Mechanism of surface microcracking of matrix in glass-reinforced polyester by artificial weathering

The first stage in the deterioration of glass-fibre reinforced polyester (GRP) composites, fibre prominence, has been reported. The mechanism of the second stage, surface microcracking, is now described. Under controlled conditions GRP sheets were subjected to cyclic variation of moisture and temperature and to radiation. It is proposed that surface microcracking takes place under the combined action of radiation-induced tensile stresses in the surface region and physically-induced stress-fatigue. Tensile stresses in the surface region are caused by shrinkage of the matrix that results from cross-linking induced by the ultra-violet portion of radiation. Stress fatigue is imposed on the composite system by physically-induced alternating stresses produced by cyclic variation of temperature and, probably, moisture resulting from thermal and moisture gradients and inhomogeneities. Stress-fatigue probably plays a dominant role in microcracking induced by artificial weathering, whereas radiation-induced stresses in the surface region are more important in microcracking occurring in outdoor weathering.     

Mechanism of breakdown in the interface region of glass reinforced polyester by artificial weathering

Laboratory studies were conducted to elucidate the mechanism of breakdown in the interface region of glass-fibre reinforced polyester (GRP) composites on outdoor weathering. GRP composites were subjected to the effects of moisture, temperature and radiation. Breakdown in the interface region occurred when the GRP sheets were aged in the presence of water and physically-induced stress (thermally and/or by moisture). The stresses involved are complex, the most predominant being axial shear stresses. Fracture characteristics of breakdown produced during laboratory ageing were very similar to those occurring on outdoor weathering.According to the mechanism proposed, the resin in the interface region is subjected, during environmental ageing, to a stress-fatigue resulting from the differential dimensional changes between glass and matrix induced by moisture and/or temperature cyclic variations. Under the influence of alternating cyclic stresses and in conjunction with the chemical degradation of the matrix, the interface region undergoes cracking, fracture and fibre delamination. This type of breakdown may be referred to as environmental stress cracking.     

Accelerated weathering of fire-retarded wood–polypropylene composites

In this study, the influence of fire retardants, namely aluminum trihydrate, zinc borate, melamine, graphite, titanium dioxide on the durability of polypropylene-based co-extruded wood–plastic composites is studied. The composites underwent accelerated weathering under a xenon-arc lamp source during 1000 h. FTIR analysis of the composite surface revealed a degradation process which was accompanied by chemical changes, including vinyl-like and carbonyl groups accumulation; fire retardants did not influence the photo-oxidation mechanism of the composite. Fire retardant-loaded samples had smaller color change compared to the unfilled one. The tensile properties of all composites declined after the weathering. Significant changes in the surface morphology of the weathered composites were observed with a scan electron microscope.     

无机纳米粒子改性环氧树脂复合材料研究进展

综述了近年来无机纳米粒子改性环氧树脂复合材料的研究现状,概括了纳米粒子改性环氧树脂的方法,详细介绍了氧化硅、氧化铝、蒙脱土、氧化钛、碳酸钙等纳米粉体以及碳纳米管等改性环氧树脂复合材料取得的研究进展,展望了此类复合材料的发展趋势及应用前景。     

Accelerated weathering effects on the mechanical and surface properties of CFRP composites

This study provides data regarding the characteristics of three carbon fiber reinforced polymer (FRP) systems with three types of resins exposed to accelerated natural weathering conditions according to ASTM G-154-00a for 1000 h. The FRP system consists of three-layered CFRP sheets bonded together with an adhesive. Degussa, Sika and Fosroc were the providers of the three FRP systems. Three-point bend testing according to ASTM D790 showed that ultra-violet (UV) light exposure decreased the flexural modulus of the three-layered FRP samples by 10%, 36% and 2% for Deagussa, Fosroc and Sika samples, respectively. While the reduction for the resin samples was 11.8%, 8.3% and 14.3% for Deagussa, Fosroc and Sika samples, respectively. SEM analysis showed cracking and deterioration in the resin and FRP systems as a result of the exposure to natural weathering.     

茶粉/聚丙烯复合材料自然老化性能

为高值化利用茶产业剩余物,拓宽木塑复合材料中植物纤维来源,以茶粉（TD）为有机增强相,聚丙烯（PP）为基体,采用开炼-注塑工艺制备了TD/PP复合材料;为研究其户外应用及老化机制,采用户外自然老化形式考察了老化时间对TD/PP复合材料力学性能、材色及表面形貌的影响,并用FTIR分析了其老化机制。结果表明:与木材纤维相比,TD中纤维素含量更少,而各种形式的抽提物含量要高很多,纤维长径比更小。户外自然老化8个月后,TD/PP复合材料的亮度变化值、红绿轴色品指数变化值、黄蓝轴色品指数变化值及色差值分别为37.64、-10.40、-3.38和39.19;TD/PP复合材料的弯曲、缺口冲击强度、弯曲模量及硬度值分别降低了38.7%、42.4%、41.4%和9.14%。FTIR分析表明,在户外自然老化环境中,TD/PP复合材料中的PP分子链和TD中木质素组分发生了光照降解。SEM照片显示,随户外自然老化时间延长,TD/PP复合材料表面出现裂纹,且裂纹数量逐渐增多,深度和宽度增大,并且局部地方出现粉化现象。      

人工模拟菌类条件对塑木复合材性能的影响

研究了人工模拟菌类条件下塑木复合材重量的变化情况,采用差示扫描量热技术对腐朽后的塑木复合材进行了检测分析,并进行了扫描电镜的观察试验.     

Moisture absorption behavior of sugar palm fiber reinforced epoxy composites

In engineering practice, moisture absorption test is generally used for quality control purposes and to measure the degradation of the quality for the composite materials. The objectives of this study are to investigate the value of Fickian diffusivity constant, moisture equilibrium content and correction factor for the natural fiber composites. Tests were carried out on composite plates, which was a combination of sugar palm fiber and epoxy resins and two different fiber compositions have been chosen which were 10% and 20% by weight. Pure epoxy plates have been used for the control measures. The specimens were oven dried for 60 h in an air-circulating oven operated with 108 °C before being immersed in the constant temperature water bath, which the distilled water was set at 40 °C for the moisture absorption behavior test for 33 days. From this study, plates with 20% fiber loading possessed the highest amount of moisture prior to the moisture absorption behavior test, which is 0.93%. In moisture absorption behavior test, the corrected value of Fickian diffusivity constant for the 20% fiber loading is 3.76 × 10⁻⁷ mm²/s, which is the highest among other composites. It is shown that, for composite plates that contain higher fiber composition, the moisture absorption rate is even higher.     

Surface degradation of unsaturated polyester resin in Xe artificial weathering environment

Unsaturated polyester (UP) resin samples were exposed to artificial weathering environment produced by a xenon lamp light exposure and weathering equipment for different time intervals. The change of appearance, optical property, chemical structure and surface microstructure before and after exposure in this environment was monitored by spectrophotometer, gloss meter, Specular Reflection Fourier Transform Infrared (SR-FTIR) spectroscopy and three-dimensional digital microscope. The results showed that the surface of UP resin turned darker and yellower in the whole weathering stage. Gloss decreased with increasing aging time due to the increase of surface roughness. SR-FTIR results showed that ester bonds in polymeric chains broke in the aging process. With increasing aging time, small voids can be observed on UP surface and expanded gradually to cracks.     

Prediction of the fatigue life of natural rubber composites by artificial neural network approaches

A back-propagation artificial neural network (BP-ANN) model was established to predict fatigue property of natural rubber (NR) composites. The mechanical properties (stress at 100%, tensile strength, elongation at break) and viscoelasticity property (tan δ at 7% strain) of natural rubber composites were utilized as the input vectors while fatigue property (tensile fatigue life) as the output vector of the BP-ANN. The average prediction accuracy of the established ANN was 97.3%. Moreover, the sensitivity matrixes of the input vectors were calculated to analyze the varied affecting degrees of mechanical properties and viscoelasticity on fatigue property. Sensitivity analysis indicated that stress at 100% is the most important factor, and tan δ at 7% strain, elongation at break almost the same affecting degree on fatigue life, while tensile strength contributes least.     

Brittleness estimation of ceramic particulate composites

The possibility of calculating the brittleness parameter of ceramic particulate composites from indentation test data is investigated. Three different concepts are considered — based on threshold crack length, transition scratch depth and simple ratio of hardness to toughness. Threshold crack length based models predict higher values compared to the other two.     

Degradation evaluation of ethylene-propylene-diene monomer (EPDM) rubber in artificial weathering environment by principal component analysis

Artificial weathering tests on ethylene-propylene-diene monomer (EPDM) containing 5-ethylidene-2-norbornene (ENB) as diene were conducted in a xenon lamp light exposure and weathering equipment for different time periods. Principal component analysis (PCA) was used to evaluate the 12 degradation parameters of EPDM including surface properties, crosslink density and mechanical properties. The results showed that the combined evaluating parameter Z of EPDM degradation increased quickly in the first 12 days of exposure and then leveled off. After 45 days, it began to increase rapidly again. Among the 12 degradation parameters, the crosslink density is strongly associated with tensile stress at 300% elongation and tear strength. The surface tension is also well correlated with the color aberration.     

MXene基水凝胶复合材料的研究进展

MXenes是一类新型二维纳米片,随着MXenes材料的迅速发展,近年来,兴起了一种新型材料,即MXene基水凝胶复合材料,其在生物医学、能源、电磁干扰屏蔽、传感器等方面均具有广泛的应用前景.但目前MXene基水凝胶复合材料的制备和应用仍处于起步阶段.本文主要回顾MXene基水凝胶复合材料的最新进展,详细梳理MXene...     

聚合物基介电复合材料研究进展

以聚合物为基体的介电复合材料在微电子等领域具有重要的应用前景,因而得到了广泛关注。从聚合物基介电复合材料的不同结构类型及其研究进展、介电理论模型,及其主要应用领域三个方面对国内外的相关研究进行了综述。     

Effect of wood species on property and weathering performance of wood plastic composites

The effects of wood species on mechanical, thermal and accelerated weathering behaviors of high-density polyethylene based wood plastic composites (WPC) were investigated. The selected wood species are poplar, Douglas-fir, black locust, white oak, and ponderosa pine. Hybrid poplar and Douglas-fir based composite ranked highly in flexural properties. Thermogravimetric analysis indicated that thermal stability of the WPC is species dependent. The final decomposition of hemicelluloses in the hardwoods occurred at higher temperatures compared to that of softwoods. Color and chemical changes that occurred due to accelerated weathering were monitored using colorimetry and Fourier transform infrared spectroscopy. Weathered WPCs showed that color change and lightness increased with exposure time; degree of increase depends on wood species. WPC oxidation by weathering was assessed by carbonyl group concentration and was shown to increase with exposure. Hybrid poplar and ponderosa pine were shown to have good color stability among the wood species examined for WPC.     

The potential of using date palm fibres as reinforcement for polymeric composites

Interfacial adhesion of natural fibres as reinforcement for fibre polymeric composites is the key parameter in designing composites. In the current study, interfacial adhesion of date palm fibre with epoxy matrix is experimentally investigated using single fibre pull out technique. The influence of NaOH treatment concentrations (0–9%), fibre embedded length and fibre diameter on the interfacial adhesion property was considered in this study. Scanning Electron Microscopy (SEM) was used to observe the surface morphology and damage feature on the fibre and bonding area before and after conducting the experiments. The results revealed that 6% concentration of NaOH is the optimum solution for treating the date palm fibre to maintain high interfacial adhesion and strength with epoxy matrix. The embedded length of the fibre controlled the interfacial adhesion property, where 10 mm embedded length was the optimum fibre length.     

地外人工光合成材料研究进展

太空探索已成为人类共同目标,重返月球、载人火星等人类历史上的重大里程碑任务已逐步实施。如何实现地外极端环境下人类生存和发展已成为载人太空探索的基本能力和基础技术。由南京大学和钱学森空间技术实验室提出的地外人工光合成技术,模拟地球绿色植物的自然光合作用,利用密闭空间废弃资源或地外天体环境中丰富的资源,通过光电催化方法原位、加速、可控地将二氧化碳转化成为氧气和含碳燃料,大幅度降低载人航天器的物资供应需求,支撑可承受、可持续的载人深空探索。本文回顾了近年来国际航空航天领域利用二氧化碳转换生成氧气和碳氢燃料的现有方法,并深入探讨面向地外原位资源利用的人工光合成材料研究进展,期望深化对地外人工光合成材料与技术的认知,有力支撑载人航天发展。     

纤维增强复合材料磨削制孔加工技术研究进展

纤维增强复合材料具有优良的物理、化学和力学性能,在航空航天、汽车、新能源等高新技术领域应用广泛。相比传统钻铣刀具,磨料工具在纤维增强复合材料制孔时,加工后的分层、毛刺、撕裂及热损伤等缺陷更小,且磨料工具可以稳定加工硬度更高的纤维增强陶瓷基复合材料。首先,综述了纤维增强复合材料在磨削制孔过程中的切屑形成、磨削轴向力、磨削温度等磨削加工机制;其次,探讨了近年来国内外在纤维增强复合材料磨削制孔技术中的制孔加工缺陷及其评价方法;然后,分析了纤维增强复合材料磨削制孔质量及其影响因素;此外,综述了纤维增强复合材料磨削制孔刀具及其磨损机制等方面的研究现状;最后,对纤维增强复合材料磨削制孔加工技术研究进行了总结和展望。      

Theoretical estimation of fracture toughness of fibrous composites

A method of estimating the fracture surface energy of fibre-reinforced materials is discussed. The surface work is shown to increase with increasing fibre content, strength and diameter, and decrease with increasing fibre modulus and matrix flow stress (or hardness).Relatively short fibres should be used if high toughness is required, and the maximum toughness that can be achieved is limited by the amount of crack opening that can be permitted. Under certain conditions, incorporation of fibres into a material can lead to embrittlement.Symbols used c half length of crack - d fibre diameter - D average separation of nearest neighbour fibres - E Young's modulus - G shear modulus of matrix - K fracture toughness - L length of fibre on either side of crack - n number of fibres per unit area - P force on fibre - R pressure exerted by matrix on fibres - u displacement - U work - x distance Greek symbols 8 G/d ² E _fr log(2/p 3) - surface energy or work - tensile strain - _fm E _mr/E_fr R - tensile stress - shear stress - coefficient of friction - Poisson's ratio Suffixes c composite - e elastic - f fibre - m matrix     

Data assimilation-based state estimation of composites during molding

Monitoring methods used to observe the molding process of carbon fiber-reinforced plastic (CFRP) facilitate the development of high-quality products. However, current monitoring methods provide only localized state information, and estimating the state of the entire material is difficult. This study proposes a method for estimating the overall state of CFRP during molding based on data assimilation, which integrates theoretical and experimental values. In this method, three types of specimens with different thermal conductivities were considered, and the temperatures throughout each specimen during molding were estimated by data assimilation of surface temperatures measured using thermocouples. To validate the accuracy of this method, the temperatures estimated by data assimilation were compared with those obtained using numerical simulations without data assimilation. The experimental results of all the specimens showed that data assimilation can be used to accurately estimate the surface temperature. The proposed method is therefore effective for monitoring composite molding processes.