Development of adhesive bonds with reduced strength as ultrasonic NDE benchmarks

A method for producing adhesive-bonded aluminum joints with predictable strength loss was developed and evaluated. The method uses an open-face specimen geometry and a humid high-temperature environment to promote adhesive degradation. The rate of degradation was greatly increased over previous accelerated degradation schemes through the use of MgSO4 as contaminant. The contaminant was applied in a purpose-built aerosol contaminating duct. Specimens were prepared and fractured using a DCB loading jig. Two studies which examined the effects of various exposure and contamination parameters were conducted. It was shown that contaminant surface concentration is a strong determinant of residual adhesive strength. Exposure to the aggressive environment was shown to have little effect beyond an initial threshold. Standard ultrasonic imaging techniques were incapable of differentiating between exposed and unexposed specimens. FESEM and EDX analysis of the fracture surfaces showed residual aluminum, suggesting an intra-oxide locus of failure consistent with other accelerated degradation methods.