5,466 research outputs found
Nonlinearly stable equilibria in the Sun-Jupiter-Trojan-Spacecraft four body problem
No full textThe Trojan asteroids have been highlighted as a main target for future discovery missions, which will enable key questions about the formation of our Solar system to be answered. Programs like the Japanese Jupiter and Trojan Asteroids Exploration Programme are already testing technology demonstrators like the IKAROS spacecraft to enable future interplanetary missions to Jupiter and the Trojans. In this paper an analytic analysis of the stability of the Low thrust Sun Jupiter Asteroid Spacecraft system, is presented, from a Hamiltonian point of view. Setting the three primaries in the stable Lagrangian equilateral triangle configuration, eight natural (i.e. with zero thrust) equilibrium points are identified, four of which are close to the asteroid, two stable and two unstable, when considering as primaries the Sun and any other two bodies of the Solar System. Artificial equilibria, which can be seen as low thrust perturbations of the natural ones, are then taken into account with the aim of identifying their linearly stable subset. The Lyapunov stability of these marginally stable points is then analysed using basic KAM (Kolmogorov Arnold Moser) theory and Arnold’s stability theorem. In order to apply such theorem an iterative procedure to reduce the Hamiltonian into Birkhoff’s Normal Form is applied up to fourth order, explicitly defining, at each step, the generating function of a symplectic transformation. Despite the complexity of this process, Normal Forms are a fundamental, necessary step for any application of KAM theory; such theory, transforming a non-integrable system into a sum of perturbed integrable ones, enables the computation of a high order analytical approximation of the system dynamics, plus an estimation of the discrepancy from the initial model. As an application of KAM theory, a proof of the nonlinear stability for the low thrust generated equilibrium points under non resonant conditions is found using Arnold’s stability theorem. Results show that Lyapunov stability is guaranteed along the linearly stable domain with the exception of a set of points with zero measure where the conditions to apply Arnold‘s theorem are not satisfied
Determination of a cohesive law for polymers from three-point bending experiments
No full textIl comportamento a frattura dei materiali polimerici può essere
efficacemente descritto mediante l’uso di modelli coesivi, basati
su una legge che correla sforzo e apertura della cricca. In questo
lavoro si applica un metodo per l’individuazione della legge
coesiva di due polimeri amorfi a partire da prove sperimentali di
flessione a tre punti
Effect of loading history and material’s structural state on the yield onset of a polymethylmethacrylate
No full textConstant strain rate tests at different rates and stress-relaxation tests at different intensities of the applied strain were performed on a polymethylmethacrylate under uniaxial loading conditions. Shear yielding onset was properly determined for each of the applied loading histories. Following previous works, different quantities were evaluated at the onset of yielding to identify a parameter, which could be fit for the definition of a yield criterion that takes into account the effect of the mechanical history. The influence of the material structural state on the parameters’ values at the onset of yielding was examined too
Dilatometric Behavior and Glass Transition in a Styrene-Acrylonitrile Copolymer
No full textEquilibrium and glass transition behavior of a styrene-acrylonitrile copolymer (SAN) under different thermobaric histories were studied by means of a PVT dilatometer. Equilibrium behavior, as determined by isothermal and isobaric measurements, could be satisfactorily described using the Simha-Somcynsky and Tait equations of state. Glass transition behavior depended upon the applied transformation path from the liquid-equilibrium state to the glassy state. From isobaric cooling ramps performed at constant rate and at several pressures, it was possible to determine the glass transition temperature and its dependence upon pressure; whereas from isothermal compressions at various temperatures, it was possible to determine a glass transition pressure and its temperature dependence. Both the dependences were linear, and a correlation was observed between the slopes of the fitting lines. A possible interpretation of this correlation is provided in terms of free volume determined at the glass transition point by applying the Simha-Somcynsky theory
The standardisation of the EWF test
No full textA description of the background to the essential work of fracture (EWF) test method is given and its relation to J tests. The scheme used in the proposed standard is outlined including the various constraints which have evolved during the round-robin testing exercise. Also given are criteria for excluding data points based on stresses and deviations from linear fits. Two sets of data from eight groups are then reported and the problem of significant variations in the extrapolated values of the essential work we is explored. Apparently satisfactory data gave values outside ±S, the standard error of the average. It is suggested that a notch sharpness criteria (20lm) and either an S value (3kJ/m2) or a correlation coefficient R2 value (0.98) needs to be imposed to give a satisfactory procedure
Shear yielding threshold and viscoelasticity in an amorphous glassy polymer: a study on a styrene-acrylonitrile copolymer
No full textThe framework of the present research is the study of the effects of viscoelasticity on the yield behaviour of amorphous glassy polymers. The research regards the shear yielding mechanism in an amorphous glassy polymer and the identification of a yield criterion, which takes into account the effect of mechanical history on the yield point (defined as the onset of plastic strain), is set as its final goal. To this end, constant strain-rate, creep and stress relaxation tests were carried out at room temperature under uniaxial compression conditions on a styrene-acrylonitrile copolymer. The results obtained emphasise that yield onset cannot be identified by a critical value of the applied stress or strain. Referring to models of the plastic deformation mechanism previously proposed in the literature, the anelastic strain, the anelastically stored energy and the viscoelastic energy were suggested to reach a threshold value at the yield point. All three parameters were evaluated at the yield threshold and showed fairly constant values irrespective of the mechanical history. Nevertheless, the data dispersion and the simplicity of the analogical model used to estimate these quantities did not allow to identify which of them actually controls the yield process
Fracture of rubber-toughened poly(methylmethacrylate): measurement and study of cohesive zone parameters
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