the maximum moisture uptake (M_∞). The moisture uptake for the hybrid composites generally exhibited Fickian behavior (no hybridization effects), behaving much like non-hybrid composites. A two-dimensional diffusion model was employed to calculate moisture diffusivities in the longitudinal direction. Interfaces and thermally-induced residual stresses affected the moisture diffusion. In addition, the effect of hygrothermal aging on glass transition temperature (T_g), short beam shear strength (SBS), and tensile strength was determined for hygrothermal exposure at 60 °C and 85% relative humidity (RH). Property retention and reversibility of property degradation were also measured. Microscopic inspection revealed no evidence of damage.     

炭纤维强化板及其加固混凝土梁的力学性能

为探寻涂胶对炭纤维布力学性能的影响 , 深入了解其加固混凝土结构的力学行为 , 对制备的炭纤维强化板(CFRP)及其加固混凝土梁 , 分别进行了轴向拉伸和预制裂缝四点弯曲加载实验。对比有胶与无胶纤维布拉伸实验 , 表明环氧树脂不仅粘接纤维布 , 对纤维布有保护作用 , 而且能够显著提高其拉伸强度。通过对加固混凝土梁的 FRP外贴应变片的加载过程进行电测跟踪测试 , 得到了材料结构损伤破坏过程曲线。由力学模型和曲线点拟合的最小二乘法 , 对 CFRP板所受轴力及混凝土的界面剪应力分布进行分析 , 得到轴力及剪应力分布。在载荷的变化过程中 , FRP与混凝土的界面逐渐脱粘 , 使得 FRP的应力集中程度降低 , 有时会出现应变回弹现象。      

Adhesive repair of bismaleimide/carbon fiber composites with bisphenol E cyanate ester

The adhesive strength and repair efficiency of bisphenol E cyanate ester (BECy) is investigated for the injection repair of high temperature polymer-matrix composites (PMCs) by lap shear (LS), short beam shear bending (SBSB), and double-cantilever beam (DCB) tests. Bismaleimide/carbon fiber (BMI-cf) composites were chosen as a model substrate. The BECy resin showed similar strength at room temperature to a benchmark epoxy adhesive and outperformed the epoxy at high temperature (200 °C) in all mechanical tests performed. The influence of moisture content of the PMC substrate on the adhesive strength of BECy was systematically investigated. Drying of PMC before repair was necessary for excellent repair performance. Both the flexural strength of repaired SBSB specimens and the inter-laminar fracture toughness of repaired DCB specimens were significantly higher than that of the control composites and stable over a broad temperature range.     

Approximate elastic-plastic analysis of the static and impact behaviour of polymer composite sandwich beams

An elastic-plastic beam bending model has been developed to simulate the post-upper skin failure energy absorption behaviour of polymer composite sandwich beams under three-point bending. The beam skins consist of woven and chopped strand glass, while the core is a resin impregnated non-woven polyester material known as Coremat. A polyester resin was used for the construction. The theoretical model consists of a central hinge dominated by a crushing core and tensile elastic strains in the lower skin. Experimental measurements of the non-linear force-deflection characteristics for the beam are compared to the theoretical predictions from the model, and it is shown that the shear crushing of the core has an important effect on the behaviour of the beam. The model shows that the most important material properties are the lower skin tensile failure strain and the core crushing strength. Dynamic effects are included in the model in the form of a strain rate dependence of the core crushing stress and the strain rate dependence of the failure strain in the lower skin. The increase in material strength with strain rate gives rise to an improved energy absorption capacity for the beam under impact loading.     

Effects of exposure to elevated temperatures and subsequent immersion in water or alkaline solution on the mechanical properties of pultruded BFRP plates

In this article, a pultruded unidirectional basalt fiber-reinforced polymer (BFRP) plate was thermally aged at 135 °C and 300 °C for 4 h, and subsequently immersed in distilled water or strong alkaline solution (simulating concrete pore water, pH = 12.6–13) for 3 months. The variation of the tensile and interlaminar shear (ILSS) properties of the BFRP plates was studied. Thermal aging exhibited a slight effect on both the longitudinal tensile properties and the interlaminar shear strength, although thermal decomposition of the resin matrix started at 300 °C and brought in a high void content (4.8%). FTIR and DMTA results indicate that thermal aging lead to postcuring and oxidation of the resin matrix, leading to an increase of the glass transition temperatures. Thermal aging accelerated the degradation of the BFRP plates in distilled water or alkaline solution at 20, 40 and 60 °C. In the studied hash immersion conditions of 60 °C alkaline solution for 3 months, the unaged, 135 °C aged and 300 °C aged BFRP samples showed reduction in the tensile strength by 43.2%, 62.3% and 74.1%, respectively. The higher the thermal aging and immersion temperatures, the more deterioration of the mechanical properties occurred. Alkaline solution immersion showed more adverse effects compared to the distilled water. The detrimental effects of the thermal aging were attributed to the formation of voids and cracks through which water or alkaline solution tended to easily penetrate into the BFRPs. The degradation of the resin due to thermal aging and immersion was analyzed with dynamic mechanical thermal analysis and scanning electron microscopy analysis. The long term variation of the tensile strength of BFRPs was evaluated based on the Arrhenius equation.     

Al颗粒夹层CFRP复合材料力学及电磁屏蔽性能EI北大核心

CFRP复合材料具有优异的力学性能,在航空航天装备中有广泛应用,但是因其单层铺层内部的结构各向异性,单向纤维铺层对于垂直极化波的电磁屏蔽效能较弱。为应对日益复杂的电磁环境,保护电子元器件不受干扰,增强复合材料的电磁屏蔽效能显得尤为重要,本工作利用非连续Al颗粒在层间面内紧密排列,构建了一种层间面内含连续Al屏蔽层的CFRP复合材料,并研究了不同Al颗粒含量对复合材料电磁屏蔽效能和力学性能的影响规律。结果表明,随着Al颗粒含量的增加,CFRP复合材料的导电性和电磁屏蔽效能也随之增加,当聚合物中Al颗粒质量分数达到33.3%时,复合材料的面内电导率提高了3个数量级,在垂直于纤维方向上对频率为3~17 GHz的电磁波的电磁屏蔽效能提高了10 dB以上。随着Al颗粒含量的增加,复合材料层间剪切强度与弯曲强度出现先上升后下降的变化规律,当树脂中Al质量分数为33.3%时,复合材料的层间剪切性能提高了5.2%达到80.5 MPa,当树脂中Al质量分数为50%时,复合材料的弯曲强度提高了20%至1441.0 MPa,弯曲模量提高了10.2%达到101.83 GPa。由此可见,Al颗粒夹层CFRP复合材料可以实现力学性能和电磁屏蔽效能的同步提升,是一种具有广泛应用前景的结构-电磁屏蔽一体化复合材料。     

Experimental investigation of salt fog effect on the CFRP laminates

To investigate the effect of an ocean environmental condition to Carbon Fiber Reinforced Plastics (CFRP) specimen, salt fog tests were conducted on long immersion hours. After salt fog tests, tensile test was performed to examine the durability of specimens under salt fog condition. Fabric types, fiber orientations, and different manufacturing methods were also investigated to look into effects of salt fog and tensile strength. Diverse fabric types, resin types, and manufacturing methods exhibited different experimental results. Finally, the experimental results showed the salt fog reduced the tensile strength of CFRP laminates.     

电热作用对碳纤维树脂基复合材料力学性能的影响

采用复合材料电热实验平台,测试碳纤维树脂基复合材料（Carbon Fiber Reinforced Polymer,CFRP）电热作用下温度场变化规律,同时从单丝拉伸断裂界面剪切强度、短梁剪切性能变化和剪切断口等多方面揭示电热作用对CFRP力学性能的影响机制。结果表明:电热作用会使CFRP整体温度迅速升高,在约4 min时达到稳态温度,随着电流强度的增大,CFRP层板表面温度越高,当电流强度为8 A（0.44 A/mm2）时,CFRP的表面温度达到151℃;单丝拉伸和短梁剪切界面强度都随着电流强度增加呈现先增加后降低的趋势;小电流时,电热作用产生较少的焦耳热,优化界面性能,提高界面剪切强度,大电流时,电热作用产生的焦耳热过大,对界面产生烧蚀等不可逆损伤,降低了界面结合性能。