{ "cells": [ { "cell_type": "markdown", "metadata": {}, "source": [ "# Análisis no-lineal aproximado" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Introducción" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "Entre las aproximaciones fundamentales de la teoría linealizada de la elasticidad se asume que no hay diferencias importantes entre la configuración original y la configuración deformada de una estructura.\n", "\n", "Nótese que si se desprecia esta hipótesis se hace necesario conocer la configuración deformada antes de poder plantear las ecuaciones de equilibrio pero, a la vez, para poder conocer la configuración deformada es necesario cargar el sistema y resolver las ecuaciones de equilibrio. En consecuencia el problema es no lineal.\n", "\n", "Este puede verse en la matriz de rigidez local de un elemento cercha:\n", "\n", "$$\n", "\\begin{Bmatrix}f_1\\\\f_2\\end{Bmatrix} = \\frac{AE}{l}\n", "\\begin{bmatrix}\n", "1&-1\\\\\n", "-1&1\\end{bmatrix}\n", "\\begin{Bmatrix}u_1\\\\u_2\\end{Bmatrix}\\, ,\n", "$$\n", "\n", "de la cual se aprecia que la rigidez depende de la longitud del elemento, la cual depende a la vez de la deformación del mismo.\n", "\n", "En este notebook utilizaremos la implementación de los elementos tipo cercha (desarrollada en NB anteriores) para resolver una estructura considerando de manera aproximada la no linealidad.\n", "\n", "En particular, abordaremos el problema de una cercha de von Mises (ver figura), sometida a una carga vertical $F$. \n", "\n", "
\n", " \"Cercha\n", "
\n", "\n", "\n", "Esta cercha tiene la particularidad de que dependiendo del valor de la carga la estructura puede desarrollar un colapso por pandeo subito e invertir su configuración. Para determinar el valor de la carga de colapso es necesario:\n", "\n", "1. Resolver el problema incrementalmente dividiendo la aplicación de la carga en varios pasos o incrementos $\\triangle F$.\n", "\n", "2. Considerar el efecto de la deformación en la solución del problema de equilibrio." ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Método de solución" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "Para resolver el problema se asumirá, en cada incremento, que la rigidez de los elementos, y por ende la rigidez de la estructura, corresponde a la configuración original antes de imponer el incremento de carga. Denotemos esta configuración y su rigidez asociada como ${}^0X$ y ${}^0K$ respectivamente y escribamos la ecuación de equilibrio en esta configuración como:\n", "\n", "$$\n", "[^0K] \\{\\triangle U\\} = \\{\\triangle F \\}\n", "$$\n", "\n", "donde $\\triangle U$ sería el desplazamiento producido por $\\triangle F$ si es que la rigidez fuera ${}^0K$.\n", "\n", "
\n", " \n", "¿De qué depende que las rigideces correspondientes a la configuración original ${}^0K$ y deformada ${}^1K$ sean diferentes?\n", "\n", "
\n", "\n", "Para verificar que tanto se esta violando la condición de equilibrio actualicemos el sistema a la configuración deformada de acuerdo con:\n", "\n", "\n", "$$\n", "X_i \\leftarrow{}^0X+\\triangle U\n", "$$\n", "\n", "y en la cual $X_i$ denota la aproximación $i$ a la configuración deformada.\n", "\n", "Recalculemos ahora la rigidez $K=K(X_i)$ en función de la nueva configuración $X_i$ y determinemos también los residuales o fuerzas sin equilibrar dadas por:\n", "\n", "$$\n", "R = \\triangle F-K(X_i)\\triangle U\n", "$$\n", "\n", "\n", "Una configuración de equilibrio se habrá encontrado cuando $R');\n", " this._root_extra_style(this.root)\n", " this.root.attr('style', 'display: inline-block');\n", "\n", " $(parent_element).append(this.root);\n", "\n", " this._init_header(this);\n", " this._init_canvas(this);\n", " this._init_toolbar(this);\n", "\n", " var fig = this;\n", "\n", " this.waiting = false;\n", "\n", " this.ws.onopen = function () {\n", " fig.send_message(\"supports_binary\", {value: fig.supports_binary});\n", " fig.send_message(\"send_image_mode\", {});\n", " if (mpl.ratio != 1) {\n", " fig.send_message(\"set_dpi_ratio\", {'dpi_ratio': mpl.ratio});\n", " }\n", " fig.send_message(\"refresh\", {});\n", " }\n", "\n", " this.imageObj.onload = function() {\n", " if (fig.image_mode == 'full') {\n", " // Full images could contain transparency (where diff images\n", " // almost always do), so we need to clear the canvas so that\n", " // there is no ghosting.\n", " fig.context.clearRect(0, 0, fig.canvas.width, fig.canvas.height);\n", " }\n", " fig.context.drawImage(fig.imageObj, 0, 0);\n", " };\n", "\n", " this.imageObj.onunload = function() {\n", " fig.ws.close();\n", " }\n", "\n", " this.ws.onmessage = this._make_on_message_function(this);\n", "\n", " this.ondownload = ondownload;\n", "}\n", "\n", "mpl.figure.prototype._init_header = function() {\n", " var titlebar = $(\n", " '
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