Roberto Montanini

In vitro biomechanical evaluation of antegrade femoral nailing at early and late postoperative stages

The present study addresses the question of evaluating, by combining both experimental and numerical methods, the stress/strain distribution within a standardized composite femur implanted with an anterograde intramedullary nail. A transverse diaphyseal fracture has been introduced in order to evaluate the implant response in the early postoperative clinical stage. By comparing these experimental data with those obtained in the fully healed stage, in which the bone continuity had been recovered, it was possible to get information on load sharing between the bone and the intramedullary nail, location of high strain concentrations, bone relative motion at the fracture site, and stiffness reduction caused by bone discontinuity. Experimental data were correlated with those predicted by a validated 3D finite element model of the complete implant/femur assembly to investigate the full field stress distribution either in the cortical bone, in the nail or in the locking screws. The obtained results suggest that full weight bearing in the immediate post-operating stage should not be allowed since high stress levels are generated in the outer shell of the cortical bone either around the proximal screw hole or the upper locking screw hole. Long-term implant reliability should be guaranteed instead, since after fracture consolidation equivalent von Mises stresses never exceed critical levels neither in the bone nor in the implant.

On the influence of friction in the calculation of conical disk springs

An analytical solution is proposed by the authors in order to take into account the effects of friction in the calculation of conical disk springs. The new formulation allows a more accurate estimate of the load corresponding to a given displacement, but it implies the knowledge of the friction coefficient f between the spring and the supporting surfaces. The reported numerical examples show that, disregarding friction effects, the maximum error committed in the evaluation of the load is in the range 2-5%, with f = 0.14 (average friction coefficient value determined experimentally on commercial conical disk springs with different geometry). Comparison with both experimental and finite element calculations show a very good agreement of the analytical prediction.

Non-contact measurement of linear thermal expansion coefficients of solid materials by infrared image correlation

A new non-contact optical method (IIC, infrared image correlation) for the determination of the coefficients of thermal expansion of solid materials is presented. The proposed method is based on performing a digital image correlation between thermal images recorded at different temperatures by means of an infrared camera. It allows the coefficient of thermal expansion of both isotropic and anisotropic solid materials to be determined by measuring simultaneously the fractional increase in length and the actual thermal field over a small region of interest in which a dual-emissivity stochastic speckle pattern has been created. The results reported in this paper prove the effectiveness of the proposed method that can be applied either to carry out reference measurements in laboratory or to evaluate thermal stresses and strains on structural components in-field.

Structural health monitoring of reinforced concrete beams by means of embedded fiber Bragg grating sensors

This study aims to investigate the mechanical behavior of FRCM composite-strengthened concrete beams using embedded FBG sensors. FBG sensors were installed both on the tensioned surface of the concrete beam and on the PBO mesh woven, that had been applied using cementitious mortar without any epoxy resin. Conventional strain gauges were used to compare results measured from the FBG sensors. Under three-point bending, a marked difference between strains measured in the concrete and those gotten on the reinforcement net was observed. A theoretical model is presented to explain the observed discrepancy.

Wavelength-encoded optical psychrometer for relative humidity measurement

In this article an optical psychrometer, in which temperature measurements are performed by means of two fiber Bragg grating sensors used as dry-bulb and wet-bulb thermometers, is introduced. The adopted design exploits both the high accuracy of psychrometric-based relative humidity measurements with acknowledged advantages of wavelength-encoded fiber optic sensing. Important metrological issues that have been addressed in the experimental work include calibration of the fiber Bragg grating temperature sensors, evaluation of response time, sensitivity, hysteresis, linearity, and accuracy. The calibration results give confidence that, with the current experimental setup, measurement of temperature can be done with an uncertainty of +/- 0.2 degrees C and a resolution of 0.1 degrees C. A detailed uncertainty analysis is also presented in the article to investigate the effects produced by different sources of error on the combined standard uncertainty uc(U) of the relative humidity measurement, which has been estimated to be roughly within +/-2% in the range close to saturation.

Experimental characterization of cantilever-type piezoelectric generator operating at resonance for vibration energy harvesting

A cantilever-type resonant piezoelectric generator (RPG) has been designed by gluing a PZT patch working in d31 mode onto a glass fibre reinforced composite cantilever beam with a discrete mass applied on its free end. The electrical and dynamic behaviour of the RPG prototype has been investigated by carrying out laboratory tests aimed to assess the effect of definite design parameters, specifically the electric resistance load and the excitation frequency. Results showed that an optimum resistance load exists, at which power generation is maximized. Moreover, it has been showed that power generation is strongly influenced by the vibration frequency highlighting that, at resonance, output power can be increased by more than one order of magnitude. Possible applications include inertial resonant harvester for energy recovery from vibrating machines, sea waves or wind flux and self-powering of wireless sensor nodes.

Design and calibration of a fibre Bragg grating-type linear displacement sensor

In this paper the design and testing of a novel displacement sensor, based on optical in-fibre Bragg Grating sensing (FBGs), is described. The principle of operation is based on the conversion of the relative displacement into optical fibre strains by means of an elastic spring: its design allows to measure linear displacements with high accuracy and extended linear range. Owing to the key features of fibre optic sensors, as high sensitivity and immunity to electro-magnetic noise, this packaged sensor has been devoted for monitoring Pixel Vertex Detectors linear displacements of BTeV, an High Energy Physics experiment to be run at the Fermilab Tevatron.

Strain Sharing Assessment in Woven Fiber Reinforced Concrete Beams Using Fiber Bragg Grating Sensors.

Embedded fiber Bragg grating sensors have been extensively used worldwide for health monitoring of smart structures. In civil engineering, they provide a powerful method for monitoring the performance of composite reinforcements used for concrete structure rehabilitation and retrofitting. This paper discusses the problem of investigating the strain transfer mechanism in composite strengthened concrete beams subjected to three-point bending tests. Fiber Bragg grating sensors were embedded both in the concrete tensioned surface and in the woven fiber reinforcement. It has been shown that, if interface decoupling occurs, strain in the concrete can be up to 3.8 times higher than that developed in the reinforcement. A zero friction slipping model was developed which fitted very well the experimental data.

Development of a fibre-optic sensor for position monitoring of vertex detectors in high energy physics experiments

The demanding task of monitoring the spatial position of vertex detectors used in high energy physics experiments performed in particle accelerators requires sensors able to measure linear displacements with high resolution under strong magnetic and radio frequency fields. Due to the severe environmental conditions, traditional electric transducers are hindered from working satisfactorily. This paper describes the development of a packaged opto-mechanical sensor which can overcome these problems. The principle of operation relies on the measurement of the tangential strains of a CFRP Ω-shaped elastic element by means of an optical in-fibre Bragg grating (FBG): its design allows us to measure linear displacements with high accuracy and extended linear range. The sensor is self-compensated in temperature in order to take into account wavelength shifts due to apparent strains and to increase its intrinsic accuracy. The latter has been evaluated by calibration performed in the laboratory to be within ±3 pm, corresponding to ±13 µm, which is close to the resolution of the interrogation system used to detect the FBG reflected wavelength shift.

Development of a film sensor for static and dynamic force measurement

In this work an innovative double-layer film sensor for the measurement of forces is presented. The sensor is a thin film (thickness below 1 mm) based on a “sandwich” structure composed of two sensing elements glued together: one layer is a capacitive film and the other is a piezoelectric film. Both the layers are sensitive to compression loads, but they are suitable for working in different frequency ranges. In fact, while the capacitive element is capable of measuring from dc up to about 400 Hz, on the contrary the piezoelectric film works in the high frequency range. The output signals of both the sensors are acquired and then filtered and processed in order to achieve a single output signal. The piezocapacitive sensor has been developed in order to synthesize, in a small and cheap device, the capability to measure compression forces in a wide range of frequencies. The sensor is very small and has many potential applications, such as in the field of modal analysis. In particular, the very small thickne...