1,721,073 research outputs found
Parameter Calibration of a Social Force Model for the Crowd-Induced Vibrations of Footbridges
No full textA reliable prediction of the human-induced vibrations of footbridges relies on an accurate representation of the pedestrian excitation for different loading scenario. Particularly, the modeling of crowd-induced dynamic loading is a critical issue for the serviceability assessment of footbridges. At the design stage, the modeling of crowd loading is often derived from single pedestrian models, neglecting the effect of the structural vibrations as well as the interactions among pedestrians. A detailed description of the crowd behavior can be achieved employing a social force model that describes the different influences affecting individual pedestrian motion. These models are widely adopted to describe the crowd behavior especially in the field of evacuation of public buildings, public safety and transport station management while applications in the serviceability assessment of footbridges are less common. To simulate unidirectional pedestrian flows on footbridges, this paper proposes a parameter calibration of the Helbing’s social force model performed adopting the response surface methodology. Parameters of the social force model are calibrated so as to represent the fundamental relation between mean walking speed and density of the pedestrian crowd. The crowd-induced vibrations are then simulated by modeling each pedestrian in the crowd as a vertical load that crosses the footbridge with time varying trajectory and velocity estimated from the calibrated social force model. Finally, results are compared to those obtained from a multiplication factor approach proposed in literature. This considers the crowd as a uniform distribution of pedestrians with constant speed and given synchronization level and the footbridge response is evaluated as the response to a single pedestrian scaled by a proper enhancement factor
Dynamic Behaviour of a steel footbridge under pedestrian loads
No full textFootbridges are generally effective structures concerning the static behavior, since they are subjected to a limited level of live loads. Nevertheless, the frequency range of the pedestrian dynamic actions may fall within the natural frequency interval of the structure, giving high dynamic amplifications. Therefore, dynamic properties of footbridges and effects of pedestrian loads need to be analyzed, comparing experimental and numerical results.
This paper is part of a research that aims to characterize the dynamic behavior of a steel footbridge with reference to pedestrian dynamic amplifications. The structure, located in Reggio Emilia (Italy), is about 170 meters long and composed of 5 simple-supported spans, linked at lower-floor level. To investigate the dynamic behavior of the footbridge, an experimental campaign has been first performed. Accelerations due to ambient vibrations (wind) and to pedestrian dynamic actions were recorded. In particular, a wide number of pedestrian dynamic loading conditions have been considered, such as excitations induced by people jumping, running and walking with different passing frequencies. Accelerations were acquired by an advanced MEMS-based system. 10 biaxial MEMS sensors were arranged in 3 different setups in order to identify as many natural modes as possible and to investigate the vibration level in several components of the footbridge.
The post-processing of experimental data allows to determine both the dynamic properties of the structure (frequencies, mode shapes and damping ratios) and the maximum accelerations caused by pedestrian actions. The dynamic characteristics are identified by means of the classic Enhanced Frequency Domain Decomposition (EFDD) method that is based on the diagonalization of the spectral density matrix.
Then, a finite element model is built and calibrated such that the analytical dynamic predictions agree with the experimental modal properties. Finally, the measured accelerations caused by pedestrian dynamic actions are compared with those given by the numerical model, considering different dynamic load models
Direct Metal Rapid Casting: mechanical optimization and tolerance calculation
No full textPurpose - The purpose of this paper is to optimize the mechanical performances of parts produced by the ZCast Direct Metal Casting process varying the thermal treatment parameters. Adopting the optimized settings, a specific dimensional evaluation is planned to calculate the international tolerance (IT) grade ensured by the process. Design/methodology/approach - Cylindrical ZCast samples are manufactured and heat treated varying time and temperature. The baked parts underwent compression tests and the rupture surfaces are observed using the scanning electron microscopy. A regression analysis is performed on the results to optimize the baking process. For the dimensional assessment, a specific benchmark is designed, built and treated. It is measured before and after baking using a coordinate measuring machine and the results are processed to obtain the IT grade. Findings - The results proved that in the heat treatment of ZCast parts time has a negligible effect on the compressive strength, whereas temperature can be optimized for best mechanical response. The IT grade is calculated for green and baked parts; separately in all three directions in space. Tolerance is proved to be fundamentally the same in every direction and independent on the heat treatment. The considered rapid casting process can be classified in IT15 grade. Originality/value - The paper suggests an original approach to improve knowledge of the ZCast process. The study of the building phenomena is combined with macroscopic measurements to develop a solid understanding of the expected performances, which is fundamental in order to support the industrial application of the technolog
Experimental investigation and optimisation of laser direct part marking of Inconel 718
Get PDFMany industries need product identification; laser marking is suitable to produce any kind of symbol, data matrix or bar code on parts. Consequently, the most important requirement for marking acceptance is, doubtless, the mark readability. The present study deals on the effect of process parameters on the laser marking of Inconel 718, with the aim to find a relation between the process parameters and mark characteristics in term of both mark geometry and readability. To this aim, laser markings, under different process conditions, were performed on Inconel 718 sheets, adopting a 30 W Q-switched Yb:YAG laser. The mark geometry was acquired by a 3D surface profiling system. Optical microscopy and SEM analysis were also performed on groove sections. In order to evaluate the readability of the marks, Weber contrast was calculated and adopted. The mark characteristics have been investigated by mean of statistical methodology (ANalysis of VAriance and Response Surface Method) and related to the process parameters. Furthermore, Master Response Optimisation methodology was adopted to individuate the optimal process conditions. It was found that mark geometry and the Weber contrast are mainly affected by the average power and the energy input per mark-length. Moreover, operative conditions allowing for maximum readability, yet without excessive increase in burr height, were also determined
Bayesian and deterministic surrogate-assisted approaches for model updating of historical masonry towers
No full textReinforcement effectiveness on mechanical performances of composites obtained by powder bed fusion
No full textNew material formulations to be used in Additive Manufacturing machines are one of the major interests in this fast growing field. The possibility to tune functional and mechanical properties, by the addition of reinforcements to a polymeric matrix, is hindered by the low provisional capability of the additive manufactured composite. The inherent anisotropy of layer manufacturing combines with mechanisms of filler dispersion and of filler/matrix adhesion in a complex scenario. The paper entails a critical evaluation of mechanical properties measured for several polymeric composites produced by Powder Bed Fusion, in the perspective of provisional models commonly accepted for composite materials. The models are reviewed versus experimental and literature data. The provisional effectiveness is generally good, except for the case of nanometric or surface treated fillers, or of specific anisotropic microstructures obtained by layer manufacturing
Repercussions of powder contamination on the fatigue life of additive manufactured maraging steel
Get PDFA wide range of materials is suitable for processing by powder bed fusion (PBF) techniques. Among the latest formulations, maraging steel 18Ni-300, which is a martensite-hardenable alloy, is often used when both high fracture toughness and high strength are required, or if dimensional changes need to be minimised. In direct tooling, 18Ni-300 can be successfully employed in numerous applications, for example in the production of dies for injection moulding and for casting of aluminium alloys; moreover, it is particularly valuable for high-performance engineering parts. Even though bibliographic data are available on the effects that parameters, employed in PBF processes, have on the obtained density, roughness, hardness and microstructure of 18Ni-300, there is still a lack of knowledge on the fatigue life of PBF manufactured parts. This paper describes the fatigue behaviour of 18Ni-300 steel manufactured by PBF, as compared by forging. Relevant negative effects of the cross-contamination of the raw material are originally identified in this paper, which emphasizes the inadequacy of current acceptability protocols for PBF powders. In the absence of contamination, endurance achieved by PBF is found equal to that by forging and consistent with tooling requirements as set out by industrial partners, based on injection moulding process modelling
High-temperature tensile behavior of AlSi7Mg parts built by LPBF under high-productivity conditions
No full textAs additive manufacturing of metals gains traction for demanding applications, more comprehensive material cards covering mechanical response across a broader spectrum of operating conditions are needed. The integration of additive manufacturing into industries that rely on aluminum alloys, notably automotive and aerospace, underscores the imperative of a profound comprehension of how these materials respond to mechanical loading at elevated temperatures. Such insights are not only important for powertrain components but also for parts that combine structural and functional purposes, such as heat exchangers. At the same time, automotive applications need to target the production of large parts with sufficiently high productivity. This study addresses the intricate interplay between microstructural evolution, plastic deformation and mechanical response of AlSi7Mg parts fabricated by laser powder bed fusion under high-productivity conditions, spanning a testing temperature range of 25-300 degrees C. Above 150 degrees C, a significant decrease in proof and tensile strength is measured, accompanied by localized necking and the formation of dimples on the rupture surfaces. At 300 degrees C, the pronounced plasticization leads to yielding and failure at stress values 30-40% lower than at room temperature, with triple ductility. Work-hardening coefficients were calculated to describe the plastic regime. Furthermore, an investigation into density, hardness, microstructure, and fracture surfaces was conducted to corroborate the mechanical response. The outcomes enabled the quantification of mechanical property variations across the 6 temperature intervals, thereby constructing a map that empowers industry to unlock the full potential of additive manufacturing aluminum alloys
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