Non-linear vibration technique for crack detection in beam structures using frequency mixing

This paper describes a new technique for detection of fatigue cracks in beam-like structures. It is based on the mixing of two excitations frequencies caused by the inherent nonlinearity of the crack. As a result, response components at frequencies equal to the sum and difference of the excitation frequencies are generated. It is shown that the amplitude of the response at the difference frequency increases with increasing crack depth. By employing bispectral analysis a relation between bispectrum value and crack size is established, which can be used for crack size evaluation. Experimental results, in good agreement with predictions, show that cracks of 7% relative crack depth can be reliably detected. The proposed technique is superior compared to linear vibration methods exhibiting high detection sensitivity and can be used for crack detection on early stage.