1,721,016 research outputs found
Effect of medium-high temperature conditioning on the mechanical properties of single quartz fibres
Full text linkThis work adopted multi-scale characterizations to investigate the effects of exposure to medium-high temperatures (600–800 °C) on the mechanical properties of single quartz fibres, focusing on whether bulk property changes, i.e., Young’s modulus, density and fracture toughness, occur and can be related to the thermal strength loss, herein quantified as 75% up to 86% with increasing temperature. Investigation of the fracture surfaces through scanning electron microscopy revealed that failure originated from the fibre surface regardless of heat treatment. Neither bulk crystallization was highlighted through X-ray diffraction nor relevant changes or gradients of Young’s modulus and hardness were disclosed over the fibre cross-sections through high-speed nanoindentation mapping. A 9% increase in fracture toughness measured through micro-pillar splitting revealed a slightly improved crack propagation resistance that cannot compensate for the drastic effect responsible for strength reduction, which is discussed in terms of surface-controlled mechanisms involving the development and growth of surface flaws
Temperature, strain rate and anisotropy effects on compressive response of natural and synthetic cellular core materials
Full text linkThe remarkable flexural properties of sandwich structures hinge on the selection of performing core materials with suitable out of plane mechanical properties, i.e. compressive ones. For this reason, this work compares the compressive behaviour of a synthetic foam (polyvinyl chloride) and an environmentally friendly agglomerated cork as a function of density, strain rate, temperature and anisotropy. The strain rate sensitivity of these cellular materials was investigated in a wide range of velocity conditions by using drop weight tower and Split Hopkinson Pressure Bar dynamic compression tests. The results highlighted a remarkable strain rate sensitivity of both materials because of their viscoelastic nature and, in particular, an increase in compressive properties with increasing strain rate. This increment was more pronounced in the medium–high strain rate range than in the low-medium one. An embrittlement effect with decreasing temperature was detected, which compromises core materials crashworthiness determining a reduction of the percentage absorbed energy. Despite a remarkable anisotropy induced by the production processes, this work confirms the feasibility of agglomerated cork as a sustainable alternative to petroleum-based cellular core materials especially in consideration of the significant recovery capabilities that ensure a higher dimensional stability of the sandwich structure
The compressive behavior and crashworthiness of cork. A review
Full text linkCork, a natural material from renewable resources, is currently attracting increasing interest in different industrial fields because of its cellular structure and the presence of the flexible suberin as its main chemical component. In an agglomerated form, it proved to be a compelling product not only as a thermal and acoustic insulator, but also as core material in sandwich structures and as a liner or padding in energy absorbing equipment. From this perspective, the assessment of its compressive response is fundamental to ensure the right out-of-plane stiffness required to a core material and the proper crashworthiness in the safety devices. Considering the complex nature of cork and the resulting peculiar compressive response, the present review article provides an overview of this paramount property, assessing the main parameters (anisotropy, temperature, strain rate, etc.) and the peculiar features (near-zero Poisson’s ratio and unique dimensional recovery) that characterize it in its natural state. Furthermore, considering its massive exploitation in the agglomerated form, the design parameters that allow its compressive behavior to be tailored and the operating parameters that can affect its crashworthiness were assessed, reporting some potential industrial applications
The effects of water absorption and salt fog exposure on agglomerated cork compressive response
No full textThe replacement of synthetic foams with agglomerated cork in sandwich composites can meet the increasing environmental concerns. Its peculiar morphology and chemical composition lead to outstanding dimensional recovery that endorsed a broad investigation of its compressive behavior. The knowledge of neat material response is fundamental to obtain a reliable design dataset, but it is necessary to consider all the environmental factors (water, moisture and sunlight) that significantly modify material mechanical properties. In view of this, the present work investigates the effect of distilled and seawater absorption and salt fog exposure on the compressive behavior of two agglomerated corks with different densities to simulate their potential employment in marine environment. The results were suitably compared with the ones of a traditional PVC foam used as benchmark. A dependence of water uptake and diffusivity on cork density and water type was detected. The less dense cork displayed a water uptake between 36.7 and 46.5% higher than the denser cork, and seawater uptake was between 21.8 and 44.4% lower than distilled water one. Concerning the compressive response, water and fog moisture plasticizing effect in wet conditions and a partial healing after drying due to salt crystal deposits were identified. Water plasticizing effect determined a reduction in the compressive modulus between 35.1 and 37.9% for the lighter cork and between 17.7 and 21% for the denser cork whereas fog moisture induced a reduction between 52 and 74% for the lighter cork and between 24 and 76.1% for the denser one
Mechanical behavior and damage degree of hybrid glass/carbon composites at low temperature
No full textHybrid composite samples with different carbon and glass fiber layers arrangement were produced by resin transfer molding (RTM) in an attempt to disclose the effects of low temperature on their flexural and low velocity impact behavior. Flexural tests were carried out at T = −50°C and the failure modes were examined, while impact tests, always at T = −50°C, were performed at penetration and at indentation in the range from 10 J to 30 J. The damage extension was accurately evaluated by ultrasonic non-destructive testing. The results were then compared with the room temperature ones obtained in a previous paper to highlight the influence of the temperature. The compression after impact tests were carried out at room temperature for samples previously impacted at both room and low temperature to assess the damage tolerance of the different hybrid configurations. Stacking sequence was found to govern both the flexural and impact behavior of laminates. At low temperature, the flexural strength and modulus values improved over the corresponding room temperature values irrespective of the stacking sequence, while an easier damage propagation and less residual strength were detected in dynamic loading
3D Printing of Low-Filled Basalt PA12 and PP Filaments for Automotive Components
Full text linkFused Deposition Modeling (FDM) enables many advantages compared to traditional manufacturing techniques, but the lower mechanical performance due to the higher porosity still hinders its industrial spread in key sectors like the automotive industry. PP and PA12 filaments filled with low amounts of basalt fibers were produced in the present work to improve the poor mechanical properties inherited from the additive manufacturing technique. For both matrices, the introduction of 5 wt.% of basalt fibers allows us to achieve stiffness values comparable to injection molding ones without modifying the final weight of the manufactured components. The increased filament density compared with the neat polymers, upon the introduction of basalt fibers, is counterbalanced by the intrinsic porosity of the manufacturing technique. In particular, the final components are characterized by a 0.88 g/cm3 density for PP and 1.01 g/cm3 for PA12 basalt-filled composites, which are comparable to the 0.91 g/cm3 and 1.01 g/cm3, respectively, of the related neat matrix used in injection molding. Some efforts are still needed to fill the gap of 15–28% for PP and of 26.5% for PA12 in tensile strength compared to injection-molded counterparts, but the improvement of the fiber/matrix interface by fiber surface modification or coupling agent employment could be a feasible solution
Influence of reprocessing cycles on the morphological, thermal and mechanical properties of flax/basalt hybrid polypropylene composites
No full textVegetable fibers hybridization with basalt ones is a suitable way to exploit the environmental advantages of vegetable fibers while preserving composites mechanical properties. Nevertheless, there are no studies available on the mechanical recycling of short fiber hybrid reinforced polymer composites. In light of this, the present work addressed the mechanical recycling of flax/basalt hybrid polypropylene composites up to seven reproc-essing cycles, evaluating the effect on microstructure, thermal and mechanical behavior and providing a direct comparison with flax composites. The results proved that the interaction of flax and basalt fibers promotes a faster degradation of flax fibers length inducing a significant decrease in hybrid mechanical properties already at the second reprocessing cycle. Despite this, hybrids are able to ensure a higher flexural stiffness and flexural strength by 5.2% and 7.7%, respectively, and an impact strength by 29.8%, 24.0% and 16.6% higher than flax at-50 degrees C, room temperature and + 50 degrees C after the second reprocessing cycle, respectively. Considering the easier processability of hybrid composites thanks to a higher Melt Volume Flow Rate (MVR), they can be conveniently repurposed and mechanically recycled to produce components traditionally manufactured with flax fibers
Application of DIC to Static and Dynamic Testing of Agglomerated Cork Material
No full textIn this work, experimental compression tests have been performed on parallelepiped specimens cut from an agglomerated cork slab. The tests have been performed both using a quasi-static testing machine and a polymeric Split Hopkinson Bar, in order to assess the sensitivity of the material to the strain rate. A standard and a high-speed digital camera have been used to collect frames of the samples during the tests. 2D DIC analyses have been conducted on the pictures of lateral faces of the specimens in order to evaluate the actual strain distributions, which showed a significant heterogeneity within each sample. Moreover, the DIC analyses on the dynamic tests have been used for evaluating the local accelerations and to compute the inertia stresses. The latter may affect the global response that can be measured by following the standard Hopkinson bar procedures, and are responsible for the fluctuations in the force histories observed in the tests at highest strain rates
Effect of temperature and MWCNTs on low velocity impact response of CFRP laminates
No full textThis experimental work addresses the effect of temperature and MWCNTs on the response of carbon fibre reinforced epoxy matrix (CFRP) laminates under low impact velocity. The test temperature ranged from +80 °C down to -40 °C while two sample configurations were examined, namely cross-ply and quasi-isotropic. Results showed the influence of temperature and of stacking sequence on the impact response of CFRP. The presence of carbon nanotubes, despite a larger delaminated area, provided the quasi-isotropic structure with an increased damage tolerance, ascribed to the enhancement of mode II interlaminar fracture toughness
Effect of basalt intraply hybridization on the damage tolerance of flax laminates. Experimental analysis and analytical modeling under low-velocity impact
No full textSince natural fiber composites have a high potential as an alternative to synthetic materials, their mechanical properties must be investigated under different loading modes. In this paper, flax, basalt and hybrid flax-basalt/epoxy resin composite laminates are experimentally characterized and their behavior under low-velocity impact conditions is investigated. Analytical models, on their part, represent a low-cost and low-time consuming tool to provide initial considerations on the mechanical behavior of these relatively new composites in load-bearing applications. Therefore, in this work, analytical models previously introduced for synthetic fiber laminates are used to provide an approximation of the load–displacement curve resulting from a low-velocity impact on flax, basalt and flax/basalt hybrid laminates. In particular, an attempt is made to predict the descending phase of the load–displacement curve. To validate the theoretical results, an experimental campaign was carried out with different impact energies from 2.5 J to 10 J at room temperature. The experimental results showed a significant improvement in the quasi-static mechanical properties and damage tolerance of the hybrid composites compared to flax-based laminates. The analytical results confirmed that the presented models are well able to predict the response of natural fiber composites during loading and unloading phases for all considered material configurations
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