Adhesive bonding joint efficiency

It is noted that the residual tensile strength depends primarily on the strength of the thermoplastic particles and the thickness of the thermoplastic layer within the narrowed crack. The reason is that the molten thermoplastic forms a thin film in the narrowed crack under the recovery pressure by the SMPFs, similar to the adhesive layer in an adhesively bonded joint (here the fractured two half beams serve as the adherends). It has been well demonstrated that the tensile and shear resistance of adhesively bonded joints highly depends on the adhesive thickness. A thinner adhesive layer usually leads to higher peel and shear resistance [28-32]. This is one reason why a 100% pre-strained SMPF leads to a higher healing efficiency because... 

In order to evaluate the healing efficiency for the Mode II fracture test, end-notched flexure (ENF) specimens can be used (see a schematic in Figure 8.11). Recently, Ouyang and Li [57] derived a formulation of a Mode II energy release rate for the ENF joints bonded with dissimilar materials. When the adhesively bonded joint becomes a standard ENF specimen, that is, the two adherends have identical thickness and are made of identical material, we have [57]... 

Given the requirement for knowledge of how to specify and preserve the properties of adhesive-bonded joints and for efficient methods of reducing the intem d stresses, let us consider the distribution of internal stresses throughout the cross-section of a nontransparent adhesive interlayer both perpendicular and parallel to the cemented surfaces. 

An adhesively bonded joint is typically more structurally efficient than a mechanically fastened joint. Fasteners introduce discrete, or at least concentrated, points at which load is transferred from one component to another. Therefore the peak stress at each fastener is much higher than overall joint stress and the joint must be suitably designed to withstand these peak stresses. Compounding this is the fact that aerospace components are typically quite thin with proportionately low fastener bearing strength. The most efficient way to mechanically fasten thin members is with numerous small fasteners which add weight and cost. 

It may be noticed that adhesively bonded joints and laminated joints appear to have similar maximum loading densities, i.e. load per unit width, as indicated in Figure 4, while the joint efficiencies of laminated joints are typically higher than those of adhesively bonded joints (Figures 5 to 7). One possible explanation of this behaviour is that in laminated strap joints the bond surface of the strap had a woven roving surface layer, while in adhesively bonded strap joints a laminate having a mat surface layer was used as straps. 

Figure 5 Joint efficiencies of adhesively bonded joints with A300 RTM adherents.

The effect of the adhesives and resins on the joint efficiency is shown in Figure 9. A possible explanation for the better performance of laminated Joints compared to adhesively bonded joints is the one given above concerning the strap lay-up sequence. However, that does not explain the behaviour of the laminated single-lap joint. 

Oplinger DW (1994) Effects of adherend deflection on single lap joints. Int J Solids Struct 31(18) 2565-2587 Renton WJ, Vinson (1975) The efficient design of adhesive bonded joints. J Adhes 7(3) 175-193 Shahin K, Kember G, Taheri F (2008) An asymptotic solution for evaluation of stresses in balanced and unbalanced adhesively bonded joints. Mech Adv Mater Struct 15(2) 88-103... 

However, most of these improvement methods are traditionally based on the mechanical stiffening m ods. They cannot adaptively adjust the stress distribution. In order to smartly and efficiently improve the joint strength, Cheng and his-coworkers have recently introduced a smart joint concept into the traditional adhesively bonded joint by integrating with piezoelectric materials, which have been confirmed as a very effective method to smartly reduce the stress concentration in the adhesive layer. Here, we will give an overview on the state-of-the-art of the adhesively bonded smart beam-like and tubular joints [9-13]. 

As mentioned above, aircraft structure is typically quite thin with numerous small fasteners to achieve efficient load transfer through joints. Mechanical fasteners are laborious to install. Adhesive bonding of large area doublers and joints can be accomplished at significant labor hour savings over equivalent mechanically fastened designs. 

Sound knowledge of the joint behavior is required for a successful design of bonded joints. To characterize the bonded joint, the loading in the joint and the mechanical properties of the substrates and of the adhesives must be properly defined. The behavior of the bonded joint is investigated by finite element (FE) analysis methods. While for the design of large structures a cost-efficient modeling method is necessary, the nonlinear finite element methods with a hyperelastic material model are required for the detailed joint analysis. Our experience of joint analysis is presented below, and compared with test results for mass transportation applications. 

In adhesive bonding, shear is a major type of stress when one substrate is forced to move parallel and relative to the other substrate. The entire bonded area is efficiently used when joints are stressed in shear. Thus tensile-shear overlap design is a common joint design used in adhesive bonding. (See adhesives tests. Fig. A.6.)... 

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