The role of curvatures in damage identification

Natural frequencies are the observed quantities in most of the pioneering works in vibration-based damage identification; they can be reliably measured, but as a global property exhibit a limited sensitivity to local damage, mainly in the early damage state. Thus, other modal quantities - modal shapes and modal curvatures - increasingly became the focus of attention. In particular, curvatures have always been very attractive because they are directly related to damage and show notable variations in the damaged area. However, their use is not as simple as it first appears for several reasons. This paper is devoted to better understand the role of curvatures in the identification procedure of localized (notch-type) damage in beams, based on strain measurements. The following numerical and experimental issues related to the use of modal curvatures are dealt with: the definition of the measured quantities by means of local strain sensors, the evaluation and normalization of experimental quantities, their contribution to locate damage in baseline-free procedures, and finally the uniqueness of the solution of the inverse problem of damage identification based on modal curvatures variations only. This investigation is supported by experiments carried out on a beam with an open crack with different depths.