Low weight additive manufacturing FBG accelerometer: design, characterization and testing

Structural Health Monitoring is considered the process of damage detection and structural characterization by any type of on-board sensors. Fibre Bragg Gratings (FBG) are increasing their popularity due to their many advantages like easy multiplexing, negligible weight and size, high sensitivity, inert to electromagnetic fields, etc. FBGs allow obtaining directly strain and temperature, and other magnitudes can also be measured by the adaptation of the Bragg condition. In particular, the acceleration is of special importance for dynamic analysis. In this work, a low weight accelerometer has been developed using a FBG. It consists in a hexagonal lattice hollow cylinder designed with a resonance frequency above 500 Hz. A Finite Element Model (FEM) was used to analyse dynamic behaviour of the sensor. Then, it was modelled in a CAD software and exported to additive manufacturing machines. Finally, a characterization test campaign was carried out obtaining a sensitivity of 19.65 pm/g. As a case study, this paper presents the experimental modal analysis of the wing of an Unmanned Aerial Vehicle. The measurements from piezoelectric, MEMS accelerometers, embedded FBGs sensors and the developed FBG accelerometer are compared.Ministerio de Economía y Competitividad BIA2013-43085-P y BIA2016-75042-C2-1-

Fiber Bragg grating application to study an unmanned aerial system composite wing

Structural health monitoring consists of structural integrity assessment by means of data acquisition and analysis from on-board sensors. Fiber Bragg grating–based monitoring is increasingly attracting the scientific community working on structural health monitoring due to its multiple advantages such as electromagnetic immunity, negligible weight and size, and multiplexing availability. However, the integration of fiber optics within a structure still requires new procedures and signal treatment techniques for increasing technology reliability and exploiting its full potential. In this article, five embedded Fiber Bragg grating sensors are installed in an unmanned aerial system wing for correlating operational conditions with structural strain in real time. Sensor locations are determined by a finite element model accounting for manufacturing limitations of the fiber line. The developed Fiber Bragg grating system and processing techniques are used in static and dynamic tests showing the capacities of this powerful technology. The assessment includes deflection shape estimation, strain cycles counting, audible and visual strain alarms, aileron control based on strain levels, and structural resonance response detection.Ministerio de Economía y Competitividad BIA2016-75042-C2-1-RMinisterio de Economía y Competitividad BIA2013-43085 -

Fatigue behaviour of PBF additive manufactured TI6AL4V alloy after shot and laser peening

Article number 106536Additive manufacturing (AM) of metallic parts is a relatively new manufacturing procedure. Many industry sectors, such as the aerospace or automotive sectors, have started to apply this technology to produce some elements, thus reducing costs and weight. Several metallic alloys have been employed for AM. Due to the high strength-to-density ratio, Ti6Al4V alloy is probably the alloy most used for AM in the aerospace industry. This alloy usually shows good static strength properties. However, the presence of internal defects and the surface roughness result in a fatigue strength that is clearly lower than that of materials produced by traditional pro cesses. Moreover, the scatter of the fatigue results is generally higher than in the case of wrought pieces. Different treatments have been proposed to improve the fatigue behavior by reducing internal defects and roughness or generating a favorable residual stress field. In this work, selected surface treatments were considered to improve the fatigue strength of AM parts, including shot and laser peening as well as a combination of shot peening plus chemical assisted surface enhancement (CASE®). Three groups of specimens, each with one of the surface treatments, were fatigue tested to compare the results produced by these treatments. The residual stresses, roughness and hardness produced by the treatments were analyzed. After testing, the fracture surfaces were also analyzed to better understand the fatigue process of the different groups of specimens. The results indicate that laser peening produced the best results, followed by shot peening plus CASE and shot peening. In all three cases, the fatigue strength was much higher than that of the reference group without surface treatment. It was also observed that all failures initiated from an interior defect in the shot peening plus CASE group, four out of six failures in the laser peened group, but only one failure in the case of shot peened group and none in the reference group. Failures of specimens with initiation from internal defects started from defects located deeper than the compressive residual stress layer produced by the treatments

The role of Si on the microstructure of Al casting alloys and short fibre composites

Zsfassung in dt. SpracheDie thermomechanischen Eigenschaften von Al-Si Legierungen sind für deren Einsatz als Komponenten für Verbrennungsmotoren und Bremsen von Bedeutung. Diesbezügliche Verbesserungen sind gefragt, da die Anforderungsprofile stetig anspruchsvoller werden. Mit gesteigerter Präzision in der Fertigung erhält die Formstabilität der Werkstoffe erhöhte Aufmerksamkeit. Die neuen Untersuchungsmethoden zur dreidimensionalen Gefüge-Architektur heterogener Werkstoffe eröffnen weitere Entwicklungsmöglichkeiten sowohl der konventionellen Legierungen als auch der Metallmatrix-Verbundwerkstoffe. Hierfür werden hier grundlegende Untersuchungen der Legierungen AlSi1.1, 1.7, 7, und 12 beschrieben, sowie der 20 vol% Saffil(d-Al2O3)-kurzfaserverstärkten Matrixlegierungen AlSi1.1, 7, 12, und 18 (SFRM). Die thermische Ausdehnung dieser Legierungen und dieser SFRM nimmt mit zunehmendem Si-Gehalt ab. Die in den Proben in einer Ebene statistisch verteilt orientierten Kurzfasern reduzieren die thermische Ausdehnung besonders stark in der Faserebene. Senkrecht zur Faserorientierung steigt jedoch der physikalische, thermische Ausdehnungskoeffizient mit steigender Temperatur über den der unverstärkten Matrix. Bis zu 1.6 % Si sind in Aluminium löslich, jedoch sinkt die Löslichkeit bis 300°C stark ab, sodass in lösungsgeglühten, abgeschreckten Proben Si-Ausscheidungen erzeugt werden können. Die Ausscheidungskinetik wird mittels Differential Scanning Calorimetry (DSC) untersucht und mit Ausdehnungseffekten, die dilatometrisch gemessen werden korreliert. Die Ausscheidung bewirkt einen zusätzlichen Expansionseffekt wegen der vom Al unterschiedlichen Kristallstruktur der Si-Ausscheidungen, während deren Auflösung zu einer Reduktion der thermischen Expansion führt.Plättchenförmige und globulare Si-Ausscheidungen wurden mittels Transmissionselektronenmikroskopie (TEM) identifiziert, die auf unterschiedliche Keimbildungsmechanismen zurückgeführt werden. Mit diesen Gefügekorrelationen kann die Dilatometrie zur Bestimmung der Ausscheidungskinetik herangezogen werden.Um die Wirkung der eutektisch gebildeten Si-Strukturen zu studieren, wurde die dreidimensionale Anordnung und deren Stabilität an AlSi 7 und AlSi12 Legierungen mit und ohne Sr-Veredelung untersucht. Mittels Tiefätzung bzw. chemischer Auflösung der a-Al-Phase wird gezeigt, dass die eutektischen Si-Strukturen im Gusszustand der Proben dreidimensional zusammenhängende Netzwerke bilden. Die Veredelung durch Sr verfeinert das eutektische Gefüge wesentlich und es sind korallenartige Strukturen erkennbar. Durch eine Lösungsglühung bei 540°C formen sich die Si-Strukturen ein und verlieren den dreidimensionalen Zusammenhang. Im Detail werden die Si-Strukturen mittels einer schichtweisen Abtragung mit einem Focussed Ion Beam (FIB) im Rasterelektronenmikroskop mit Details von einigen zehn nm dargestellt, wobei auch die elementspezifische Abbildung mittels energiedispersiver charakteristischer Röntgenstrahlung unterstützt wird (SEM-EDX).In den kurzfaserverstärkten Materialien verbindet das eutektische Si die Kurzfasern und es bildet sich ein dreidimensionales Si-Kurzfaser-Netzwerk. Eine Lösungsglühung bewirkt auch in den SFRM eine Vergröberung der Si-Strukturen, aber diese bewirken weiterhin den Zusammenhalt der Kurzfaserverteilung. Mit steigendem Si-Gehalt nimmt auch der Elastizitätsmodul und die Druckfestigkeit bei erhöhter Temperatur zu. Die beobachtete Entfestigung ab Stauchungen von 4% wird auf Bruchvorgänge in der Verstärkungsstruktur zurückgeführt. Bei thermozyklischen Kriechversuchen zeigen die SFRM-Proben der übereutektischen AlSi18-Matrix eine doppelt so hohe Lebensdauer wie die eutektische und die untereutektische Matrix. Die Lebensdauer sinkt nach einer Lösungsglühung, dafür nimmt die Duktilität deutlich zu. Die Si-Ausscheidungsvorgänge werden auch in den SFRM beobachtet und die Art der Vernetzung der Kurzfasern durch das eutektische Si beeinflusst die mechanischen Kennwerte.Thermal stability and the improvement of the mechanical properties of Al-Si alloys and their composites are important for their application in engine and break components. The study of the three-dimensional microstructure of the materials would be a significant tool for further alloy and composite development by understanding the influence of the 3D architecture of their heterogeneities.In this work the thermal expansion of different AlSi1.1, -1.7, -7, -12 alloys and reinforced AlSi1.1, 7, 12, 18 alloys with 20%vol. of Al2O3-short fibres is investigated. A reduction of the thermal expansion is observed by increasing the Si content in both unreinforced and reinforced Al-Si alloys. The addition of random planar alumina fibres reduces the in plane thermal expansion, but the transverse expansion is significantly increased with respect to the matrix. The precipitation kinetics of Si is studied by means of Differential Scanning Calorimetry (DSC). The measured exothermic and endothermic effects produced by the precipitation and dissolution of Si are correlated with an increase and reduction of the coefficient of thermal expansion (CTE), respectively.The results provide a method to use dilatometry for studies of precipitation kinetics of Si in aluminium.A detailed study of the 3D morphology of different eutectic architectures of unmodified and Sr-modified AlSi7-12 alloys and composite materials is been conducted, Scanning Electron Imaging (SEI) of deep etched samples and a new Focused Ion Beam - Energy Dispersive Spectroscopy (FIB-EDX) method are applied. The 3D structures of the silicon phases can be reconstructed on sub-µm scale.Al-Si alloys, with 7% and higher content of Si, present a percolating Si-network in the as cast condition. The addition of Strontium produces morphological changes of the network from lamellar Si to a fine dispersed coral fibrous like architecture. Spheroidisation and coarsening of the eutectic Si architecture is studied after different exposure times at 540°C. The disintegration of the eutectic Si network in AlSi foundry alloys by the solution treatment is in contrast to the stabilization of the Si short fibre network in the SFRMs.The addition of alumina fibres into the Al-Si matrices has avoided the disintegration of the Si-structure by forming an Al2O3-Si-network.During solution treatment, Si diffuses from fibre free-zones to the fibre surface by forming coarse Si particles.Both, elastic modulus and maximum strength (from compression tests) show the improvement of the properties by increasing the Si content. Strength vs. true strain curves of compression tests depict a softening in the composite materials starting at about 4% of strain (independent of the Si content), which is related to the fracture of the alumina fibre network. Thermal cycling tests depict an increment of the cycling life of composite by the hypereutectic addition of Si by a factor of ~2 comparing the AlSi18/Al2O3/20s composite with AlSi7/Al2O3/20s or AlSi12/Al2O3/20s materials. The ductility of materials is increased during solution treatment but with a considerable reduction in the number of cycles of thermal cycling creep tests.13