Wavelets Transform is a method which converts a time response presentation in 1D- space into a time-frequency response in 2D-space. The main advantages of this transformation are its amplified functional and its 2D analysis presentation. The present paper describes the application of this method for the detection of nonlinearities and damages in structural dynamic systems. The main advantages of this new detection method can be summarized as: 1) It is important that the occurrence of an initial damage be identified for a structural systems. In general, the initial damages are very small the proposed method has the advantage of "high sensitivity" in detecting very small changes; 2) It is assumed that a structure behaves linearly prior to damage, but afterward it may change to a nonlinear structure. However, this method can be applied to detect either strong or weak nonlinearity existed in the structure based on input/output data of the system. In general, it is more important to detect weak nonlinearity since it is the case at initial damages.The proposed method is illustrated by a simulated five-degree-of-freedom structural system. It is assumed that the original state of the structural system is linear before it is damaged, then it is changed to a Duffing' s nonlinear system after it is damaged. It is worth to note that Duffings nonlinearity can be used to simulate the yielding phenomenon of materials.
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