Limitations of modal domain techniques in SHM

Modal methods are not very sensitive to small size delaminations which are of practical interest and can be very cumbersome and computationally expensive while implementing in practice for on-line health monitoring. In most of the methods based on modal parameters, it is assumed that the modes under consideration are affected by damages. As pointed out by Ratcliffe [2000], the changes in individual natural frequencies due to small damage may become insignificant and may fall within measurement error. In practical situation, this can considerably reduce the effectiveness of the prediction. The relative insensitivity of lower modes to damage becomes a significant problem when only a few lower modes are used in SHM. Change of structural dynamic performance caused by structural damage that is less than 1 per cent of the total structural size is unnoticeable. Yan and Yam [2002] have pointed out that when the crack length in a composite plate equals 1 per cent of the plate length, the relative variation of structural natural frequency is only about 0.01 to 0.1 per cent. Therefore, using vibration modal parameters, e.g., natural frequencies, displacement or strain mode shapes and modal damping are generally ineffective in identifying small and incipient structural damage.