Using non-linear vibration techniques to detect damage in concrete bridges
There has been much work published in recent years on the use of vibration characteristics to detect damage in bridges. Almost all of this work has been based on the assumption that the vibration is linear, i.e. the natural frequencies are not dependent on the amplitude of oscillation. The aim of the work presented here was to investigate the possibility of using changes in the non-linear vibration characteristics to detect damage in reinforced concrete bridges. These changes in the non-linear vibration characteristics were studied by conducting impact excitation vibration tests o reinforced concrete beams. The non-linearities were detected by examining the changes in fundamental frequency over time (and hence over amplitude of vibration). Several time-frequency distribution estimation tools are discussed including the discrete Fourier Transform moving window, the auto-regressive model moving window, harmonic wavelets and examples of the Cohen class of bilinear time-frequency distributions. A detailed investigation into these various distribution predictors was conducted to assess which is most suitable for analysing the vibration signals to detect changes in frequency with time. To understand the non-linearities in the vibration characteristics, a time-stepping model was described. The model is capable of including damage in the form of a moment-rotation relationship over the cracked region. It was validated for linear vibrations against theoretical values and the representation of a non-linear mechanism using the model was compared with experimental data. Static load tests were also conducted on the beams at various damage levels. They involved the use of vibrating wire strain gauges to investigate the moment-rotation behaviour over the cracked region. Several possible non-linear crack mechanisms are discussed and two of them are assessed using the vibration and the static load tests. Future experimental work is proposed to study the possible non-linear mechanisms further. The beam tests demonstrated that there is a change in non-linear vibration behaviour with damage. The change is greatest at low levels of damage and after the beam has been loaded to 30% of the failure load in three-point loading there is a reversal in the trend and a slight reduction in non-linearity with further damage.