1,721,059 research outputs found
Random short sisal fiber biocomposites: Optimal manufacturing process and reliable theoretical models
No full textAlthough several works have been published in literature on agave fibers and their biocomposites, accurate information about the choice of both the fibers and the manufacturing processes that allow the user to optimize the biocomposites properties in terms of strength and stiffness are not yet available; also, no theoretical models that can be used for an accurate evaluation of the mechanical properties of these biocomposites, are reported. To this aim, the present work intends to give a contribution by considering green epoxy biocomposites reinforced by both short and discontinuous sisal agave fibers arranged in proper MAT-type fabrics. In particular, an optimized manufacturing process that allows to obtain good quality biocomposites, is proposed. A detailed analysis of the experimental results, obtained through preliminary pull-out and tensile tests carried out, along with an accurate analysis of the damage process performed by SEM micrographs, have allowed to develop reliable theoretical models that permit the mechanical properties evaluation of the analyzed biocomposites. Finally, the comparison with the most performing short/discontinuous fiber biocomposites reported in literature has shown how the implemented biocomposites exhibit comparable tensile strength and significantly higher stiffness, also respect to biocomposites reinforced by more stiff and more expensive fibers (flax, hemp, etc.)
An analysis of through-thickness residual stresses in aluminium FSW butt joints
No full textIn the paper, the results of a wide experimental campaign on friction stir welding (FSW) of aluminum alloys are reported. The attention was focused on the through-thickness residual stresses that occur on aluminum joints, after the welding process. In detail, using the hole-drilling method the residual stresses distribution in the zone close to the tool shoulder border of the joint advancing side, has been investigated; four different aluminum alloys and three different process conditions have been considered. The experimental analysis has shown that unlike traditional welding processes, the residual stresses are negative in the surface of the examined zone, and increase with depth until values of about 100–150 MPa that occur at a depth of about 0.5–1.0 mm. As expected, the maximum value of the residual stresses induced by the FSW process influences the mechanical behavior of the joint significantly, as it has been observed for the AA6082-T6 aluminum alloy by considering its static and fatigue resistance.
Such results corroborate that the hole-drilling method, widely employed in the industrial field due to its simplicity and low cost, can be used for an accurate estimation of the maximum residual stresses that occur in an aluminum butt joint obtained by friction stir welding
Numerical model for the characterization of biocomposites reinforced by sisal fibres
No full textAlthough several works have been recently published in literature about biocomposites, i.e. on innovative and ecofriendly polymer matrix composites reinforced by natural fibers, there are not studies on the influence of the waviness that various natural fiber present after their extraction. In order to give a contribution to the knowledge of the effects of the fiber waviness on the main mechanical properties of biocomposites, as the longitudinal Young modulus, in the present study a systematic numerical analysis has been carried out by using parametric models properly developed, that let the user to consider the effects of the key influence parameters as the fiber concentrations and the fiber curvature. Successive experimental studies have allowed to corroborate the accuracy of the numerical results, as well as to highlight the local effects due to the fiber waviness, that in some cases can become more significant than the global effects analyzed by the numerical approach
Optimal manufacturing and mechanical characterization of high performance biocomposites reinforced by sisal fibers
No full textThe increasing interest about eco-sustainable materials in the industrial production (automotive, civil construction, packaging), has led to the increase of the research works dealing with biocomposites. However, until now the most attention has been devoted to the development of short fiber biocomposites for non-structural applications, whereas only a few works have considered high performance biocomposites for structural applications. Consequently, the development of structural biocomposites from robust natural fibers, as sisal fibers, is a result expected from the scientific community, but not yet achieved. In order to give a contribution to the implementation of high performance biocomposites constituted by a green matrix reinforced by sisal fibers, the present work proposes a manufacturing process that allows to obtain good quality unidirectional biocomposites with fiber volume fraction up to 70%. In detail, it uses unidirectional “stitched” fabrics, properly obtained in laboratory from optimized fibers, and a curing under a proper pressure cycle. The comparison with independent data reported in literature, has evidenced how the proposed biocomposites exhibit mechanical properties higher than most of biocomposites described in literature, so that they can advantageously substitute not only materials as steel, aluminum and glass fiber reinforced plastics, but also other biocomposites reinforced by more expensive fibers
Mode I translaminar fracture toughness of high performance laminated biocomposites reinforced by sisal fibers: Accurate measurement approach and lay-up effects
Full text linkThe present work performs a systematic experimental analysis of the translaminar fracture behavior of high performance biocomposites constituted by green epoxy reinforced by sisal fibers, by varying the main influence parameters as fiber concentration and lay-up. Despite the corrective function properly introduced to take into account the anisotropy as well as the use of the equivalent crack length, the study shows that the LEFM does not give accurate estimations of the fracture toughness, because the extension of the near tip damaged zone is higher than the singular dominated one. Accurate estimations can be obtained instead by the proposed modified area method that takes into account both the local damage and the fiber bridging that occurs during crack propagation, that lead to R-curves whose asymptotic values constitute the true fracture toughness of the biocomposites examined. The constancy of the damage mechanisms observed by varying the fiber concentration, allows the user to compute the fracture toughness of a generic laminate from the specific fracture energy of the unidirectional lamina. Finally, the relatively high fracture toughness of the examined laminates allows to state that they can advantageously replace not only other composites having lower toughness, but also metals as steel, aluminum and titanium
RESISTENZA ALLA FRATTURA TRANSLAMINARE DI BIOCOMPOSITI RINFORZATI CON FIBRE DI AGAVE
No full textData la crescente attenzione nei confronti dell’ambiente, le sempre più restrittive norme in materia di
salvaguardia ambientale e di riciclo dei materiali hanno portato ad un notevole interesse dei ricercatori
verso i biocompositi, materiali costituiti da rinforzi di origine naturale e matrici a basso impatto
ambientale. Nonostante molteplici studi siano stati indirizzati a tali materiali innovativi, allo stato
attuale poche ricerche hanno riguardato l’analisi della tenacità alla frattura di laminati biocompositi. Il
presente lavoro propone pertanto uno studio sperimentale del comportamento alla frattura
translaminare in modo I di laminati biocompositi in fibre di agave e matrice epossidica green,
valutando in particolare l’effetto della variazione percentuale volumetrica del rinforzo e della sequenza
di impacchettamento (unidirezionali, cross-ply, angle-ply ecc.). Le prove di resistenza alla frattura a
trazione in modo I sono state eseguite su campioni con configurazione Compact Tension (CT) al fine
di determinare sia il fattore critico di intensificazione delle tensioni, sia la Critical Strain Energy
Release Rate (CSERR). In particolare, i diversi provini CT sono stati ottenuti attraverso laminazione
manuale e successivo processo di compression-moulding, a partire da tessuti unidirezionali di tipo
stitched appositamente prodotti in laboratorio. Al fine di individuare il metodo ottimale per una
accurata valutazione sperimentale della CSERR, sono stati confrontati i risultati ottenuti con diversi
metodi tra cui l’Area Method ed il Compliance CalibrationThe recent attention toward the environmental protection, as well as to the restrictive regulations in term of material recycling, have led to a noticeable interest of the scientific research for biocomposites, i.e. materials constituted by natural fibers and eco-friendly matrix. Although various research works have been focused on such innovative materials, only a few articles have been devoted to the their fracture strenght (thoughness).
The present work regards the experimental study of the translaminar fracture behavior (in mode I) of biocomposite laminates constituted by green epoxy matrix reinforced by optimized agave sisalana fibers, varying the main influence parameters as the fiber concentration and the lay-up (unidirectional, cross-ply, angle-ply etc.).
The fracture tests in mode I, have been performed by using Compact Tension (CT) specimens, in order to determine both the critical Stress Intensity Factor (SIFc) and the so called Critical Strain Energy Release Rate (CSERR).
In more detail, several CT specimens have been manufactured by hand lay-up and successive compression-moulding process whose parameters have been optimized in previous study of the same authors. To this aim proper stitched fabrics have been previously manufactured in laboratory. In order to detect the optimal method for an accurate experimental analysis of the CSERR, the results provided by the Area Method and the Compliance Calibration, have been compared
VALUTAZIONE DEL COMPORTAMENTO ALL’IMPATTO DI BIOCOMPOSITI RINFORZATI CON FIBRE DI AGAVE
No full textThe growing attention on environmental issues has led to a recent research interest in eco-sustainable and renewable materials, among which biocomposites play an important role. Biocomposites are materials consisting of a matrix with low environmental impact or renewable, reinforced with natural fibres. Several research activities reported in literature, deal mainly with the static mechanical properties. Just few works are instead devoted to the assessment of their dynamic properties, such as fatigue strength, impact strength, etc. In order to give a contribution to the knowledge of the impact behaviour of green epoxy matrix biocomposites reinforced with agave fibres, in this paper a systematic study is carried out to evaluate the impact strength of different laminates (single layer, cross-ply, quasi-isotropic), by low velocity impact test. The various selected laminates, allowed to assess the effects of the main influence parameters as fibre distribution (unidirectional and random), fibre concentration and lay-up
First lamina hybridization of high performance CFRP with Kevlar fibers: Effect on impact behavior and nondestructive evaluation
Full text linkThe impact behavior of a carbon-Kevlar hybrid composite, widely used in sport car manufacturing, was evaluated. To highlight the hybridization effect, comparative analyses were performed with the basic CFRP laminate having the same lay-up. Tensile, bending and low velocity impact tests, followed by nondestructive inspections, highlighted that Kevlar first lamina hybridization leads to an increment in specific impact strength, up to 55%. To assess the most reliable technique to detect the impact damage, nondestructive evaluation was performed by pulsed thermography, phased array ultrasonic technique, computed tomography and digital radiography. Phased array ultrasonic technique can be considered the most appropriate technique
The influence of the quarter wave plates in automated photoelasticity
No full textDuring the last decades, several methods have been proposed to automate photoelastic analyses. Some procedures are based on the circularly polarised light by using quarter wave plates. However, quarter wave plates are typically matched for a specific wavelength, and an error is introduced at different wavelengths. The error of quarter wave plates affects the measurement of isochromatic and isoclinic data. In this paper, the influence of the errors of quarter wave plates in some of the most common automated photoelastic methods is reviewed. The errors in the photoelastic data are given and the procedures to reduce, or eliminate, them are also suggested. (C) 2002 Elsevier Science Ltd. All rights reserved
EFFETTI DI NANOTUBI SULLA RESISTENZA A FATICA DI BIOCOMPOSITI A FIBRE CORTE RANDOM
No full textIn order to comply with the recent regulations in terms of environmental protection, biocomposites reinforced with natural fibers are increasingly used in the automotive sector, for the replacement of synthetic materials in semi-structural applications as panels, filling material, coating, dashboards, etc. The good mechanical performances of these materials, mixed with low weight and low cost, also make it possible to minimize the weight of the vehicle, and therefore consumption, but also the relative production costs. These are composites with random short fiber appreciated for their good stiffness, generally well above that of the matrix alone. Due to the peculiar "transverse" damage phenomena, often also influenced by debonding and pull-out phenomena related to the limited fiber-matrix adhesion, the mechanical, static and fatigue resistance of these materials is instead generally comparable to that of the matrix alone. This prevents their use in more mechanically demanding applications. For this reason, several chemical treatments on the fibers have been proposed in the literature by various authors, although still not fully satisfactory results have been obtained in terms of improving resistance. Taking into account the positive effects of the nanotubes on the fatigue resistance of structural polymeric adhesives and polymer matrix composites reinforced with synthetic fibers, in this work the effects of nanotubes on the fatigue resistance of biocomposites reinforced with agave fibers are analyzed, the fibers are obtained with a special compression molding process. The tests carried out on green epoxy matrix biocomposites in which nanotubes with different concentrations in volume were previously dispersed, showed that the static strenght is significantly influenced by the presence of nanotubes, although more appreciable improvements are found in the fatigue performance
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