33 Fatigue Behavior of Novel Hybrid Fastening System with Adhesive Inserts 275 Fig. 33.8 Load-displacement diagram of as-fabricated and pre-cycled conventional and hybrid bolted joints 0 10 20 30 40 50 60 0 5 10 15 20 25 30 35 Force, kN Displacement, mm As-fabricated conventional bolt As-fabricated hybrid GCbolt Pre-cycled conventional bolt Pre-cycled hybrid GCbolt for possibly having lower residual strength. On the other hand, unlike the conventional joint, the hybrid bolted joint exhibited surprising improvement of residual joint strength after fatigue cycling by about 8 %. This finding could be attributed to many reasons. Firstly, the bolt is tightly assembled with the laminates through the injected resin. This arrangement would help the bolt to carry the applied load uniformly, so that the bolt and the washer could be treated favorably through work hardening and their proof loads/yield strengths increased. Secondly, after one million of cycles, the resin may act as a cushioning media between the laminate and the bolt that would allow the load to transfer smoothly during tensile shear loading. Finally, the residual strength of the pre-cycled hybrid bolted joint is seen to be about 12 % higher than that of the pre-cycled conventional joint. Also, the fiber reinforced laminates used in the adherends could also be contributing to increase in stiffness. The microdegradation of the matrix within the adherends may cause fibers to slightly re-align along the load-direction. Post-fatigue quasi-static testing would have aligned fibers relative to the non-cycled specimens. This could explain the increase in stiffness for both the conventional and hybrid bolted joint. Similar joints with all metal adherends would confirm this hypothesis and will be reported in our next work. The results are preliminary and statistically significant tests along with insights from numerical simulations need to be performed to fully understand the observed results. Nevertheless, the results show promise in use of the hybrid joints in a wide range of structural applications. 33.6 Conclusion In this work, the static and fatigue behavior of conventional and the most basic form of novel hybrid bolted joints were investigated. In the hybrid bolted joint, SC-15 epoxy resin was used as the structural insert. This most basic configuration of the hybrid bolt consists of a bolt that has a channel machined through the bolt-shaft that allows injection of an insert compound that fills the hole-clearance of the work-pieces and acts a structural component. The observed results from quasistatic and fatigue tests can be summarized as follows: • It is well known that the presence of slip between the joined parts leads to rapid loosening, as a result, the reliability of bolted joint is significantly affected. The hybrid joint eliminates the presence of slippage. If failure is defined as the onset of delamination, then the hybrid joint is three times better than the conventional one. • On the other hand, if the ultimate strength is considered as design criteria (which is not practically adopted by different industries), the conventional joint shows a slightly better performance. This phenomenon could be explained by the fact that the presence of clearance in the joint causes excessive laminate damage by the concentrated load and forces the absorption of more energy by delamination and localized fiber buckling/breakage. As a result, the work done by the plastic deformation of the bolt is delayed, which eventually enhances the load carrying capacity of the joint. • The hybrid joint exhibits inferior mechanical performance and lower fatigue life at higher load rates. Similar to the quasistatic loading condition, a large clearance between the bolt and the hole results in more concentrated loads on the laminate, causing more excessive laminate damage around the bolt. Hence, more energy is being absorbed by the laminate, which leads to extended life of the joint and allows the bolt to absorb less energy through plastic deformation. • Residual test results of pre-cycled conventional and hybrid bolted joints show that the as-fabricated conventional bolted joint exhibits higher loads than the pre-cycled specimens. • The residual strength of the cycled hybrid joint is about 12 % higher than that of the pre-cycled conventional joint.
RkJQdWJsaXNoZXIy MTMzNzEzMQ==