Modal Analysis for Damage Detection in Structures

Nondestructive inspection of structures by modal analysis of vibration response is reported. The dependence of natural frequencies and modal damping coefficients on deterioration in structures is examined. The magnitude of change in natural frequencies is a function of the severity and of the location of deterioration in structures. Ratios of changes in natural frequencies normalized with respect to the largest frequency change are independent of severity for small deterioration and can serve to indicate the location of deterioration directly. Specific deterioration events have an associated characteristic ensemble of ratios of natural frequency changes that may be compiled in advance to form a data base for later interpretation of observed modal parameter changes. The modal analysis method is demonstrated in experiments on a welded steel frame exposed to fatigue loading, and on wire ropes damaged by sawcuts. The method holds promise as a condition monitoring tool for bridges and other skeletal structures.