Moisture absorption by epoxy/montmorillonite nanocomposite

The peculiarities of moisture absorption of epoxy–nanoclay composite are estimated in the paper. Second Fick’s law of diffusion was used to predict moisture diffusivity and equilibrium moisture content using accelerated analytical procedure. It was experimentally confirmed that sorption process in NC passes more slowly than in pure epoxy resin, for the highest filler content diffusivity reduces about half of diffusivity as for epoxy resin. The deviation from mixture rule was obtained for the equilibrium moisture content and the estimation of interphase content in composite was undertaken. It was determined that the higher content of interphase consistently leads to greater moisture absorption.     

Helium gas permeability of montmorillonite/epoxy nanocomposites

Helium gas permeability of silicate clay (montmorillonite) particles/epoxy nanocomposites was examined. The incorporation of increasing amounts of montmorillonite particles reduced the helium gas permeability. Based on Fick’s law, gas permeation behavior of the nanocomposite was evaluated. With the increase of montmorillonite loading, gas diffusivity decreased, while gas solubility increased. Helium diffusion behavior is in agreement to the numerical results based on the Hatta–Taya–Eshelby theory. It has been revealed that dispersion of nanoscale platelets in polymer is effective in improving gas barrier property.     

Rapidly cured epoxy/anhydride composites: Effect of residual stress on laminate shear strength

The drive towards rapid cure thermosetting composites requires a better understanding of the residual stresses that develop during curing. This study investigates the impact of residual stresses on the interlaminar shear strength of resin-infused epoxy/anhydride carbon-fibre laminates. The magnitude of the residual stress was varied by changing the initial injection cure temperature between 75 °C and 145 °C. The corresponding cycle times and the final glass transition temperature of the resin were also measured. The experimentally measured chemical shrinkage and thermal expansion properties of the resin after vitrification were used as inputs to a finite element analysis to calculate the peak residual stresses in the composite. An increase in the initial cure temperature from 85 to 135 °C resulted in an increase of 25% in the residual stress, which led to an experimentally measured reduction in the composite’s short beam shear strength of approximately 16% (8 MPa), in good agreement with model prediction.     

Study on the curing process for carbon/epoxy composites to reduce thermal residual stress

In this work, a cure monitoring system using dielectrometry and a fiber Bragg grating (FBG) sensor, was devised to measure the dissipation factor and thermal residual stress of carbon fiber-reinforced epoxy composite materials. Three rapid-cooling points, which were based on the cure initiation point, were chosen as test variables to investigate the effect of cure cycle on process-induced internal strain. The internal strains generated in the composite specimens were measured using embedded FBG sensors. Three-point bending tests were conducted to investigate the effect of thermal residual stress on the flexural strength of the composite specimens.     

Study on fracture behavior of surface treated montmorillonite/epoxy nanocomposites

It is known that the mechanical properties of clay-reinforced nanocomposites are significantly affected by the dispersion of clay particles in the matrix. In this study, the effect of surface-treatment of Montmorillonite (MMT) on the fracture behavior of MMT/epoxy nanocomposite was investigated. For this purpose, fracture tests were performed using samples with three different clay concentration level. After fracture tests, SEM analysis was made on the fracture surfaces to examine the fracture mechanism. It was found that the MMT treatment using 3-aminopropyltriethoxysilane enhanced the fracture toughness increased of the MMT/epoxy nanocomposite. This is due to the improved intercalation effect and interfacial strength between MMT and epoxy matrix.     

Impact of volume fiber on distribution of thermal stress residual near fiber/epoxy interface using axisymmetric model

Thermal residual stresses are important in composite materials. The aim of this work is the computation of thermal residual stresses by finite element method and the effect of volume fraction on their distribution. In this work, two cases are considered by using an epoxy matrix with respectively glass and carbon fibers with a different volume fiber using an axisymmetric model. From the results of the numerical calculation, it is shown that the stresses are important and thus should be taken into account. The interface is affected by thermal stresses. The normal stresses and shear stress value have an influence on the behavior of the material. Hence, on the performance of composites during service, is an information of significance for the designers.     

碳纳米管-有机化蒙脱土多维纳米界面构筑及其对环氧树脂的增韧机制

分别采用混酸、环氧树脂（EP）和硅烷偶联剂对碳纳米管（CNTs）进行功能化处理,用十八烷基三甲基氯化铵对蒙脱土（MMT）进行有机化处理,将具有一维纳米尺度的CNTs和二维纳米尺度的有机化蒙脱土（OMMT）复合引入EP酸酐固化体系,通过溶液共混法制备纳米OMMT/EP、CNTs/EP、CNTs-OMMT/EP复合材料。使用简支梁冲击试验仪测试三种复合材料的冲击强度,并利用SEM观察纳米复合材料的冲击断面形貌。实验结果表明,当OMMT的含量为4wt%时,纳米OMMT/EP复合材料的冲击强度比未掺杂纳米组分的EP提高了16.7%。经硅烷偶联剂处理后的CNTs（Si-CNTs）能与EP基体形成良好界面,当Si-CNTs的含量为0.9wt%时,纳米Si-CNTs/EP复合材料冲击强度比未掺杂纳米组分的EP提高了84.0%。当OMMT的含量为4wt%、Si-CNTs的含量为0.9wt%时,纳米Si-CNTs-OMMT/EP复合材料的冲击强度比未掺杂纳米组分的EP提高了135.4%。管状CNTs和片层结构OMMT对EP的韧性具有协同提高作用。     

Fatigue damage mechanisms and residual properties of graphite/epoxy laminates

Damage mechanisms and accumulation, and associated stiffness and residual strength reductions were studied in cross-ply graphite/epoxy laminates under cyclic tensile loading. Stress-life data were fitted by a two-parameter wearout model and by a second-degree polynomial on a log-log scale. The fatigue sensitivity is highest for the unidirectional laminates and it decreases for the crossply laminates with increasing number of contiguous 90° plies. Five different damage mechanisms were observed: transverse matrix cracking, dispersed longitudinal cracking, localized longitudinal cracking, delaminations along transverse cracks, and local delaminations at the intersection of longitudinal and transverse cracks. Failure patterns vary with cyclic stress level and number of cycles to failure. Under monotonie loading, failure is brittle-like and concentrated. At high stress amplitudes and short fatigue lives failure results from few localized flaws, whereas at lower stress amplitudes and longer fatigue lives failure results from more dispersed flaws. The residual modulus shows a sharp reduction initially, followed by a more gradual decrease up to failure. The residual strength showed a sharp reduction initially, followed by a plateau or even some increase in the middle part of the fatigue life, and a rapid decrease in the last part of the fatigue life. A tentative cumulative damage model is proposed based on residual strength and the concept of equal damage curves.     

环氧树脂/聚氨酯梯度材料的残余热应力分析

采用有限元方(Finite Element Method)对环氧树脂/聚氨酯(EP/PU)梯度材料在制备过程中产生的残余热应力进行了分析.详细讨论了梯度层数目对应力大小的影响.研究结果表明:当体系中EP和PU组成相同时,梯度材料缓和热应力的效果比双层材料显著.且梯度材料的热应力缓和效果随梯度层数增加而增加.     

Reduction of residual stress and deformation in electron beam welding by using multiple beam technique

The electron beam could be controlled by magnetic field for fast deflection, in which way multi-beam could be produced by deflection technique. The multi-beams run simultaneously for material processing with different heat input and positions. Therefore, it is possible to adjust the thermal effects and optimize the process. In this paper, the generation of multi-beams in electron beam welding (EBW) was investigated, and the processes of EBW with multi-beams were also investigated by both the numerical simulation methods, i.e., finite element analysis (FEA), and the experiments. The result shows that the residual stress of EBW could be minimized by using the multiple beam technique, and at the same time the welding deformation could also be reduced with the optimized parameters.     

Reduction of residual stress and deformation in electron beam welding by using multiple beam technique

The electron beam could be controlled by magnetic field for fast deflection, in which way multi-beam could be produced by deflection technique. The multi-beams run simultaneously for material processing with different heat input and positions. Therefore, it is possible to adjust the thermal effects and optimize the process. In this paper, the generation of multi-beams in electron beam welding (EBW) was investigated, and the processes of EBW with multi-beams were also investigated by both the numerical simulation methods, i.e., finite element analysis (FEA), and the experiments. The result shows that the residual stress of EBW could be minimized by using the multiple beam technique, and at the same time the welding deformation could also be reduced with the optimized parameters.     

The sensitivity of the superconducting transition temperature to residual stress in high-T c intermetallic compounds

Residual-stress-induced superconductivity is reported to occur only in the high-T _c members of the C14 and C16 crystal systems. Second transitions are also seen in ThIr₂(C15) and MoRe₃(A12). The change inT _c , as a result of introducing large residual stresses, increases with the bulkT _c .Research sponsored by the Air Force Office of Scientific Research, Office of Aerospace Research, U.S. Air Force, under AFOSR Grant No. AF-AFOSR-631-67-A.     

Thermophysical analysis of plasticized epoxy compounds

A study has been made concerning the effect of dibutylphthalate and thiocol plasticizers in various concentrations on the structure as well as the thermophysical and the mechanical properties of grade ÉD-5 epoxy resin. It is shown that the entire complex of analyzed thermophysical properties changes appreciably, depending on the kind and on the concentration of the plasticizer. Conclusions are drawn pertaining to the mechanism of epoxy resin plasticization by these two additives at various concentration levels.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 24, No. 3, pp. 499–506, March, 1973.     

热残余应力对考虑微观孔隙碳纤维增强环氧树脂复合材料横向拉伸性能的影响

为研究由于材料固化产生的热残余应力对碳纤维增强环氧树脂复合材料横向拉伸性能预测结果的影响,发展了一种基于摄动算法的纤维和孔洞随机分布代表性体积单元（RVE）生成方法,建立更加接近真实材料微观结构的RVE模型。通过施加周期性边界条件,并赋予组分（纤维、基体和界面）材料本构关系,进而实现温度和机械荷载下模型的热残余应力和损伤失效分析。从结果中发现,材料固化过程会在纤维之间产生残余压应力,在模型孔隙周围产生沿加载方向的残余拉应力。所建立不含孔隙RVE模型的失效均是由于界面脱黏引起,材料固化在纤维之间产生的残余压应力会增加模型的预测强度。含有孔隙的RVE模型失效起始于孔隙周围的基体中,而材料固化在模型孔隙周围产生的热残余拉应力对含孔隙RVE模型预测的失效强度有降低作用。对于具有不同孔隙尺寸的RVE模型,模型的失效强度随着孔隙尺寸的增加而不断降低,但是热残余应力减弱了孔隙尺寸对模型预测结果的降低作用。对于具有不同长宽比椭圆形孔隙的RVE模型,热残余应力增加了孔隙长宽比对模型强度的降低作用。      

管状陶瓷金属封接残余应力计算比较

陶瓷/金属封接过程中由于温差容易产生残余应力,将影响到封接强度和产品的可靠性.应用薄壳理论公式和ANSYS有限元分析软件对典型筒状陶瓷/金属对封结构的残余应力分布进行了计算比较,指出了薄壁理论公式的适用范同,同时讨论了异材界面端存在应力奇异性的问题.     

Incorporation of montmorillonite in epoxy to obtain a protective layer prepared by electrophoretic deposition

An electrophoretic deposition (EPD) process was successfully used to obtain a composite epoxy coating containing montmorillonite. To disperse and obtain the intercalation of montmorillonite particles, a suitable procedure was optimized. This procedure did not require the use of additional chemicals and it was performed at room temperature.The microstructural analysis (TEM and XRD) confirmed that the intercalation of montmorillonite layers with epoxy was achieved. Functional characterisations (EIS and TMA) demonstrated the improved properties of the reinforced epoxy coating with respect to the simple epoxy coating.Moreover, the EPD process resulted more efficient in obtaining a montmorillonite-reinforced epoxy coating than a simple epoxy coating.     

Thermal stress hysteresis and stress relaxation in an epoxy film

Thermal cycling of an epoxy coating on silicon through the glass transition temperature (T _g) revealed a large stress hysteresis on the first thermal cycle through T _g and a change in the stress–temperature slope at T _g resulting from the change in the epoxy elastic properties due to the glass transition. This stress hysteresis was not observed on subsequent thermal cycles through T _g. However, after the coating was annealed (aged) below T _g (for hours or longer)—during which the stress relaxed exponentially with time—the stress hysteresis returned. The magnitude of stress hysteresis, on cycling through T _g, was found to correlate to the magnitude of long-time relaxation that occurred during annealing at temperatures below T _g.     

Texture and residual stress in FeMn/Ni80Fe20 multilayers

The texture and residual stress in FeMn/Ni₈₀Fe₂₀ multilayers were studied using conventional sin²ψ method. The results show that the FeMn and Ni₈₀Fe₂₀ layers are both (111) textured. The sin²ψ plots are nonlinear, which indicates a strong residual stress gradient through the depth of the sample. Fitting of the sin²ψ plots give the residual stress in the samples. The in-plane residual stress is tensile and decreases from the surface of the sample to the substrate. The out-of-plane residual stress cannot be neglected because of the strong texture. In the two samples with the same total thickness but different periods, the FeMn layers have the same texture and residuals stress; while for Ni₈₀Fe₂₀ layer, as the thickness of each period increases, the texture becomes stronger and the residual stress becomes larger. It appears that the in-plane residual stress increases as the texture becomes stronger.     

Evaluation of residual strains in epoxy with different nano/micro-fillers using embedded fiber Bragg grating sensor

The influence of SiO₂ nanoparticles and rubber micro-fillers on the mechanical and thermal responses of an epoxy based composite is investigated using classical quantitative thermo-mechanical testing (tensile tests, DMTA, TMA), microstructural analysis (Micro-CT, TEM, SEM microscopy) as well as distributed optical sensing in order to determine different residual strain fields generated during processing. The results show that the tensile modulus of the compounds increases with the addition of SiO₂ and decreases with the rubber content, following estimates of the Hashin–Shtrikman model. The coefficient of thermal expansion appears to be insensitive to the particles’ content in the temperature range investigated. The residual strains generated during processing are influenced by the rubber content that introduces a strong relief, with respect to the one generated by the neat resin, whereas the silica content tends to increase their level.