1,721,133 research outputs found
Estimating Koopman operators for nonlinear dynamical systems: A nonparametric approach
Full text linkThe Koopman operator provides a linear description of non-linear systems exploiting an embedding into an infinite dimensional space. Dynamic Mode Decomposition and Extended Dynamic Mode Decomposition are amongst the most popular finite dimensional approximations of the Koopman Operator. In this paper we capture their core essence as a dual version of the same problem, embedding them into the Kernel framework. To do so, we leverage the RKHS as a suitable space for learning the Koopman dynamics. Learning from finite length data automatically provides a finite dimensional approximation induced by data. Simulations and comparison with standard procedures are included
Reference object for traceability establishment in X-ray computed tomography measurements of fiber length in fiber-reinforced polymeric materials
No full textX-ray computed tomography (CT) is an advanced non-destructive measuring technique that enables holistic three-dimensional measurements of outer as well as inner geometries and micro-scale features. In the field of fiber-reinforced polymeric (FRP) materials, CT can overcome the limitations of conventional methods typically based on time-consuming and destructive operations used for the measurement of fiber orientation and length, which are two characteristics that have a great impact on physical and mechanical properties of FRP parts. This work specifically addresses the CT measurement of fiber length, which is particularly complex because – differently from the fiber orientation analysis – it requires the individual fibers to be properly identified and segmented. Moreover, the traceability of such measurement has not been achieved yet. This work investigates an experimental methodology for traceability establishment and measurement uncertainty determination, based on the substitution approach. In particular, a task-specific reference object, including a selection of fibers with different lengths and configurations, was designed, manufactured and calibrated for the purpose
Accuracy of fiber length measurements using X-ray computed tomography for the analysis of composite materials
No full textX-ray computed tomography is capable of measuring non-destructively the fiber geometrical characteristics that deeply affect both physical and mechanical properties of fiber-reinforced polymeric products. In particular, X-ray computed tomography can overcome the limitations of conventional methods used for fiber orientation analysis and fiber length measurement, which are mainly based on time-consuming and destructive optical analyses. This work especially addresses the case of fiber length measurement, which is particularly complex because – differently from the case of fiber orientation and concentration analyses – it needs the individual fibers to be identified and segmented. Moreover, the accuracy of such measurements has not been thoroughly investigated so far. In this work, a new method for accuracy evaluation is proposed. The method is based on the use of a newly developed task-specific reference object, which has been designed, manufactured and calibrated with respect to the length of a selection of fibers with different lengths and different configurations
X-ray computed tomography for dimensional measurements of threaded parts
No full textX-ray computed tomography (CT) is and advanced measuring technique capable of performing holistic dimensional verification of both internal and external threaded elements. The contactless and non-destructive nature of CT measurements makes them suited for evaluating also screw threads and threaded features that are internal, small in size, or made of easily deformable materials, overcoming the limitations of conventional contact and optical measuring techniques. This work presents a CT-based methodology for measuring the geometry and local form errors of threaded products applied to (i) additively manufactured metal dental implants (characterized by significant form errors and complex surface topography) and (ii) injection molded polymeric threaded components (fragile, deformable and characterized by high form errors). The accuracy of CT dimensional measurements is investigated through comparison with reference measurements
Improving surface and porosity analyses of laser powder bed fusion parts through CT-based three-dimensional metal powder characterization
No full textX-ray computed tomography (CT) can be used to perform three-dimensional (3D) measurements of both size and shape of metal powder used as feedstock material in laser powder bed fusion (LPBF). Such powder characteristics – which can be altered when employing particles recovered after being used although not processed in previous processes – are known to have a significant influence on typical LPBF process issues and product flaws, such as the presence of internal defects and complex surface texture. This work investigates the possibility to exploit the CT-based 3D powder characterization to guide and improve the evaluation of surface texture and internal pores by aiding the identification of surface features due to unstable process conditions and the discrimination between pores caused by insufficient fusion and voids caused by the presence of spatter formation
Enhancing fiber length measurements performed by X-ray computed tomography for improving the production quality of composite materials
Full text linkX-ray computed tomography (CT) is a non-destructive measuring technique that allows performing measurements of outer as well as inner geometries and features. This work addresses the application of CT in the field of fiber-reinforced polymers, which are increasingly used in industry to manufacture products with enhanced mechanical properties and lightweight. In particular, when fiber-reinforced components are fabricated for example using the injection molding process, the presence of long and well-oriented fibers is necessary to achieve good mechanical properties of the products, but the process itself often leads to relevant fiber breakage and complex fiber orientation. In this context, for optimizing the process, adequate and accurate measuring techniques are needed to correlate the injection molding process parameters with the fiber geometrical characteristics. Metrological CT is the only available three-dimensional measuring technique capable of evaluating in a nondestructive way the relevant fiber geometrical characteristics, including fiber length and fiber orientation. The conventional methods are in fact commonly based on optical measurements, which require destructive operations. CT data are already successfully used to evaluate the fiber orientation, whereas the fiber length measurement is more complex because it needs the individual fibers to be identified and segmented. Despite the inherent difficulty of the latter operations, there are already software tools able to measure the fiber length from high-resolution CT data. However, the accuracy of CT fiber length measurements has not been thoroughly investigated so far. This work proposes an experimental methodology for the accuracy evaluation and enhancement of fiber length measurements performed by means of X-ray computed tomography on injection molded components characterized by a polymer matrix reinforced with glass fibers. The work lays the foundations for establishing CT as a tool to be effectively used for quality improvement of injection molding processes and products, as well as for enhancing process simulations and modelling in the Industry 4.0 context
Value-function estimation and uncertainty propagation in Reinforcement Learning: a Koopman operator approach
No full textEnabling X-ray computed tomography measurement of contact surfaces in mono-material assemblies and titanium dental implant systems
Full text linkSince X-ray computed tomography (CT) can analyse non-accessible features and parts in a non-destructive way, it enables the inspection of the individual components of assemblies in the assembled state. Such unique inspection capability is of great relevance because, even when each part of an assembly meets the specified tolerances, the proper functioning of the assembled system is not guaranteed, as variations can occur during the assembly operations, for instance due to clamping and/or assembly forces. Besides the dimensional evaluation of components in the assembly state, contact surfaces/interfaces and micro-gaps between coupled assembled components are in many cases important to be accurately identified and measured. These evaluations can be more easily conducted on multi-material assemblies in which the materials of the single components are characterized by different densities and characteristics, thus enabling their mutual distinction and segmentation within the CT reconstructed volume. In the case of mono-material assemblies, instead, due to the single X-ray attenuation coefficient to be considered, the three-dimensional model of the assembly resulting after CT reconstruction would appear as a unique and continuous volume in regions where components are in contact, with serious difficulties in evaluating their interface. To overcome this limit, the present research was aimed at developing new CT-based methods to enable a complete evaluation of mono-material assemblies. Such methods were applied to titanium dental implant systems, for which an accurate analysis of contact surfaces and interfaces between the assembled components is crucial to ensure perfect sealing and prevent mechanical failures as well as microbiological leakage and consequent bacteria infiltrations. Finally, the accuracy of the obtained measurement results is evaluated using a reference object made of titanium, featuring different calibrated micro-gaps
L’impatto del turismo sul commercio al dettaglio. Il caso di Venezia
No full textThe tourism is one of the most important elements able to transform the urban structure of a city. This is particular true regarding the commercial structures, especially during the affirmation of the mass tourism. For this reason, a multivariate analysis through the Principal Components is presented in order to identify the urban structure of the Venetian city as well as the areas interested by this phenomena. Furthermore, in order to understand deeply the transformation occurred as a consequence of the tourism for the entire urban system, an investigation and comparison of shops typologies in a period of thirty years has been made. Finally, an identification of the most important pathways interested by the tourism phenomena has been identified and investigated
New experimental approach for local measurements of effective layer thickness, powder bed density and volumetric energy density to enhance metal laser powder bed fusion
No full textThis work presents an experimental approach for simultaneous measurements of effective layer thickness, powder bed density, and volumetric energy density, useful to improve the analysis of process dynamics and enhance precision in metal laser powder bed fusion. The approach is based on a special building platform, including removable inserts with reference geometries, and high-resolution X-ray computed tomography. The local variability in layer thickness and energy density are evaluated and used as indicators of process stability and energy transmission efficiency. The contextual measurement of powder bed density offers additional insights into potential process-related influences such as spatter formation and denudation
- …
