ANALISI NUMERICO-SPERIMENTALE SUL COMPORTAMENTO A FATICA DI COMPOSITI CARALL RIVETTATI

Nel lavoro è descritto un modello di danneggiamento per provini CARALL (CArbon Reinforced ALLuminium), rivettati e non rivettati, sottoposti a prove di fatica mediante flessione pulsatoria a tre punti in controllo di spostamento. I provini sono realizzati sovrapponendo 8 lamine di tessuto intrecciato di carbonio [0°/90°] e resina epossidica con interposta una lamina in lega di alluminio 6061-T6 dello spessore di 0.8mm. Le prove sono state condotte su provini non rivettati (Tipo 1), su provini con due rivetti disposti simmetricamente ad una distanza d=22 mm dalla mezzeria (Tipo 2) e su provini con 2 rivetti disposti ad una distanza d=44 mm dalla mezzeria (Tipo3). L’analisi dei risultati sperimentali ha mostrato come l’operazione di rivettatura introduca delle microcricche in prossimità del rivetto favorendo la nucleazione e propagazione della delaminazione

Numerical experimental analysis of hybrid double lap aluminum-CFRP joints

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) require a preliminary drilling of the elements to be joined, that may cause localized material damage and stress concentration, especially for anisotropic laminates characterized by high stress concentration factors and easy drilling damaging, with significant decrease of the load-carrying capacity of the joined elements. In order to exploit the advantages of the bonded joints and those of the mechanical joints, both industrial manufacturing and research activity have been focused recently on the so called hybrid joints, obtained by the superposition of a mechanical joint to a simple adhesively bonded joint. In order to give a contribution to the knowledge of the mechanical behavior of hybrid bonded/riveted joints, in the present work a numerical–experimental study of bonded/riveted double-lap joints between aluminum and carbon fiber reinforced polymer (CFRP) laminates, has been carried out. It has permitted to highlight both the static and the fatigue performance of such joints obtained by using aluminum and steel rivets, as well as to known the particular damage mechanisms related also to the premature localized delamination of the CFRP laminate due to the riveting process

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 the contrary, the riveted joints require a preliminary drilling of the elements to be joined and, consequently, may cause localized material damage, especially in presence of strongly anisotropic laminates which are characterized by high stress concentration. In the present work a numerical and experimental study of hybrid double-lap joints between aluminum and CFRP laminates, have permitted to highlight both the static and the fatigue performance of such joints, also with reference to the particular damage mechanism of the adhesive layer and to the interlaminar damage of the CFRP, due to the riveting process

Preliminary design of a magnetorheological brake for automotive use

After an initial study of the characteristics of magnetorheological fluids based on a thorough search in literature, a preliminary configuration of MR brake is proposed. It comes from the evaluation of the main factors influencing the design of an appropriate magnetic circuit, and then the performance obtained in terms of brake torque. The analytical study of the brake has allowed a first preliminary sizing. Through the subsequent execution of an electromagnetic finite element model, created in ANSYS, it was possible to assess more accurately the distribution of the magnetic field inside the MR fluid and hence the resistance to relative motion between rotor and stator. The work needs an accurate optimization due to insufficient value of the braking torque and high mass of the device

ANALISI DEGLI EFFETTI DEL CAMPO MAGNETICO SULLO STATO TENSIONALE INDOTTO IN PIASTRE DI ACCIAIO FORATE

Si è considerata una piastra forata di materiale ferromagnetico opportunamente accoppiata da bobine di filo di rame smaltato alimentato da un alimentatore DC a 30V. Essa è stata analizzata agli Elementi Finiti e successivamente sono state eseguite sperimentazioni per la validazione dei risultati ottenuti. Dopo aver levigato la piastra sono stati installati due estensimetri per misurare la deformazione lungo l’asse x e lungo l’asse z. Si sono ottenuti i valori di deformazione di 5µm in direzione z e -2µm in direzione x. Il confronto numerico-sperimentale mostra che i risultati di laboratorio sono più bassi di quelli numerici, mentre segni e ordini di grandezza sono gli stessi. Si conclude che i risultati dell’analisi FEM possono essere considerati accettabili a causa dell’ortogonalità del piano delle spire rispetto al piano della piastra e a causa del coefficiente di impacchettamento del rame

EFFETTO DELLA PRESSURIZZAZIONE DEI FORI E DELL’INDENTAZIONE ANULARE SULLA VITA A FATICA DI PROVINI IN LEGA DI ALLUMINIO

Nel lavoro è investigato il comportamento a fatica di lastre forate in lega di alluminio. In particolare è valutata l‟influenza combinata dell‟espansione a freddo dei fori e dell‟indentazione sul miglioramento della vita a fatica. A tal fine sono state condotte diverse prove di fatica aventi lo scopo di valutare il numero di cicli a rottura e la conseguente curva di Wöhler. I risultati sperimentali sono stati confrontati con quelli ottenuti senza la realizzazione di alcun processo di espansione e di espansione-indentazione. Lo studio ha permesso di concludere che il processo di espansione ed indentazione anulare determina un ritardo nella nascita della cricca e della conseguente rottura a fatica

Effect of suturing the femoral portion of a four-strand graft during an ACL reconstruction.

Purpose A suture passed along the part of the graft that will be inserted into the femoral tunnel is widely used by surgeons, because it could prevent the graft sliding on the femoral fixation device during pulling from the tibial side. The aim of this study was to evaluate the biomechanical effects of suturing the intratunnel femoral part of the graft during an anterior cruciate ligament (ACL) reconstruction. Methods Bovine digital extensor tendons and tibias were harvested from 20 fresh-frozen mature bovine knees ranging in age from 18 to 24 months. Quadruple-strand bovine tendons were passed through the tibial tunnel and secured distally with a bioabsorbable interference screw. In one half of all grafts (N = 10), the looped-over part of the graft was sutured in a whipstitch technique over a distance of 30 mm (Group 1). In one half of all grafts (N = 10), the looped-over part was left free from any suture (Group 2). The grafts were preconditioned at 50 N for 10 min, fol- lowed by cyclic loading at 1 Hz between 50 N and 250 N for 1,000 cycles. Load-to-failure test was then carried out at a rate of 1 mm/s. Results There was no statistically significant difference between mean stiffness at pullout and yield load between the two groups. In all specimens on Group 1, failure occurred following to partial breaking and then slipping of the tendons between the screw and the tunnel. Concerning Group 2, in six cases failure occurred as described for Group 1 specimens. In the remaining four cases, failure occurred entirely through the ligament mid-substance. Conclusions Suturing in a whipstitch fashion the femoral portion of the graft doesn’t affect the mechanical proprie- ties of the ACL graft. When suspension fixation device is used, suturing the looped-over part of the graft could be helpful in order to provide equal tension in all of the strands of the graft at time of tibial fixation