ABSTRACT of the Doctoral Thesis

Lightweight Structural Methods for Increasing the Buckling Load and Fundamental Frequency of Thin-Walled Structures

by Christian Bilik (2011)

Stability loss of a structure demonstrates a mode of failure which preferentially occurs for slender or thin-walled structures as typical in the lightweight design. For increasing critical loads or enhancing safety margins for given loads, constructive measures, especially the usage of stiffeners, are often required. Similar holds for the eigenfrequency representing a dynamic characteristic of a structure. Under usage structural eigenfrequencies of a structure should not be excited in order to avoid resonance phenomena. Also, in this case, design solutions often comprise the application of stiffeners to increase the eigenfrequencies and shift them to higher frequency-domains, which are not being excited by operating states. Typically an installation of stiffeners results in a mass increase as well as a geometry change of the concerned structure.

Aim of the thesis at hand is the investigation and evaluation of different possibilities of structural modifications to enhance the buckling stability. Further goal is a raise of the eigenfrequency in terms of increasing a structure's fundamental frequency. Both of these aims should be achived without mass increase and with either no or very small changes in geometry. Finally the manufacturing process roll forming is investigated in terms of stability of the resulting product and the effects of process-related residual stresses present in the final product.

It can be shown that residual stresses, contained in a part or structure, can act as stiffeners. Structures containing advantageous residual stresses (in present case laser irradiation is used as thermal treatement to introduce residual stresses) are stiffened according to their buckling behaviour and in terms of an enhancement of their eigenfrequency without the necessity of adding additional mass. Different kinds of laser treatment result in different residual stress distributions within the affected structure. Deformations during and after the laser treatment should be kept as small as possible. Further investigations are comparisons with experimentally found results.

An introduction of beads, representing a classical, geometry-changing stiffening method for thin-walled structures is also investigated within the current work. Presently geometries in that case are predominantly gained by trial and error principle or found in literature. A bead laying algorithm for finding suitable geometry pathes for the beads is described and applied to different structures.

During the roll forming process, representing a continuous forming process of thin metall strips, the undesired occurence of instabilities during the forming process as well as in the deformed semi-finished product is possible. Additionally, residual stresses within the finished product can result in significant changes in its stability- and eigenfrequency behaviour. Both is investigated in the present work under usage of numerical methods.

revised 110922 (hjb)