Advanced Materials Research Vol. 275

Abstract: Ultrafine grained Al-4wt%Cu-(2.5-10) vol.% SiC metal matrix composite powders were produced from a mixture of Al, Cu and SiC powders using high energy mechanical milling (HEMM). The composite powders produced were first hot pressed at 300°C with a pressure of 240 MPa to produce cylindrical powder compacts with a relative density in the range of 80-94% which decreased with increasing the SiC volume fraction. Powder compact forging was utilized to consolidate the powder compacts into nearly fully dense forged disks. With increasing the volume fraction of SiC from 2.5% to 10%, the average microhardness of the forged disks increased from 73HV to 162HV. The fracture strength of the forged disks increased from 225 to 412 MPa with increasing the volume fraction of SiC particles from 2.5 to 10%. The Al-4wt%Cu-2.5vol.%SiC forged disk did not show any macroscopic plastic yielding, while the Al-4wt%Cu-(7.5 and 10)vol.% SiC forged disk showed macroscopic plastic yielding with a small plastic strain to fracture (~1%).

Abstract: AZ61 alloy is an important magnesium (Mg) alloy which can be rolled at elevated temperature. However, the insufficient ductility of as-extruded alloys limits their design and applications. Pb or Pb+Sn additions lead to a high ductility (~18% at room temperature) and good tensile strength (~250 MPa). This paper discusses the mechanism of property enhancement and the influence of Pb addition on the solidification process of AZ61 alloy.

Abstract: The effect of the curing temperature and NaOH content on cracking tendency and compressive strength for NaOH-slag geopolymers is reported in relation to 29Si and MAS NMR structural analyses and XRD phase quantitative analysis.

Abstract: Low-energy 14N+ ions were implanted with 23 keV under normal incidence into C-axis (0001) sapphire at room temperature. DYNAMIC-TRIM calculations were performed to calculate the N depth profiles for the various fluences from 1x1016 to 1017 cm-2. Electron Beam Annealing (EBA) was performed at a sample temperature of 700 °C for 10 min to allow the implanted and substrate atoms in the implanted layer to move to energetically preferable positions. Nuclear Reaction Analysis revealed the implanted nitrogen ion concentrations. Atomic Force Microscopy and Scanning Electron Microscopy show some nanostructures at the surface of the sapphire substrate exhibiting an average width of 139 ± 25 nm and height of 37 ± 7 nm using the lowest fluence of 1x1016 ions cm-2. Notably for samples implanted with the highest fluence of 1x1017 ions cm-2, bubble/holes like structures appeared after EBA due to out-diffusion of nitrogen that causes blistering and exfoliation effects.

Abstract: Polyamide 6 (PA6) based nanocomposites toughened with 20 wt% maleated styrene-ethylene-butylene-stryrene (mSEBS) reinforced with 1-7 wt% silicon carbide nanoparticles (SiCp) were fabricated via melt blending followed by injection molding. Tensile results showed that SiCp additions improve the Young’s modulus and tensile strength of PA6/mSEBS blends but decrease their tensile ductility and impact strength. EWF test revealed that the SiCp additions reduce both the specific essential work of fracture and specific non-essential plastic work of fracture. Thus SiCp additions are detrimental to the fracture toughness of PA6/mSEBS blend.

Abstract: Inserts are used to transfer localized loads to structures made of sandwich composites. Stress concentrations near inserts are known to cause failures in sandwich panels. Experimental insert pull-out tests show that the load to failure can vary by 20% between batches of sandwich panels. Clearly, uncertainties in the mechanical properties of core and adhesive potting materials have to be accounted for in the optimal design of inserts in sandwich composites. In this paper, we use an one-dimensional computational model of an insert in a homogenized honeycomb sandwich panel to explore the utility of reliability methods in design. We show that the first-order reliability method (FORM) produces accurate estimates of loads that lead to low failure probabilities. We also observe that FORM is sensitive to the failure criteria and may not converge if the failure surface is not smooth and convex.

Abstract: An experimental study on the strengthening of steel structures with FRP (Fiber Reinforced Plastic) is presented in this paper. Test coupons were prepared by applying FRP patches on both sides of steel coupons. Standard tensile tests were conducted to the test coupons. Two types of CFRP (Carbon Fiber Reinforced Plastic) and one type of GFRP (Glass Fiber Reinforced Plastic) were studied. The load and strain data were recorded, and the stiffness and strength were derived. The results show that CFRP provides better strengthening than GFRP, but there is no significant difference between PAN graphite/epoxy and pitch graphite/epoxy laminates.

Abstract: This research addresses the influence of various processing parameters on the post cure quality of carbon fibre composites. Four processing parameters were investigated in the study, in terms of their impact on void content and overall compressive strength. The first parameter distinguishes between laminates cured in a vacuum oven and those cured in an autoclave under high positive pressure. The second parameter describes the impact on voids of differing fibre architectures, comparing a unidirectional fibre structure to that of woven cloth. Thirdly, the influence of compaction during manufacture is analysed and lastly, variation in cure temperature was tested to determine its effect on final laminate quality. The quality of the cured laminate samples was assessed from visual inspection, and in terms of compressive strength and void fraction calculated by Micro-CT X-ray Tomography. The results show that autoclave-cured samples feature significant quality improvements in terms of void fraction and compressive strength when compared to oven-cured samples. Unidirectional laminates incur higher sensitivity to void inclusion than cloth laminates due to the influence of fibre wrinkling. Compaction has no effect on laminate strength; it does however reduce variability in certain cases. Temperature affects different fibre structures in different ways, these being highly dependent on curing method. Finally, it was discerned that curing by autoclave was the dominant processing parameter. Thus, regardless of other manufacturing techniques, the autoclave samples featured almost zero voids and were consequently of the highest quality.

Abstract: The present paper deals with the optimum processing condition of abaca fiber-reinforced green composites, followed by the examination of their strength and fracture behavior. Tensile properties of heat-treated abaca fibers were examined. It was found that the tensile strength of abaca fiber depended strongly on the heat-treatment conditions and that there was a critical condition which the strength of heat-treated abaca fiber was unchanged. After that, two types of abaca fiber-reinforced green composites were fabricated by a hot-pressing method, namely unidirectional and cross-ply composites. These green composites were made using woven abaca fiber as a reinforcement and dispersion-type, starch-based biodegradable resin as a matrix. The mechanical and fracture properties of the abaca green composites have been evaluated as a function of fiber content. These properties are comparable to those of glass fiber reinforced plastics. The tensile strength of abaca fiber reinforced green composites was affected by weft materials; namely the strength of green composites with soft cotton weft was higher than that of the composites with hard abaca weft. This phenomenon was derived from the damage in warp which was introduced during the hot-pressing.

Abstract: With the rapid development of aerospace and automobile industries, metal matrix composites (MMCs) have attracted much attention because of its excellent performance. In this paper, Ni-Cr/AC8A composites reinforced with porous Ni-Cr preform were manufactured by low pressure infiltration process, infiltration temperatures are 700oC~850oC. The microstructure and phase composition of composites were evaluated using optical microscope, X-ray diffraction (XRD) and electro-probe microanalysis (EPMA), It's found that they're intermetallic compounds generated in the composites. Recently, intermetallic compounds have attracted much attention as high-temperature material. We study the hardness of Ni-Cr/AC8A composites, the results show the Ni-Cr/AC8A composite has high hardness due to the intermetallic compounds exist.