Integrating evolutionary programming and electro-mechanical impedance method for damage identification

The electro-mechanical impedance (EMI) method is a relatively new nondestructive evaluation (NDE) method which uses the electro-mechanical coupling effect of piezoceramic Lead Zirconate Titanate (PZT) transducers to measure the force impedance of the structure. Due to the electro-mechanical coupling effect of the transducers, any changes in the mechanical impedance of the structure caused by the damage will affect the electrical impedance of the transducer. By comparing the impedance spectra of the damaged structure with the baseline, i.e., the impedance spectra for the pristine structure, the damage in the structure can be assessed. The quantity of the damage is assessed by a statistically calculated damage index, such as root mean square deviation or mean absolute percentage deviation or covariance change or correlation coefficient deviation. The EMI method has shown some advantages over the conventional NDE methods. However, it is not able to simultaneously identify the location and quantify the damage. This paper presents a technique integrating evolutionary programming and the EMI method to overcome this limitation. Essentially, the proposed technique first makes use of the EMI method to measure the variations of the electro-mechanical impedance of the structure. The damage is then identified by a system identification technique which is generally employed in the vibration-based method. Due to the numerous local optima in the search space, the traditional optimization strategies may not be able to find the correct solution. Evolutionary programming (EP) is then used as the system identification technique to find the global optimum. Thus, both the location and the quantity of the damage can be simultaneously identified. In order to enhance the integrated EP and EMI method, a fitness function, which can be generally applied to the other methods, is proposed to discriminate the variations caused by damages from the discrepancies caused by modelling errors. An experimental test is carried out on a cylindrical shell specimen to verify the damage detection results. The experiment demonstrated that both the location and the extent of damage can be simultaneously identified by the integrated EP-EMI method.