Measurement of Bond Line Fracture Toughness in Adhesively Bonded Composite Structures by Nanoindentation

Abstract

This study explores a probable correlation between the degradation of bond line toughness in adhesively bonded joints and the degradation/changes in measurable mechanical properties of the adhesive layer by indentation hardness due to contamination. The proposed framework utilizes the large scale bridging of interfacial fracture proposed by Tvergaard and Hutchinson (Philos Mag A 70:641–656, 1994). A typical adhesive/adherend material system (EA9394/Hexcel IM7-G/8552) exposed to different contaminants at the same concentration was examined. Nano indentation technique, considered as a non-destructive testing methodology compared to the bond line thickness, was utilized to measure the adhesive mechanical properties. In addition, macroscopic mode-I fracture toughness was independently measured by double cantilever beam test. Finite element method employing cohesive zone method was used to rationalize the experimental results and the prospective scaling-laws. The combined experimental results of macroscopic properties and the numerical results of the interfacial properties suggest a scaling between the interfacial cohesive fracture toughness and the measurable flow stress. While the proposed scaling is verified to a common adhesive-adherend system in aerospace industry, with additional examination of other systems, the proposed scaling law facilitates the utilization of the non-destructively evaluated indentation hardness to serve as an indicator for the bond line macroscopic fracture toughness.