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A novel composite tandem sandwich structure made of aramid fabric and aramid honeycomb, impregnated in a matrix of epoxy resin combined with poly (vinyl butyral), PVB, is proposed. The composite was additionally reinforced with a small concentration of titanium diboride nanoparticles, TiB2, to study its effect on impact resistance. Sandwich composite samples were fabricated by impregnating the aramid fabrics and honeycomb in subsequent layers with the system epoxy/PVB/TiB2. Their mechanical behavior was examined through three-point bending test, Charpy impact test, and compressive test. The chemical reaction between PVB and epoxy resin, and chemical inertness between the nanoparticles and the other constituents were confirmed by FTIR analysis. The flexural strength and stiffness increased by 27.6% and 62.5%, respectively, in the presence of TiB2 reinforcement. Impact tests showed increase of 104% in impact toughness in the case of samples with TiB2. Nano TiB2 did not increase compressive strength of the proposed sandwich structure, but it improved its stiffness and reduced the permanent deformation. The achieved improvements in mechanical resistance make this new composite structure a promising candidate for automotive, airspace, naval, civil engineering, sports equipment industry, and for use in defense technologies. [ABSTRACT FROM AUTHOR]

Honeycomb cores made of phenol resin impregnated paper are widely used in sandwich structures of aircraft. Owing to the rather weak core material, this kind of sandwich is prone to a range of damages as a result of impact loading which may accidentally occur during operation. When employed in aircraft these structures have to be damage tolerant. This means barely visible damages have to be considered in the structural design process. In the standard design process for damage tolerant aircraft structures especially the effect of impacts has to be considered but is mostly investigated on unloaded sandwich components. However, aircraft structures are not completely stress free, even when the aircraft is in parking position. At least the structural weight will cause a permanent static preload. In the literature there are only few investigations dealing with the impact behaviour of preloaded honeycomb sandwich structures. In general, studies on composite structures have shown that tension preloads lead to higher structural impact resistance, while prestressing in compression reduces mechanical properties such as impact strength and stiffness of the structure. For this reason, a compression preload is considered as the more critical load case. In order to evaluate the effect of static compression preloads an experimental study on five different honeycomb sandwich configurations was conducted. The experimental investigations showed that even low preloads cause a higher structural damage area, which results in a lower residual strength of the structure. The results indicated that it is recommendable to consider structural compression preloads in the aircraft structural design process. [ABSTRACT FROM AUTHOR]

Apart from the inherent anomalous behaviour under tensile and compressive structures, auxetic structures have shown improved energy absorption characteristics that are of prime interest to various fields of study. This is further exemplified by additive manufacturing (AM) techniques and polymer composites to tailor the shape, geometry and form of these structures. Consequently, this paper aims to characterise the in-plane compressive behaviour and negative Poisson's ratio (NPR) of the most prominent auxetic structures fabricated additively used polymer nanocomposite materials. The study incorporates the use of glycol-modified polyethylene terephthalate (PETG) and nanocomposites of PETG filled with organically modified montmorillonite (OMMT) nanoclay particles to produce auxetic structures fabricated through fused filament fabrication (FFF). Different structures such as hexagonal re-entrant honeycomb structures, peanut-shaped honeycombs, chiral honeycomb structures and missing rib structures are characterised for their compressive performance through experimental approaches involving mechanical testing and digital image correlation (DIC). Different parameters such as the peak crushing strength, average crushing strength, NPR, specific energy absorption (SEA), and crush force efficiency (CFE) of these structures are evaluated at different strain rates/loading rates for varying concentrations of nanoclay and PETG. It is observed that higher loadings of nanoclay particles lower the compressive strength of the structures. Additionally, the NPR decreases with increasing strain rates and is also influenced by the composition and the resultant stiffness. Moreover, the geometrical parameters of the structure largely influence its strain energy absorption. The results have shown that such material-structure combinations can produce structures of high-performance capabilities suitable for aerospace applications. [ABSTRACT FROM AUTHOR]

The trapped air in unvented honeycomb core and the surrounding environment can induce a pressure differential across the facesheet during launch ascent. Combined with the presence of a defect, the pressure differential across a facesheet in combination with external loads can cause facesheet failure or unstable facesheet debond growth. While honeycomb core venting is the recommended solution, there might be practical limitations in implementing this solution or the possibility that a structure intended to be vented is partially unvented due to manufacturing issues. A methodology for accepting composite sandwich structures for space applications is presented, which heavily relies on the building block approach to characterize the facesheet strength and interfacial fracture toughness. To demonstrate the approach, an allowable flaw size is defined for a hypothetical flight condition using analyses anchored to subscale tests. [ABSTRACT FROM AUTHOR]

A new type of pier anti-collision composite structure composed of honeycomb steel and polyurethane (PU) elastomer was proposed in this study. The impacts of the shape and filling materials of inner core cells on the failure mode, load–displacement cure, bearing capacity, structural stability, and energy absorption were studied by conducting uniaxial compression tests on device segments. Test results showed that the bearing capacity, structural stability, and energy absorption of honeycomb steel structure were significantly improved by PU elastomer filling. Besides, when compared with the square honeycomb structure and the regular hexagon honeycomb structure, the maximum values of average load, total energy absorption (TEA), and specific energy absorption (SEA), which were 69.6 kN, 1986.1 J, and 1300 J/kg, respectively, for the regular triangle honeycomb structure without PU filling, increased to 459.3%, 376.38%, and 212.5%, respectively, for the regular hexagonal core cell structure with PU filling, which was proved to be the most suitable core structure for pier anti-collision device. [ABSTRACT FROM AUTHOR]

The paper composite sandwich structure with corrugation and honeycomb cores has been widely used in civil and national defense industries, and the cushioning energy absorption characteristic is a key indicator to evaluate the performance of this composite structure. Therefore, this paper is focused on the influences of honeycomb thickness on the shock acceleration response and deformation characteristics to analyze cushioning energy absorption performance of the composite structure by various experimental tests. The experimental result shows that the paper corrugation layer firstly comes into crushed, and then the paper honeycomb layer is crushed. Additionally, the large honeycomb thickness may cause the secondary collapse of paper honeycomb layer. Under the same impact energy or impact mass, the cushioning energy absorption of the single-sided composite sandwich structure is better than that of the double-sided structure with the same honeycomb thickness. However, the impact resistance of the double-sided composite structure is better than that of the single-sided structure. For the paper composite sandwich structures with the honeycomb thicknesses 10, 15, 20, and 25 mm, the increase of honeycomb thickness would decrease the cushioning energy absorption of the whole structure under the drop impact with low energy. However, under the drop impact with high energy, the influence of honeycomb thickness on cushioning energy absorption is contrary. For the paper composite sandwich structure, the specific energy absorption, unit volume energy absorption, and stroke efficiency for the honeycomb thicknesses 10, 15, 20, and 25 mm are higher than those for the honeycomb thickness 70 mm. Therefore, the low honeycomb thickness is more advantageous for the cushioning energy absorption of paper composite sandwich structure. [ABSTRACT FROM AUTHOR]

Composite sandwich structures are commonly considered in large-scale aerospace applications due to their performance on a per mass basis. The nature of a large-scale sandwich structure generally necessitates the use of multiple sections of core to fill out the structural form. These core sections must be spliced together to ensure that shear loads are appropriately transmitted throughout the core. Because core installation in a large-scale component is a challenging operation, core splice joint width can be difficult to control in manufacturing. As such, the effects of core splice joint width on sandwich structure performance should be well understood. This study examines the effects of core splice joint width in honeycomb sandwich structures via mechanical testing and post-failure analysis. A threshold core splice joint width is shown to exist with respect to core shear, while the integrity of the facesheet-to-core interface is shown to degrade with increasing core splice joint width. [ABSTRACT FROM AUTHOR]

This paper presents the effects of ballistic impact damage on the mechanical behaviour of composite honeycomb sandwich panels. The panels are composed of facesheets made of carbon fibre reinforced laminates and the Nomex honeycomb core. Ballistic tests were conducted in a shooting range using live projectiles of three different calibres. The specimens were subjected to both normal and oblique impacts to simulate the damage to composite sandwich structures that can be inflicted by the use of small weapons or by fragments of anti-aircraft ammunition. The modes and extent of the damage induced have been analysed using optical microscopy, X-ray inspections and computer tomography methods. Structural performance of the intact and ballistically damaged panels has been investigated experimentally using in-plane compression tests, and numerically, based on the finite element modelling and analysis. The reduction in the load-carrying ability caused by the ballistic damage has been assessed for the panels under consideration and compared with that predicted by numerical simulations. [ABSTRACT FROM AUTHOR]

In view of cushioning energy absorption of paper composite sandwich structures with sinusoidal corrugation and hexagonal honeycomb cores, the static yield strength and compression deformation were firstly analyzed by experiments, then the unit volume energy absorption, unit area energy absorption, and specific energy absorption were evaluated. Further, the influences of static compression rate, corrugation types, and honeycomb thickness on the cushioning energy absorption were respectively studied. The experimental observations show that the paper composite sandwich structures are crushed layer by layer due to the difference in the yield strength of sandwiches, and the honeycomb core is crushed after the collapsing of corrugation core. The three evaluation indices of cushioning energy absorption have similar change rule with the compression rate, corrugation types, and honeycomb thickness. The influence of static compression rate on the yield strength and cushioning energy absorption of the paper composite sandwich structure is not obvious. The composite sandwich structures consisting of paper corrugation sandwich with large inertia moment and honeycomb sandwich with large thickness have more excellent cushioning energy absorption, the B-corrugation can increase the unit volume energy absorption of paper composite sandwich structures by up to 18% than C-corrugation, and the 3-honeycomb can increase the unit volume energy absorption of paper composite sandwich structures by up to 32% than 2-honeycomb. [ABSTRACT FROM AUTHOR]

Microcracking in composite honeycomb sandwich structure and its effect on mechanical properties are studied in this paper. A methodology is presented to study the extent of mechanical strength degradation of composite sandwich structure, subjected to thermal fatigue. The material under study is used for spacecraft structural applications. The test coupons were exposed to thermal cycling at elevated temperature as high as +150°C inside the oven and cryogenic temperature of −190°C by dipping in liquid nitrogen, which is comparable to the thermal environment experienced by spacecraft structures. After each thermal cycle, coupons were inspected for microcracks under an optical microscope at the cross section. The microcracks were then quantified using parameters like crack length and crack density with increase in the number of cycles. Flatwise tensile test was conducted on the coupons after every 10 thermal cycles, up to 60 cycles, to make a correlation between crack density and mechanical strength. It was observed that by increasing the number of thermal cycles, the crack density increases and the flatwise tensile strength decreases up to a specific number of cycles. Finite element analysis was performed to predict the possible location of microcracks formation and compared with experimental observation. Good correlation was observed. [ABSTRACT FROM AUTHOR]

In this study, the behaviour of aluminium honeycomb structures under low-speed impact has been investigated by experimental and finite element analysis method. The production of aluminium honeycomb composite structures was carried out using different adhesives of different widths and heights. The low-velocity impact experiments of the honeycomb structures produced were performed according to ASTM D7766 standard. As a result of the experiment, the force values which caused damage to the structure were measured according to the time, and the maximum force values were taken. Findings have shown that the decrease in cell width, the increase in cell height, and depletion of multiwall carbon nanotubes increase the maximum impact force values in honeycomb composite structures. It is seen that the results obtained by the finite element method and the experimental results are approximately 85% in agreement. There was no statistically significant difference between the results as a result of conducted t-test. [ABSTRACT FROM AUTHOR]

Sandwich structures with composite honeycomb core and skin have been mostly used for a variety of engineering applications. There are different man-made honeycomb structures with metal, polymer, and paper honeycomb cell geometry which provide minimum weight, least material content and minimal material cost. In this paper, the textile fiber-based 3 D woven honeycomb structure is engineered by using fabric geometrical parameters and mathematical expressions have been developed to calculate the repeat unit weight, fiber volume fraction (FVF) and specific weight of 3 D woven honeycomb structures. Five 3 D woven honeycomb fabric samples with different cell sizes were produced using model-based construction parameters on a customized rapier weaving machine. Fabric dimensional parameters were determined experimentally to validate the model value with actual results. A reasonably high agreement was observed between experimental and theoretical values. The model can be used as a tool to engineer woven honeycomb reinforcement architecture to produce lightweight structural composite materials. [ABSTRACT FROM AUTHOR]

Structural properties and chemical bonding of new composites (ZnMnO2 and activated carbon) with honeycomb structure have been studied by using X-ray diffraction and Fourier transform infrared. The composition of the powder of ZnMnO2 has been studied by X-ray fluorescence in waste derived from a conventional battery. X-ray diffraction data show that the average crystallite size and the dislocation density for the composite are 90.17 nm and 12.30 × 1013 m−2, respectively. The C–O and C=O bonding for composites seen shifted to the lower wave number of pure ZnMnO2 and activated carbon indicated size reduction as an effect of Zn or Mn in the site of activated carbon. In addition, the reflection loss was analyzed by vector network analyzer and shows reflection loss of about −21.72 dB, the frequency range being 4.40–4.59 GHz for a thickness of 6 nm. The honeycomb structure of composite ZnMnO2–AC in this study promises new types of composite materials for various applications. [ABSTRACT FROM AUTHOR]