A baseline free approach for multiple damage detection in beams

Abstract

The presence of a single damage in structure tends to change its modal parameters (natural frequency and mode shapes).This fact is used by many researchers to predict the damage parameters i.e. damage depth and its location by measuring the changes in the modal parameters, by establishing a functional relationship. But in many cases, the severity of damage has to be large enough to be detected, thus making minute damages difficult to detect. In addition, the absence of baseline data poses problem for structures constructed long back when the concept of structural health monitoring was not in mainstream. The objective of this work is to develop and present a technique for identifying the location of multiple structural damages in a beam using the present modal data only. The method operates solely on the mode shape from the damaged structure and does not require a priori knowledge of the undamaged structure, thus developing a baseline free procedure. Multiple damages are induced for a simply supported beam by considering flexural vibrations and including two hairline crack of 0.08 mm at the bottom of the beam. Modal analysis has been done by carrying out parametric studies using ANSYS software to evaluate the natural frequencies and the corresponding mode shapes for different damage parameters (severity and location) of the simply supported beams. Secondly, curvature mode shapes are determined using finite difference approximation from displacement mode shapes. These curvature mode shapes can locate the damage when its thickness reduction is greater than 10%. However, for less severe damage, further processing of curvature mode shapes is required before damage can be located. Curve smoothening is done by use of cubic polynomial to the finite difference approximation to get the healthy signature of the beam. Post-processing of the damaged signature by determining the value of the intercept between the damaged signature and the cubic curve gives us the difference function which can be used to quantify the damage and compare its extent with other cases of damage. It is also concluded that this procedure is best suited to the mode shape obtained from the fundamental natural frequency. The mode shapes from lower natural frequencies are more sensitive to detect the damage as compared to higher frequencies modes. Experimental studies are also under way on real size I shaped simply supported steel beam to verify the simulation findings.