Key Engineering Materials Vols. 297-300

Abstract: This investigation aims at doing some durability and damage tolerant (DaDT) tests with 2124-T851 aluminum specimens having corner cracks under a random history, correlating with the simulation results from AFGROW and then drawing some conclusions from those comparisons. Two hydraulic actuators and a homemade Wood’s alloy grip are employed to do the test. The surface crack lengths are measured by a traveling microscope and used as a reference for the later fractography. The crack penetration and the total lives are about 17 and 27 blocks, respectively. The crack length and depth are evaluated by the fractography after completing each test and used to grasp the shape change. The Forman equation is used to simulate the fatigue crack growth behavior according to a bearing stress ratio extensively. The crack penetration life is decreased as the bearing stress ratio increases. The crack aspect ratio is very dependent on the bearing stress ratio. The LBH loading accelerates the fatigue crack growth in the crack depth direction but decelerates that in the length direction until crack penetration.

Abstract: SiC materials have excellent high temperature strength, low coefficient of thermal expansion, good resistance to oxidation and good thermal and chemical stability etc. However, the brittle characteristics of SiC such as low fracture toughness and low strain-to fracture still impose a severe limitation on practical applications of SiC materials. Therefore, in the interests of safety, we are required to measure fracture toughness of materials. In the present work, monolithic Liquid Phase Sintered SiC (LPS-SiC) was fabricated by hot pressing method under 20MPa using sintering additives at different temperature such as 1760oC, 1780oC, 1800oC and 1820oC. The starting powder was high purity β-SiC nano-powder with an average particle size of 30nm. Compositions of sintering additives were Al2O3 / Y2O3 = 0.7 and 1.5 (wt. %). Monolithic LPS-SiC was evaluated in terms of sintering density, hardness and fracture toughness through indentation fracture method by the Vickers hardness tester. Sintered density, hardness and fracture toughness of fabricated LPS-SiC increased with the increase of sintering temperature. They are higher than those of fabricated SiC by the chemical vapor deposition method.

Abstract: The carbon fiber reinforced carbon-carbon composite (C/C composite) is one of the candidates due to its excellent thermal stability as well as high strength. A two-dimensional C/C composite has great anisotropy in those properties in with- and across- fiber directions. It is, therefore, important to consider the anisotropy for the stress evaluation and for the fracture probability of the components. In the present study, FEM analyses on deformation and stress of the component were carried out taking account of the anisotropy. In addition, the fracture probability of the components was evaluated by the statistical fracture theory. It was found that anisotropy affect the thermal stress and the risk of rupture.

Abstract: The strong continuous fiber reinforced metal matrix composites (MMCs) are recently used in aerospace and transportation applications as an advanced material due to its high strength and light weight. However, MMC is significantly affected by the interface under the transverse loading. Furthermore, the crack at the interface induces weakness of the characteristics of the overall mechanical response and strength of the MMCs. In order to be able to utilize these MMCs effectively and with safety, it must be determined their elastic plastic fracture behaviors at the interface. The influence of different regular fiber arrangement as like square and hexagonal arrangement on the strength of transversely loaded fiber reinforced matrix is analyzed. And the interface of fiber and matrix is modeled as thin multi layers with properties linearly gradient to distinguish the interface from the fiber and matrix. Different fiber arrangement of square and hexagon type is studied. And fiber volume fraction is changed for several kinds (5%-60%).

Abstract: WC-TiC-Co cermet and CuZnNi alloy composite coatings were produced on mild steel by a high temperature inside-furnace brazing technique. The microstructure, phase constituents and interfacial diffusion behavior of cermet and Cu-based alloy were investigated by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron probe microanalysis (EPMA) and X-ray diffraction. The tensile strength and wear property of composite coatings were also investigated. The results show that crème particles were not decomposed severely during the inside- furnace brazing process. The microstructure of the matrix is α and β phases. Diffusion occurred at the cermet/Cu-based alloy interface. The tensile strength of the composite coatings reached 240-300MPa, which depended on the brazing temperature and was far higher than that of the flame hardfacing layers. Cermet fracture was basically a brittle fracture in nature and matrix involves ductile fracture.

Abstract: Aluminum or CFRP (Carbon Fiber Reinforced Plastics) tube subjected to axial loading sustains load, and absorbs energy during axial collapse. The aluminum tube absorbs energy by stable plastic deformation, while the CFRP tube with higher specific strength and stiffness than those of the aluminum tube absorbs energy by unstable brittle failure. To achieve a synergy effect by combining the two members, aluminum/CFRP compound tubes were manufactured, which are composed of aluminum tubes wrapped with CFRP outside aluminum tubes with different fiber orientation angle of CFRP and number of plies. The axial quasi-static collapse tests were performed for the tubes. The collapse characteristics of the tubes were compared with those of respective aluminum tube and CFRP circular tube. Test results showed that the collapse of the aluminum/CFRP tubes complemented unstable brittle failure of the CFRP tube due to the ductile characteristics of the inner aluminum tube. The collapse modes and the absorbed energy were influenced by the fiber orientation angle of CFRP and the number of plies. The absorbed energy per unit mass, which is in the light-weigh aspect, was higher in the aluminum/CFRP compound tube than in the aluminum tube or the CFRP tube alone.

Abstract: This paper presents a modified Split Hopkinson Pressure Bar (SHPB) technique to obtain compressive stress-strain data for rubber materials. An experimental technique that modifies the conventional SHPB has been developed for measuring the dynamic compressive stress-strain responses of rubber materials with low mechanical impedance and low compressive strengths. This paper introduces an all-polymeric pressure bar set-up which achieves a closer impedance match between the pressure bar and the specimen materials. In addition, a pulse shaper is utilized to lengthen the rising time of the incident wave which helps the stress equilibrium and homogeneous deformation of rubber materials. It is found that the modified technique can determine the dynamic deformation behavior of NR and NBR rubber more accurately than those from the conventional SHPB technique.

Abstract: We experimentally investigated compressive behaviors of a paper and a paper coated by polyethylene resin. The initial thickness of sheets of paper measured under a compressive stress of 4.5kPa was widely distributed in comparison with the basis weight, although the results based on the ISO 534 standard (under compressive stress of 100kPa) had relatively little dispersion. For compressive deformation, the thickness immediately decreased under low compressive stress, because the gap between fibers in the paper collapsed. After that, the variation of the thickness under higher compression was small due to the compression of fibers. We found that although the resin-coat layers did not have an insignificant effect on compressive compliance above 250kPa, the compliance of the coated paper was larger than that of uncoated paper.

Abstract: CFRP composite materials widely used as structural materials for airplanes, ships and aero space vehicles because of their high strength and stiffness. This study aims to investigate the effects of curvature and stacking sequence on the penetration characteristics of CFRP laminated shell. They are stacked to [02/902]S, [03/903]S and [02/903/0]S, [0/902/0]S, and their interlaminar numbers are two and four. We are manufactured to composite laminated shells with curvature radii of 100, 150, 200mm and ∞ (flat-plate), respectively. They were cure to the appropriate hardening temperature (130°C) at the vacuum bag of an autoclave. Composite laminated shells of dimensions 100㎜×140㎜ were prepared. The velocity of the steel ball that transversely impacts a specimen was measured both before and after impact by determining the time for the ball to pass two ballistic-screen sensors located at a known distance apart. In general, the kinetic energy after impact-kinetic energy before impact increased in all specimens. Absorbed energy increased as the curvature increased [03/903]S and [02/902]S which is small interlaminar number, was higher than four interlaminar specimen.