Study on Adhesive Bonds Influence on EMI Signatures

Abstract

The reported research was focused on carbon fibre reinforced polymer (CFRP) material. It has been used automotive, aviation and other branches of industry. One of the joining methods of the CFRP elements is based on adhesive bonding. This bonding method is either used in manufacturing and for composite repair patches. The performance of adhesive bonds depends on the physico-chemical properties of the surfaces to be joint. The weak bond can be caused by improper preparation of these surfaces and the joint itself. Contamination or thermal treatment can lead to weak bonds that can endanger the integrity of the structure. The crucial issue is to assess the bond quality after bonding and while the structure with adhesive bonds is in service. In this research we employ the electromechanical impedance (EMI) method for detection and assessment of the adhesive bonds. Measurements were conducted using HIOKI Impedance Analyzer IM3570. The piezoelectric sensor mounted on the sample surface was used for excitation and sensing of the samples. Due to piezoelectric effect the electrical response of a piezoelectric transducer is related to mechanical response of the sample to which the transducers is attached to. The impedance signatures gathered by the sensor were measured up to 5 MHz. The investigation comprised of three cases: (1) study of EMI signatures of free sensors, (2) study of EMI signatures of sensors bonded to samples with good bonds, and (3) study of EMI signatures for sensors bonded to samples with weak bonds. The changes in registered spectra caused by transducer bonding were observed and analysed. Next the study was focused on the spectra changes due to presence of weak bonds. Moreover cases of varying level of weak adhesive bonds were investigated in order to study the sensitivity of the method to the level of bond degradation. For the comparison and analysis of the gathered EMI spectra numerical indexes were proposed. doi: 10.12783/SHM2015/29