Estimate residual strength of degraded bonded joints by combining analytical models with non-destructive evaluations

Durability issues associated with adhesive degradation have hindered widespread use of bonding in aircraft applications and contributed to several bond-related in-service accidents, fostering extensive related research. While the non-destructive evaluation (NDE) of bondline quality remains elusive, some authors suggested that NDE could establish bond strength thresholds, supporting the theoretical concept of a residual effective bond overlap (REBO). These authors related strength thresholds to NDE of specimens with artificial weak bonds only phenomenologically. This work proposes two physical-based approaches, combining NDE with analytical models for stress analysis, to estimate the residual strength of environmentally degraded metallic bonded joints considering an approximated REBO. These approaches incorporate fundamental characteristics of two physically distinct NDE techniques and a water-related aging mechanism. Statistically treated experimental data for verification was obtained from aluminum-epoxy bonded joints typical of aeronautic application. Two sets of these joints were exposed in a hot/wet controlled environment – to induce representative degradations – for different periods, non-destructively inspected, and mechanically tested. Each set of specimens differed only in the surface preparation to prompt dissimilar failure modes – adhesion and cohesion failures. Estimated residual strengths for different levels of degradation and failure modes were in agreement with quasi-static loads in a conservative fashion.