Investigation of cohesive and interfacial properties of structural adhesive materials by advanced acoustic methods

This research study has examined the feasibility of using acoustic methods for evaluation of the adhesive mechanical properties. The first method is based on the measurement of bulk longitudinal sound velocity during the process of the adhesive cure reaction. Glass transition temperature Tg depends on the extent of reaction of adhesive polymerization; acoustic parameters reflect viscoelastic properties of the material. Acoustic characteristics such us sound velocity or attenuation reflect changes in the adhesive mechanical properties and predict cohesive strength of the adhesive joint. Experimental results show the validity of this assumption. Methodology for monitoring the viscoelastic properties of the adhesive was developed. It was shown that sound velocity in epoxy adhesive correlates with the cohesive strength of the adhesive.;The second method is scanning acoustic microscopy which quantitatively allows visualization of the intact adhesive/steel interface. Changes in the microstructure on the intact metal-adhesive interface were investigated. Two dimensional Fourier transforms allow us to determine the main sizes of the granular structure which is 200μm. It was shown that changes in brightness of the images correspond to changes in the reflection coefficient on the adhesive/metal during polymerization reaction. Adhesive adjacent to the interface has Young's modulus slightly higher than the adhesive in the middle of the layer. Conditions optimal for visualization of the major defects of the adhesive structure were determined. The capability of scanning acoustic microscopy to detect and dynamically monitor small changes in both structure of the metal/epoxy interface and bulk adhesive material was demonstrated.