Sensitivity analysis of local/global modal parameters for identification of a crack in a beam

A crack in a structure will affect the modal parameters both locally as well as globally. The present work discusses the studies carried out using those parameters that get changed locally and globally as well as a procedure to identify the crack location. Even though the crack can be located exactly by monitoring changes that occur in the parameters locally such as strain variation, etc., this method has limitations in terms of extracting these locally varying modal parameters. A new technique is presented to identify the presence of a crack and its location, from the changes that occur in a few of its lowest natural frequencies, one of the modal parameters that changes globally. The study is carried out both analytically and experimentally and the results are presented in this paper. A cantilever beam was considered and modelled with flat plate bending elements so as to introduce the crack and analyze the propagation of the crack. The crack is a through-thickness one, growing along the breadth of the beam starting from one edge. The location of the crack is also moved from the fixed end to the free end along its length. The changes in natural frequencies are observed from analytical study, due to the presence of the crack at different locations and depths, and the percentage change in frequency values are calculated. These results are confirmed by the experiments. From these observations a method is suggested to identify the location of the crack. (C) 1999 Academic Press.