Non-baseline detection of small damages from changes in strain energy mode shapes

Abstract

Several methods for damage detection based on identifying changes in strain energy mode shapes (SEMS) have been recently described in the literature. Most of these methods require knowing strain energy distribution for the undamaged structure (baseline SEMS). This is especially true for detection of small damages, where changes in the SEMS cannot be observed otherwise. Usually, the mode shapes from the structure under test should be compared to the baseline mode shapes to provide sufficient data for damage detection. However, these methods do not cover damage detection on structures where baseline mode shapes cannot be readily obtained, for example, structures with preexisting damage. Conventional methods, like building a finite element (FE) model of a structure to be used as a baseline might be an expensive and time-consuming task that can be impossible for complex structures. This paper suggests a method for extraction of localized changes (damage peaks) from SEMS based on Fourier analysis of the strain energy distribution. A detailed analytical proof is given for the case of a pinned–pinned beam and a numerical proof for the free–free beam. The analytical predictions have been confirmed both by the FE model and impact testing experiments on a free–free aluminum beam, including single and multiple damage scenarios.