Mechanical behavior of chemically-treated hemp fibers reinforced composites subjected to moisture absorption

Natural Fibers Reinforced Composites (NFRC) are finding much interest as substitutes for glass- or carbon-reinforced composites thanks to their lightness, easy handling, processing and recyclability. However, their polarity makes them incompatible with hydrophobic thermoplastic matrices, leading to extended moisture adsorption which causes the debonding between fibers and matrix, affecting, thus, the mechanical properties of NFRCs. In the present work, NFRCs were manufactured using hemp fibers previously chemically treated with NaOH alkali solutions or (3-Glycidyloxypropyl) trimethoxysilane (GPTMS) solutions of various concentrations. To assess the effectiveness of the used chemical treatments in hindering the moisture adsorption and the entailed mechanical failure of the NFRCs, untreated and treated hemp fibers based composites were subjected to moisture adsorption test and then to tensile testing as a function of the chemical treatment, temperature and concentration of reagents. The results show that the treatments with 5 wt% of both NaOH and GPTMS are the most effective, reducing composites' water uptake from 7.74% to 6.46% and 5.58% respectively at room temperature, and from 9.67% to 8.19% and 8.13% respectively at 50 °C. Moreover, the comparison between the mechanical testing results carried out before and after the moisture adsorption test, shows that the water uptake induces mainly a stiffness decrease (about 50% when alkali treatments were used and about 60% using silane treatment), while not significantly affect the loading capability of the composites regardless of chemical treatment. However, the specimen obtained using 5 wt% GPTMS is more effective in the prevent the failure of the composite induced by water uptake

3D strip model for continuous roll-forming process simulation

Abstract The paper addresses the complexities for a reliable numerical simulation of the roll forming process. During the process, the material is progressively bent accumulating plastic deformation at each forming step. Strain hardening limits the material formability and may causes flaws of the final shape. A simplified method for the FEM modeling of the process has been developed introducing a narrow-strip 3D model. This approach leads better performance than the classical modeling method, in terms of results reliability and low computational time. In order to verify the proposed model, an experimental campaign of testing, for a specific roll forming production process, was carried out. On the quasi-static regime, the post necking behavior of the sheet metal was characterized. The Vickers hardness and the plastic strain of uniaxial tests were empirically correlated. By the hardness correlation, the plastic strain accumulated at different stages of the process was evaluated and compared with the numerical results. Further possible improvements of the method are highlighted

Numerical FEM Evaluation for the Structural Behaviour of a Hybrid (bonded/bolted) Single-lap Composite Joint

Abstract The structural behaviour of a single-lap hybrid (bonded/bolted) composite joint subjected to a tensile external load was evaluated by means of the Finite Element Method (FEM). In particular, the distribution of stresses acting in its adhesive layer was compared with that relative to the case of a simply adhesive bonded joint. Furthermore, the load transferred by the bolt was determined at different characteristics of the adhesive and of the applied external tensile load, corresponding to both single and double bolt configuration. The obtained values were in turn compared with experimental data found in literature, so validating the produced numerical simulations

FEM and BEM Analysis of a Human Mandible with Added Temporomandibular Joints

Mathematical modelling of human mandible and its temporomandibular joints (TMJs) is one of the most important steps for developing a powerful forecasting tool to analyse the stress/strain behaviour of a human masticatory system under occlusal loads. In this work the structural behaviour of a mandible with articular discs, undergoing a unilateral occlusion, is numerically analysed by means of both Finite Element Method (FEM) and Boundary Element Method (BEM). The mandible is considered as completely edentulous and its anisotropic and non-homogeneous bone material behaviour is modelled. The material behaviour of the articular discs was assumed to be either elastic or hyper-elastic. The loads applied to the mandible are related to the active muscle groups during a unilateral occlusion. The results of FEM and BEM analyses are presented mainly in terms of stress distribution on the mandible and on the articular discs. Due to the uncertainty in the determination of the biological parameters, a sensitivity analysis is provided, which demonstrates the impact of the variation of articular disc stiffness and TMJ friction coefficient on the mandible stress peaks and on the occlusal loads (for a given intensity of muscle loads). Moreover a comparison between the effectiveness of the BEM and FEM numerical approaches on this kind of problem is provided

Fatigue behavior of hybrid and bonded single lap joints made of composite material

Joining of composite materials can be performed with different techniques and, in particular, trough mechanical fasteners, bonding, hybrid solutions. In last years, hybrid (bolted/bonded) joints are attracting the interest of several companies and scientific community, since the use of both techniques permit to overcome some critical aspects connected to the separate usage of adhesive and bolts, i.e., negative effects of the environmental conditions on adhesive, localized stresses at the notch. This paper aims to improve the knowledge about the fatigue behavior of hybrid CFRP (Carbon Fiber Reinforced Polymer) joints. For the purpose, experimental fatigue and static tests are performed on hybrid and bonded joints and the results herein discussed. Results are post-processed with the main goal to highlight the benefits led to the hybrid technique with respect to the bonding one

Stress Relaxation Behavior of Additively Manufactured Polylactic Acid (PLA)

In this work, the stress relaxation behavior of 3D printed PLA was experimentally investigated and analytically modeled. First, a quasi-static tensile characterization of additively manufactured samples was conducted by considering the effect of printing parameters like the material infill orientation and the outer wall presence. The effect of two thermal conditioning treatments on the material tensile properties was also investigated. Successively, stress relaxation tests were conducted, on both treated and unconditioned specimens, undergoing three different strains levels. Analytical predictive models of the viscous behavior of additive manufactured material were compared, highlighting and discussing the effects of considered printing parameters

study of two alternative cooling systems of a mold insert used in die casting process of light alloy components

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Stress Analysis of an Endosseus Dental Implant by BEM and FEM

In this work the Boundary Element Method (BEM) and the Finite Element Method (FEM) have been used for an elastic-static analysis of both a Branemark dental implant and a generic conic threaded implant, modelled either in the complete mandible or in a mandibular segment, under axial and lateral loading conditions. Two different hypotheses are considered with reference to degree of osteo-integration between the implant and the mandibular bone: perfect and partial osteointegration. The BEM analysis takes advantage of the submodelling technique, applied on the region surrounding the implant. Such region is extracted from the overall mandible and the boundary conditions for such submodel are obtained from the stress analysis realised on the complete mandible. The obtained results provide the localisation of the most stressed areas at the bone-implant interface and at the mandibular canal (containing the alveolar nerve) which represent the most critical areas during mastication. This methodology, enriched with the tools necessary for the numerical mandible reconstruction, is useful to realise sensitivity analysis of the stress field against a variation of the localisation, inclination and typology of the considered implant, in order to assess the optimal implant conditions for each patient under treatment. Due to the high flexibility in the pre- and post-processing phase and accuracy in reproducing superficial stress gradients, BEM is more efficient than FEM in facing this kind of problem, especially when a linear elastic constitutive material law is adopted

FEM Substructuring for the Vibrational Characterization of a Petrol Engine

In this work the vibrational behavior of a 4-cylinder, 4-stroke, petrol engine has been simulated by leveraging on a reduced modelling strategy, based on the Component Mode Synthesis (CMS), adopted to reduce the size of the full FEM model of the engine. The FEM model of the engine, comprising all of its sub-components, has been preliminary characterized from the vibrational standpoint; subsequently, the CMS has been adopted in order to reduce the FEM model size. Frequency Response Function (FRF) analyses have been used to identify the resonant frequencies and mode shapes of the different FEM models, and the so-obtained results have been compared showing a very good agreement. The reduced model has been able to reproduce with a high accuracy the vibration response at the engine mounts. The adopted reduced modelling strategy turned out to be effective in lowering the computational burden, keeping, at the same time, an accurate replication of the engine vibrational behavior. Runtimes have been significantly reduced from 24 hours for the full FEM model to less than 2 hours for the reduced model

Matrici di pressatura di ingranaggi cilindrici a denti diritti: analisi numerica FEM sull’influenza del modulo e del numero di denti sugli stati tensionali

Dopo una premessa sulla competitività della metallurgia delle polveri per la realizzazione di componentimeccanici di geometria complicata, si espongono i dati impostati per uno studio sistematico sull’influenza deiparametri di progetto, della ruota dentata e dello stampo, sul livello tensionale che si può raggiungere a finepressatura, ipotizzando una pressione radiale di 400 MPa, orientativamente corrispondente ad una pressioneassiale di almeno 700 MPa. Nell’indagine sono stati considerati 8 valori del modulo, 3 valori del rapporto fraraggio di raccordo di testa e modulo, 4 altezze di matrice, 3 spessori di parete dei nuclei, 3 materiali per inuclei (acciaio rapido tradizionale, Vanadis 60 della Böhler-Uddeholm e metallo duro con 10% Co). L’analisiFEM ha evidenziato come un aumento del raggio di raccordo di testa consente una diminuzione dei livellimassimi di tensione nel nucleo atta a garantire il rispetto delle condizioni di sicurezza. L’interferenza relativafra nucleo e anello è la variabile di progetto cui spetta la necessità di attenta valutazione, connessa – secondoi risultati di un’analisi “Taguchi” - alla scelta del materiale del nucleo, specialmente per le matrici più alte. Èstata considerata anche l’influenza dello spessore relativo del nucleo, rapportato cioè al diametro esterno delladentatura. Le condizioni di sollecitazione degli anelli di cinturazione, per i quali è stato imposto un valore 4 delrapporto fra diametri, non implicano mai condizioni di insufficiente sicurezza delle matrici cinturate. Non siriportano i risultati dell’analisi FEM per moduli m = 4 e m = 5 poiché la verifica delle condizioni d’ingranamentoha fatto rilevare la necessità di modificare i profili dei denti, in varia misura, rispetto alle classiche evolventi.I risultati ottenuti possono portare a progetti più affidabili sia delle ruote dentate – da produrre mediantemetallurgia delle polveri – sia degli stampi di formatura