Fracture mechanism characterization of cross-ply carbon-fiber composites using acoustic emission analysis

The sequence of microscopic fracture mechanisms in locally loaded cross-ply carbon-fiber composites was studied by analyzing acoustic emission (AE) signals in combination with the modal analysis of Lamb waves, using microscopic and ultrasonic examination of the specimen after load interruption. The first 70 AE events were analyzed, which were detected during the initial loading segment when the first sudden load drop and gradual load recovery were observed. Characteristics of the detected waves were compared with the S₀- and A₀-mode Lamb waves produced by a spot- or line-focused YAG laser. The internal damage progression of the composite specimen was determined to be the fiber fracture in the front lamina, transverse cracks in the mid-lamina, delamination and splitting.     

New environmental barrier coating system on carbon-fiber reinforced silicon carbide composites

Carbon-fiber-reinforced silicon carbide composites (C/SiC) are promising materials for high-temperature, light weight structural components. However, a protective coating and environmental barrier coating (EBC) are necessary to prevent the oxidation of the carbon and the reaction of the formed silica scale with water vapor. Current EBC systems use multiple layers, each serving unique requirements. However, any mismatch in the coefficients of thermal expansion (CTE) creates internal stresses and might lead to crack formation. In this case, oxygen and water vapor penetrate through the EBC, reducing the lifetime of the component. Mullite (Al₆Si₂O₁₃) is used in many known EBC systems on silicon-based ceramics either as an EBC itself or as a bondcoat. Due to its low CTE and its sufficient thermal cycling behavior, mullite was chosen in this investigation as a first layer. As mullite suffers loss of SiO₂ when exposed to water vapor at high temperatures, an additional protective top coat is needed to complete the EBC system. Different oxides were evaluated to serve as top coat, especially high-temperature oxides with low coefficients of thermal expansion (LCTE). An EBC containing mullite as bondcoat and the LCTE oxide La₂Hf₂O₇ as a top coat is proposed. Both layers were applied via atmospheric plasma spraying. In this paper, results of the influence of processing conditions on the microstructure of single mullite and LCTE oxide layers as well as mullite/LCTE oxide systems are presented. Special emphasis was directed toward the crystallinity of the mullite layer and, in the top layer, toward low porosity and reduced crack density.     

Processing and microstructure of alumina-based composites

A study of powder structure and its effect on the sintering tendency of certain alumina-based ceramic systems, that is, Al₂O₃-SiO₂ and Al₂O₃-ZrO₂, was carried out to improve their mechanical strength and fracture toughness. The compacting behavior and the sintering characteristics were optimized through control of various parameters such as composition, compaction pressure, sintering temperature, and time. Best densification was obtained for mixtures prepared using very fine and deagglomerated alumina powders.     

Metal-matrix composites for space applications

From the onset of the space era, both organic-matrix and metal-matrix composites (MMCs), with high specific stiffness and near-zero coefficient of thermal expansion (CTE), have been developed for space applications. Of the organic-matrix composites, graphite/epoxy (Gr/Ep) has been used in space for truss elements, bus panels, antennas, wave guides, and parabolic reflectors in the past 30 years. MMCs possess high-temperature capability, high thermal conductivity, low CTE, and high specific stiffness and strength. Those potential benefits generated optimism for MMCs for critical space system applications in the late 1980s.^1,2 The purpose of this article is to detail the history, status, and opportunities of MMCs for space applications. Editor’s Note: A hypertext-enhanced version of this article can be found at www.tms.org/pubs/journals/JOM/0104/Rawal_0104.html. For more information, contact S.P. Rawal, Lockheed Martin Space Systems-Aeronautics Operations, Advanced Structures and Materials and Thermal Control Group, Denver, Colorado.     

Polypyrrole composites for shielding applications

This article highlights the physical properties of polypyrrole (PPy) coated over MnZn ferrite (MZF), nickel coated over PPy, and PPy coated over Ni-MZF magnetic core particles. The commercial ferrite and Ni-ferrite particles are primarily used, and the PPy-ferrite particles with composite structure are synthesized both via an oxidative polymerization of pyrrole in an aqueous solution, which contains well-dispersed ferrite particles, and electrochemical polymerization technique in acetonitrile (ACN). The materials have been processed in the form of coatings, films, and sheets by blending of conventional polymer such as polyurethane (PU) wherein the composites retain the mechanical properties of the conventional polymers and the electrical conductivity of the conducting polymers. The influence of ferrite content with respect to the electrical and ferromagnetic properties of PPy composites were investigated by electrochemical impedance spectroscopy (EIS) and dc field-cooled, zero-field cooled susceptibilities and M–H loop measurements. Their structural characterization is also discussed based on Fourier transform infrared (FTIR) and the X-ray diffraction (XRD) measurements. The shielding effectiveness (SE) properties will be reported in our future studies.     

Zirconium-based composites for orthopedic applications

A relatively new composite material consisting of a ZrC matrix reinforced by ZrB₂ platelets and containing residual zirconium exhibits a combination of high strength, fracture toughness, and specific stiffness. In preliminary tests, this material compared favorably to the commonly used implant alloys Ti-6Al-4V and Co-Cr-Mo. In addition, the results of wear tests on this material against ultrahigh-molecular-weight polyethylene indicate promising results for use in orthopedic devices.     

Aluminum composites for automotive applications: A global perspective

The advancement of metal-matrix composites in the automotive market is still hampered by the low-volume usage of these materials, which is caused by their high cost in comparison with aluminum alloys and, in some cases, by the lack of theoretically predicted properties. Many significant challenges must be met as these materials reach maturity and the technology is scaled-up for automotive-component fabrication. The successful commercialization of metal-matrix composites will ultimately depend on their cost effectiveness for different applications. This requires optimum methods of processing, machining, and recycling, including some very new and advanced forming routes. For more information, contact V.M. Kevorkijan, Stampal SB d.o.o., Partizanska 38, Slovenska Bistrica 2310, Slovenia; telephone 386 62 632 567; fax 386 62 636 660; e-mail kevorkijan.varuzan@amis.net.     

Processing and microstructure of SiC laminar composites

Porous laminar materials and alternate laminates of dense and porous layers in silicon carbide have been elaborated by tape casting and liquid phase sintering (YAG-alumina eutectic) processing. Porosity was introduced by the incorporation of pore forming agents (PFA) (5–50 vol%) in the slurry. Two types of PFA with a narrow size distribution have been used (corn starch and polyamide powders). The effects of size, content, type of PFA on tape casting processing, sintering characteristics, porosity control and microstructure are investigated. For each PFA, the porosity attains a maximum value dependent on the PFA nature (41 vol% with corn starch). Only for starch, the volumetric shrinkage was unaffected by the PFA content up to 45 vol%. Homogeneous distribution of porosity has been obtained for both monolithic and composite laminates. An equiaxed and homogeneous silicon carbide microstructure has been obtained and was unaffected by PFA. Layered structures without defects have been obtained with parallel layers and uniform thickness (dense layer: 70 μm; porous layer: 80 μm).     

钢铁工业烧结烟尘电除尘技术的特点及应用

针对钢铁工业烧结机机头、机尾烟气的不同特点,结合烧结烟尘排放标准,从电除尘电场特性、气流分布以及振打等方面,讨论了烧结机电除尘器选型设计和技术改造中应考虑的几个问题,并给出应用实例。     

Fabrication of RGO/Cu composites based on electrostatic adsorption

To address the issues of reduced graphene oxide (RGO) dispersion in copper (Cu) matrix and interface bonding between RGO and Cu, an electrostatic adsorption method with interface transition phase design was employed to prepare the RGO/Cu based composites. Cu-Ti alloy powder was employed to improve the combination by forming carbides at the RGO-Cu interface. It was noted that the mechanical property of 0.3wt.%RGO/Cu-Ti composite was increased by 60% compared with that of the matrix. Strengthening mechanism analysis suggested that the enhancement of the mechanical property was ascribed to the load transfer and second phase strengthening which were from the improved dispersion of RGO and the in-situ formed titanium carbide phase.     

Processing ceramic-matrix composites using electrophoretic deposition

A novel, cost-effective processing technique for manufacturing ceramic-matrix composites (CMCs) containing two-dimensional woven-fiber reinforcement has been developed. The technique relies on the electrophoretic deposition (EPD) of ceramic sols on/into the fiber preforms to achieve the required impregnation. The laboratory-scale results achieved thus far indicate that the processing approach offers great potential for the manufacture of high-quality, composite products with dual-component oxide matrices for high-temperature structural applications.     

Processing high-temperature refractory-metal silicide in-situ composites

High-temperature, refractory-metal, intermetallic, in-situ composites consist of high-strength, niobium-based silicides with a niobium-based metallic toughening phase. A variety of processing schemes have been used to generate these in-situ composites, including solidification and vapor phase processes. Secondary processing, such as forging and extrusion, has also been employed. These composites offer an excellent balance of high-and low-temperature mechanical properties with promising environmental resistance at temperatures above 1,100°C. Authors’ Note: All compositions are given in atom percent. For more information, contact B.P. Bewlay, General Electric Company, Corporate Research and Development, Schenectady, New York 12301; (518) 387-6121; fax (518) 387-5576.     

Natural-fiber-reinforced polymer composites in automotive applications

In the past decade, natural-fiber composites with thermoplastic and thermoset matrices have been embraced by European car manufacturers and suppliers for door panels, seat backs, headliners, package trays, dashboards, and interior parts. Natural fibers such as kenaf, hemp, flax, jute, and sisal offer such benefits as reductions in weight, cost, and CO₂, less reliance on foreign oil sources, and recyclability. However, several major technical considerations must be addressed before the engineering, scientific, and commercial communities gain the confidence to enable wide-scale acceptance, particularly in exterior parts where a Class A surface finish is required. Challenges include the homogenization of the fiber's properties and a full understanding of the degree of polymerization and crystallization, adhesion between the fiber and matrix, moisture repellence, and flame-retardant properties, to name but a few.     

Cast aluminum-matrix composites for automotive applications

The potential automotive applications of metal-matrix composites, particularly aluminum-matrix composites, are numerous. Although some composite components have reached the demonstrator stage, there is still much work to do and many barriers to conquer before widespread application can be expected. These challenges include such issues as processing for specific properties, compiling property databases and addressing recyclability.     

Polymer composites for long-term structural applications

The potential new markets for polymer-matrix fiber composites (PMCs) are in the commercial aircraft, transportation, machinery, marine, and public works industries. A common requirement for many of the envisioned applications is the need to withstand structural loading for extended periods of time. Although good design using PMCs usually implies exploiting the high strength and stiffness properties of the fiber reinforcement, this article demonstrates that the matrix and interface properties play primary roles in the long-term durability of these materials.     

碳纳米管-银复合材料的制备工艺和电导率

采用粉末冶金方法制备碳纳米管-银复合材料,研究了制备工艺、碳纳米管含量对碳纳米管-银基复合材料密度、硬度、抗弯强度、电导率的影响.结果表明:采用复压烧结,烧结温度为700℃时,复合材料的性能较好;碳纳米管和银的弱界面结合,使得碳纳米管对复合材料的强化效果不明显;当碳纳米管的体积含量大于10%时,碳纳米管在晶界上发生偏聚,碳纳米管-银界面对电子产生散射,导致复合材料的电阻率迅速增加.     

Processing iron-aluminide composites containing carbides or borides

Iron-aluminide composites containing 30–90 vol. % carbides or borides can be processed to near full density (greater than 97% theoretical density). This wide range of ceramic contents enables the tailoring of the composite properties to a variety of applications requiring a combination of the corrosion and oxidation resistance of iron aluminides and the hardness and wear resistance of the ceramic phases. The composites are processed by conventional liquid-phase sintering of mixed powders as well as pressureless melt infiltration. Typical mechanical properties such as hardness, flexure strength, and fracture toughness were evaluated for composites containing different volume fractions of carbide or boride particulates. Furthermore, evaluations of the wear resistance, oxidation resistance, aqueous corrosion resistance, and thermal expansion of the iron-aluminide composites suggest many potential applications for these new materials.