1,720,972 research outputs found
A Numerical Scheme for Integrating the Rate Plasticity Equations with an a Priori Error Control
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Validation of a numerical model for the mechanical behavior of a continuous positive airway pressure mask
Full text linkFinite Element models (FEM) are developed for the analysis of the contact pressures exerted by a Continuous Positive Airway Pressure (CPAP) mask applied to a dummy head. This is seen as a preliminary step in the analysis of the mechanical effects of CPAP masks applied to human faces, such as recently employed for the care of COVID-19 patients, or other purposes. These mechanical effects can range from negligible, in the case of correct positioning, sufficiently light tension in the headgear, correct mask design, etc., to the possible development of device-related pressure ulcers and/or dentofacial deformations, especially in children. The results of Finite Element analyses are compared, for their validation, with experimental ones. The numerical analysis tool appears able to predict, at an acceptable cost, both the intensity and the area distribution of the contact pressures, as well as the force-displacement relationship occurring in the headgear. This might help the design and the production of more effective and tolerable CPAP masks
Finite element analysis of fluid phase nonlinearity effects on the undrained dynamic behaviour of nearly saturated porous media
No full textA simple time integration scheme is presented, able to take into account a particular nonlinearity of the fluid phase in a dynamically loaded, nearly saturated porous medium. The analysis is
confined to the dynamic undrained case, under the assumptions of small displacements and linear elastic behaviour for the solid phase. A mixed finite element approach is adopted, in conjunction with an explicit time integration scheme. The resulting overall algorithm may be theoretically framed within the context of the Linear Inequalities. Some applications indicate that the nonlinear behaviour of the fluid phase alone may play an important role in the global two-phase medium response. The results obtained by means of more traditional approaches in some cases may be un conservative
On the construction of extended problems and related functionals for general nonlinear equations
No full textStarting from existing methods for the symmetrisation of general nonlinear, nonpotential operators (Tonti, Int. J. Engng. Sci. 22 (11–12) (1984) 1343–1371; Auchmuty, Nonlinear Anal. Theory Methods Appl. 12 (5) (1988) 531–564) this work discusses some alternative formulations and illustrates some significant implications of such methods, which should make them more suited to practical application. Further, a new class of the so-called “extended” functionals is proposed, much simpler to construct than the preceding ones. Even if the definition of the functionals requires the doubling of the unknown functions, the new unknowns have a precise physical meaning in the solution of the problem, which may help in the actual solution process. The application of the new method is illustrated by means of two examples in the field of continuum mechanics: the nonassociated elastic–plastic rate constitutive equations, and the nonlinear continuum dynamics equations with initial conditions
Time domain analytical bounds to the homogenized viscous kernels of linear viscoelastic composites
Full text linkNew extremum principles in linear viscoelasticity are derived from general stationarity ones proposed in Carini and Mattei (2015), exploiting suitable selections of the admissible fields in the associated convolutive functionals. These new extremum principles have therefore a restricted validity. Analytical bounds to the homogenized viscous kernels of linear viscoelastic composites are derived in the time domain. In the restricted case of macroscopically isotropic composite materials, six new bounds are obtained from the new extremum principles. These bounds can be derived exploiting the choice of Representative Volume Elements (RVEs) loaded in a purely deviatoric way only. Two strict lower bounds to the homogenized viscous kernels, of the Reuss type, are also derived. One of these was already proposed in Huet (1995), and is valid for generic linear viscoelastic composites under general stress and strain states. The other Reuss-type strict lower bound is new, but has the same limited validity as the first six ones. The new upper bounds for isotropic composites, obtained both in terms of viscous kernels and of their time rates, as well as the strict lower bounds, are extensions to viscoelasticity of the Voigt and Reuss bounds for linear elastic composites. The performance of the obtained bounds is checked by comparison with numerical solutions. It is worth remarking that the use of the new extremum theorems for the purpose of deriving bounds is not possible in a general RVE stress case, i.e., deviatoric plus volumetric. As a consequence, the non-strict bounds holding for the case of deviatoric RVE loading are not valid for the volumetric case. The reason for this difference is not yet fully understood
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
On the effectiveness of convolutive type variational principles in the numerical solution of viscoelastic problems
Full text linkWith reference to the nonaging linear viscoelastic problem, three convolutive type variational formulations existing in the literature are critically reviewed: the Gurtin formulation, the split Gurtin formulation and the Huet formulation. The formulations are used for the numerical solution of the hereditary viscoelastic problem through spatial and temporal discretizations considering both a finite time range and using a step-by-step method of time marching. Several numerical examples are included and numerical results are compared with the aim of investigating the effectiveness of the variational formulations in the numerical solution of the viscoelastic problem
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