Key Engineering Materials Vol. 425

Abstract: Composite materials have shown their efficiency in improving the mechanical properties of concrete structures, in addition to ensuring better resistance to environmental conditions. Reinforced concrete structures are often very sensitive to accidental loads, leading to deterioration, failures and human life fatalities. The reinforcement of concrete columns by composite materials, judiciously integrated in the concrete matrix, has the advantage of offering sufficient rigidity and strength to prevent overall collapse, on one hand, and, to preserve external and esthetic aspects of reinforced concrete works, on the other hand. The experimental and numerical studies in the present work represent a promising revelation regarding the effectiveness of the proposed confinement process by integrating a composite grid inside the reinforced concrete matrix. The concepts of single and double confinement are developed and discussed on the basis of experimental results for concrete specimens.

Abstract: The demand for lightweight structures in the automotive and aerospace industry increases permanently, and the importance of lightweight design principles is also increasing in other industrial branches, aiming towards improved energy efficiency and sustainability. Light metals are promising candidates to realize security relevant lightweight components because of their high specific strength; and amongst them, aluminum alloys are the most interesting materials due to their high plasticity and strain to failure, good processability, passivation in oxygen containing atmosphere, and low cost. However, for many applications, their stiffness as well as strength and fatigue behavior at elevated temperature are insufficient. Metal matrix composite (MMC) formation by integration of reinforcements in the form of continuous or discontinuous (short) fibers can yield a high increase in the alloys’ specific mechanical properties at room temperature and at elevated temperature. The integration of fibers with conventional manufacturing techniques like squeeze casting, hot pressing or diffusion bonding leads to restrictions in the component’s geometry. Moreover, these techniques result in elevated process costs mainly caused by long cycle times and the need of additional protective fiber coatings. In the present paper, an alternative method for the manufacturing of aluminum matrix composites is described, combining thermal spraying and semisolid forming (thixoforging) technologies for the formation of fiber prepregs and subsequent forming with simultaneous densification. Therefore, prepregs with the matrix alloy as a thick surface coating on the reinforcement fibers are manufactured in a fast, automated coating process, while reheating, densification and shaping are performed in a separate process, allowing an optimization of both processes towards cycle times and resulting material properties. Continuous fiber and short fiber reinforced aluminum matrix composites are manufactured using woven or parallel arranged continuous fibers, or short fibers as a fleece or fiber paper material. For the coating process, twin-wire electric arc spraying is applied as a well established, cost efficient thermal spray technology. The coating process is optimized towards microstructure of the matrix alloy prior to semisolid forming, which requires a globular alloy microstructure, and reduced fiber damage during the high-temperature liquid melt deposition. The thermally sprayed fine-grained matrix material enables semisolid forming at liquid contents of 40-60 vol% of the alloy, with short flow paths, reduced mechanical loads and short cycle times. Thus, limited fiber damage and residual stresses will occur, leading to good mechanical material properties. A production line for industrial-scale coating of fiber fabric coils in a continuous process is introduced in order to provide prepregs of various fiber-reinforcement materials and fiber architectures; moreover, a winding equipment for simultaneous fiber winding and coating is presented that enables local reinforcement for components with adapted, tailored composite material design.

Abstract: Carbon nanotubes are one of the most exciting discoveries of nanosized materials in the 20th century. Challenges to create materials applicable for industrial applications involve both the incorporation of the carbon nanotubes into the material and to ensure that they do not agglomerate. Aluminium and magnesium based materials are among the metals that can benefit from the incorporation of carbon nanotubes. The fabrication of Aluminium carbon nanotube composites has challenges from reactivity and degradation of the carbon nanotube additions; hence the powder metallurgy route is preferred. Magnesium based materials on the other hand do not have this limitation and both the powder metallurgical route and the casting route are viable. Among the benefits of adding carbon nanotubes are increased yield strength and stiffness. Here is important that the effect is significant already at very low addition levels. This makes it possible to increase strength without having a significant detrimental effect on ductility. In fact, for magnesium alloys ductility can be improved due to the activation of additional slip planes improving the normally low ductility of HCP structure materials.

Abstract: The composite, consisting of Ti6Al4V matrix reinforced by unidirectional SiC fibres (SCS-6), has been investigated by mechanical spectroscopy at temperatures up to 1,173 K. For comparison, the same experiments have been performed on the corresponding monolithic alloy. The internal friction (IF) spectrum of the composite exhibits a new relaxation peak superimposed to an exponentially increasing background. This peak, which is not present in the monolithic alloy, has an activation energy H = 186 kJ mol-1 and a relaxation time 0 = 2.3 x 10-15 s. The phenomenon has been attributed to a reorientation of interstitial-substitutional pairs in the  phase of Ti6Al4V matrix around the fibres. This explanation is supported by the results of micro-chemical characterization carried out by X-ray photoelectron spectroscopy (XPS) combined with Ar ion sputtering.

Abstract: The press joining rolling process used for the production of metal/polymer/metal systems is introduced. In the first step three-layer sandwich sheet, 316L/polypropylene- polyethylene/316L (316L/PP-PE/316L) with and without local reinforcement, were processed by roll bonding at approx. 250°C of two steel sheets with a pre-rolled PP-PE - core sheet. Mechanical and forming behaviour of the parts had been investigated by tensile, bending and deep drawing tests. It could be shown that for moderate drawing depths deep drawing behaviour is close to the one of the mono-material.