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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. [ABSTRACT FROM AUTHOR]

The use of woods with a low density and good structural uniformity has intensified in recent decades, paving new ways for their superior valorization. This research study aimed to examine the wood of three deciduous wood species with low densities and great uniformity of structure, namely linden wood, with an oven-dried density of 461 kg/m3, paulownia wood, with an oven-dried density of 304 kg/m3, and balsa wood, with an oven-dried density of 118 kg/m3. The physical and mechanical properties of these species were studied using various methodologies. The obtained results show that, although they have significant differences in terms of densities and mechanical characteristics, the coloristic properties of the three analyzed species obtained using the CIELab are almost similar. As a general conclusion, based on all the properties found in this research, it can be concluded that paulownia wood is more appropriate than linden or balsa for use, being able to replace linden in its uses. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This study aimed to evaluate the effect of ammonium persulfate's (APS) oxidation time on the characteristics of the cellulose nanocrystals (CNCs) of balsa and kapok fibers after delignification pretreatment with sodium chlorite/acetic acid. This two-step method is important for increasing the zeta potential value and achieving higher thermal stability. The fibers were partially delignified using acidified sodium chlorite for four cycles, followed by APS oxidation at 60 °C for 8, 12, and 16 h. The isolated CNCs with a rod-like structure showed an average diameter in the range of 5.5–12.6 nm and an aspect ratio of 14.7–28.2. Increasing the reaction time resulted in a gradual reduction in the CNC dimensions. The higher surface charge of the balsa and kapok CNCs was observed at a longer oxidation time. The CNCs prepared from kapok had the highest colloid stability after oxidation for 16 h (−62.27 mV). The CNCs with higher crystallinity had longer oxidation times. Thermogravimetric analysis revealed that the CNCs with a higher thermal stability had longer oxidation times. All of the parameters were influenced by the oxidation time. This study indicates that APS oxidation for 8–16 h can produce CNCs from delignified balsa and kapok with satisfactory zeta potential values and thermal stabilities. [ABSTRACT FROM AUTHOR]

Natural fiber and wood are environmentally friendly materials with multiscale microstructures. The sound absorption performance of flax fiber and its reinforced composite, as well as balsa wood, were evaluated using the two-microphone transfer function technique with an impedance tube system. The microstructures of natural materials were studied through scanning electrical microscope in order to reveal their complex acoustical dissipation mechanisms. The sound absorption coefficients of flax fiber fabric were predicted using a double-porosity model, which showed relatively accurate results. The integrated natural materials sandwich structure was found to provide a superior sound absorption performance compared to the synthetic-materials-based sandwich structure composite due to the contribution of their multiscale structures to sound wave attenuation and energy dissipation. It was concluded that the natural-materials-based sandwich structure has the potential of being used as a sound absorption structure, especially at high frequency. [ABSTRACT FROM AUTHOR]