Some technological properties of wood–styrofoam composite panels

Synthetic resins are widely used in wood based composites manufacturing. Besides their many advantages, most of them contain formaldehyde and a chemical agents that cause environmental problems. Styrofoam known as expanded polystyrene, is used all over the world for various purposes including thermal insulation, packing, coffee cups, fabrication of car parts etc. This study investigated the evaluation possibilities of styrofoam wastes in plywood production as a bonding material. Pine (Pinus pinea) and poplar (Populus deltoides I-77/51) veneers were used to produce wood–styrofoam composite (WSC) and traditional plywood. Urea-formaldehyde adhesive was used as bonding material for traditional plywood panels. Two different types of styrofoam having high density (25 kg/m³) and low density (10 kg/m³) were used as binder in the manufacturing of WSC panels. Bonding and bending strength, modulus of elasticity, density and thermal conductivity of plywood and WSC panels were investigated. Experimental results showed that mechanical properties of panels manufactured with low density styrofoam type were higher than those of panels manufactured with high density styrofoam type. The lowest thermal conductivity among the all panels was found for poplar panels manufactured with high density styrofoam.     

Fabrication of nature-inspired bulk laminar composites by a powder processing

Among many kinds of “nano-laminar” composites inspired by the brick and mortar structure of nacre in mollusk shells, a bulky, dense and ceramics–base composite has been a missing piece despite its importance. Here we report that such a composite with a submicron-order layered structure can be fabricated by a simple method, sintering aligned flake-like inorganic powder coated with ductile matrix material. The composites fabricated by this method had crack extension resistance by interface delamination, crack deflection, and ligament bridging by the ductile matrix. They showed non-brittle fracture behavior in a bending test despite a quite high flake volume fraction of over 80%, and had a work of fracture (WOF) of more than 300 J/m², several hundreds times as large as that of monolithic glass.     

Global shared-layer blending method for stacking sequence optimization design and blending of composite structures

A global shared-layer blending (GSLB) method is proposed for obtaining manufacturable stacking sequence of composite structures with blending and design rules. The method combines the traditional SLB technique with an evaluation algorithm of spatial variation of panels, where the manufacturability of laminates is enhanced by identifying and minimizing the ply-drops, and controlling the laminate transition drop boundaries. In addition, a blended design scheme is also proposed, which is achieved by using the stacking sequence table technique. A composite wing structure is selected to validate the efficiency and accuracy of the proposed method. Results show that the GSLB method can be used for generating more manufacturable designs of large-scale composite structure with multiple engineering constraints.     

Experimental identification of yield surface of Al–Cu bimetallic sheet

Design of structures made form metal layered composite with a gradient variation of physical properties requires knowledge of their behaviour in the small of elasto-plastic strain. The aim of the research was the experimental investigation of these behaviours. Preliminary tests were carried out on standard flat specimens made from aluminium-copper layered composite, which was obtained by rolling process. Each component (layer) in the state before connecting into a composite was tested independently. The test specimens were cut from metal sheets in different directions (in the range 0–90°). The primary strength tests showed a large anisotropy of mechanical properties. Further studies, in the main part, were associated with the investigations of evolution of yield surfaces for Al–Cu bimetal and components in the range of strains from proportional limit to 0.3%. The investigations were realised by monotonic tensile tests of mini specimens, which were cut out in different directions from the large-size specimens and put to the initial deformation 0.75% in the direction of rolling. The method gave possibility to realising tests in the plane stress state and build for aluminium, copper and Al–Cu bimetal experimental yield surfaces. Evolution of yield surface with increasing levels of plastic deformation was studied. Analysis of their shape showed that aluminium, copper and Al–Cu bimetal had isotropic hardening. It was shown that the law of mixtures applied to the determination of the yield surface of Al–Cu bimetal was applicable only in a short range of elasto-plastic deformation (0.05–0.2%) and for specimens cut at an angle 0–45° from the large-size specimen.     

Mechanical properties of graphene films enhanced by homo-telechelic functionalized polymer fillers via π–π stacking interactions

Most researches on graphene/polymer composites are focusing on improving the mechanical and electrical properties of polymers at low graphene content instead of paying attention to constructing graphene’s macroscopic structures. In current study the homo-telechelic functionalized polyethylene glycols (FPEGs) were tailored with π-orbital-rich groups (namely phenyl, pyrene and di-pyrene) via esterification reactions, which enhanced the interaction between polyethylene glycol (PEG) molecules and chemical reduced graphene oxide (RGO) sheets. The π–π stacking interactions between graphene sheets and π-orbital-rich groups endowed the composite films with enhanced tensile strength and tunable electrical conductivity. The formation of graphene network structure mediated by the FPEGs fillers via π–π stacking non-covalent interactions should account for the experimental results. The experimental investigations were also complemented with theoretical calculation using a density functional theory. Atomic force microscope (AFM), scanning electron microscope (SEM), X-ray diffraction (XRD), nuclear magnetic resonance (NMR), thermal gravimetric analysis (TGA), UV–vis and fluorescence spectroscopy were used to monitor the step-wise preparation of graphene composite films.     

Interlaminar stress recovery for three-dimensional finite elements

An accurate evaluation of interlaminar stresses in multilayer composite laminates is crucial for the correct prediction of the onset of delamination. In general, three-dimensional finite element models are required for acceptable accuracy, especially near free edges and stress concentrations. Interlaminar stresses are continuous both across and along layer interfaces. Nonetheless, the continuity of interlaminar stresses is difficult to enforce in C⁰$C^0$ interpolated elements. Nodal values of the stresses are usually retrieved using extrapolation techniques from super-convergent points, if known, or Gauss points inside the element. Stress fields within an element can be deduced using either constitutive relations or variationally consistent procedures. In either case, spurious oscillations in stress fields may be encountered leading to a reduced accuracy of the recovered stresses at nodes. In this paper, an efficient interlaminar stress recovery procedure for three-dimensional finite element formulations is presented. The proposed procedure does not rely on extrapolation techniques from super-convergent or integration points. Interlaminar stress values are retrieved directly at nodes and stress continuity at the inter-element boundary is automatically satisfied. Several benchmark problems were analysed. Comparisons with finite element software and available solutions in the literature confirmed the accuracy of the procedure. Accurate interlaminar stresses were obtained using coarser meshes compared to customary recovery procedures.     

Enhanced mechanical,thermal and flame retardant properties by combining graphene nanosheets and metal hydroxide nanorods for Acrylonitrile–Butadiene–Styrene copolymer composite

Three metal hydroxide nanorods (MHR) with uniform diameters were synthesized, and then combined with graphene nanosheets (GNS) to prepare acrylonitrile–butadiene–styrene (ABS) copolymer composites. An excellent dispersion of exfoliated two-dimensional (2-D) GNS and 1-D MHR in the ABS matrix was achieved. The effects of combined GNS and MHR on the mechanical, thermal and flame retardant properties of the ABS composites were investigated. With the addition of 2 wt% GNS and 4 wt% Co(OH)₂, the tensile strength, bending strength and storage modulus of the ABS composites were increased by 45.1%, 40.5% and 42.3% respectively. The ABS/GNS/Co(OH)₂ ternary composite shows the lowest maximum weight loss rate and highest residue yield. Noticeable reduction in the flammability was achieved with the addition of GNS and Co(OH)₂, due to the formation of more continuous and compact charred layers that retarded the mass and heat transfer between the flame and the polymer matrix.     

Low-velocity impact and static behaviors of high-resilience thermal-bonding inter/intra-ply hybrid composites

This study prepared inter/intra-ply hybrid composites reinforced with sandwich-structure recycled Kevlar nonwoven/glass woven compound fabric. Negative-depth needle punching and thermal bonding were applied to strengthen the structure with two compound cover plies and a fluffy cushioning center ply. The effects of center ply areal density, needle punching depth, and fiber blending ratio on the static and dynamic impact resistance behaviors of the composites were investigated. The results indicated that areal density significantly influenced the static and dynamic impact behaviors, which were both enhanced by the promotion of thermal-bonding points. As the needle punching deepened, the static and dynamic puncture resistances represented opposite tendencies because of different failure mechanisms. Static friction was the dominant factor for static puncture resistance, whereas kinetic friction was the dominant factor for dynamic puncture resistance. A similar phenomenon was observed when fiber blending ratio was varied. In terms of the non-penetrating dynamic cushioning test, areal density was the most distinct influence factor on cushioning behavior and the hybrid composites sample with an areal density of 700 g/m² could eliminate up to 66.5% of the incident force. Therefore, the inter/intra-ply hybrid composites showed high impact resistance and excellent dynamic cushioning property.     

Static analyses of FGM beams by various theories and finite elements

The 1D Carrera Unified Formulation (CUF) is here used to perform static analyses of functionally graded (FG) structures. The hierarchical feature of CUF allows one to automatically generate an infinite number of displacement theories that may include any kind of functions of the cross-section coordinates (x, z), among which those used to describe the variation of the mechanical properties of FG materials. The governing equations are derived by means of the Principle of Virtual Displacements in a weak form and solved by means of the Finite Element method (FEM). The equations are written in terms of “fundamental nuclei”, whose forms do not depend on the used expansions. Trigonometric, polynomial, exponential and miscellaneous expansions are here used and evaluated for various structural problems. Resulting theories are assessed by considering several aspect-ratios, gradation laws, loading and boundary conditions. The results are compared with 1-, 2- and 3-D solutions both in terms of displacements and stress distributions. The comparisons confirm that the 1D CUF elements are valuable tools for the study of FG structures.     

Wood–plastics composites with better fire retardancy and durability performance

This study concerns the preparation and study of wood–plastic composites (WPCs). The matrix used was high density polyethylene. Results showed that the addition of wood fibres increased mechanical properties (tensile, flexural and compression) of the neat plastic remarkably. Additives such as fire retardants and light stabilizers were added to improve properties like fire retardancy and durability performance. The addition of fire retardants could lead to auto-extinguishing materials when ammonium polyphosphate or aluminium hydroxide were used. Outdoor durability depended on both the light stabilizer and the fire retardant added to the formulation. The fire retardant worsened the outdoor durability. However, stabilized fire retarded-WPCs showed much lower fading than non-stabilized non-fire retarded composites and several industrial samples. Stabilized composites with aluminium hydroxide as fire retardant showed the best overall results with a fading degree even lower than the stabilized non-fire retarded composite.     

Characterization of polypropylene–polyethylene blends made of waste materials with compatibilizer and nano-filler

Waste polypropylene and polyethylene were blended by a twin-screw extruder with two compatibilizers (PE-g-MAH and EPDM) and an additive (O-MMT). The mechanical properties were measured firstly. By adding O-MMT, the tensile strength showed a decline while the impact strength made a promotion. The phase morphology was observed by scanning electron microscopy (SEM) to explore the fracture toughness of blends. The blend with EPDM had a better compatibilization than PE-g-MAH. X-ray diffraction was used to investigate the crystallization behavior and the result showed no change by blending. Moreover, further measurements such as thermogravimetric (TGA) and differential scanning calorimetry (DSC) were taken to show the thermal stability and crystallization temperature of the blend. Additionally, the storage modulus and loss modulus are measured by dynamic mechanical analysis (DMA), the presence of O-MMT caused the increases of the storage modulus and loss modulus.