Damage Assessment from the Dynamic Response of Deteriorating Structures

Abstract

The demand for the residual life determination of a structure is a topic growing in importance, due to the high rehabilitation costs in the case of late damage detection. To this end condition monitoring techniques reveal an essential tool for planning repair or strenghtening and, as long as non destructive evaluation (NDE) methods are applicable, they are usually prefereable with respect to those requiring more or less extensive sampling. Among the NDE methods special attention seems to be paid to dynamic testing, because of its execution speed and capability to work with ambient vibrations [1],[2]. The paper subject pertains to this latter field and aims to present a monitoring technique devoted to provide a synthetic index of the structural state, whose value, if critical, must address towards more refined and oriented investigations. Within this context, it has found advantageous not to try to identify the remaining load carrying capacity directly, but to split the problem into two stages. Initially the dynamic behaviour of the structure is exploited and global damage symptoms are introduced in order to identify the damage effects at least in an overall sense; subsequently further problem dependent quantities, responsible for the damage origin and growth, are derived from the damage indicators and used for safety assessment purposes. Since this latter stage corresponds to a conventional design process, the critical point in the safety assessment problem is reduced to the identification of the dynamic properties of the structure. To this end, signal analysis procedures able to deal directly with the dynamic responses arising from almost arbitrary nonconservative, nonlinear and time — variant systems are used. A time domain approach is followed in either case and the Hilbert transform or the Wigner distribution is used depending whether the system allows for the uncoupling of the vibrating contributions or not. Accuracy and reliablity of the two approaches are discussed by the help of some numerical simulations, whereas their effectiveness is tested against an actual case study.