Search Results (13)

To bridge the mechanical performance gap between polyethylene terephthalate (PET) foam cores and balsa wood in wind turbine blades, this study proposes a hierarchical groove-perforation design for structural optimization. A finite element model integrating PET foam and epoxy resin was developed and validated against experimental shear modulus data (α < 0.5%). Machine learning combined with a multi-island genetic algorithm (MIGA) optimized groove parameters (spacing: 7.5–30 mm, width: 0.9–2 mm, depth: 0–23.5 mm, perforation angle: 45–90°) under constant resin infusion. The optimal configuration (width: 1 mm, spacing: 15 mm, angle: 65°) increased the shear modulus by 9.2% (from 125 MPa to 137.1 MPa) and enhanced compressive/tensile modulus by 10.7% compared to conventional designs, without increasing core mass. Stress distribution analysis demonstrated that secondary grooves improved resin infiltration uniformity and interfacial stress transfer, reducing localized strain concentration. Further integration of machine learning with MIGA for parameter optimization enabled the shear modulus to reach 150 MPa while minimizing weight gain, achieving a balance between structural performance and material efficiency. This hierarchical optimization strategy offers a cost-effective and lightweight alternative to balsa, promoting broader application of PET foam cores in wind energy and other high-performance composite structures. Full article

The growing demand for sustainable composites has increased interest in natural fiber reinforcements as alternatives to synthetic materials. This study evaluates the bending properties of sandwich structures with flax fibers and 3D-printed lightweight foaming PLA cores compared to conventional designs using glass fibers and traditional cores. Three-point bending tests (EN 310) and density profile analysis showed that, despite its lower density, the 3D-printed foaming PLA core achieved a modulus of elasticity of 2269.19 MPa and a bending strength of 31.46 MPa, demonstrating its potential for lightweight applications. However, natural fibers influenced resin absorption, affecting core saturation compared to glass fibers. The use of bio-based epoxy and foaming PLA contributes to a lower environmental footprint, while 3D printing enables precise material optimization. These findings confirm that 3D-printed cores offer a competitive and sustainable alternative, with future research focusing on further optimization of internal structure to enhance mechanical performance. Full article

Interlayered composites with three types of cores were fabricated and tested. Quasi-static penetration tests (QSPTs), bending tests, and impact tests were conducted on the fabricated composites with carbon fiber epoxy laminate facings. Penetration test procedures were carried out until the composite was perforated and completely punctured. A 9 mm diameter rounded-tip punch was used; the diameter of the support hole was 45 mm. To determine the mechanical properties in the bending tests, three-point bending was carried out at a speed of 2 mm/min. Impact tests were also carried out using a Charpy impact test and a hammer with an energy of 2 J. Our findings indicate that the core material plays a crucial role in determining a composite’s mechanical behavior. Balsa cores offer the best properties in the QSPT test and bending strength and stiffness (57 MPa and 7.4 GPa, respectively), while Rohacell^® cores provide excellent impact resistance (12 kJ/m²). Nomex^® cores demonstrate high bending stiffness (5.3 GPa) but perform worse than Balsa. The choice of core material is application-dependent; Balsa cores are optimal for bending and point loads, and Rohacell^® cores are optimal for impact-dominated scenarios. Full article

Within the range of composite laminates for structural applications, sandwich laminates are a special category intended for applications characterized by high flexural stresses. As it is well known from the technical literature, structural sandwich laminates have a simple configuration consisting of two skins of very strong material, to which the flexural strength is delegated, between which an inner layer (core) of light material with sufficient shear strength is interposed. As an example, a sandwich configuration widely used in civil, naval, and mechanical engineering is that obtained with fiberglass skins and a core of various materials, such as polyurethane foam or another lightweight material, depending on the application. Increasingly stringent regulations aimed at protecting the environment by reducing harmful emissions of carbon dioxide and carbon monoxide have directed recent research towards the development of new composites and new sandwiches characterized by low environmental impact. Among the various green composite solutions proposed in the literature, a very promising category is that of high-performance biocomposites, which use bio-based matrices reinforced by fiber reinforcements. This approach can also be used to develop green sandwiches for structural applications, consisting of biocomposite skins and cores made by low-environmental impact or renewable materials. In order to make a contribution to this field, a structural sandwich consisting of high-performance sisal–epoxy biocomposite skins and an innovative renewable core made of balsa wood laminates with appropriate lay-ups has been developed and then properly characterized in this work. Through a systematic theoretical–experimental analysis of three distinct core configurations, the unidirectional natural core, the cross-ply type, and the angle-ply type, it has been shown how the use of natural balsa gives rise to inefficient sandwiches, whereas performance optimization is fully achieved by considering the angle-ply core type [±45/90]. Finally, the subsequent comparison with literature data of similar sandwiches has shown how the optimal configuration proposed can be advantageously used to replace synthetic glass–resin sandwiches widely used in various industrial sectors (mechanical engineering, shipbuilding, etc.) and in civil engineering. Full article

To contribute to the development of sustainable composites, this work investigates the effects of moisture on the key AE characteristics related to the damage mechanisms of a bio-based balsa wood core sandwich in 4-point bending tests, including cumulative counts, amplitude, peak frequency, and duration. Novel triple dog-bone balsa wood core sandwich specimens with different MC (moisture content) were studied by comparing microscopic observations and a proposed two-step clustering approach in AE analysis. Three MC states, i.e., dry, 50% MC, and 120% MC, are discussed. GFRP (glass-fiber-reinforced polymer) laminate skin damages were found to be predominant in most GFRP–balsa sandwich specimens, but balsa wood core damages play a more important role as MC increases. The degradation of the bending stiffness of the sandwich was proven to be faster in the first linear stage of the moisture absorption curve, while the decrease in bending strength was more pronounced at the MC saturation level. Finally, for all of the dry and wet sandwich specimens, peak frequency and duration were proven to be more helpful in identifying damages associated with the lighter bio-based balsa wood core, such as balsa core damages and skin/core debonding. Full article

Sandwich composites are often used as primary load-bearing structures in various industries like aviation, wind, and marine due to their high strength-to-weight and stiffness-to-weight ratios, but they are vulnerable to damage from Low-velocity-impact (LVI) events like dropped tools, hail, and birdstrikes. This often manifests in the form of Facesheet-Core-Debonding (FCD) and is often termed Barely-Visible-Impact-Damage (BVID), which is difficult to detect and can considerably reduce mechanical properties. In general, a balsa core sandwich is especially vulnerable to FCD under LVI as it has poorer adhesion than synthetic core materials. A cork core sandwich does show promise in absorbing LVI with low permanent indentation depth. This paper also reviews surface treatment/modification as a means of improving the adhesion of composite core and fiber materials: key concepts involved, a comparison of surface free energies of various materials, and research literature on surface modification of cork, glass, and carbon fibers. Since both balsa and cork have a relatively low surface free energy compared to other materials, this paper concludes that it may be possible to use surface modification techniques to boost adhesion and thus FCD on balsa or cork sandwich composites under LVI, which has not been covered by existing research literature. Full article

This study investigated the capability of honeycomb core structures made of kenaf fibre-reinforced polylactic acid (PLA) composite. Two types of kenaf fibre were used in this study, these being woven kenaf and non-woven cotton/kenaf. Initially, the corrugated shape panel was manufactured using a hot moulding compression method. The panel was then cut into corrugated strips, bonded together using epoxy resin to form the honeycomb core structure, and balsa wood used as their skins. The effects of core height and crosshead displacement rate were investigated. The honeycomb core consisted of 20 mm, 30 mm and 40 mm core heights, and the crosshead displacement rate ranged from 2 mm/min to 500 min/min. Of all the samples, core structure with a height of 20 mm tested at 500 mm/min offered the highest value of compressive strength and specific energy absorption, which were 6.23 MPa and 12.36 kJ/kg, respectively. It was also discovered that the core height and loading rate have significant effects on the mechanical properties of the kenaf/PLA honeycomb core structure. Full article

The aim of this study is to analyze the properties of Paulownia tomentosa x elongata plantation wood from Serbia, considering the influence of the stem height (0 to 1 m and 4.5 to 6 m above soil level—height spot) and radial position from the pith to bark (in the core, near the bark, and in between these zones—cross-section spot). The results show that most properties are improved when the samples were taken from upper parts of the tree (height spot) and from the near bark spot (cross-section spot). The mean density measured 275 kg/m³ at the stem height between 4.5–6 m and 245 kg/m³ for the samples collected from 0–1 m trunk height. The density had the highest value on the spot near bark (290 kg/m³), for the mature wood at a height of 4.5–6 m, and near pith had a mean density of 230 kg/m³. The Brinell hardness exhibited highest values in the axial direction (23 N/mm²) and near bark (28 N/mm²). The bending strength was 41 N/mm² for the trunk’s height range of 4.5–6 m and 45 N/mm² in the cross-section, close to cambium. The three-point modulus of elasticity (MOR) of the samples taken at a stem height of 4.5 to 6 m was up to 5000 N/mm², and on the spot near bark, the MOR measured 5250 N/mm². Regarding compressive strength, in the cross-section, near the pith, the mean value was the highest with 23 N/mm² (4.5–6 m), whilst it was 19 N/mm² near bark. The tensile strength was, on average, 40 N/mm² for both 0–1 m and 4.5–6 m trunk height levels and 49 N/mm² between bark and pith. The screw withdrawal resistance measured 58 N/mm for the samples extracted at a stem height of 4.5 to 6 m and 92 N/mm for the specimens collected near pith. This study stresses the influence, in short-rotation Paulownia timber, of indicators, such as juvenile and mature wood (difference emphasized after the fifth year of growth) and height variation, on the physical and mechanical properties of sawn wood. This study will help utilize more efficient sustainable resources, such as Paulownia plantation wood. This fast-growing hardwood species from Europe is adequate as a core material in sandwich applications for furniture, transport, sport articles, and lightweight composites, being considered the European Balsa. Full article

This cutting-edge review highlights the fundamentals, design, and manufacturing strategies used for sandwich composites. Sandwich composite structures have the advantages of light weight, high strength, impact resistance, stability, and other superior features for advanced applications. In this regard, different core materials have been used in the sandwich composite structures, such as cellular polymer foam, metallic foam, honeycomb, balsa, tubular, and other core geometries. Among these, honeycomb sandwich composite materials have been effectively applied in space engineering, marine engineering, and construction applications. The foremost manufacturing techniques used for sandwiched composite structures include hand lay-up, press method, prepreg method, vacuum bagging/autoclave, vacuum assisted resin infusion, resin transfer molding, compression molding, pultrusion, three-dimensional (3D) printing, four-dimensional (4D) printing, etc. In advanced composite manufacturing, autoclave processes have been the method of choice for the aerospace industry due to less delamination between plies and easy control of thickness dimensions. Moreover, machining processes used for sandwich composites are discussed in this article. In addition to aerospace, the high-performance significance of sandwiched composite structures is covered mainly in relation to automobile engineering and energy absorption applications. The structure-, fabrication-, and application-related challenges and probable future research directions are also discussed in this article. Full article

Climatic chamber testing conditions are becoming more demanding. A wide range of temperatures is used to check the quality of products and materials, since they are constantly being improved. However, there is no literature on how the components of the climatic chamber panels react under high temperatures. The present work therefore sets out to perform a thermal and mechanical characterisation of four core materials often used in sandwich panels: balsa wood, mineral wool, and polyethylene terephthalate and polyurethane rigid foams. The thermal characterisation focused on thermal conductivity and the specific heat was characterised using an indirect method developed previously by the authors to simulate a real application scenario where one surface of the sandwich panels was subjected to high temperature, while the opposite surface was kept at room temperature. Steady and unsteady conditions were analysed up to 200 °C. Balsa and mineral wool exhibited a nonlinear increase in thermal conductivity with temperature, and the polymeric foams showed linear behaviour. The specific heat results also increased with temperature, and the relation was nonlinear for all the tested materials except for polyethylene terephthalate, which showed linear behaviour. Higher temperatures had the least effect on the specific heat for balsa wood and mineral wool. The polyethylene terephthalate foams were the most affected by temperature. Temperature variation was tested using the impulse excitation technique. The polymeric foams and balsa wood were studied up to 100 °C and 160 °C, respectively. The elastic modulus decreased with temperature. After 24 h of cooling, the tests were repeated and the elastic modulus had regained or even increased its initial value, for all the materials. Full article

Fiber-reinforced polymer matrix composites continue to attract scientific and industrial interest since they offer superior strength-, stiffness-, and toughness-to-weight ratios. The research herein characterizes two sets of E-Glass/Epoxy composite skins: stressed and unstressed. The stressed samples were previously installed in an underground power distribution vault and were exposed to fire while the unstressed composite skins were newly fabricated and never-deployed samples. The mechanical, morphological, and elemental composition of the samples were methodically studied using a dynamic mechanical analyzer, a scanning electron microscope (SEM), and an x-ray diffractometer, respectively. Sandwich composite panels consisting of E-glass/Epoxy skin and balsa wood core were originally received, and the balsa wood was removed before any further investigations. Skin-only specimens with dimensions of ~12.5 mm wide, ~70 mm long, and ~6 mm thick were tested in a Dynamic Mechanical Analyzer in a dual-cantilever beam configuration at 5 Hz and 10 Hz from room temperature to 210 °C. Micrographic analysis using the SEM indicated a slight change in morphology due to the fire event but confirmed the effectiveness of the fire-retardant agents in quickly suppressing the fire. Accompanying Fourier transform infrared and energy dispersive X-ray spectroscopy studies corroborated the mechanical and morphological results. Finally, X-ray diffraction showed that the fire event consumed the surface level fire-retardant and the structural attributes of the E-Glass/Epoxy remained mainly intact. The results suggest the panels can continue field deployment, even after short fire incident. Full article

An advanced fire thermal model was developed to predict the evolution of the temperature and decomposition gradient across a sandwich composite structure when exposed to high temperatures (fire). This model allows the prediction of a large numbers of parameters, such as thermal expansion, gas mass storage, porosity, permeability, density, and internal pressure. The highlight of this model is that we consider, in the sandwich constituents (core and skins), additional parameters, such as changing volume porosities, other coupled constituents (as infused resin in the balsa core), and what make the main originality of the present approach: moisture content (free and bounded water). The time dependence of many parameters, i.e., among others, the combustion advancing front and mechanical properties, can be predicted in a large number of material and fire scenarios. The proposed approach was validated in the case of sandwich panels, with glass/polyester or glass/vinyl ester skins and balsa core, exposed to high temperatures up to 750 °C. The influence of water on the thermal and mechanical responses is also highlighted. Full article

In this study, the hygro–thermo–mechanical responses of balsa core sandwich structured composite was investigated by using experimental, analytical and numerical results. These investigations were performed on two types of specimen conditions: dry and moisture saturation sandwich composite specimens that are composed of E-glass/polyester skins bonded to a balsa core. The wet specimens were immersed in distilled water at 40 °C until saturated with water. The both dry and wet sandwich composite specimens were heated by fire. The mass loss kinetic and the mechanical properties were investigated by using a cone calorimeter following the ISO 5660 standard and three-point bending mechanical test device. Experimental data show that the permeability and fire resistance of the sandwich structure are controlled by two composite skins. Obtained results allow us to understand the Hygro–Thermo–Mechanical Responses of the sandwich structured composite under application conditions. Full article