Procedia IUTAM最新文献

The influence of the laser-beam radial distribution of the energy flux density is theoretically studied for the Gaussian distribution (mode TEM₀₀), and doughnut distribution of TEM_01* mode for the values of the Peclet number from 0 to 3. The model of linear thermal conduction in the target indicates that profile TEM₀₀ is the best for thermo-activated treatment processes that can be accomplished in a wide temperature range and profile TEM_01* can be advantageous for a narrow range of the permissible processing temperature. If the phase transitions of melting/solidification and evaporation are included into the model, the estimate of the width of the laser-treated band is reduced but the tendencies predicted by the linear model are not changed.

In rigid-body mechanics, models that capture collisional contact as an instantaneous exchange of momentum may exhibit dynamics that include infinite sequences of impacts accumulating in finite time to a state of persistent contact, often referred to as chatter. In this paper, we review theoretical tools for the analysis of transient and steady-state behavior in the vicinity of critical periodic orbits for which chatter terminates at a point corresponding to the imminent release from persistent contact, and illustrate the application of this theory to a simplified model of a mechanical pressure relief valve. A general theory for single-degree-of-freedom impact oscillators, previously described in an unpublished manuscript by Nordmark and Kisitu¹, is shown to yield both qualitative and quantitative agreement with model simulation results. The predicted bifurcation structure shows that the border orbit unfolds supercritically into a universal cascade of local attractors with nontrivial scaling relationships.

Study of networks with multiple agent dynamics has been a central focus in systems and control community, with numerous results. Among them is the study of how time delays in sensing and actuation could affect network stability, and/or control design for the agents. However, little is known in published work regarding how to design agents’ protocols analytically to achieve fast stability reaching. Here, an approach is developed to optimize the rightmost roots of a class of large scale LTI network dynamics via Lambert W function, in order to accomplish this. Results indicate that a delay explicitly present in the controller as a coefficient can create conditions for arbitrary rightmost root assignment. This feature is scalable, can be implemented with ease, and applied effectively for relatively large/small delays.

Accurate forecasts of extreme storm surge water levels are vital for operators of major ports. Existing regional tide-surge models perform well at the open coast but their low spatial resolution makes their forecasts less reliable for ports located in estuaries. In December 2013, a tidal surge in the North Sea with an estimated return period of 760 years partially flooded the Port of Immingham in the Humber estuary, on the UK east coast. Damage to critical infrastructure caused several weeks of disruption to vital supply chains and highlighted a need for additional forecasting tools to supplement national surge warnings. In this paper, we show that Artificial Neural Networks (ANNs) can generate better short-term forecasts of extreme water levels at estuarine ports. Using Immingham as a test case, an ANN is configured to simulate the tidal surge residual using an input vector that includes observations of surge at distant tide gauges in NW Scotland, wind and atmospheric pressure, and the predicted astronomical tide at Immingham. The forecast surge time-series, combined with the astronomical tide, provides a boundary condition for a local high-resolution 2D hydrodynamic model that predicts flood extent and damage potential across the port. Although the forecasting horizon of the ANN is limited, 6 to 24 hour forecasts at Immingham achieve an accuracy comparable to or better than the UK national tide-surge model and at far less computational cost. Use of a local rather than a larger regional hydrodynamic model means that potential inundation can be simulated very rapidly at high spatial resolution. Validation against the 2013 surge shows that the hybrid ANN-hydrodynamic model generates realistic flood extents that can inform port resilience planning.

The interactions between plasticity and damage mechanisms are not clearly established concerning the fracture of ductile sheet materials (e.g., flat to slant transition). The question addressed herein is to elucidate which mechanism is responsible for localized phenomena leading to the final failure. A mechanical test carried out on a notched plate made of 2139-T3 aluminum alloy is imaged thanks to synchrotron laminography at micrometer resolution. Ductile damage (i.e., void nucleation, growth and coalescence) is analyzed via reconstructed volumes. Although the low volume fraction of secondary phases in the tested alloy is challenging, digital volume correlation is also utilized to measure displacement fields and estimate strain fields in the bulk of the alloy during the whole test. In the first part of this study, the resolution of the measurement technique is assessed under such conditions. Then strained bands are shown to occur very early on in what will be the slant region of the fracture path. Conversely, damage grows at very late loading steps.

The structural and mechanical properties of ZnO nanowires (NWs) have been systematically investigated by using molecular dynamic simulations based on the empirical Buckingham potential. Under tensile loading in <0001> direction, ZnO NWs undergo four-stage deformation: elastic stretching of initial Wurtzite structure, Wurtzite to body-centered tetragonal (BCT) phase transformation, stretching of the resulting BCT structure and eventually brittle fracture. The entire deformation process is significantly size dependent. As the NW size decreases, the Young's modulus dramatically increases. The critical stress for both phase transformation and fracture decreases while the critical strain increases with increasing the NW size; both converge to constant values when the size is sufficiently large. The strain energy density for the initiation of phase transformation appears independent of the size, which implies that the size-dependent phase transformation is dominated by the size effect of the Young's modulus.

A system of optical monitoring and diagnostics of selective laser melting process with alternative beam power density distributions is developed. The experimental work with input parameters variation showed the correlation between obtained power density distribution and geometrical parameters of single tracks. Technological gaps of stable track formation for different distributions have been detected, and high-speed process photography have been realized.

During machining, the use of variable helix tools can potentially improve the system's stability to regenerative chatter. However, this configuration of tool has a distributed time delay, which makes the stability analysis more complex. The analysis is further exacerbated by the time-periodic coefficients that occur during milling. The present contribution demonstrates how the Fourier transform and harmonic transfer function approach can be used to analyse the system stability. This provides new insight into the stability of these tools, based on a mathematically elegant approach that makes extensive use of the shift theorem.

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Experiments designed by students at any level is a fun way to learn physics. Laboratory activities can help students acquire, integrate and construct knowledge in a friendly way. For the last 15 years, we have promoted experimental work as projects in theoretical courses. The enthusiasm students have shown, even though they have to spend many extra hours, contrasts their dislike for traditional laboratory courses where they follow recipes. In this paper, we present the original project and its influence in other courses and in new curricula. We will not address the epistemological aspects [1] and give only a slight overview of pedagogical advantages of this approach.