Fastener load distribution

Friberg M. Fastener load distribution and interlaminar stresses in composite laminates [Licentiate thesis]. Stockholm Royal Institute of Technology 2000. 

P(l) The conditions of equilibrium shall always be fulfilled when determining fastener load distribution, far field stress distribution, and the stress distribution in the vicinity of fastener holes. 

P(2) The stiffness properties (elasticity and geometry) of the joined members and fasteners shall, directly or indirectly, be taken into account when determining the fastener load distribution and far field load distribution. 

Table 5.3 Fastener load distribution in multi-row joint (as proportion of average fastener load).

P(3) In general, the fastener load distribution shall be determined from the equilibrium conditions and the genetic compatibility of the Joint in conjunction with a constitutive relation for the fasteners. 

To determine the load distribution in a bolted connection accurately, taking into account the elastic properties of the joined members and the fasteners explicitly, it would be necessary to use a numerical method such as the finite element method (FEM). To greatly simplily the calculation of fastener load distribution it is assumed that the members are macroscopically rigid and that the elasticity is limited to local areas in the vicinity of the fasteners. It may furthermore be assumed that the load versus deformation response of an individual fastener is linear. Thus, the effect of member stiffness on fastener load distribution is taken into account by means of special correction factors. 

Figure 5.11 Fastener load distribution due to concentric load.

A major limitation of the simple method of determining the fastener load distribution is that the stiffness (elasticity and geometry) of the joined members is not explicitly taken into account. Another limitation of the simple method is that the specific elasticity properties of the analysed member is not considered when determining the fastener hole stress distribution. These limitations can effectively be overcome by adopting a numerical method such as the finite element method (FEM) for evaluation of the load distribution. 

Load distribution in iarge Joint assemblies and effects of missing fasteners... 

The influence of member stiffness (elasticity and geometry) on the bolt load distribution is illustrated by a simple example. Figures 5.7 and 5.8 show schematically a single-lap joint with uniform lap thickness loaded in tension. The upper and lower parts represent the joined members and the layer between represents a row of fasteners. When the load is applied, the members deform concentrically and the fasteners in shear. If the members were rigid, the fasteners would transfer equal amounts of the load, and the shear deformation would be equal in all fasteners (Figure 5.7). 

Figure 5.13 (a) shows the bolt load distribution for two equally thick (3.9 mm) glass fibre laminates joined together by 2, 3 and 4 tandem rows of fasteners. In the two-row joint (Wx=40 mm and 120 mm, respectively) the load is evenly distributed among the two rows. In the three row joint (Wx=40 mm and 60 mm, respectively) the fasteners at the end of the overlap transfer about 7% higher bolt load than that of an even distribution, whereas the bolt in the middle transfers a load 14% less than that of an even distribution. In the four-row configuration (Wx=40 mm) the bolts at the end of the overlap transfer about 20% higher load than that of an even distribution. Hence, the two bolts in the middle transfer about 20% less load than that of an even distribution. 

Figure 5.13 (c) shows the bolt load distribution for a 3.9 mm thick glassfibre laminate Joined to an equally thick steel plate by 2, 3, and 4 tandem fastener rows. For all configurations the bolts at the end of the overlap transfer substantially higher load than the bolts in the beginning of the overlap. 

Stress distribution Local stress points in structure Fairly good stress distribution Points of high stress at fasteners Good uniform load distribution over joint area (except in peel)... 

Most structures require the joining of one or more similar or dissimilar materials in their fabrication. Conventional joining techniques have employed mechanical fasteners, such as screws, nails, bolts, and rivets. These joining techniques result in large stresses near the fastener, produced by nonuniform load distributions. 

From a design standpoint, quality control is needed at every phase during the formation of the adhesive bond to help insure reliability. Yet, thinner and lightweight substrates can be bonded due to the more uniform stress distributions in bonded assemblies while stress concentrations are reduced by the elimination of fasteners. Joint design is more critical with adhesives to insure proper load distributions in the various applications. 

In the side view of a bolted connection under load, the stress peaks around the bolt appear very clearly (Fig. 32.2). The unstressed zones of the component appear yellow, e.g., in the upper right-hand corner. Moving from this unstressed area toward the bolt, a series of colored lines are crossed until the zone of greatest stress is reached around the shaft of the bolt. A plan view of the connection reveals a similar picture a high concentration of stresses around the bolt (Fig. 32.3). At these points the component is vulnerable to damage. A similar pattern of stress distribution is observed in components that are fastened together with rivets or spot welds. 

Adhesives as a class of materials are designed to hold substrates together by surface attachment. The products distribute a load over an entire bondline rather than concentrating stress at specific points like mechanical fasteners. Adhesive bonding also can significantly reduce part weight and assembly time compared to mechanical fasteners. 

As mentioned, large commercial aircraft contain about three million fasteners, and so a multitude of bolted joint configurations exist. However, despite this diversity, there is a basic commonality in the stresses set up by the fastener or bolt load at a particular bolthole, and the effect of other loaded bolt-holes on this stress distribution [1], This allows simplified coupon tests to be used in establishing design criteria and data for these various joint configurations. The intent is to then combine a suitable number of test/ model results to represent the actual situation. 

Washers distribute the tightening load over a wider area than does a bolt head, screw head or nut. They also keep the surface of the work from being damaged by the fastener. 

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