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Structure evolution during deformation of isotropic auxetic foams is investigated by simple compression experiments. It is shown that the main feature observed in the experimental data can be accurately described by an isotropic hyperelastic model with an Ogden-type strain energy function. The model can be easily implemented in a finite element code and, hence, can be used to simulate foams undergoing complex structural deformations. [ABSTRACT FROM AUTHOR]

This study focuses on the chemical and thermo-physical characterization of date palm fiber, which can be used in the reinforcement of green composites. Alkaline solutions with different concentrations of NaOH (0.5, 1, 2 and 3%) and various immersion times (0.5, 1, 2, 4, 6 and 8h) at room temperature were used to treat fibers obtained from date palm fruit branches. In total, 750 samples were tested, and the results obtained show that the chemical treatment of the fibers can lead to an increase, up to 3 times, of their mechanical properties (strength and Young's modulus) in quasi-static tensile tests. Due to the dispersion in the experimental data, a statistical approach is required to evaluate the mechanical properties. To better understand the phenomena related to the physico-chemical properties of these fibers, several characterization techniques have been used, namely Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and X-ray diffraction. The data have been compared with results reported in the literature for other lignocellulosic fibers. [ABSTRACT FROM AUTHOR]

This work evaluates an epoxy composite reinforced with curauá fibres and/or organophilic clay. Epoxy polymers reinforced with nano-clays are also assessed. Composites are manufactured by cold pressing using fibres in pristine and treated conditions. Three levels of the factors NaOH solution (2.5, 5 and 10%) and immersion time (2, 4 and 10 hours) are investigated. Nano-clays are incorporated at the levels of 2.5, 5 and 10 wt.%. The morphology and crystallinity of the treated fibres are evaluated by scanning electron microscopy and X-ray diffraction, respectively. Tensile, three-point bending and impact tests are performed to characterise the composites. Tensile strength, flexural strength and impact resistance are increased by 24%, 44% and 47%, respectively, when compared to untreated fibre composites. The highest tensile and flexural strengths are achieved by hybrid composites containing 5 wt.% of nano-clay and 20 wt.% of curauá fibres treated with 5% NaOH for 4 hours. In contrast, the highest tensile modulus is achieved when hybrid composites are made from untreated fibres and 10 wt.% of nanoparticles. The highest impact resistance is obtained by curauá composites, without particles, composed of fibres treated with 5% NaOH for 4 hours. The inclusion of nano-clay leads to reduced impact resistance values. [ABSTRACT FROM AUTHOR]

The present work reports an experimental study on the thermal and mechanical properties of hybrid composites obtained from Petiole Date Palm Fiber (PDPF)/Glass fiber (GF) as reinforcement and vinylester resin (VE). In order to improve the fiber/matrix adhesion, palm fibers were alkali treated with 5% NaOH solution for 24 h–72h. SEM and ATR-FTIR analysis revealed that the 48 h treatment of PDFP with NaOH solution led to rough fiber surface. Vacuum assisted resin transfer molding (VARTM) system was used to produce four hybrid composites (30PDPF/0GF, 20PDPF/10GF, 10PDPF/20GF and 0PDPF/30GF) where the weight ratio of total fiber reinforcement was kept 30%. The treated palm fibers were arranged as a nonwoven mat and placed between woven glass fabrics. Tensile, flexural, dynamic mechanical thermal analysis (DMTA), and thermogravimetric (TGA) were carried out to evaluate the performance of the hybrid composites. The flexural strength and modulus for pure PDPF composite were found to be 60 MPa and 3.87 GPa respectively. Addition of 20%wt glass fiber led an increase in the thermal stability and an enhancement in the tensile and flexural strength by 71.72% and 74.51%, respectively compared to pure PDPF composite. However, the incorporation of 10% of PDPF increases the damping factor from 0.2 for the composite glass/VE to 0.54 for the hybrid 10PDPF/20GF. According to findings of this study, PDPF based composites can be used as non-structural parts in automotive and boat industries. [ABSTRACT FROM AUTHOR]

Stress concentrations pose a significant challenge in designing and optimizing composite components, often resulting in structural alterations and crack formation. This study delves into a detailed numerical and analytical analysis of stress concentration factors in composite materials, with a specific focus on the influence of negative Poisson's ratio and out‐of‐plane modulus. By exploring the interaction of these material properties, the objective is to devise effective strategies for alleviating stress concentrations in composite structures. Through analytical and numerical simulations, a robust correlation between the customized Poisson's ratio and modulus and the mitigation of stress concentration factors, particularly in epoxy/graphite‐reinforced composites are established. This tailored approach has the potential to enhance energy absorption capabilities and consequently reduce the risk of crack propagation in perforated laminated composite plates. This research findings offer valuable insights into composite material design, presenting innovative solutions for enhancing structural integrity and reducing susceptibility to stress‐related issues. [ABSTRACT FROM AUTHOR]

Cactus fibres have previously shown unusual mechanical properties in terms of bending and axial stiffness due to their hierarchical structural morphology. Bioinspiration from those cactus fibres could potentially generate architected materials with exciting properties. To that end we have built bioinspired artificial analogues of cactus fibres to evaluate their mechanical properties. We have generated 3D printed specimens from rendered models of the cactus structure using two different printing techniques to assess the reproducibility of the structural topology. Bioinspired additive manufactured materials with unusual mechanical properties constitute an ever-evolving field for applications ranging from novel wing designs to lightweight plant-inspired analogues. The cactus-inspired 3D printed specimens developed here demonstrate an unusually high bending to axial stiffness ratios regardless of the manufacturing method used. Moreover, when compared to their equivalent beam analogues the cactus specimens demonstrate a significant potential in terms of specific (weight averaged) flexural modulus. Imaging of the artificial cactus reinforcements has enabled the generation of a one-dimensional reduced order finite element model of the cactus structure, with a distribution of cross sections along the length that simulate the inertia and mechanical behaviour of the cactus topology. The novel bioinspired material structure shows an excellent reproducibility across different manufacturing methods and suggest that the tree-like topology of the cactus fibre could be very suited to applications where high bending to axial stiffness ratios are critical., (© 2020 IOP Publishing Ltd.)