Detection of Adhesive Layer Degradation in Acoustic Emission Sensors using the Electromechanical Impedance for Self-Diagnosis

Acoustic emission (AE) sensors are commonly employed for testing and monitoring purposes, thereby necessitating the development of effective field-testing methods to assess their functionality during use. The ideal outcome would be a method for self-diagnosis. A novel approach to field-testing is to measure the electromechanical impedance (EMI) of the AE sensor. This method requires only minimal additional equipment and eliminates the need for human interaction, which is required for current field tests such as pencil-lead break tests. Prior research conducted by the authors demonstrated the general applicability of this approach for sensors of a specific type in various unknown conditions, as well as the influence of variations in the coupling-specific and structural parameters. To advance the use of the EMI for self-diagnosis in the field, it is essential to investigate specific common types of sensor defects. One such defect is the degradation of the adhesive layer between the piezoelectric element and the sensor's wear plate or the casing, depending on the sensor design. This work presents the findings of an experimental study, analyzing the effect of degradation of the adhesive layer using two Vallen VS150-M sensors, and a numerical study focusing on the same defect. The defects were induced by subjecting the sensors to temperatures outside of their operating temperature range. The numerical model is employed to induce a known defect in the adhesive layer and to facilitate a comparison of the resulting data with that obtained from the experimental study. Moreover, the experimental and numerical results were employed to evaluate the efficacy of self-diagnosis using the EMI spectrum for the aforementioned sensors of the type VS150-M.