Structural applications of adhesive bonding have been increasing in recent years due to improvements in the types of adhesives available and in improved knowledge of bondline procedures. Consequently, there exists a demand for precise numerical modeling of adhesive joint behavior, particularly along bondline interfaces where low surface energy adhesives contact high surface energy metallic oxides. Recent experimental studies have shown that adhesive bond strength for steel substrates which had been cathodically electroprimed (ELPO-primed) before bonding were significantly higher than strengths for unprimed steel substrates. Subsequent finite element studies showed a correlation with the experimental results in that peak stress values at the midplane of single-lap-shear (SLS) joints were reduced for primed surfaces. The present study provides a qualitative examination of debond behavior at primed and unprimed bonded surfaces by applying finite element analyses to compute opening (Mode I) and shear (Mode II) components of the strain energy release rate for small debond lengths at selected locations through the bondline thickness of the SLS joint. The bondline-thickness locations where the total strain energy release rate was a minimum correlate well with experimentally observed failure surfaces. In addition, a significant reduction in Mode I was computed for the primed compared to the unprimed joint. This reduction in peel response is consistent with the enhanced strength of the primed joints.

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