C. Borsellino

Failure Map of Composite Laminate Mechanical Joint

The aim of this research is to investigate the pin/hole contact stress of a composite laminate and failure modes when submitted to tensile bearing tests. The limit loads and failure modes are evaluated as a function of pin diameter and hole position. Analyzing the joint geometry effect on the fracture mechanisms, a failure map is obtained, identifying three regions of typical failure modes of mechanically fastened joints. A theoretical approach is proposed to identify the field of each fracture mode to obtain a simple experimental methodology to support the design of a particular joint laminate. In addition, a simplified numerical model is proposed to evaluate near the hole the stress/strain distribution under tensile bearing load. This allows one to better understand the relevant dependence of the failure modes from the geometry of the joint for a given composite laminate.

Influence of bracket-slot design on the forces released by superelastic nickel-titanium alignment wires in different deflection configurations.

OBJECTIVE To evaluate how different bracket-slot design characteristics affect the forces released by superelastic nickel-titanium (NiTi) alignment wires at different amounts of wire deflection. MATERIALS AND METHODS A three-bracket bending and a classic-three point bending testing apparatus were used to investigate the load-deflection properties of one superelastic 0.014-inch NiTi alignment wire in different experimental conditions. The selected NiTi archwire was tested in association with three bracket systems: (1) conventional twin brackets with a 0.018-inch slot, (2) a self-ligating bracket with a 0.018-inch slot, and (3) a self-ligating bracket with a 0.022-inch slot. Wire specimens were deflected at 2 mm and 4 mm. RESULTS Use of a 0.018-inch slot bracket system, in comparison with use of a 0.022-inch system, increases the force exerted by the superelastic NiTi wires at a 2-mm deflection. Use of a self-ligating bracket system increases the force released by NiTi wires in comparison with the conventional ligated bracket system. NiTi wires deflected to a different maximum deflection (2 mm and 4 mm) release different forces at the same unloading data point (1.5 mm). CONCLUSION Bracket design, type of experimental test, and amount of wire deflection significantly affected the amount of forces released by superelastic NiTi wires (P<.05). This phenomenon offers clinicians the possibility to manipulate the wires load during alignment.

Effect of galvanic corrosion on durability of aluminium/steel self-piercing rivet joints

Abstract Galvanic corrosion of steel/aluminium hybrid joints, obtained by self-piercing riveting technique, was investigated. Potentiodynamic polarisation tests, performed in 3·5 wt-% NaCl solution, evidenced the anodic and cathodic behaviour of metal constituent of the joints. Furthermore, long term aging tests were performed to evaluate the relationship among failure mechanism, joint configuration and corrosion damage in salt spray environment. The experimental results evidenced that the degradation phenomena influenced significantly performances and failure mechanisms of the joints, inducing premature failure of the joint at lower stress level at increasing aging time. Furthermore a theoretical model was proposed to forecast failure modes. Based on this model a simplified map of failure mechanisms promoted by corrosion phenomena has been drawn.

Study of new joining technique: flat clinching

In this paper a development of clinching, called flat clinching, is presented. After a press clinching process, the joined sheets have been deformed by a punch with a lower diameter against a flat die. In this way a new configuration is created with a geometry that has no discontinuity on the external surface (bottom). A new procedure has also been tested: the second step is perfomed by pressing the joint between two flat dies. This second case has revealed itself to be very effective.Tensile tests have been done to compare the joints strength among the various joining techniques. Moreover some joints have been cut to analyse the changing of the contact line shape and how its characteristics parameters can influence the bonding performances. To optimise the process a finite element analysis has been performed.

Electrochemical Impedance Spectroscopy Behaviour of NdFeB Coated Magnets in Saliva Solution for Orthodontic Applications

In this work the assessment of the corrosion performances in saliva solution of NdFeB magnets coated with silane layers was studied for its application in orthodontic brackets. The silane film, deposited by dip coating technique, has been prepared with varying dipping steps, with the purpose to identify the number of layers able to achieve an optimal protective action. Corrosion protection performance, during immersion in Fusayama synthetic saliva solution, was evaluated by means electrochemical impedance spectroscopy (EIS). The silane coatings evidenced good barrier properties resulting in an improvement of the anti-corrosion performances of the magnets. Better results were observed for samples with at least 15 layers of silane, that evidenced still acceptable protective action after three days of immersion in a Fusayama synthetic saliva solution.

Evaluation of the Clinical Impact of ISO 4049 in Comparison with Miniflexural Test on Mechanical Performances of Resin Based Composite

The aim of this study was to evaluate the effect of different specimens dimensions on the mechanical properties of a commercial microfilled resin composite by using a modified ISO 4049 standard protocol, that generally provides specimen dimensions of 25 mm length × 2 mm width × 2 mm height; these standard dimensions are not clinically realistic considering the teeth diameter and length average. Furthermore, the overlapping irradiations required lead to specimens that are not homogeneous with the presence of some flaws due to packaging steps. For this reason, a miniflexural test was employed in this work both to simulate clinically realistic dimensions and to concentrate fewer defects. The flexural tests were performed at varying span length, in the range between 18.5 mm as stated by the ISO 4049 flexural test (IFT) and 10.5 mm according to the miniflexural test (MFT), at the increasing of layers with a 1 mm buildup multilayering technique. The results evidenced the impact of specimen dimensions on mechanical performances and consequently stability of resin-based composite with the formation of an asymmetrical structure which possesses higher stiffness and strength at increasing layering steps.

Study of Snowboard Sandwich Structures

The aim of the present research is to extend the knowledge of mechanical properties both on single components and on complete structure employed for snowboard. Flexural and torsion tests are performed to acquire important comparison parameters between snowboard sandwich structures that differ for the core material employed (wood, PVC foam core). A simplified FEM model is proposed to simulate the flexural tests of the sandwich structure showing good predictive capability.

Effect of Chemical Etching on Adhesively Bonded Aluminum AA6082

The efforts of new automotive industry are mainly directed towards the substitution of aluminum for steel in the body structure because the aluminum structures are lighter than traditional steel ones and meet the requirements, in terms of both vehicle design and manufacture. However, this substitution is not so automatic, but it is important to study the material properties and the structure design, focusing the attention on the methods of joining. Welding, typical technique to joint steel parts, is particularly difficult when applied on aluminum ones and then, in many cases, the adhesive bonding is preferred. To optimise the joint performances it is necessary to pre-treat the metal surface, not only to remove contaminants, but also to provide the intimate contact needed for the adhesive to successfully bond with the adherent surface. The mere cleaning of aluminum surfaces is not suitable for their bonding due to the oxide layer that naturally occurs on exposure to air so we need to apply a more effective treatment (mechanical or chemical etching) to increase the adhesion capability of the substrates. In this work different adhesive joint configurations (single lap) between aluminum substrates are studied. Two different resins are employed to evaluate the influence of the adhesive on the joint performances. Moreover the aluminum sheets are treated with a chemical etching with two different procedures.

Hydroxyapatite Whiskers Based Resin Composite versus Commercial Dental Composites: Mechanical and Biocompatibility Characterization

A systematic evaluation of mechanical and biocompatibility properties of different volume fractions of hydroxyapatite whiskers in comparison with three commercial dental composites filled with micro- and nanosilica particles was carried out. Six groups with different hydroxyapatite whiskers mass fractions were taken into account in order to be compared with the performances of silica particles based composites group. Flexural properties were evaluated via a universal testing machine (2.5 kN Zwick Line) with a 2 kN load-cell (sensitivity 0.001 N). The test was replicated 10 times for the seven experimental groups to better identify statically the significance of the mechanical performances data. MTT quantitative colorimetric assay was performed in order to evaluate the mitochondrial activity of living cells exposed to different resin composites. Data obtained show better interfacial interaction with filler/matrix until 20 wt% of hydroxyapatite whiskers partially replaced silica particles filler. After this threshold, the mechanical performances decrease dramatically due to both the hydroxyapatite agglomerates formation and the low degree of resin conversion. In addition, biocompatibility test showed less cytotoxic effect with the addition of 20 wt% of hydroxyapatite in comparison with higher rates.

Three-Point Flexural Properties of Bonded Reinforcement Elements for Pleasure Craft Decks

AbstractThe aim of this work was both to study the performances of pleasure craft reinforced components, bonded using a structural adhesive, and to compare them with those obtained using over-lamination as joining system, typically employed in the shipbuilding. With such aim, two different lots of components were prepared: in the first lot, the reinforcement structures were laminated directly on the investigated composite components and, in the second one; they were made separately in a mould and, then, bonded to the composite components. This last method allowed to evaluate the introduction of a product/process innovation in a field typically unwilling to innovation, still tied to craft, and non-standardized procedures. The results of bending tests, performed in order to evaluate the mechanical behaviour of the reinforced components, evidenced the goodness of this innovative design choice. Finally, a finite element analysis was performed. Graphical AbstractGraphical abstract of the work: Left) Ship 85 feet long; Center) Manufacturing procedures and sections of the reinforcement elements; Right) Test apparatus and typical failure modes