On the Use of Mode Shapes in the Identification of Structural Damage

The random decrement technique is an averaging technique that can be used to extract the free decay response of a structure from its stationary random vibratory response. The free decay response can then be used to identify the vibratory characteristics of the structure. The main advantage of the technique is that the identification of the parameters of the structure is achieved without previous knowledge of the excitation forces. This paper extends the random decrement technique to obtain the mode shapes of the structure using the concept of a multichannel random decrement technique (MCRD). This technique is based on extracting simultaneous random decrement records from measurements made at several points on the structure. The change in mode shapes as a result of occurrence of damage to the structure is then used to detect the presence and location of damage. The method is illustrated and validated using two case studies and finite element modeling. Excellent agreement between mode shapes obtained using the MCRD and those obtained from classical modal analysis techniques was found. The use of changes in the mode shapes of the structure to detect the presence and location of damage was successful.