A Review of Structural Adhesive Joints in Hybrid Joining Processes

Hybrid joining (HJ) is the combination of two or more joining techniques to produce joints with enhanced properties in comparison to those obtained from their parent techniques. Their adoption is widespread (metal to metal joint, composite to composite and composite to metal) and is present in a vast range of applications including all industrial sectors, from automotive to aerospace, including naval, construction, mechanical and utilities. The objective of this literature review is to summarize the existing research on hybrid joining processes incorporating structural adhesives highlighting their field of application and to present the recent development in this field. To achieve this goal, the first part presents an introduction on the main class of adhesives, subdivided by their chemical nature (epoxy, polyurethane, acrylic and cyanoacrylate, anaerobic and high-temperature adhesives) The second part describes the most commonly used Hybrid Joining (HJ) techniques (mechanical fastening and adhesive bonding, welding processes and adhesive bonding) The third part of the review is about the application of adhesives in dependence of performance, advantage and disadvantage in the hybrid joining processes. Finally, conclusions and an outlook on critical challenges, future perspectives and research activities are summarized. It was concluded that the use of hybrid joining technology could be considered as a potential solution in various industries, in order to reduce the mass as well as the manufacturing cost.

A Study on the Shape Selection of Mechanical Fastening for the Repair of Fighter Wing

A study on optimal shape selection of a mechanical fastening for the repair of crack defect of ROK Air Force F-5 fighter wing was conducted. The crack defect occurred in the spar of the wing, and the technical manual does not specify the repair method. However, ROK Air Force decided to develop a repair technology for this defect in consideration of various logistic conditions. Three repair shapes for the proper repair were devised and the finite element analysis was performed to examine the structural safety of these three connection members. As a result of the structural safety review, two connection members except one were structurally safe with safety margins over zero because the calculated stress values were at or below the yield strength level. Therefore, two connection members were determined to be able to use for repair under the condition that the aircraft operated within the design limit load. The results of this study would be very useful if the same defect occurs in long-term aircraft operated by the ROK Air Force.

Frictional damping in hollow beam structures joined by bolts, rivets, and adhesive

The objective of this work was to investigate how different joining techniques affect the level of damping in structures. Beams were constructed from four different joining techniques, bolting, riveting, adhesive bonding, and brazing by joining two lengths of steel each with a ‘U’-shaped cross-section. They were joined such that the edges of the ‘U’ overlapped to form a tube. The damping of each beam was determined by flexural vibration. The bolted beam had a series of bolts along its length. The effect of removing bolts was investigated. It was found that removing bolts increased damping. When bolts were removed successively from holes at the end of the beam, the damping increased more than when bolts were removed from holes in the middle of the beam. A further objective of this project was to investigate the effect of introducing penetrant between two surfaces. WD-40 was introduced between the contacting surfaces for the beams joined by mechanical fastening. The penetrant had the effect of increasing damping. This may be because the penetrant has the effect of increasing the relative displacement between the two beams, leading to greater energy dissipation. Introducing penetrant also changed the order of which beam had the greatest damping, with the bolted beam now having greater damping than the riveted beam. The effect of increasing bolt tension on the bolted beam was also investigated. When the beams were dry, increasing bolt tension reduced the damping, but when penetrant was introduced increasing the bolt tension increased the damping. A comparison between the damping properties from different joining techniques was made. The conclusions could be applied in industry by engineers constructing beams of a similar fashion.

Forming a Flanged Hole When Quenching Press-Hardened Steel for Mechanical Fastening

The recent stringent regulations on vehicle safety and reducing CO2 emissions have led to a continuous increase in the application of press-hardened steel (PHS) in automobiles. Similar to other high-strength steels, assembling PHS components using the common welding techniques employed in automotive production lines is significantly difficult because of the surface coating layers and the additives within. This difficulty in post-processing, attributed to its high strength, also limits the mechanical fastening of PHS components. Therefore, this study aims to develop a process for forming a structure enabling mechanical fastening by sequentially applying piercing and hole-flanging operations during the hot stamping process. Our experimental apparatus was designed to perform the hole-flanging operation after the piercing operation within a single stroke at a specific temperature during the quenching process of PHS. At high temperatures of 440 °C or higher, the hole-flanging process was conducted in a direction opposite to that of the piercing operation for creating the pilot hole. An extruded collar with a height of 8.0 mm and a diameter of 17.5 mm was achieved, which is hole expansion ratio(HER) of 82.5%.

A State-of-the-Art Review on Advanced Joining Processes for Metal-Composite and Metal-Polymer Hybrid Structures

Multi-materials of metal-polymer and metal-composite hybrid structures (MMHSs) are highly demanded in several fields including land, air and sea transportation, infrastructure construction, and healthcare. The adoption of MMHSs in transportation industries represents a pivotal opportunity to reduce the product’s weight without compromising structural performance. This enables a dramatic reduction in fuel consumption for vehicles driven by internal combustion engines as well as an increase in fuel efficiency for electric vehicles. The main challenge for manufacturing MMHSs lies in the lack of robust joining solutions. Conventional joining processes, e.g., mechanical fastening and adhesive bonding involve several issues. Several emerging technologies have been developed for MMHSs’ manufacturing. Different from recently published review articles where the focus is only on specific categories of joining processes, this review is aimed at providing a broader and systematic view of the emerging opportunities for hybrid thin-walled structure manufacturing. The present review paper discusses the main limitations of conventional joining processes and describes the joining mechanisms, the main differences, advantages, and limitations of new joining processes. Three reference clusters were identified: fast mechanical joining processes, thermomechanical interlocking processes, and thermomechanical joining processes. This new classification is aimed at providing a compass to better orient within the broad horizon of new joining processes for MMHSs with an outlook for future trends.

Deformation and Anchoring of AA 2024-T3 rivets within thin printed circuit boards

This work evaluates the viability of applying Friction Riveting as an alternative for the assembly of components on printed circuit boards (PCBs). The popular press-fit technology for assembling components on PCBs consists of a pin inserted tightly into a relatively smaller hole, resulting in good electrical and mechanical properties. However, some limitations are highlighted, such as numerous processing steps and the need for predrilled holes. Friction Riveting is based on mechanical fastening and friction welding principles, where polymeric components are joined with metallic rivets through frictional heating and pressure. The main benefits of using Friction Riveting in PCBs compared with fit-press are (i) a reduced number of processing steps and (ii) shorter joining cycles, because there is no pre-drilling involved with fasteners anchored within the PCB in a single step. The joints were manufactured using 5 mm diameter AA-2024-T3 rivets and 1.5 mm thick glass-fiber-reinforced epoxy laminates (FR4-PCB). It is shown for the first time that it is possible to deform metallic rivets within thin composite plates at a reduced diameterto-thickness ratio. The feasibility study followed a one-factor-a-time approach for parameter screening and optical microscopy assessed joint formation of the deformed rivets inside the laminates through volumetric ratio (VR). The joints present significant deformation (VR=0.5) at the tip of the rivet inserted into overlapped PCBs plates, with thicknesses below 3.0 mm, which is considered the lowest achieved so far with Friction Riveting.