Andrea Brotzu

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.

Characterization of Bronze Corrosion Products on Exposition to Sulphur Dioxide

In the main frame of the research aimed to model the corrosion growth on bronze surface, the objective of the work here reported has been to characterize the corrosion products formed on laboratory samples of bronze alloy (Cu Sn12), during the early stage of exposure to moist air with sulfur dioxide. A cycling corrosion cabinet was used to control 200 ppm gas concentration, relative humidity (RH) and temperature, according to the DIN 50018 (Kesternich test).The method is designed to evaluate how well the surface resists to sulfur dioxide corrosion; the test cycle consists of 8 hours exposure to sulfur dioxide at 40°C temperature and 100% relative humidity, followed by 12 hours drying at room condition. Weight variation, Spectrophotometer, Scanning Electron Microscopy with X-ray microanalysis (SEM-EDS), X-ray Diffraction (XRD) analysis were carried out for the tarnish products characterization. Some of the compound identified were brochantite (Cu4(OH)6SO4), chalcanthite (CuSO4•5H2O) cuprite (Cu2O), cassiterite (SnO2) and ottemannite (Sn2O3).

Effects of the manufacturing process on fracture behaviour of cast TiAl intermetallic alloys

The ? -TiAl based intermetallic alloys are interesting candidate materials for high-temperature applications with the efforts being directed toward the replacement of Ni-based superalloys. TiAl-based alloys are characterised by a density (3.5-4 g/cm3) which is less than half of that of Ni-based superalloys, and therefore these alloys have attracted broad attention as potential candidate for high-temperature structural applications. Specific composition/microstructure combinations should be attained with the aim of obtaining good mechanical properties while maintaining satisfactory oxidation resistance, creep resistance and high temperature strength for targeted applications. Different casting methods have been used for producing TiAl based alloys. In our experimental work, specimens were produced by means of centrifugal casting. Tests carried out on several samples characterised by different alloy compositions highlighted that solidification shrinkage and solid metal contraction during cooling produce the development of relevant residual stresses that are sufficient to fracture the castings during cooling or to produce a delayed fracture. In this work, crack initiation and growth have been analysed in order to identify the factors causing the very high residual stresses that often produce explosive crack propagation throughout the casting.

Use of Fiber Optic Sensors for Monitoring Crack Growth in Fatigue and Corrosion-Fatigue Tests

Fibre Bragg Gratings (FBGs) are optical strain gages manufactured directly inside the fiber core. They provide several advantages with respect to conventional strain gages. In particular it is possible to put several FBGs along the same fibre (multiplexing), they are immune to corrosive environments and to electromagnetic interferences. They can be embedded in almost all types of materials and are very useful in Structural Health Monitoring. An innovative approach for testing specimens in this area is reported in this work. An aluminium alloy 2024-T3 CT specimen has been manufactured with one small feed-through hole and a superficial groove. Two FBG sensors, multiplexed on the same fibre, have been glued one inside the hole and one inside the groove. Fatigue test has been carried out monitoring the crack length both with standard measure system and with the FBG strain sensors placed in front of the crack tip.

Investigation on some factors affecting crack formation in high resistance aluminum alloys

Aluminum alloys having good mechanical properties are Al-Zn- Mg alloys (7xxx) and Al-Cu-Li alloys (Weldalite®). These alloys may be subjected to stress corrosion cracking. In order to overcome this problem the Al 7050 alloy has been developed and it is widely used for aerospace applications. Despite that, some components made of this alloy cracked during the manufacturing process including machining and chemical anodization. In a previous work cracked Al 7050 components have been analyzed in order to identify possible causes of crack formation. In this work the susceptibility of this alloy to intergranular corrosion has been analysed and compared with that of other high resistance aluminum alloys.

Fatigue crack micromechanisms in a Cu-Zn-Al shape memory alloy with pseudo-elastic behavior

Shape memory property characterizes the behavior of many Ti based and Cu based alloys (SMAs). In Cu-Zn-Al SMAs, the original shape recovering is due to a bcc phase that is stable at high temperature. After an appropriate cooling process, this phase (?-phase or austenitic phase) transforms reversibly into a B2 structure (transition phase) and, after a further cooling process or a plastic deformation, it transforms into a DO3 phase (martensitic phase). In ?-Cu-Zn-Al SMAs, the martensitic transformation due to plastic deformation is not stable at room temperature: a high temperature “austenitization” process followed by a high speed cooling process allow to obtain a martensitic phase with a higher stability. In this work, a Cu-Zn-Al SMA in “as cast” conditions has been microstructurally and metallographically characterized by means of X-Ray diffraction and Light Optical Microscope (LOM) observations. Fatigue crack propagation resistance and damaging micromechanisms have been investigated corresponding to three different load ratios (R=0.10, 0.50 and 0.75).

Design, Prototyping and Preliminary Testing of a Ti-Al Gas Turbine Blade

This work investigates the feasibility of manufacturing a turbine blade made of a Ti-Al intermetallic alloy by means of investment casting. The work is based on a multidisciplinary approach that combines a conventional CFD analysis of the flow field around the statoric and rotoric blades with the results of several metallurgical studies aimed at the optimization of the alloy composition by finding the best compromise among fracture toughness, oxidation resistance at high temperatures and mechanical properties. The combination of the two techniques lead to an iterative procedure (of which only the first two steps are reported in this paper): a conventional blade is first modeled and the corresponding investment cast is produced via a 3D printing technique; a first version of the blade is built; a modified blade shape is then obtained by a refined CFD study; as a last step the final version of the blade is cast. On the basis of standard operational specifications representative of modern gas turbines, a turbine blade was therefore designed, tested by CFD (ANSYS-FLUENT) to ensure proper fluid dynamic performance, and its levels of thermo-mechanical stress under working conditions were calculated via a commercial CAD software (ANSYS). The fully 3D version of the component was subsequently prototyped by means of fused deposition modeling. A full-scale set of blades (blade height approximately 7 cm, blade chord approximately 5 cm) was produced by means of investment casting in an induction furnace. The produced items showed acceptable characteristics in terms of shape and soundness. The blade alloy was analyzed by performing metallographic investigations and some preliminary mechanical tests. At the same time, the geometry was refined by a complete and more complex CFD study, and a slightly modified shape was obtained. Its final testing under operative conditions is left for a later study. The paper describes the spec-to-final product procedure and discusses some critical aspects of this manufacturing process such as the considerable reactivity between the molten metal and the mold material, the resistance of the ceramic shell to the molten metal impact at temperatures as high as 2073 K and the limit mold porosity that may compromise the component surface finish. Furthermore, a detailed account is provided for the CFD results that led to the modification of the original commercial shape: pressure, velocity and temperature fields in the statoric and rotoric channels are described in some detail, and a preliminary performance assessment of the turbine stage is presented and discussed.Copyright

Fatigue behavior of lubricated Ni-Ti endodontic rotary instruments

The use of Ni-Ti alloys in the practice of endodontic comes from their important properties such as shape memory and superelasticity phenomena, good corrosion resistance and high compatibility with biological tissues. In the last twenty years a great variety of nickel-titanium rotary instruments, with various sections and taper, have been developed and marketed. Although they have many advantages and despite their increasing popularity, a major concern with the use of Ni-Ti rotary instruments is the possibility of unexpected failure in use due to several reasons: novice operator handling, presence manufacturing defects, fatigue etc. Recently, the use of an aqueous gel during experimental tests showed a longer duration of the instruments. The aim of the present work is to contribute to the study of the fracture behavior of these endodontic rotary instruments particularly assessing whether the use of the aqueous lubricant gel can extend their operative life stating its reasons. A finite element model (FEM) has been developed to support the experimental results. The results were rather contradictory, also because the Perspex (Poly-methyl methacrylate, PMMA) cannot simulate completely the dentin mechanical behavior; however the results highlight some interesting points which are discussed in the paper.

Use of FBG sensors for monitoring cracks of the equestrian statue of Bartolomeo Colleoni in Venice

The Bartolomeo Colleoni monument suffered for years damage from the local climate. The process of restoring the Colleoni equestrian statue, started in 2003, allowed to understand how the bronze statue was originally cast and manufactured and the techniques used in its construction. During this process a relevant crack on the right foreleg was investigated in correspondence of the cast-on joining the right foreleg to the front portion of the horse body. The crack was investigated experimentally by Fiber Bragg Grating (FBG) sensors, avoiding any modelling because of the very complex structure of the statue. An array of FBG sensors connected in series was glued on the crack with the aim of capturing live information about the effect of applying stress on the crack opening. The monitoring system was successfully tested during repositioning of the RIDER on the horse and is available for long term inspection of the crack opening evolution.

Wear surface damage of a Stainless Steel EN 3358 aeronautical component subjected to sliding

The present paper describes the failure analysis of an aircraft component subjected to several episodes of in service failure, resulted in loss of the aircraft safety. Modern aircrafts are provided with mechanical systems which have the task to open not pressurized hatches during landing. The components of such systems are subject to considerable mechanical stresses in harsh environment (presence of moisture and pollutants, significant and sudden temperature variations). The system is constituted by a sliding piston, a related nipple and by a locking system consisting of 4 steel spheres which are forced into a countersink machined on the piston when the hatches is open. The whole system is activated by a preloaded spring. The machined parts, nipple and piston, are made of EN3358 steel (X3CrNiMo13-8-2), a precipitation hardening stainless steel with very low content of carbon often used in the aerospace. The samples provided by the manufacturer present different types of damage all referable to phenomena relative to the sliding of the piston inside the nipple. The present paper describes the different damage observed and the microstructure of the material, then are reported the results obtained from the characterization of the material of the samples by means of optical and electronic microscopy, carried out to define the mechanisms involved in the system seizure. In order to define the primary cause of failure and to propose solutions to be adopted, also analyzing the criticality of using this PH stainless steel for this application, the results of different tests were compared with system design and working data.