In the context of structural health monitoring, the Fast Fourier Transform (FFT) is an algorithm used to analyze the frequency content of a measured signal, such as vibration data collected from a structure. The FFT can be used to identify the natural frequencies of a structure, which can indicate the presence of damage or defects. It is a computational tool to convert time domain data into frequency domain data.
In the graph below we are looking at accelerometer data form an accelerometer on a suspension bridge hanger. From the data it can be hard to tell at which frequency the largest movements are occurring in the hanger. Using an FFT analysis the data now shows energy in the movement at each frequency step. In Figure 2 we see how there are spikes at every frequency with more energy showing the different natural frequencies of the hanger.
In the acceleration data from the hanger in Figure 2 the FFT suggests that the first mode is at roughly 0.6 Hz. The reason the first mode is not the largest peak on the graph is due to the placement of the accelerometer close to the bottom of the cable.