Fernando G. Flores

Buckling of short tanks due to hurricanes

Abstract Buckling of thin-walled, short tanks, under severe wind conditions, is investigated using numerical methods. The shells are representative of cylindrical tanks that failed during hurricane Marilyn in the Caribbean islands in 1995, with radius/thickness ratio of 1900 and radius/height ratio of 5. Several models are employed to study instability: bifurcation buckling from a linear fundamental path, nonlinear analysis of imperfect tank, and dynamic response. Wind is modeled using several pressure distributions in the circumferential direction, and the results are compared with those due to axisymmetric pressure. The results show that for the present very thin-walled short tanks, bifurcation buckling produces good estimates of the critical state; however, the structure is imperfection sensitive and this load is reduced in the order of 30–50%.

Post-buckling of elastic cone-cylinder and sphere-cylinder complex shells

Abstract A study is presented of the elastic buckling of pressurized complex shells of revolution which are formed by the intersection of cylindrical with spherical or conical shells. The studies are based on the general theory of elastic stability using an asymptotic approximation to the secondary path. An axisymmetric finite element is used for the discretization of the shell. First, the behaviour of the individual components is presented for simply supported and clamped boundary conditions. Next, the complex shells are investigated in terms of the buckling load, the meridional and circumferential shape of the buckling mode, and the initial characteristics of the post-buckling path. The results show that the bifurcation loads of complex shells are lower than those of the individual components; but the post-critical path does not decrease as severely as in the isolated shells.

Linear versus non-linear analysis of imperfect spherical pressure vessels

Abstract A pressurized thin spherical shell with deviations in shape is analysed using a geometrically linear and a non-linear version of a finite element formulation. A local geometric imperfection, described as a cosine function, is assumed, and the influence of different parameters on the non-linear behaviour is considered. The results show the importance of the non-linear terms in the analysis of slender shells, and that a linear analysis can be highly conservative, depending on the geometric parameters which define the problem.

Stresses in Thin Spherical Shells with Imperfections. Part I: Influence of Axisymmetric Imperfections

Abstract A finite element model of axisymmetric geometry is used to obtain stress and moment fields in the region of an imperfection in thin, spherical shells. In Part I the studies are restricted to axisymmetric imperfections with a cosine variation along the meridian. Parametric studies are carried out to identify the main parameters controlling the response for internal pressure and gravity load. The results show that the behaviour of the shell is similar to the imperfect cylindrical shell with the same radius as in the spherical shell.

Stresses in thin spherical shells with imperfections. Part II: Influence of local imperfections☆

Abstract The changes in stress resultants in thin spherical shells, associated with a local imperfection introducing curvature errors in all directions, are investigated. An axisymmetric finite element model of the shell and imperfection is employed to carry out the linear elastic analysis. Parametric studies have been performed, to identify the main parameters controlling the response, for the case of internal pressure. The results are compared with those obtained in Part I for axisymmetric imperfections, and bounds for maximum elastic stress resultants are established to cover the possibility of both local and axisymmetric imperfections.

Larvicidal activity of crude extracts from Larrea cuneifolia (Zygophyllaceae) and of its metabolite nordihydroguaiaretic acid against the vector Culex quinquefasciatus (Diptera: Culicidae)

INTRODUCTION The aim of the present study was to analyze the larvicidal activity of different crude extracts of Larrea cuneifolia and its most abundant lignan, nordihydroguaiaretic acid (NDGA), against Culex quinquefasciatus. METHODS Chloroform, methanol, and aqueous extracts from L. cuneifolia and NDGA were tested against larvae of Cx. quinquefasciatus under laboratory conditions. RESULTS The chloroform extract showed the highest larvicidal effect, with an estimated LC50 of 0.062 mg/ml. NDGA also demonstrated significant larvicidal activity with an estimated LC50 of 0.092 mg/ml. CONCLUSIONS These results indicate that the chloroform extract of L. cuneifolia and NDGA are promising insecticides of botanical origin that could be useful for controlling Cx. quinquefasciatus.

Applications of a Rotation-Free Triangular Element for Finite Strain Analysis of Thin Shells and Membranes

This paper shows applications of a recently developed shell element to the analysis of thin shell and membrane structures. The element is a three node triangle with only translational DOFs (rotation free) that uses the configuration of the three adjacent elements to evaluate the strains. This allows to compute (constant) bending strains and (linear) membrane strains. A total Lagrangian formulation is used. Strains are defined in terms of the principal stretches. This allows to consider rubber materials and other type of materials using the Hencky stress-strain pair. An explicit central difference scheme is used to integrate the momentum equations. Several examples, including inflation and deflation of membranes show the excellent convergence properties and robustness of the element for large strain analysis of thin shells and membranes.

Thickness changes in pressurised shells

A pressurised thin spherical shell with reductions in thickness is analysed using a geometrically linear and a non-linear version of a finite element formulation. The change in thickness is assumed to have a circular shape and is symmetric about the middle surface of the shell. The influence of two parameters is considered on the non-linear behaviour: the change in thickness and the extent of the zone affected. The results show that the most critical stresses arise from the hoop membrane action at the edges of the zone affected. It is also shown that a linear analysis can be highly conservative in terms of bending action.

Enhanced Rotation-Free Basic Shell Triangle. Applications to Sheet Metal Forming

An enhanced rotation-free three node triangular shell element (termed EBST) is presented. The element formulation is based on a quadratic interpolation of the geometry in terms of the six nodes of a patch of four triangles associated to each triangular element. This allows to compute an assumed constant curvature field and an assumed linear membrane strain field which improves the in-plane behaviour of the element. A simple and economic version of the element using a single integration point is presented. The implementation of the element into an explicit dynamic scheme is described. The efficiency and accuracy of the EBST element and the explicit dynamic scheme are demonstrated in many examples of application including the analysis of a cylindrical panel under impulse loading and sheet metal stamping problems.

Triangular prismatic solid-shell element with generalised deformation description

The solid-shells are an attractive kind of element for the simulation of f orming processes, due to the fact that any kind of generic 3D constitutive law can be employed without any kind of additional modification, besides the thermomechanic problem is formulated without additional assumptions. Additionally, this type of element allows the three-dimensional description of the deformable body, thus contact on both sides of the element can be treated easily. The present work consists in the development of a triangular prism element as a solid-shell, for the analysis of thin/thick shell, undergoing large deformations. The element is formulated in total Lagrangian formulation, and employs the neighbour (adjacent) elements to perform a local patch to enrich the displacement field. In the original formulation by Flores, a modified right Cauchy-Green deformation tensor () is obtained; in the present work a modified deformation gradient () is obtained, which allows to generalise the methodology and allows to employ a wide range of constitutive laws. The element is based in three modifications: (a) a classical assumed strain approach for transverse shear strains (b) an assumed strain approach for the in-plane components using information from neighbour elements and (c) an averaging of the volumetric strain over the element. The objective is to use this type of elements for the simulation of shells avoiding transverse shear locking, improving the membrane behaviour of the in-plane triangle and to handle quasi-incompressible materials or materials with isochoric plastic flow. Some examples have been evaluated to show the good performance of the element and results.