The tensile behaviour of a commercial grade of isotactic polypropylene was tested in a temperature range between 20 and 150 °C with a video-controlled testing system which is capable of imposing a constant true strain-rate within the neck automatically. The results are displayed in the form of effective stress-strain curves and modelled by a constitutive equation in a multiplicative form. It is thus shown that, for each temperature, the plastic response can be described up to very large strains ( 2.0) by a set of four parameters. The assumptions introduced in this modelling are critically discussed in order to check the validity of the simplifying hypotheses (strain homogeneity, isochoric deformation, etc.). The constitutive equation thus obtained was utilized in a finite difference code in order to predict the development of stretching instabilities of polypropylene. The simulation gives access to the engineering stress-strain response of the stretched test piece and to the detailed kinetics of the incipient neck. It is found that the severity of the instabilities is less at room temperature than near the melting point because of the decrease of the strain-hardening and of the strain-rate sensitivity with temperature. 相似文献
Photo- and charge-carrier-induced ion migration is a major challenge when utilizing metal halide perovskite semiconductors for optoelectronic applications. For mixed iodide/bromide perovskites, the compositional instability due to light- or electrical bias induced phase-segregation restricts the exploitation of the entire bandgap range. Previous experimental and theoretical work suggests that excited states or charge carriers trigger the process, but the exact mechanism is still under debate. To identify the mechanism and cause of light-induced phase-segregation phenomena, the full compositional range of methylammonium lead bromide/iodide samples are investigated, MAPb(BrxI1-x)3 with x = 0…1, by simultaneous in situ X-ray diffraction (XRD) and photoluminescence (PL) spectroscopy during illumination. The quantitative comparison of composition-dependent in situ XRD and PL shows that at excitation densities of 1 sun, only the initial stage of photo-segregation is rationalized with the previously established thermodynamic models. However, a progression of the phase segregation is observed that is rationalized by considering long-lived accumulative photo-induced material alterations. It is suggested that (additional) photo-induced defects, possibly halide vacancies and interstitials, need to be considered to fully rationalize light-induced phase segregation and anticipate the findings to provide crucial insight for the development of more sophisticated models. 相似文献
Wireless Personal Communications - The Invisible Internet Project (I2P) as a secure protocol uses robust mechanisms and strong algorithms to reinforce the security and the anonymity of the... 相似文献
The joint estimation of the location vector and the shape matrix of a set of independent and identically Complex Elliptically Symmetric (CES) distributed observations is investigated from both the theoretical and computational viewpoints. This joint estimation problem is framed in the original context of semiparametric models allowing us to handle the (generally unknown) density generator as an infinite-dimensional nuisance parameter. In the first part of the paper, a computationally efficient and memory saving implementation of the robust and semiparmaetric efficient R-estimator for shape matrices is derived. Building upon this result, in the second part, a joint estimator, relying on the Tyler’s M-estimator of location and on the R-estimator of shape matrix, is proposed and its Mean Squared Error (MSE) performance compared with the Semiparametric Cramér-Rao Bound (SCRB).
Journal of Automated Reasoning - Algebraic datatypes, and among them lists and trees, have attracted a lot of interest in automated reasoning and Satisfiability Modulo Theories (SMT). Since its... 相似文献
Recent advances in design theory help clarify the logic, forms and conditions of generativity. In particular, the formal model of forcing predicts that high-level generativity (so-called generic generativity) can only be reached if the knowledge structure meets the ‘splitting condition’. We test this hypothesis for the case of Bauhaus (1919–1933), where we can expect strong generativity and where we have access to the structures of knowledge provided by teaching. We analyse teaching at Bauhaus by focusing on the courses of Itten and Klee. We show that these courses aimed to increase students’ creative design capabilities by providing the students with methods of building a knowledge base with two critical features: (1) a knowledge structure that is characterized by non-determinism and non-modularity and (2) a design process that helps students progressively ‘superimpose’ languages on the object. From the results of the study, we confirm the hypothesis deduced from design theory; we reveal unexpected conditions on the knowledge structure required for generativity and show that the structure is different from the knowledge structure and design process of engineering systematic design and show that the conditions required for generativity, which can appear as a limit on generativity, can also be positively interpreted. The example of Bauhaus shows that enabling a splitting condition is a powerful way to increase designers’ generativity. 相似文献
This paper proposes to decompose the nonlinear dynamic of a chaotic system with Chebyshev polynomials to improve performances
of its estimator. More widely than synchronization of chaotic systems, this algorithm is compared to other nonlinear stochastic
estimator such as Extended Kalman Filter (EKF) and Unscented Kalman Filter (UKF). Chebyshev polynomials orthogonality properties
is used to fit a polynomial to a nonlinear function. This polynomial is then used in an Exact Polynomial Kalman Filter (ExPKF)
to run real time state estimation. The ExPKF offers mean square error optimality because it can estimate exact statistics
of transformed variables through the polynomial function. Analytical expressions of those statistics are derived so as to
lower ExPKF algorithm computation complexity and allow real time applications. Simulations under the Additive White Gaussian
Noise (AWGN) hypothesis, show relevant performances of this algorithm compared to classical nonlinear estimators. 相似文献
Authentication and authorization for Grids is a challenging security issue. In this paper, key issues for the establishment
of Grid authentication and authorization infrastructures are discussed, and an overview of major Grid authentication and authorization
technologies is presented. Related to this, recent developments in Grid authentication and authorization infrastructures suggest
adoption of the Shibboleth technology which offers advantages in terms of usability, confidentiality, scalability and manageability.
When combined with advanced authorization technologies, Shibboleth-based authentication and authorization infrastructures
provide role-based, fine-grained authorization. We share our experience in constructing a Shibboleth-based authentication
and authorization infrastructure and believe that such infrastructure provides a promising solution for the security of many
application domains. 相似文献
Many causes can be at the origin of hip osteoarthritis (e.g., cam/pincer impingements), but the exact pathogenesis for idiopathic
osteoarthritis has not yet been clearly delineated. The aim of the present work is to analyze the consequences of repetitive
extreme hip motion on the labrum cartilage. Our hypothesis is that extreme movements can induce excessive labral deformations
and lead to early arthritis. To verify this hypothesis, an optical motion capture system is used to estimate the kinematics
of patient-specific hip joint, while soft tissue artifacts are reduced with an effective correction method. Subsequently,
a physical simulation system is used during motion to compute accurate labral deformations and to assess the global pressure
of the labrum, as well as any local pressure excess that may be physiologically damageable. Results show that peak contact
pressures occur at extreme hip flexion/abduction and that the pressure distribution corresponds with radiologically observed
damage zones in the labrum.