Thank you for replying!..I collected both time and voltage (scaled to acceleration) data. Hence I do have data about time interval also. But I have used 1024 datapoints to calculate FFT in Matlab. Then I calculated FFT magnitude, i.e. absolute of FFT. The amplitudes you are seeing in the figure...
I have collected time-domain acceleration signals from the test rig through an accelerometer. Voltage signals are scaled to acceleration signals based on voltage sensitivity of accelerometer. Please refer the link given at the last to get the link for PDF attachment to see the time domain...