F. Felli

Experimental modal analysis of an aircraft model wing by embedded fiber Bragg grating sensors

A critical issue in practical structural health monitoring is related to the capability of proper sensing systems integrated within the host structures to detect, identify, and localize damage generation. To this aim, many techniques have been proposed involving dynamic measurements such as modal analysis, acoustic emission, and ultrasonics. This paper relies on the use of embedded fiber Bragg grating sensors for performing an experimental modal analysis on a wing of an aircraft model. Time domain response of the embedded fiber-optic sensors induced by hammer impacts were acquired and transformed into the frequency domain. Using a classical technique based on the frequency transfer function, the first displacement and strain mode shapes of the wing have been retrieved in terms of natural frequencies and amplitudes. Experimental results confirm the excellent performances of this class of sensing devices to determine the modal behavior within complex structures compared with conventional accelerometer-based detection systems.

Metal coating for enhancing the sensitivity of fibre Bragg grating sensors at cryogenic temperature

Fibre Bragg grating (FBG) sensors that are immune to electromagnetic interference could advantageously perform cryogenic temperature monitoring in superconducting magnetic fields, but their intrinsic temperature sensitivity is quite poor and must be enhanced. In fact, the low thermal expansion coefficient of silica limits the temperature sensitivity of bare FBG sensors at cryogenic temperature. In this paper the possibility of improving the temperature sensitivity of FBG sensors by metal coating is investigated. Specifically, zinc and copper coating depositions are performed by the traditional electrowinning process, after aluminium pre-coating of the sensor. Coated FBG sensors are inspected by both optical and metallographic techniques. SEM metallographic investigations show that a homogeneous deposit is obtained, with good metal adhesion to the FBG sensor. Optical testing shows that the optical properties of the coated FBG sensors are slightly affected: aluminium pre-coating produces appreciable modification of the diffraction spectrum in both peak width and peak shift, while zinc coating produces a moderate peak shift and copper coating just enlarges the peak width. Results presented in this paper show that both metals appreciably increase the temperature sensitivity of the FBG sensors. Zinc coating provides the highest sensitivity and high-resolution temperature measurements are possible at temperatures as low as 15 K.

Improving FBG Sensor Sensitivity at Cryogenic Temperature by Metal Coating

Commercially available fiber Bragg grating (FBG) sensors cannot be used for measuring cryogenic temperatures because they are made of silica the thermal expansion coefficient of which tends to zero when approaching 4 K. Because of the many advantages of fiber optic sensors with respect to conventional ones, in this paper it is shown how to circumvent such a limitation by applying a proper metal coating. This approach drastically increases temperature measuring capability of FBGs at cryogenic environments typically encountered in application involving liquid gases or in space. Various metals have been deposited by electro winning on the external fiber surface previously treated with an aluminum precoating. Also, a special casting process has been developed. The explored temperature region was 4.2-40 K. The paper reports the characterization of FBG sensors coated with different metals and shows the validity of this new temperature sensor with respect to conventional ones.

Study of gas purifiers for the CMS RPC detector

Abstract The CMS RPC muon detector utilizes a gas recirculation system called closed loop (CL) to cope with large gas mixture volumes and costs. A systematic study of CL gas purifiers has been carried out over 400 days between July 2008 and August 2009 at CERN in a low-radiation test area, with the use of RPC chambers with currents monitoring, and gas analysis sampling points. The study aimed to fully clarify the presence of pollutants, the chemistry of purifiers used in the CL, and the regeneration procedure. Preliminary results on contaminants release and purifier characterization are reported.

The upgrade of the CMS RPC system during the first LHC long shutdown

The CMS muon system includes in both the barrel and endcap region Resistive Plate Chambers (RPC). They mainly serve as trigger detectors and also improve the reconstruction of muon parameters. Over the years, the instantaneous luminosity of the Large Hadron Collider gradually increases. During the LHC Phase 1 ( ~ first 10 years of operation) an ultimate luminosity is expected above its design value of 1034 cm−2s−1 at 14 TeV. To prepare the machine and also the experiments for this, two long shutdown periods are scheduled for 2013-2014 and 2018-2019. The CMS Collaboration is planning several detector upgrades during these long shutdowns. In particular, the muon detection system should be able to maintain a low-pT threshold for an efficient Level-1 Muon Trigger at high particle rates. One of the measures to ensure this, is to extend the present RPC system with the addition of a 4th layer in both endcap regions. During the first long shutdown, these two new stations will be equipped in the region |η| < 1.6 with 144 High Pressure Laminate (HPL) double-layer RPCs operating in avalanche mode, with a similar design as the existing CMS endcap chambers. Here, we present the upgrade plans for the CMS RPC system for the fist long shutdown, including trigger simulation studies for the extended system, and details on the new HPL production, the chamber assembly and the quality control procedures.

Moisture content and strain relation in wood by Bragg grating sensor and unilateral NMR

The paper describes a low-invasive experimental apparatus designed for the accurate determination of mechanical deformation and moisture content (MC) relationship on macroscopic wood samples. The device is particularly indicated for monitoring wooden handwork whose mechanical deformation is especially critical in relation to its role such as, for example, works of art or architectural works. The MC of wood is measured with a portable single-sided NMR probe and mechanical deformation by a fiber Bragg grating optical sensor. The data obtained are of high accuracy, despite the dimensions of the sample. The methodology provides an effective tool for investigating the dynamic relation between environmental relative humidity, MC, and shrinking–swelling of wood. Adsorption results collected for longitudinal deformation in silver fir (Abies alba Mill.) are presented to show the sensitivity of the optical sensor. Interesting findings include the detection of two different mechanisms of elongation and the time evolution of water mobility versus hydration and strain.

An optical system for cryogenic temperature measurements

Fiber optics with a photo-imprinted Bragg grating have been studied for potential use as temperature sensors in cryogenic applications. Sensors were coupled with different metallic materials to improve the measurement sensitivity in the very low temperature region, i.e. 4.2–30 K. Various coatings were deposited on the external fiber surface, and full characterization of the tested sensors is given in the paper.

An RPC-based Technical Trigger for the CMS Experiment

In the CMS experiment, sub-detectors may send special trigger signals, called “Technical Triggers”, for purposes like test and calibration. The Resistive Plate Chambers are part of the Muon Trigger System of the experiment, but might also produce a cosmic muon trigger to be used during the commissioning of the detectors, the CMS Magnet Test-Cosmic Challenge and the later running of CMS. The proposed implementation is based on the development of a new board, the RPC Balcony Collector (RBC); the test results on prototypes and their performance during the recent CMS Cosmic Challenge are presented.

Fiber optic sensors for space missions

Fiber optic sensors offer several advantages over conventional ones such as the immunity from electromagnetic interferences and the possibility of multiplexing many of them along the same fiber. Such sensors can be used to monitor local deformations on or inside the structures. In the paper will be mentioned techniques that can be used for embedding those sensors into several types of materials and possible applications in the field of Structural Health Monitoring (SHM). However the main objective is to show applications is in the field of scientific space experiments where precise position monitoring of particle detectors or of optical instrument can be crucial for the success of the mission. A real time position monitoring system based on fiber optic sensors is presented. The experimental results are compared with a conventional optical technique.

Cyclic oxidation behavior of co-rolled al-ferritic stainless steel

The lifetime of thin-walled catalyst-carrying foils is directly linked to the overall amount of aluminum present in the base alloy. Ferritic stainless steel containing more than 7 wt. % of Al represents an interesting material for the production of substrates in catalytic converters because of its good corrosion resistance at high temperature. Despite that, the considerable brittleness of the alloy makes production by means of traditional metallurgical processes difficult. Enriching the steel surface with aluminum avoids these problems and due to the high superficial aluminum concentration, enhances the Al2O3 scale formation in the early stages of the oxidation process. Among several techniques, the co-rolling process appears to be a promising technology for enriching ferritic stainless steel surfaces with aluminum without affecting the structural and mechanical properties of the substrate. This process involves cold rolling the annealed AISI 430 ferritic stainless steel together with aluminum foil to form an Al-steel-Al sandwich. After a preoxidation treatment the resultant assemblies were subjected to cyclic oxidation in air. Their oxidation behavior was then characterized by the weight gain versus the number of cycle curves. Oxides were examined by SEM and the phases were identified by X-ray diffraction (XRD). The results highlight that in the lower range of temperatures (T