Damage assessment in concrete beams using non-linear analysis of vibration measurements

Many researchers have proposed methods of assessing structural damage by analysis of vibration measurements. Most approaches involve comparing vibration signals measured on the structure before and after damage, with both signals analysed using linear methods. For concrete structures, there is evidence that the actual behaviour is non-linear even at very low amplitudes of excitation. A possible alternative method is therefore to assess damage by estimating the degree of non-linearity present within a given response measurement. In this paper, tests are reported on simply supported, reinforced concrete beams subjected to increasing levels of flexural cracking at midspan. At each, damage level, the degree of non-linearity is assessed by examining the variation of measured natural frequency with vibration amplitude, as determined by time-frequency analysis of transient vibrations. The results showed a sharp increase in non-linearity due to initial cracking, but then only quite small changes as the cracks grew. At very high damage levels there appeared to be a reduction in the degree of non-linearity, indicating that the non-linear mechanism present in the system was breaking down. On the basis of these findings, it is concluded that measures of non-linearity may be useful in detecting the initiation of damage, but are unlikely to be helpful in differentiating between intermediate damage levels.