Aging characterization of adhesives and bonded joints by non-destructive damping measurements

Long term performance of adhesive joints in various environments is the main concern of bonding technology in both military and civil applications. Joint strength can be greatly degraded by aging. Periodic non-destructive strength evaluation of bonded devices is, thus, a critical necessity for quality assurance. Developing a tool for non-destructive testing of joint strength in bonded devices is the long-term goal of the present study. The method revealed here is application of vibration analysis, concentrating on the damping behavior of the tested object. Contrary to other non-destructive evaluation methods, which indicate only the presence of defects, this method is sensitive mainly to the material properties, thus can assess material degradation, In this case, the property of interest is the strength. In stage 1 of this research it was shown that aging of adhesives and bonded joints can be evaluated non-destructively by vibration analysis, based on the internal friction (damping) effect. Good correlation between the specific damping capacity (SDC) and the shear strength of bonded joints was determined. In stage 2, an additional step towards the target application was achieved, by broadening the range of the tested specimen. Additional adhesives and adherends as well as various geometries and sizes of the bonded joints were tested. The effect of the following factors was established: adhesive aging or shear strength, adherends, temperature, sample size [up to diameter of 356 mm (14 inches)] and sample shapes (flats and rings). The results show that the method is most promising. For more reliable wide-ranging guidelines, theoretical analysis by numerical simulation of vibrational stresses is required.