Traditional structural health monitoring (SHM) techniques are vulnerable to factors such as temperature change and vibration noise that are not relevant to structural damages. To overcome these drawbacks, this paper develops a reference-free crack detection method based on Lamb wave mode conversion. Neither baseline data nor damage threshold is required in this method. According to PZT polarization characteristics, feature signals which contained crack-lead Lamb wave converted modes are obtained from PZT components of two sets of side-by-side, and their amplitudes are obtained in the frequency domain to represent signal energy. Whether there being a crack is judged by comparing energy of each feature signal. Simulation and experiments show that the proposed method is not sensitive to optimal excitation frequency and sampling time, therefore it has strong robustness and applicability
