Bonded-bolted joints

The two major classes of laminate joints are bonded joints as in Figure 7-39 and bolted joints as in Figure 7-40. Often, the two classes are combined, for example, as in the bonded-bolted joint of Figure 7-41. Joints involving composite materials are often bonded because of the natural presence of resin in the composite and are often also bolted for... 

Bonded-bolted joints generally have better performance than either bonded or bolted joints. The bonding results in reduction of the usual tendency of a bolted joint to shear out. The bolting decreases the likelihood of a bonded joint debonding in an interfacial shear mode. The usual mode of failure for a bonded-bolted joint is either a tension failure through a section including a fastener or an interlaminar shear failure in the composite material or a combination of both. 

Bonded-bolted joints have good load distribution and are generally designed so that the bolts take all the load. Then, the bolts would take all the load after the bond breaks (because the bolts do not receive load until the bond slips). The bond provides a change in failure mode and a sizable margin against fatigue failure. 

Figure 7-45 Complex Bonded-Bolted Joint (Courtesy of...

Bolted, bon and bonded-bolted joints have barely been introduced. Further befTavioral and design information is available in the book Joining Fibre-Reinforced Plastics edited by F. L. Mathews [7-11], which has chapters written by various authors including two chapters on design by L. J. Hart-Smith. 

P(l) Bonded-riveted joints are, from the design point of view, identicai to bonded-bolted Joints and shall be dealt with accordingly. 

A bonded-bolted Joint combines adhesive bonding and mechanical fastening. Functions of both elements and possible applications are discussed in this section. 

Figure 5.44 Load distribution in a bonded-bolted joint with a joint end debond.

In lightly loaded bonded-bolted joints the bolts can provide a fail-safe load path in the event of adhesive failure. In heavily loaded structures mechanical fasteners can effectively prevent the spreading of debonding or delaminations from local damage or initial faults. 

When mechanical fasteners are used with bonding, a better damage tolerance is obtained. However, the weight of the joint structure is also increased, joining becomes more complicated and expensive, and even reductions in joint strength are possible. All these aspects have to be considered when a bonded-bolted joint is to be used instead of a bonded joint. 

There are no simple analytical methods for analysing bonded-bolted joints. The static strength of an undamaged bonded-bolted joint can be analysed identically as for the corresponding bonded joint. When the effect of defects needs to be evaluated, computer codes or finite element (FE) packages are required. Hart-Smith (reference 5.45) has developed a FORTRAN code called A4EK for analysing intact and flawed bonded-bolted step-lap joints with linearly elastic adherend deformations. 

Finite element methods can also be used to analyse bonded-bolted joints. A high stress concentration exists in angular corners of the adherends. This phenomenon is identical to that encountered in the analysis of bonded joints. Non-linearities in the adhesive and adherends have to be considered. With an FE analysis a more detailed picture of the behaviour of a bonded-bolted joint may be reached than with closed-form analytical methods. However, perfoming FE analyses can be time consuming and their accuracy is dependent on the accuracy of the model, as discussed in 5.3.2.2. 

Hart-Smith, L.J., Design methodology for bonded bolted composite Joints, USAF Contract Report AFWAL-TR-81-3154, Vol, 1, February, 1982. 

Composites may be joined by bonding, bolting or both. Scarf, stepped lap, supported single-lap joints are the most common, but single-lap or double-lap types are used (Shear tests). 

If a circuit conductor is connected to earth or to any other reference point nothing which could break electrical continuity or introduce high impedance, for example, fuse, thyristor or transistor, is allowed in the conductor unless suitable precautions are taken. Permitted devices would include a joint or bolted link, but not a removable link or manually operated knife switch without bonding of all exposed metal work and multiple earthing. 

Hart-Smith, L.J., Design methodology for bonded-bolted composite joints, USAF Contract Report AFWAL-TR-81-3154, February 1982. Summarized in Bonded-Bolted Composite Joints, Douglas Paper 7398. Presented to AIAA/ASME/ASCE/AHS 25th Structures, Structural Dynamics and Materials Conference, Palm Springs, CA, May 14-16, 1984. Published in J. Aircraft, 22, 993-1000 (1985). 

Chan and Vedhagiri (2001) studied the response of various configurations of single-lap joints, namely bonded, bolted, and bonded-bolted joints by three-dimensional FE method and the results were validated experimentally. The authors found that for the bonded-bolted joints, the bolts help to reduce the stresses at the edge of the overlap, especially after the initiation of failure. The same type of study was carried out by Lin and Jen (1999). 

Chan WS, Vedhagiri S (2001) Analysis of composite bonded/bolted joints used in repairing. J Compos... 

Hart-Smith, LJ (1984b) Bonded-bolted composite joints. Douglas paper 7398, presented to AIAA/ ASME/ASCE/AHS 25th structures, structural dynamics and materials conference. Palm Springs, 14-16 May 1984 published in J Aircraft 22, 995-1000(1985)... 

Transition Joints. Use of explosion-clad transition joints avoids the limitations involved in joining two incompatible materials by bolting or riveting. Many transition joints can be cut from a single large-area flat-plate clad and deflvered to limit the temperature at the bond interface so as to avoid undesirable diffusion. Conventional welding practices may be used for both similar metal welds. 

Ma.rine. In the presence of an electrolyte, eg, seawater, aluminum and steel form a galvanic cell and corrosion takes place at the interface. Because the aluminum superstmcture is bolted to the steel bulkhead in a lap joint, crevice corrosion is masked and may remain uimoticed until replacement is required. By using transition-joint strips cut from explosion-welded clads, the corrosion problem can be eliminated. Because the transition is metaHurgicaHy bonded, there is no crevice in which the electrolyte can act and galvanic action caimot take place. Steel corrosion is confined to external surfaces where it can be detected easily and corrected by simple wire bmshing and painting. 

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