Improved De-Bonding of Composite Adhesive Joints With Bondline Inserts

Abstract The demand for effective de-bondable adhesive technology enabling substrate separation under small loads has grown in recent years. Thermally Expandable Particles (TEP) can be embedded in structural adhesives to promote mechanical separation of the adherends. However, the activation of TEP additives in joints with non-metallic adherends is challenging and can result in substrate thermal damage and poor de-bonding performance, due to the low thermal conductivity and dielectric loss factor typical of plastics and polymer-matrix composites. In this study, the effect of bondline stainless steel inserts on fully composite (Carbon Fiber Reinforced Polymer, or CFRP) bonded Single Lap Joints (SLJ) mechanical and de-bonding performance is evaluated. A centrifugal mixer is used to disperse the TEP in the adhesive. TEP additives are activated using induction heating of the bondline insert, which also helps control crack initiation and propagation. SLJ de-bonding tests are run under a constant 20 lb (89 N) load, and substrate temperature is recorded with thermocouples and an infrared thermometer. Joint strength is evaluated with quasi-static lap shear tests on a servo-hydraulic tensile test apparatus. Preliminary de-bonding testing is performed on a broad initial set of 316 stainless steel insert designs. Out of those, the four best-performing insert geometries are chosen for the complete study. Two TEP enrichment levels (10% and 20% wt.) are investigated. The mechanical and de-bonding performance of SLJs with steel inserts is compared to TEP-only baseline fully-composite and multi-material (AA 6061 Aluminum Alloy + CFRP) joints. The results show that bondline inserts enable fast de-bonding of fully-composite SLJs. Insert geometry and thickness affect joint de-bonding time and reliability, and can be optimized to allow for a partial recovery of lap shear strength. 100% de-bonding reliability is achieved with “block”-type inserts, with de-bonding performance similar to TEP-enriched metallic joints. Visual inspection of the fracture surfaces shows the relationship between TEP activation and crack propagation path. Discussion and conclusions are provided.