Aluminum-epoxy bonding strength assessment based on modified butt joint sample

The bonding strength of the aluminum-epoxy interface is a key parameter for the determination of the mechanical behavior of insulating products e.g., barrier/support insulators. Therefore, it must be accurately determined by the measurements. The determination of the bonding strength of a bi-material interface is not a trivial task, because there are few standard testing procedures available. A common butt-joint sample can be considered as a bi-material notch with a 180° opening angle. The stress has a singular character at the root of the notch due to a mismatch of the mechanical properties of the involved materials. This singular stress concentration influences the bonding strength evaluation. Moreover, this weakest point could be contaminated or damaged during the manufacturing process (release agent, particles, crack). Combining, all those factors could be at the origin of premature debonding and statistical scatter of measured values. To avoid stress singularity and failure initiation on the edge, a new geometry with a radius shape is proposed. A modified butt joint sample aims to protect the critical point by introducing the stress relief notch on the epoxy side of the bi-material interface. The sample geometric arrangement was optimized by changing the diameter (ø) and the radius (r), to achieve a uniform distribution of the normal stress on the interface. According to the simulation, the proposed design removes the singularity, and the stress is uniformly distributed at the central area of the interface. Due to this improvement, the debonding occurs at the central part of the specimen, where the stress remains uninfluenced by the stress singularity or manufacturing process. The proposed improvement should reduce the statistical scatter of the measured bonding strength of epoxy-aluminum bi-materials.