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Applied Composite Materials: An International Journal for the Science and Application of Composite Materials (v.19, #2)
Static and Fatigue Behaviour of Hexagonal Honeycomb Cores under In-plane Shear Loads by Gabriel Bianchi; Guglielmo S. Aglietti; Guy Richardson (pp. 97-115).
Due to their high specific strength and high specific stiffness properties the use of honeycomb panels is particularly attractive in spacecraft structures. However, the harsh environment produced during the launch of a satellite can subject the honeycomb cores of these sandwich structures to severe quasi-static and dynamic loads, potentially leading to static or early fatigue failures. Knowledge of the static and fatigue behavior of these honeycomb cores is thus a key requirement when considering their use in spacecraft structural applications. This paper presents the findings of an experimental test campaign carried out to investigate the static and fatigue behaviors of aluminum hexagonal honeycomb cores subject to in-plane shear loads. The investigation involved carrying out both static and fatigue tests using the single block shear test method. These results are also discussed in relation to the observed damage and failure modes which have been reported for the statically tested specimens and for the fatigue tested specimens at various stages of fatigue life. As well as conducting tests for the more conventional principal cell orientations (L and W), results are also presented for tests carried out at intermediate orientations to investigate the variation of core shear strength with loading orientation. The results are further investigated using explicit non-linear finite element analysis to model the buckling failure mechanisms of the tested cores.
Keywords: Honeycomb core; Shear strength; Fatigue; Buckling; Nonlinear finite element analysis
Minimum-Weight Sandwich Structure Optimum Design Subjected to Torsional Loading by Xiang Li; Gangyan Li; Chun H. Wang; Min You (pp. 117-126).
As one of the most valued structural engineering innovations developed by the composites industry, sandwich structures are now used extensively in automotive, aerospace and civil infrastructure due to the main advantage of lightweight. This paper develops a minimum weight optimization method for sandwich structure subjected to torsion load. The design process are identified for a sandwich structure required to meet the design constraint of torsion stiffness. The optimum solutions show that at optimum design the core weight accounts for 66.7% of the whole sandwich structure. To illustrate the newly developed optimum design solutions, numerical examples are presented for sandwich structures made of either isotropic face skins or orthotropic composite face skins. Agreement between the theoretical analysis and the examples results is good.
Keywords: Sandwich structure; Lightweight; Torsion stiffness; Optimum design
Effect of a Thin Soft Core on the Impact Behaviour of CFRP Laminates by Giancarlo Caprino; Valentina Lopresto; Massimo Riccio; Claudio Leone (pp. 127-139).
Low-velocity impact tests were carried out on sandwich plates having CFRP facings and thin rubbery core. Two types of cores, differing in the material nature and thickness, were used. For comparison, similar tests were performed on the monolithic laminate. Various impact parameters, among which indentation, first failure energy, perforation energy, absorbed energy and maximum contact force, were analyzed, to highlight the effect of the core on the material response. The influence of the core on the macroscopic behaviour of the panels was quite limited, except in the elastic phase, where the lower stiffness of the sandwich configurations resulted in a higher energy at first failure. More relevant differences were found from the study of failure modes, carried out combining ultrasonic C-scan and a limited number of microscopic observations. In particular, in correspondence of the energy for barely visible impact damage, besides considerable facing-core debonding, both the facings of the sandwich structures exhibited fibre breakage at their back side.
Keywords: Polymer-matrix composites; Impact behaviour; Mechanical testing; Absorbed energy; Failure modes
Fire Burnthrough Response of CFRP Aerostructures. Numerical Investigation and Experimental Verification by Dimitris E. Sikoutris; Dimitris E. Vlachos; Vassilis Kostopoulos; Stuart Jagger; Stefan Ledin (pp. 141-159).
Aircraft structures are designed to withstand further to dynamic mechanical loadings thermal loads too. In the event of an external fire (while the aircraft is on the ground) the fuselage structure has to withstand and delay fire penetration. Prolonged burnthrough time is the design target. In the current work, a progressive fire-degradation material model is developed that links decomposition kinetics with the thermophysical properties of polymer composite materials. The material model is then implemented in a FE model to simulate the response of the flat panels under fire burnthrough conditions. Experimental investigation is performed in accordance to the ISO2685:1998 (E) Standard.
Keywords: Carbon fiber composites; High-temperature properties; Thermal properties; Progressive CFRP fire-degradation; Finite element analysis (FEA); Fire burnthrough modeling
Uniform Design of Optimizing Formulation of Friction Materials with Composite Mineral Fiber (CMF) and Their Friction and Wear Behavior by Yazhou Yang; Man Jiang; Jie Xu; Yunhai Ma; Jin Tong (pp. 161-170).
In this work, the uniform design method was applied to arrange the experimental scheme for optimizing formulation of friction materials. The friction and wear of the friction materials based on the optimized formulation was carried out on a constant speed friction tester (JF150D-II), using pad-on-disc contact mode against gray cast iron disc. The worn surfaces of the friction materials were examined by scanning electron microscopy (JSM5310) and the friction mechanism was discussed. The results showed that the uniform design method was appropriate for finding the optimum formulation of the friction materials with better properties. Compared with two conventional friction materials, the friction materials based on the optimized formulation possessed higher and stable friction coefficient and higher wear resistance, even at the disc temperature of 350°C. The adhesion, strain fatigue and abrasive wear were the main wear mechanisms of the friction materials. Tribo-chemical phenomenon and plastic deformation existed on the worn surface layer.
Keywords: Friction materials; Uniform design; Formulation; Optimization; Friction; Wear
Compressive and Tensile Behaviours of PLLA Matrix Composites Reinforced with Randomly Dispersed Flax Fibres by Fabrice Roussière; Christophe Baley; Grégory Godard; Dominique Burr (pp. 171-188).
Nowadays, the ecological footprint of a material is becoming tremendously important. The Poly l-Lactide Acid (PLLA) matrix composites reinforced by randomly scattered flax fibres have mechanical properties similar to polyester/glass composites [1], lower environmental impacts and can be compost at the end of their lives. In this study, the mechanical characterization of biocomposites has been pushed further with the determination of the compressive and tensile properties. Furthermore, the mechanical properties of single flax fibres have been measured and implemented in a micro-mechanical estimation of the composite elastic modulus. Tensile and compressive stiffness determined by the mechanical analyses show very good correlations with the mathematical estimation.
Keywords: Composite materials; PLA; Flax; Mat; Compression; Tension