Photoelastic stress pattern analysis using Fourier transform with carrier fringes: influence of quarter-wave plate error

The Fourier transform method, widely applied in photomechanics for the automated analysis of interferometric fringe patterns, has been recently extended to the photoelastic isochromatic fringe patterns analysis. Unfortunately, its use in photoelasticity involves some limitations that have not been completely highlighted in literature. This work deals with the influence of the quarter-wave plate tolerance on the evaluation of the retardation. Both theoretical and experimental analyses have shown that the quarter-wave plate error does not affect the retardation only if the principal stress directions in the model and in the carrier are aligned. In general, instead, the tolerance of the quarte…

Numerical experimental analysis of hybrid double lap aluminum-CFRP joints

Abstract Due to their reliability and ease of assembly, both the adhesively bonded and the mechanical joints are commonly used in different fields of modern industrial design and manufacturing, to joint composite materials or composites with metals. As it is well known, adhesively bonded joints are characterized by high stiffness and good fatigue life, although delamination phenomena localized near the free edges may limit their use, especially for applications where corrosive environments and/or moisture can lead to premature failure of the bonding. In these cases, a possible alternative is given by the well-known mechanical joints. On the contrary, these last joints (bolted, riveted) requ…

EXPERIMENTAL AND NUMERICAL EVALUATION OF THE MECHANICAL BEHAVIOUR OF GFRP SANDWICH PANELS

This work deals with the analysis of the mechanical behaviour of a class of sandwich structures widely employed in marine constructions, constituted by fiber-glass laminate skins over PVC foam or polyester mat cores. In detail, a systematic experimental study and numerical simulations have shown that the theoretical prediction of the strength and the actual failure mechanism of these sandwich structures can be affected by significant errors, specially in the presence of prevalent shear loading. Moreover, because of the low shear stiffness and the elastic constants mismatch of the skins and core material, failure modes and strength are strongly influenced by eventual stresses orthogonal to t…

STUDIO DI GIUNZIONI METALLO-COMPOSITO SOTTOPOSTE A CARICHI STATICI E DI FATICA

Obiettivo di questo studio è l’analisi delle giunzioni ibride metallo-composito realizzate con la tecnica co-cured, cioè incollando i materiali con la stessa resina adoperata per la manifattura del laminato composito. E’ stata studiata in particolare la giunzione fra alluminio e composito in fibra di vetro (Al/GFRP), accoppiamento che trova applicazioni in svariati ambiti del settore dei trasporti navali e terrestri. Sono stati analizzati giunti a semplice sovrapposizione (GSS) e a doppia sovrapposizione (GDS) sottoposti a carichi statici e di fatica. Tramite analisi numeriche FEM nonché prove sperimentali eseguite variando la lunghezza di sovrapposizione del giunto, si è indagato sul crite…

Elaborazione delle immagini con il metodo della trasformata di Fourier: il caso della fotoelasticità.

The carrier fringes method has been proposed in digital photoelasticity in combination with techniques such as Fourier transform and phase shifting method, without considering the influence of the isoclinics on the isochromatic patterns analysis. Unlike other optical methods as moiré and holographic interferometry, in photoelasticity the light intensity emerging from a circular polariscope is related to both the isochromatic retardation and the isoclinic parameter. As it is shown by the theoretical analysis, owing to the misalignment between the principal stresses in the model and in the carrier, the computed retardation is affected by an error which is the same for all photoelastic methods…

Studio sperimentale di giunti ibridi CFRP-alluminio

CFRP STRUCTURAL CAPACITORS: EFFECT OF DAMAGE AND MECHANICAL LOAD ON CAPACITANCE

Aim of this work is to study the effect of mechanical load and damages, on the performance of structural capacitors, made by CFRP composite laminates with a PET dielectric film (treated with sodium hydroxide) inserted at the laminate middle-plane. Such capacitors have been characterized by ILSS and tensile tests and the properties so estimated were compared to those of the simple CFRP. By measuring the capacitance before mechanical loading, under loading and after unloading, it has been observed that, due to the damage of the CFRP layers, proved also by proper fractographic analysis, at high strain level the capacitance decreases although it exhibits a complete recovery after unloading. Suc…

S113 Mapping Multiple Residual Stress Components Using the Contour Method and Superposition

Low-velocity impact behaviour of green epoxy biocomposite laminates reinforced by sisal fibers

Abstract Due to its good mechanical characteristics, low cost and high availability in the current market, sisal fiber is one of the most used for the manufacturing of biocomposites in various industrial fields (automotive, marine, civil construction etc.). The particular sub-fibrillar structure of the sisal fiber (similar to aramid fibers) and the corresponding anisotropic behavior detected by recent research activities, suggest that such biocomposites should exhibit also high impact strength, in such a way to permit their advantageously use also for the manufacturing of crashworthy components (bumpers, helmets, protection systems etc.), that are at the same time also eco-friendly, lightwe…

AN ACCURATE METHOD TO PREDICT THE STRESS CONCENTRATION IN COMPOSITE LAMINATES WITH A CIRCULAR HOLE UNDER TENSILE LOADING

The paper presents a theoretical-numerical hybrid method for determining the stresses distribution in composite laminates containing a circular hole and subjected to uniaxial tensile loading. The method is based upon an appropriate corrective function allowing a simple and rapid evaluation of stress distributions in a generic plate of finite width with a hole based on the theoretical stresses distribution in an infinite plate with the same hole geometry and material. In order to verify the accuracy of the method proposed, various numerical and experimental tests have been performed by considering different laminate lay-ups; in particular, the experimental results have shown that a combined …

Ottimizzazione di Giunti Ibridi(Spr/Incollati) Gfrp-Al

Use of Hybrid Methods (Hole-Drilling and Ring-Core) for the Analysis of the RS on Welded Joints

The hybrid methods (HMs) for the residual stress (RS) analysis, such as the well-known hole-drilling method (HDM) and the ring-core method (RCM), have been widely developed since 80’. They are mechanical methods based on the partial relaxation of the RSs that occur when a proper geometry variation of the analysed component is introduced by drilling a hole (HDM) or a proper annular groove (RCM). The RS computation is performed by measuring the strains relaxed on surface and then by combine properly such measured strains with the influence coefficients previously computed accurately by using a numerical codes that consider the geometry of the particular component to be examined. In such a man…

Complete isochromatic fringe order analysis in digital photoelasticity by Fourier Transform and load Stepping

Stiffness and Reinforcement Effect of electrical Resistance Strain Gauges

:  The increasing use of low-modulus materials, on which the reinforcement effect of the electrical resistance strain gauge is not negligible, has re-opened the research interest into this issue. This study deals with the evaluation of stiffness, and of the strain gauge Young's modulus involved in the estimation of both the global and the local reinforcement effect; the relationship between the strain gauge stiffness and the local reinforcement effect is also analysed. In particular, the experimental technique used to determine the stiffness of some commercial strain gauges is described. The results show that the strain gauge stiffness alone does not permit an accurate evaluation of the loc…

ON THE REINFORCEMENT EFFECT OF STRAIN GAGES

TOWARD A DESIGN METHOD FOR METAL-COMPOSITE CO-CURED JOINT BASED ON THE G-SIFs

Abstract In this work, a systematic study of the singular stress field in the zone where the interface intersects the free edge surfaces of bonded metal-composite co-cured joints, has been performed. The obtained theoretical, numerical and experimental results have permitted to detect the relationships between the joint configuration and the singular stress field, as well as to implement a new design method based on the so called generalised stress intensity factors. Such a proposed method allows the user to predict the static strength of a generic metal-composite co-cured joint, vs. the main influence parameters as the elastic modulus of the coupled materials, the overlap length, the taper…

ERROR AND UNCERTAINTY ANALYSIS OF RESIDUAL STRESS EVALUATION BY USINGTHE RING-CORE METHOD

The Ring-Core Method is a technique used for the experimental analysis of the residual stresses in mechanical components. For uniform and non-uniform residual stresses estimation, the use of the method leads in general to accurate results but, unfortunately at present the user does not have appropriate procedures to correct the obtained results from systematic errors as well as to estimate the uncertainty due to random errors. In order to overcome such drawbacks, in the present work, the procedures for the correction of the effects of the main error sources and for the stress uncertainty estimation, are proposed. The practical application of such procedures allow the user to highlight the r…

Analysis of the accuracy of fiber-optic strain transducers installed by using composite smart patches

The aim of this work is to investigate the accuracy of fiber-optic strain transducer related to the incomplete load transmission that can occur on composite smart patches used as base support for the installation of such transducer, commonly used for structural monitoring of concrete elements. In detail, since the dimensions of the transducer are generally very small compared to those of the structural element being monitored, attention is given not to the global reinforcement effect, which occurs when the transducer alters the entire strain field of the transversal section of the monitored element, but rather to the so-called local reinforcement effect, which is due to an incomplete load …

First lamina hybridization of high performance CFRP with Kevlar fibers: Effect on impact behavior and nondestructive evaluation

The 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 consid…

Inverting Multiple Residual Stress Components from Multiple Cuts with the Contour Method

An extension of the contour method is presented which allows the measurement of multiple stress components by making multiple cuts. In the contour method, a body is carefully cut in two using wire electric discharge machining (EDM). The contours, or shapes, of the cut surfaces are then measured and used to calculate the original residual stress normal to the cut plane using a simple finite element calculation. In the extension presented here, the two pieces from the original body can be cut again in a transverse direction, and the contours of the new cut surfaces are measured. The stresses calculated on the planes of these second cuts have been affected by the first cut. Then a simple inver…

Optimization of depth increment distribution in the ring-core method

The integral equation method is the most suitable calculation procedure for the determination of non-uniform residual stresses by semi-destructive mechanical methods such as hole and ring-core drilling. However, the high sensitivity to strain measurement errors due to the ill-conditioning of the equation sets has prevented its practical use. Examination of the influence of the strain measurement error on the calculated stresses and its propagation has shown that, for given maximum groove depth and total steps number, the error sensitivity depends on the particular depth increment distribution used. By means of an alternative matrix formulation of the equation sets the depth increment distr…

Effect of the hole-bottom fillet radius on the residual stress analysis by the hole drilling method

ANALISI DELL'EFFETTO RINFORZANTE DI PATCHES IN COMPOSITO PER L'INSTALLAZIONE DI TRASDUTTORI DI DEFORMAZIONE A FIBRA OTTICA

Experimental Tests of Fatigue Induced Delamination in Gfrp and Cfrp Laminates

This work deals with the experimental analysis of the delamination phenomena in various composite materials under different loading conditions. Quasi-static and fatigue tests are performed on specimens made of glass-fibre reinforced plastic (GFRP) and carbon-fibre reinforced plastic (CFRP). In particular, under both quasi-static and fatigue loading, single fracture modes I and II (using standard DCB and ENF test configurations) and mixed modes I+II (using the MMB test configuration) with several mode mixtures, have been analysed. Further experiments of delamination growth with mode mixture that varies with the crack length, will be performed.

Concentrazione delle tensioni e previsione della resistenza di elementi strutturali forati in materiale composito

A new calculation procedure for non-uniform residual stress analysis by the hole-drilling method

The hole-drilling method is one of the most used semi-destructive techniques for residual stress analysis in mechanical parts. In the presence of non-uniform residual stress, the stress field can be determined from the measured relaxed strains using several calculation methods, but the most used one is the so-called integral method. This method is characterized by some simplifications that lead to approximate results, especially when the residual stress varies abruptly. In this paper a new calculation procedure called the spline methods is proposed, which allows these drawbacks to be overcome. Numerical simulations and an experimental test have corroborated the best performance of the prop…

Effetti del raggio di raccordo al fondo foro nella analisi delle tensioni residue con il metodo della rosetta forata

Il metodo della rosetta forata è una delle tecniche di analisi delle tensioni residue più utilizzati in campo industriale. Il metodo è abbastanza accurato per tensioni costanti, mentre nel caso di tensioni variabili nello spessore del componente esaminato diverse cause di incertezza devono essere attentamente esaminate. Poiché nella valutazione numerica dei coefficienti di influenza il foro è considerato perfettamente cilindrico e privo di raccordo, una sorgente di errore è costituita dal raggio di raccordo al fondo foro. In questa memoria viene studiato l’effetto che il raggio di raccordo ha sulle deformazioni misurate e sulle tensioni calcolate. E’ cosi mostrato come anche piccoli raggi d…

Measuring Multiple Residual-Stress Components using the Contour Method and Multiple Cuts

The conventional contour method determines one component of residual stress over the cross section of a part. The part is cut into two, the contour (topographic shape) of the exposed surface is measured, and Bueckner’s superposition principle is analytically applied to calculate stresses. In this paper, the contour method is extended to the measurement of multiple residual-stress components by making multiple cuts with subsequent applications of superposition. The theory and limitations are described. The theory is experimentally tested on a 316L stainless steel disk with residual stresses induced by plastically indenting the central portion of the disk. The multiple-cut contour method resu…

ANALISI DEL COMPORTAMENTO MECCANICO DI SANDWICHES GREEN COSTITUITI DA PELLI IN BIOCOMPOSITO E CORE IN BALSA/SUGHERO

Negli ultimi anni l’aumento della sensibilità nei confronti della salvaguardia ambientale, anche attraverso apposite norme in materia di lotta all’inquinamento e di riciclo dei materiali a fine vita, ha portato ad un notevole interesse dei ricercatori verso i cosiddetti compositi green, materiali costituiti in genere da rinforzi di origine naturale e matrici “bio-based” a basso impatto ambientale. Una categoria interessante di materiali innovativi, caratterizzati da una elevata resistenza a flessione unita ad un basso peso specifico, è rappresentata dai cosiddetti sandwiches, costituiti in genere da robuste pelli in composito fibro-rinforzato e da un core in materiale molto leggero e poco r…

Error and Uncertainty Analysis of the Residual Stresses Computed by Using the Hole Drilling Method

:  The hole-drilling method is one of the most used techniques for the experimental analysis of the residual stresses in mechanical components. For both through-thickness uniform and non-uniform residual stress distributions, its application is standardised by the ASTM E837-08. In accordance with the ASTM limitations, the analysis of uniform residual stresses, to which the present work deals with, leads in general to results with a maximum bias of about 10%. Unfortunately, in general the user does not have appropriate procedures to estimate the actual stress error; consequently, if one or more of the experimental influence parameters fall out of the corresponding standard limitations, the c…

Local Reinforcement Effect of a Strain Gauge Installation on Low Modulus Materials

The reinforcement effect of electrical resistance strain gauges is well documented in the technical literature. In this paper the local reinforcement effect in tension is studied by using a simple theoretical model by considering a strain gauge mounted on a semi-infinite plate having the same width of the strain gauge and subjected to a uniaxial tension load. Neglecting the effect of the adhesive layer and considering the interface shear stress as an exponential distribution, the proposed model gives a closed-form solution. In detail, this model permits a simple formula to be obtained which allows the user to correct the local reinforcement effect provided that a proper calibration is perfo…

STUDIO NUMERICO-SPERIMENTALE DI GIUNZIONI IBRIDE A DOPPIA SOVRAPPOSIZIONE CFRP-ALLUMINIO

The adhesively bonded joints and the mechanical joints between structural components made by composite materials or between elements made by composite and metal, are commonly used in different fields of modern industrial design and manufacturing, since they are characterized by economy, reliability and ease of assembly. The bonded joints are characterized by high stiffness and fatigue life, although delamination phenomena localized near the edges of attack of the adherends may limit their use, especially in particular applications where corrosive environments and humidity can lead to premature failure. In these cases, a possible alternative is offered by the well known riveted joints. On th…

Measuring Inaccessible Residual Stresses Using Multiple Methods and Superposition

The traditional contour method maps a single component of residual stress by cutting a body carefully in two and measuring the contour of the cut surface. The cut also exposes previously inaccessible regions of the body to residual stress measurement using a variety of other techniques, but the stresses have been changed by the relaxation after cutting. In this paper, it is shown that superposition of stresses measured post-cutting with results from the contour method analysis can determine the original (pre-cut) residual stresses. The general superposition theory using Bueckner’s principle is developed and limitations are discussed. The procedure is experimentally demonstrated by determini…

Previsione della vita a fatica di componenti soggetti a sollecitazioni random a banda larga

ANALISI DI GIUNTI A DOPPIA SOVRAPPOSIZIONE GFRP-ALLUMINIO INCOLLATI E BULLONATI

Numerical model for the characterization of biocomposites reinforced by sisal fibres

Abstract Although 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 an…

Stress Concentration and Strength Prediction of Composite Plates with Circular Holes

Clinical diagnosis and evoluation

Valutazioni tecniche ed economiche sull'uso degli inseguitori solari

I processi di antropizzazione connessi allo sviluppo del settore energia hanno determinato delle “impronte” sul nostro pianeta ormai indelebili. Non resta che provare a ridurre i consumi energetici e affidare la produzione dell’energia ad un mix equilibrato di fonti. In questo contesto, con particolare riferimento alla produzione dell’energia elettrica, lo sfruttamento della risorsa solare ha assunto un ruolo di primo ordine. Gran parte delle attività di ricerca e sviluppo del settore si è concentrata sulle tecniche e sulle metodologie innovative di conversione della fonte in energia elettrica. Accanto a queste, a parità di tecnologia di conversione adottata, altre attività di ricerca e svi…

Analysis and optimization of hybrid double lap aluminum-GFRP joints

In this paper a systematic investigation of the mechanical performance of hybrid double-lap Al-GFRP bonded-bolted joints, has been carried out by using experimental analyses and numerical simulations. In order to detect the optimal geometric configuration, as well as to highlight the contribution of adhesive and bolts, the results relative to hybrid joints have been compared with those of simply adhesively bonded and simply bolted joints. The experimental and numerical results have shown that by using the minimum overlap length provided from theory, the bolt leads to a significant decreasing of both the maximum shear and the maximum peel stresses in the adhesive layer and, consequently, the…

Residual Stress Analysis in Orthotropic Materials by the Hole Drilling Method

Determination of Nonuniform Residual Stresses Using the Ring-Core Method

This paper considers residual stress analysis using the ring-core method. In particular, the so-called integral equation method is applied to evaluate nonuniform residual stress fields. The proposed method overcomes typical drawbacks of the incremental strain method which lead to incorrect results for strongly varying stress fields. The experimental results obtained with a specimen subjected to a bending load confirm the theoretical predictions.

OTTIMIZZAZIONE STRUTTURALE DI UNA IMBARCAZIONE A VELA DA 72 PIEDI

The influence of the quarter wave plates in automated photoelasticity

During 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.

New concept in bioderived composites: Biochar as toughening agent for improving performances and durability of agave-based epoxy biocomposites

Biocomposites are increasingly used in the industry for the replacement of synthetic materials, thanks to their good mechanical properties, being lightweight, and having low cost. Unfortunately, in several potential fields of structural application their static strength and fatigue life are not high enough. For this reason, several chemical treatments on the fibers have been proposed in literature, although still without fully satisfactory results. To overcome this drawback, in this study we present a procedure based on the addition of a carbonaceous filler to a green epoxy matrix reinforced by Agave sisalana fibers. Among all carbon-based materials, biochar was selected for its environment…

KNOWN RESIDUAL STRESS SPECIMENS USING OPPOSED INDENTATION

In order to test new theories for residual stress measurement or to test the effects of residual stress on fatigue, fracture, and stress corrosion cracking, a known stress test specimen was designed and then fabricated, modeled, and experimentally validated. To provide a unique biaxial stress state, a 60 mm diameter 10 mm thick disk of 316L stainless steel was plastically compressed through the thickness with an opposing 15 mm diameter hard steel indenters in the center of the disk. For validation, the stresses in the specimen were first mapped using time-of-flight neutron diffraction and Rietveld full pattern analysis. Next, the hoop stresses were mapped on a cross section of two disks usi…

Fatigue life prediction under wide band random loading

In this paper, a method for the high-cycle fatigue life prediction of components subjected to gaussian, stationary, wide band random loading is presented. It allows the user to evaluate the fatigue life of components subjected to uniaxial stress states directly from the stress power spectral density (PSD), avoiding onerous simulations in time domain. The proposed method can be applied to random stress processes having PSD of any shape, and the fatigue life predictions obtained are more accurate than that provided by most of the frequency domain techniques proposed in literature.

ANALISI STATICA E A FATICA DI GIUNZIONI IBRIDE CO-CURED METALLO COMPOSITO A SEMPLICE E A DOPPIA SOVRAPPOSIZIONE

Obiettivo di questo studio è l’analisi delle giunzioni ibride metallo-composito realizzate con la tecnica co-cured, cioè incollando i materiali con la stessa resina adoperata per la manifattura del laminato composito. E’ stata studiata in particolare la giunzione fra alluminio e laminato in fibra di vetro/resina epossidica (Al/GFRP), accoppiamento che trova applicazioni in svariati ambiti del settore trasporti. Le geometrie analizzate sono quella della giunzione a semplice sovrapposizione (Single Lap Joint) e a doppia sovrapposizione (Double Lap Joint), per confrontarne il diverso comportamento sotto l’applicazione di carichi monotoni e ripetuti ciclicamente. L’analisi è stata affrontata ad…

The Reinforcement Effect of Strain Gauges Embedded in Low Modulus Materials

The reinforcement effect of electrical resistance strain gauges is well-described in the literature, especially for strain gauges installed on surface. This paper considers the local reinforcement effect of strain gauges embedded within low Young modulus materials. In particular, by using a simple theoretical model, already used for strain gauges installed on the surface, it proposes a simple formula that allows the user to evaluate the local reinforcement effect of a generic strain gauge embedded on plastics, polymer composites, etc. The theoretical analysis has been integrated by numerical and experimental analyses, which confirmed the reliability of the proposed model.

Effects of matrix cracks on the mechanical behavior of polimeric matrix composites

ANALISI SPERIMENTALE DEL COMPORTAMENTO MECCANICO DI BIO-COMPOSITI RINFORZATI CON FIBRE DI AGAVE

This work deals with the study of the mechanical performance of bio-composites obtained by reinforcing both "green" thermoplastic and thermosetting matrices with natural fibers obtained by means of proper extraction processes, from two variety of agave (striata and americana) that grows in the Sicilianterritory. Through systematic experimental analyses, the mechanical performance of short fiber composites with thermoplastic matrix (PLA) obtained by hot molding, as well as composites with thermosetting matrix (tri-components "green" epoxy) obtained by hand lay-up, has been studied. Also, an innovative fiber extraction process having a zero environmental impact, have been proposed. Moreover, …

NUMERICAL-EXPERIMENTAL METHOD FOR THE ANALYSIS OF RESIDUAL STRESSES IN COLD-EXPANDED HOLES

Hole cold expansion is a technique widely used to improve the fatigue life of components with holes, e.g. bolted or riveted joints. As it has been demonstrated in literature by analytical, numerical and experimental analyses carried out by several authors, the compressive residual stresses introduced by the hole cold expansion have a beneficial influence on both the static and the fatigue strength of the treated component, because they reduce significantly the typical stress peaks around the hole due to stress concentration. In the literature, various analyses of the residual stresses introduced by the hole cold expansion have been performed by using several methods such as X-ray diffractio…

Error and Uncertainty Analysis of Non-Uniform Residual Stress Evaluation by Using the Ring-Core Method

Like other mechanical methods used for the residual stress evaluation on a generic structural component, the Ring-Core Method (RCM) is potentially subjected to various error sources that in some particular experimental conditions can lead to significant errors and/or uncertainties on the computed residual stresses. The accuracy of the residual stresses computed by the RCM can be improved by a proper error analysis that allows the user to correct the systematic errors, as well as to evaluate the remaining stress uncertainty due to the random errors. Therefore, in order to give a contribution to the accuracy of the RCM, in the present work the list of the main error sources is presented and t…

ANALISI DELL’INCERTEZZA DEL METODO DELLA ROSETTA FORATA IN PRESENZA DI TENSIONI UNIFORMI

Analisi delle Tensioni Residue in Laminati Compositi mediante il Metodo del Foro

EXPERIMENTAL TESTS OF FATIGUE INDUCED DELAMINATION IN GFRP LAMINATES

Fatigue delamination experiments on GFRP and CFRP specimens under single and mixed fracture modes

This paper deals with the experimental analysis of the delamination phenomena in composite materials under different loading conditions. Quasi-static and fatigue tests are performed on specimens made of glass-fibre (GFRP) and carbon-fibre (CFRP) reinforced plastic. In particular, experiments have been carried out under single fracture modes I and II (using standard DCB and ENF test configurations) and mixed modes I+II (using the MMB test configuration) with several mode mixtures. Results obtained for the two materials have been compared paying attention on the relationship between the parameters that describe the fatigue behaviour and the mode mixture acting during the crack propagation. © …

Optimal manufacturing and mechanical characterization of high performance biocomposites reinforced by sisal fibers

Abstract The 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 performanc…

A novel frequency domain method for predicting fatigue crack growth under wide band random loading

This work deals with the evaluation of the fatigue crack growth rate of structural components subjected to uniaxial Gaussian stationary wide band random loading. In detail, a new frequency domain method that allows the user to estimate the expected crack growth rate directly from the PSD data is proposed. Using a stochastic mean function properly, introduced and described by simple closed form relationships implemented by systematic numerical simulations of a high number of wide band random processes, the proposed method permits to avoid the onerous time domain simulations and provides in general crack growth rate predictions in a good accordance with the so-called time domain method. Pract…

Influence of the anisotropy of sisal fibers on the mechanical properties of high performance unidirectional biocomposite lamina and micromechanical models

Abstract High performance biocomposites reinforced by sisal fibers, are between the most promising materials that could be used in various fields, from automotive to civil constructions, thanks to their good mechanical performance, as well as to the low cost and the great availability of the fiber. Nevertheless, at present their practical use is prevented by the limited knowledge of their mechanical performance. The results of the present study have shown that the intimate fibrillar structure of the sisal fiber is associated with a high anisotropy involving not only the elastic parameters, but also the damage processes with typical fiber splitting phenomena, that influence noticeably the bi…

Rapid evaluation of notch stress intensity factors using the peak stress method with 3D tetrahedral finite element models: Comparison of commercial codes

The peak stress method (PSM) allows a rapid application of the notch stress intensity factor (NSIF) approach to the fatigue life assessment of welded structures, by employing the linear elastic peak stresses evaluated by FE analyses with coarse meshes. Because of the widespread adoption of 3D modeling of large and complex structures in the industry, the PSM has recently been boosted by including four-node and ten-node tetrahedral elements of Ansys FE software, which allows to discretize complex geometries. In this paper, a Round Robin among eleven Italian Universities has been performed to calibrate the PSM with seven different commercial FE software packages. Several 3D mode I, II and III …

Multiple Stress Components From Multiple Cuts for The Contour Method

Materiali Compositi Ibridi Resistenti al Fuoco

La sempre crescente richiesta nell’ambito della progettazione strutturale aeronautica/aerospaziale di materiali aventi elevate perfomance meccaniche, ha determinato negli ultimi decenni la diffusione dei materiali compositi a matrice polimerica (Polymer Matrix Composites, PMCs) rinforzati prevalentemente con fibre di carbonio (Carbon Fiber Reinforced Plastic, CFRP). Le principali limitazioni nell’uso di tali compositi sono dovute all’elevata sensibilità agli agenti ambientali ed alle basse temperature di esercizio; inoltre, tali materiali mal sopportano l’esposizione al fuoco e ciò per via della facile incendiabilità non solo della matrice ma anche delle fibre. Al fine di mettere a punto un…

Complete Isochromatic Fringe-order Analysis in Digital Photoelasticity by Fourier Transform and Load Stepping

:  In this paper, a new fully automated photoelastic technique based on both the Fourier transform and the envelope of the fringe centres, is presented. Using two images of the isochromatics acquired with different loads and a simple calculation routine, the method allows the user the complete determination of the isochromatic fringe retardation on any arbitrary line chosen over the model domain, without the need for supplementary external information or calibration. Various experiments have corroborated the accuracy of the method, estimated theoretically to about 0.07 fringe orders. As a result of its simplicity and sufficient accuracy, the method can be used in the industrial field also b…

Toward high performance renewable agave reinforced biocomposites: Optimization of fiber performance and fiber-matrix adhesion analysis

Abstract The increasing sensitivity toward the environmental pollution and the recent laws on the environmental protection, have led to an increasing attention to the so called biocomposites, i.e. to ecofriendly or renewable composite materials, obtained from biopolymers reinforced by natural fibers. Although the contribution of various works reported in literature, focused on biocomposites reinforced by agave fibers, such materials are still exclusively used in the automotive industry for non-structural applications, and the implementation of high performance biocomposites for semi-structural and structural applications, is an expected, but not yet reached objective. Therefore, the present…

OTTIMIZZAZIONE DI UNA GIUNZIONE INCOLLATA A DOPPIA SOVRAPPOSIZIONE PER SANDWICH IN COMPOSITO

Grazie alle peculiari performances meccaniche, quali soprattutto l’elevata resistenza a flessione unita ad un basso peso specifico, le strutture sandwich in composito sono sempre più frequentemente utilizzate nella moderna progettazione industriale. Nel presente lavoro, considerando un composito sandwich con skins in vetroresina e core in PVC espanso, è stato eseguito uno studio numerico e sperimentale al fine di individuare la configurazione ottimale di una giunzione incollata a doppia sovrapposizione avente quali aderendi esterni le stesse skins del sandwich e aderendo interno costituito da un inserto in metallo o in composito. In dettaglio, previa preliminare ottimizzazione teorica della…

Limitation of fourier transform photoelasticity: Influence of isoclinics

The application of the Fourier transform to photoelasticity was used in the evaluation of the retardation using a carrier system of fringes. In photoelasticity, the light intensity from the analyzer in a circular polariscope depends on both the retardation (isochromatics) and the isoclinic parameter. The theoretical analysis shows that the angle between the principal stresses in the model and in the carrier system of fringes influences the evaluation of the retardation (isochromatics), as occurs when misaligned compensators (namely, Babinet) are used. As a consequence, this method may not be applied as a full-field technique, although the error is small if the angle between the principal st…

Environmental aging effects on high-performance biocomposites reinforced by sisal fibers

Among the innovative materials, an important role is played by the so-called biocomposites, generally made by an eco-friendly matrix reinforced with natural fibers. Unfortunately, due to the degradability of the green matrixes as well as to hydrophilicity of the natural fibers, the resistance of such innovative materials to the environmental agents is, in general, relativity low, and it can significantly limit their use in outdoor conditions. To contribute to the knowledge of the effects of the leading environmental agents on the mechanical properties of highperformance biocomposites made of a green epoxy matrix reinforced by agave fibers, a systematic experimental testing campaign has been…

Experimental Characterisation and Numerical Simulation of the Mechanical Behaviour of GFRP Sandwich Panels

Optimal calculation steps for the evaluation of residual stress by the incremental hole-drilling method

The integral method is a suitable calculation procedure for the determination of nonuniform residual stresses by semidestructive mechanical methods such as the hole-drilling method and the ring-core method. However, the high sensitivity to strain measurement errors due to the ill conditioning of the equations has hindered its practical use. the analysis of the influence of the strain measurment error on the computed stresses carried out in the present work has showed that, given both maximum hole depth and number of total steps, the error sensitivity depends on the particular depth increment distribution used. By means of the matrix formulation, the depth increment distribution that optimiz…

STUDIO DI UN’APPLICAZIONE IN CAMPO AERONAUTICO DI COMPOSITI STRUTTALI IBRIDI RESISTENTI AL FUOCO

In the aeronautical/aerospace structural design the increasing demands of materials having high mechanical performance, has led to the a wide use of Polymeric Matrix Composites (PMCs) mainly reinforced by carbon fibers (Carbon Fiber Reinforced Plastics, CFRP). The main drawbacks in the use of such composites are due to high sensitivity to environmental features and low working temperatures; moreover, these materials do not tolerate the fire exposure because of the inflammability of both the matrix and the fibers. In order to develop a composite that combines the structural performance of CFRP with a good strength to high temperatures and fire, in the present work the use of a hybrid composi…

Un metodo per l’analisi delle tensioni residue nei fori espansi a freddo

The Role of Patient-Specific Morphological Features of the Left Atrial Appendage on the Thromboembolic Risk Under Atrial Fibrillation

BackgroundA large majority of thrombi causing ischemic complications under atrial fibrillation (AF) originate in the left atrial appendage (LAA), an anatomical structure departing from the left atrium, characterized by a large morphological variability between individuals. This work analyses the hemodynamics simulated for different patient-specific models of LAA by means of computational fluid–structure interaction studies, modeling the effect of the changes in contractility and shape resulting from AF.MethodsThree operating conditions were analyzed: sinus rhythm, acute atrial fibrillation, and chronic atrial fibrillation. These were simulated on four patient-specific LAA morphologies, each…

Limitation of carrier fringe methods in digital photoelasticity

The carrier fringes method has been proposed in digital photoelasticity in combination with techniques such as Fourier transform and phase shifting method, without considering the influence of the isoclinics on the isochromatic patterns analysis. Unlike other optical methods as moire and holographic interferometry, in photoelasticity the light intensity emerging from a circular polariscope is related to both the isochromatic retardation and the isoclinic parameter. As it is shown by the theoretical analysis, owing to the misalignment between the principal stresses in the model and in the carrier, the computed retardation is affected by an error which is the same for all photoelastic methods…

PROVE DI INVECCHIAMENTO SU BIOCOMPOSITI EPOSSIDICI AD ELEVATE PERFORMANCE RINFORZATI CON FIBRE DI AGAVE

La riduzione dell'impatto ambientale nella moderna produzione di materiali compositi a matrice polimerica ha attratto una significativa attività di ricerca finalizzata alla messa a punto di nuovi materiali compositi “green” caratterizzati altresì da una apprezzabile riduzione dei costi e del peso specifico. Tra questi materiali giocano un ruolo importante i cosiddetti biocompositi, materiali costituiti da matrici a basso impatto ambientale o rinnovabili, rinforzate con fibre naturali. Nel presente lavoro, attraverso una sistematica campagna di prove sperimentali sono analizzati gli effetti dei principali agenti ambientali sulle proprietà meccaniche di biocompositi ad elevate prestazioni cos…

The Reinforcement Effect of Strain Gauges Embedded in Low Modulus Material

The reinforcement effect of electrical resistance strain gauges is well-described in the literature, especially for strain gauges installed on surface. This paper considers the local reinforcement effect of strain gauges embedded within low Young modulus materials. In particular, by using a simple theoretical model, already used for strain gauges installed on the surface, it proposes a simple formula that allows the user to evaluate the local reinforcement effect of a generic strain gauge embedded on plastics, polymer composites, etc. The theoretical analysis has been integrated by numerical and experimental analyses, which confirmed the reliability of the proposed model.

Implementation of eco-sustainable biocomposite materials reinforced by optimized agave fibers

Abstract Although several works have recently been published in literature about biocomposites, i.e. about composites with polymeric matrix reinforced by natural fibers, only a few articles have been devoted to the implementation of high performance biocomposites for structural and semi-structural applications. The present study aims to give a contribution by considering biocomposites obtained by using an eco-friendly partially bio-based epoxy (green epoxy) and sisal (agave sisalana fibers) obtained by a proper optimization process. Through a systematic experimental analysis, three different types of biocomposites obtained with a suitable manufacturing process, such as random short fiber bi…

IL METODO DI PROVA “AIAS - TR” PER LA MISURA DELLE TENSIONI RESIDUE COSTANTI O VARIABILI CON LA TECNICA DEL FORO INCREMENTALE

Viene presentata una sintesi dell’attività svolta dal Gruppo di lavoro AIAS “Tensioni Residue” nel periodo dal 2006 al 2009. Lo scopo dell’attività è stata l’elaborazione del metodo AIAS - TR 01 per la misura delle tensioni residue con il metodo del foro incrementale. La normativa tecnica esistente sull’argomento (ASTM E 837-08) [1] ha un campo di applicabilità ristretto; infatti essa non considera alcuni effetti che possono influenzare l’accuratezza della misura. Il metodo di prova (AIAS - TR 01) fornisce una visione d’insieme del metodo della rosetta forata, aggiornata all’attuale stato dell’arte (2009) sia a livello tecnologico che a livello teorico. Le principali innovazioni riguardano …

L’EFFETTO RINFORZANTE DEGLI ESTENSIMETRI INGLOBATI

L’effetto rinforzante degli estensimetri elettrici a resistenza è ben documentato nella letteratura tecnica soprattutto per ciò che riguarda gli estensimetri installati in superficie. Questa memoria riguarda l’effetto rinforzante locale degli estensimetri inglobati all’interno dei materiali. In particolare si estende agli estensimetri inglobati un semplice modello già utilizzato per gli estensimetri installati in superficie. Il modello - che si basa sulle seguenti ipotesi: influenza dell’adesivo trascurabile, modulo di Young dell’estensimetro costante, distribuzione esponenziale delle tensione tangenziali e spessore della struttura uguale a quello dell’estensimetro – conduce ad una semplice…

MAPPING MULTIPLE RESIDUAL STRESS COMPONENTS USING THE CONTOUR METHOD AND SUPERPOSITION

The standard contour method determines the residual stress component normal to the plane of measurement. This talk presents extensions to the contour method to allow determination of multiple stress components. Two extensions are presented, both using additional stress measurements and then superposition to determine the original residual stresses. The first extension involves making additional cuts to determine stresses on multiple cut planes, using only the contour method. The second extension involves measuring in-plane stresses on the surface exposed by the original cut, using x-ray diffraction and hole drilling in this case. Experiments were preformed on indented disks of both 316L sta…

Analysys and optimization of an end double-lap bonded joint for GFRP composite sandwich panels

Although the best configuration of the joints used to connect a common GFRP sandwich panel with the main structures is the adhesively bonded one, that permit high static and fatigue performances as well as to avoid stress concentrations, mechanical or hybrid joints are still widely used in the industrial field. After a preliminary theoretical design, the optimal configuration of an end adhesively bonded double-lap joint constituted by a simple insert as internal adherent and the same face sheets as external adherent, has been researched. In detail, several experimental tests and successive numerical simulations under tensile and bending loading, performed by varying the main influence para…

An analysis of through-thickness residual stresses in aluminium FSW butt joints

Abstract In 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…

Rapid evaluation of notch stress intensity factors using the peak stress method: Comparison of commercial finite element codes for a range of mesh patterns

The peak stress method (PSM) is an engineering, finite element (FE)-oriented method to rapidly estimate the notch stress intensity factors by using the singular linear elastic peak stresses calculated from coarse FE analyses. The average element size adopted to generate the mesh pattern can be chosen arbitrarily within a given range. Originally, the PSM has been calibrated under pure mode I and pure mode II loadings by means of Ansys FE software. In the present contribution, a round robin between 10 Italian universities has been carried out to calibrate the PSM with 7 different commercial FE codes. To this aim, several two-dimensional mode I and mode II problems have been analysed independe…

Full field automated evaluation of the quarter wave plate retardation by phase stepping technique

Quarter wave plates are optical elements commonly used in photoelasticity to obtain circularly polarized light. They divide the incident light field into two linearly polarized orthogonal components with a phase difference of a quarter of the light wavelength. Due to the tolerance in manufacturing, however, the actual phase shifting produced by the plates is affected by an error, which noticeably influences the photoelastic measurements performed by means of various automated methods. This paper presents a technique, based on the phase stepping method, for the full field automatic evaluation of the quarter wave plate error.

ANALISI DELL’EFFETTO RINFORZANTE DEGLI ESTENSIMETRI INGLOBATI IN MATERIALI A BASSO MODULO

L’effetto rinforzante degli estensimetri elettrici a resistenza è ben documentato nella letteratura tecnica soprattutto per ciò che riguarda gli estensimetri installati in superficie. Questa memoria riguarda l’effetto rinforzante locale degli estensimetri inglobati all’interno dei materiali. In particolare si estende agli estensimetri inglobati un semplice modello già utilizzato per gli estensimetri installati in superficie. Il modello - che si basa sulle seguenti ipotesi: modulo di Young dell’estensimetro costante, distribuzione esponenziale delle tensione tangenziali all’interfaccia ER-struttura e larghezza della struttura uguale a quella dell’estensimetro – conduce ad una semplice formul…

RESIDUAL STRESS ANALYSIS OF ORTHOTROPIC MATERIALS BY THE THROUGH-HOLE DRILLING METHOD

The present study deals with the development and the application of the through-hole drilling method for the residual stress analysis in orthotropic materials. Through a systematic theoretical study of the stress field present on orthotropic plates with a circular hole, the relationships between the relaxed strains measured by a rectangular strain gauge rosette and the Cartesian components of the unknown residual stresses are obtained. The theoretical formulas of each influence coefficient allow the user an easy application of the method to the analysis of uniform-residual stresses on a generic homogeneous orthotropic material. Furthermore, to extend the method to the analysis of the residu…

Basalt Fiber Hybridization Effects on High-Performance Sisal-Reinforced Biocomposites

The increasing attention given to environmental protection, largely through specific regulations on environmental impact and the recycling of materials, has led to a considerable interest of researchers in biocomposites, materials consisting of bio-based or green polymer matrixes reinforced by natural fibers. Among the various reinforcing natural fibers, sisal fibers are particularly promising for their good mechanical properties, low specific weight and wide availability on the current market. As proven in literature by various authors, the hybridization of biocomposites by synthetical fibers or different natural fibers can lead to an interesting improvement of the mechanical properties or…

Experimental analysis and micromechanical models of high performance renewable agave reinforced biocomposites

Abstract The present work deals with the experimental study of high performance biocomposites reinforced with optimized agave fibers, as well as the successive implementation of reliable micromechanical models that can be used at the design stage. In detail, systematical experimental analyses performed on biocomposites with epoxy or PLA matrix, have permitted to highlight that for short fibers biocomposites the reinforcing leads to a significant improvement of the matrix stiffness, whereas the particular damage mechanism based essentially on the matrix failure with consequent tensile failure of the fibers aligned with the applied load, does not allow to obtain an actual reinforcing of the m…

UN MODELLO NUMERICO PER LA INTERPRETAZIONE DEL COMPORTAMENTO MECCANICO DI BIOCOMPOSITI RINFORZATI CON FIBRE DI AGAVE

Sebbene diversi lavori siano stati pubblicati recentemente in letteratura sui biocompositi, ovvero su innovativi materiali compositi a matrice polimerica rinforzati con fibre naturali, nessuno studio è stato sino ad ora condotto sugli effetti legati alla naturale forma ondulata assunta da diverse fibre naturali, come per esempio l’agave sisalana, a seguito della estrazione. Nel caso, piuttosto ricorrente nella sperimentazione, di laminazione di biocompositi a fibre lunghe senza uso di apposti tessuti unidirezionali confezionati previo accurato stiramento e allineamento delle fibre, si ottengono laminati le cui proprietà meccaniche possono essere in linea di principio influenzate significati…

Use of Automated Photoelasticity to Determine Stress Intensity Factors of Bimaterial Joints

A new systematic experimental procedure has been developed to obtain the stress intensity factors governing the singular stress field that occurs near the intersection between the interface and free edges of bimaterial joints. A preliminary theoretical study of the singular stress field is carried out by the well-known Airy stress function method. The obtained stress laws are properly combined with the basic law of photoelasticity in order to define a procedure that permits the zone dominated by the singularity to be located and the stress intensity factors (SIFs) to be computed on the basis of full field data provided from automated photoelasticity. In particular, a systematic error analys…

OTTIMIZZAZIONE DI GIUNTI IBRIDI HBB A DOPPIA SOVRAPPOSIZIONE GFRP-ALLUMINIO

In order to exploit the advantages of both mechanical joints and adhesively bonded joints, in the last year a noticeable research activity has addressed to the so called hybrid joints (Hybrid Bonded Bolted joint, HBB) that consist in combining a classical mechanical joint (bolted, riveted joint, etc..) to a traditional bonded joint or a co-cured joint. The present work shows the results of experimental and numerical analyses of double-lap HBB joints, carried out to detect the geometric configuration that permits to distribute the applied load between the two coexisting junctions and then to optimize their mechanical performance. The studied joint consists of an internal adherent made by GFR…

ON THE EFFECTS OF A CRACK PROPAGATING TOWARD THE INTERFACE OF A BIMATERIAL SYSTEM

This paper deals with the influence of matrix cracks on the failure mode of bimaterial systems and composite materials. In order to investigate such an influence, the stress field near a crack embedded into the more yielding material and propagating perpendicularly to the interface, has been analyzed by using systematic numerical simulations. Such analysis has shown that the crack propagation give rises to transversal stresses that can damage the reinforcing materials when this has low modulus, as glass fibers, or low transversal strength, such as carbon fibers. Moreover, the longitudinal stress concentration can damage the reinforcing material only if this has high stiffness, as in the cas…

Local reinforcement effect of embedded strain gauges

The reinforcement effect of strain gauges installed on low Young's modulus materials has received attention by many researchers with respect to both strain gauges installed on the surface [1,2] and embedded inside the material [3,4]. In the case of strain gauges installed on the surface, the evaluation of the local reinforcement effect gives [5] the following correction coefficient C, i.e. the ratio between the actual strain (without the strain gauge) and the strain ' measured by the strain gauge: * ' 1 sg s E C E (1) being * sg g , sg sg sg sg sg sg t L L E E L t t (2) where Esg is the Young’s modulus of the strain gauge, * Esg is a characteristic of the strain gauge which gives the strain…

Mode I translaminar fracture toughness of high performance laminated biocomposites reinforced by sisal fibers: Accurate measurement approach and lay-up effects

Abstract The 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 acc…

Static strength and fatigue life of optimized hybrid single lap aluminum–CFRP structural joints

Hybrid bolted/bonded joints are used to assemble structural components, commonly made by carbon fiber reinforced plastics (CFRP), with aluminum frames. Hence, they have become common solutions in a number of modern structural applications in the industrial fields, as well as civil constructions. Unfortunately, due to the lack of understanding of the relationships between the multiple parameters of influence that characterize their mechanical performance, only limited improvement have been achieved so far over classical bonding approaches, in terms of static and fatigue strength. As a result, further studies are needed in order to better exploit the potential of hybrid bolted/bonded joints a…

ANALISI DEL COMPORTAMENTO MECCANICO DI SANDWICH ECO-COMPATIBILI RINFORZATI CON FIBRE NATURALI

The increasing environmental sensitivity in many industrial fields, from automotive to aeronautics to naval field, has led to an increasing demand of new bio-materials or renewable structural materials. In order to give a contribution to the design of eco-compatible structural materials, in this work new sandwich materials constituted by a core made by balsa and skins made by a composite material made by a “green” epoxy reinforced by agave fibers extracted from striated and americana agave obtained in Sicily by a proper research program financed by European founds. Previous theoretical analysis followed by systematical experimental analysis of the core, of the skins and the of the sandwich,…

Analysis of the Parameters Affecting the Stiffness of Short Sisal Fiber Biocomposites Manufactured by Compression-Molding

The use of natural fiber-based composites is on the rise in many industries. Thanks to their eco-sustainability, these innovative materials make it possible to adapt the production of components, systems and machines to the increasingly stringent regulations on environmental protection, while at the same time reducing production costs, weight and operating costs. Optimizing the mechanical properties of biocomposites is an important goal of applied research. In this work, using a new numerical approach, the effects of the volume fraction, average length, distribution of orientation and curvature of fibers on the Young’s modulus of a biocomposite reinforced with short natural fibers were stud…

UN METODO INNOVATIVO PER LA PREVISIONE DELLA RESISTENZA DI GIUNTI CO-CURED IN MATERIALE COMPOSITO

IBRIDIZZAZIONE SUPERFICIALE MEDIANTE BASALTO DI BIOCOMPOSITI AD ELEVATE PERFORMANCE RINFORZATI CON FIBRE DI AGAVE

Una delle principali limitazioni nell’uso pratico dei biocompositi polimerici rinforzati con fibre naturali (lino, canapa, agave, ecc.), specie in applicazioni di tipo “outdoor”, è costituita dalla bassa resistenza all’invecchiamento prodotto dagli agenti ambientali (UV, umidita, acqua ecc.), caratteristica questa in genere strettamente derivante dalla elevata idrofilia delle fibre naturali e dalla limitata protezione offerta in tal senso dalle matrici polimeriche, specie se di tipo termoindurente. Con riferimento ai biocompositi ad elevate performance rinforzati con fibre di agave, materiali già messi a punto in letteratura e pure ampiamente caratterizzati nelle loro interessanti proprietà…

ERROR AND UNCERTAINTY ANALYSIS OF THE RESIDUAL STRESSES COMPUTED BY USING THE HOLE DRILLING METHOD

The hole-drilling method is one of the most used techniques for the experimental analysis of the residual stresses in mechanical components. For both through-thickness uniform and non-uniform residual stresses, its practical application is standardised by the ASTM E837-08. For uniform residual stresses, to which the present work deals with, the use of the method in accordance with the ASTM limitations, leads to results with a bias of about 10%. Unfortunately, although some experimental parameters can be frequently out of the corresponding standard limitation, the user does not have appropriate procedures to correct the obtained results which, in accordance with the ASTM standard, have to be…

Enhancement of Static and Fatigue Strength of Short Sisal Fiber Biocomposites by Low Fraction Nanotubes

AbstractThanks to good mechanical performances, high availability, low cost and low weight, the agave sisalana fiber allows to obtain biocomposites characterised by high specific properties, potentially very attractive for the replacement of synthetic materials in various industrial fields. Unfortunately, due to the low strength versus transversal damage processes mainly related to the matrix brittleness and/or to the low fiber/matrix adhesion, the tensile performance of random short fiber biocomposites are quite low, and to date most of the fiber treatments proposed in literature to improve the fiber-matrix adhesion, have not led to very satisfactory results. In order to overcome such a dr…

Random short sisal fiber biocomposites: Optimal manufacturing process and reliable theoretical models

Although 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…

MIXED-MODE DELAMINATION TESTS ON COMPOSITES UNDER FATIGUE LOADS

Influence of the Resin Layer Thickness at the Interface of Hybrid Metal-composite Co-cured Joints

As discussed in literature, the accurate analysis of the modern co-cured joints between composite materials or between a metal and a composite material (hybrid joints) is complicated by the influence of the interface resin layer on the interface singular stress field. In such joints, in fact, there is not a proper adhesive layer and the thickness of the resin layer, that plays the role of the adhesive, is not constant due to the presence of the reinforcing fibers in the composite adherent. For this reason several authors assume that a generic co-cured joint can not be studied as a simple bi-material joint, without considering the particular characteristics of the actual resin layer. This pr…

Validation Specimen for Contour Method Extension to Multiple Residual Stress Components

A new theoretical development of the contour method [1], that allow the user to measure the three normal residual stress components on cross sections of a generic mechanical part, is presented. To validate such a theoretical development, a residual stress test specimen was properly designed, fabricated and then tested with different experimental techniques.

On the Stiffness and the Reinforcement Effect of Electrical Resistance Strain Gauges

The reinforcement effect of a strain gauge installed on low modulus materials can be significant. The increasing use of low modulus materials requires therefore the evaluation of such effect. This paper concerns the relationship between the local reinforcement effect and the strain gauge stiffness. The conclusion is that the gauge stiffness alone does not allow the user a thorough evaluation of the reinforcement effect.