Residual Strength Estimation of Degraded Bonded Joints based on Non-Destructive Evaluations

Abstract

Pioneer designers acknowledged structural bonding advantages over other joining methods, sparking many aerospace applications. Nonetheless, durability issues, weak bonds, and the inability to non-destructively interrogate bond quality after-the-fact hindered widespread bonding use. These challenges also contributed to several aircraft accidents, fostering extensive related research. Firstly, we examined the bonding operational history in aviation. The challenge to simulate or experimentally reproduce bonded joints’ long-term behavior under real-life conditions magnifies the value of in-service data. Since 1944 the International Civil Aviation Organization has recommended investigating aircraft incidents/accidents. Among numerous investigation reports publicly available, we identified 73 bond-related occurrences involving type-certified aircraft registered in 13 countries worldwide. These occurrences were surveyed to assess potential bonding certification shortfalls. This survey observed that environmental degradation or adhesion failure is often associated with maintenance/production bonding process issues. These observations (like existing certification policies) reinforced the importance of process control and suggested that no combination of additional layers of protection (e.g., precautionary design features) can effectively guarantee the required minimum level of safety throughout the aircraft’s life in case of substandard bonding. Next, we proposed two analytical models for degraded bonded joints. Over a century, several models were created and enhanced, mostly considering adhesives in pristine, uniform conditions. Nevertheless, real-life bondlines often contain defects/degradations, reducing joints’ residual strength. The proposed continuum-mechanics-based closed-form solutions apply to complex configurations, orthotropic adherends, degraded ductile adhesives, and general loading conditions. These models generalized a model for elastic functionally graded adhesives, incorporating (linear and non-linear) plasticity theories. Comparison with finite element models indicated conservative strain distributions. Finally, we proposed methods for estimating the bonded joints’ residual strength based on non-destructive evaluations (NDEs). While NDE of bondline quality remains elusive, NDE could establish bond strength thresholds. These physics-based methods combined different NDE techniques with the proposed models, considering approximated bondline load-carrying effective regions. Typical aeronautical metallic bonded joints were environmentally aged for different periods, non-destructively inspected, and mechanically tested. Estimated residual strengths for different failure modes were conservative compared to quasi-static failure loads.