Fastener stiffness

Other important variables are patch structure (composite material, layup and thickness according to the analytical results) and fastener stiffness which should be determined by test or analysis and subsequently used in the analysis of the overall repair. Fastener tensile and shear stresses should be determined as to their adequacy for static strength and for fatigue loading. Fastener selection is addressed in Chapter 11. 

Fastener stiffness (inverse of fastener flexibility) slope of an experimentally determined curve of R as a function of... 

Jarfall (reference 5.14) suggested the constant C be called the fastener flexibility. The inverse of the fastener flexibility is denoted as the fastener stiffness K xp ... 

To determine Kffe from Equation (5.32), Kmesh and Kexp must be known. Kme is determined from a finite element analysis of the source model using rigid spring elements. Kexp is usually unknown and must be estimated from analytical or semi empirical models. There exist several semi-empirical expressions for the fastener flexibility (the inverse of the fastener stiffness Kexp) which are used in the industry. Those presented by Huth (reference 5.12) are valid for both metallic and composite (carbon FRF) members. The fastener flexibility for a single-lap joint from (reference 5.12) is ... 

For each figure the data are plotted for two different car manufacturers. The performance difference is the influence of the cars suspension systems parameters on the different elastomeric rail fasteners stiffnesses. Figure 5-75 shows clearly that there are definitely two populations, two different car manufacturers. Therefore, a system that performs well for one authority may not offer the same performance to another one. 

High stiffnesses and strengths can be attained for composite laminates. However, these characteristics are quite different from those of ordinary materials to which we often need to fasten composite laminates. Often, the full strength and stiffness characteristics of the laminate cannot be transferred through the joint without a significant weight penalty. Thus, the topic of joints or other fastening devices is critical to the successful use of composite materials. 

Assuming the C-C bonds of the prism all are the same length, determine how many mono-, dr-, and tribromine-substituted isomers are possible for 17. Compare the results with those expected for benzene with structure 13. If you have molecular models of the ball-and-stick type, these will be very helpful. A simple alternative model for 17 would be a piece of stiff paper folded and fastened as in 18 to give a prism with three equal square faces. 

In order to correct the difficulty associated with clogging of the reservoir by antimony (III) fluoride, the checker installed a stiff spiral wire to scrape the walls of the reservoir. It was fastened in the ground-glass joint cap by a rubber stopper. By rotating the ground-glass joint the wire spiral could easily be turned. No air leaks were encountered with this modification. 

Figure 3. Boundary-element model of a vertical hydrofracture propagating through ten layers of different stiffnesses. Horizontal arrows indicate the hydrofracture s fluid overpressure of 6 MPa in the lowermost layer J. Crosses indicate the fastening of the model in the lower comers. The opening displacement is exaggerated. Three vertical internal springs (through layers 1. H and G) allow the hydrofracture tip to propagate up to the bottom of the soft layer F, which makes the tip wide and blunt, dissipates most of the fracture-tip tensile stress (contours of Tj in megapascals), and may arrest the hydrofracture.

Figure 7. Boundary-element model of the aperture variation of a partly inclined hydrofracture. The crossed layers (A. C etc.) are rather stiff, the dashed layers (B, D etc.) are soft. The hydrofracture is subject to a fluid overpressure indicated by arrows) of 20 MPa in its vertical parts but 14 MPa in its inclined parts. Crosses indicate the fastening of the model in all the corners. The aperture (shown without exaggeration) is slightly smaller inside the soft layers.

Sealing shells are mostly applied in insufficiently stiff and weakly loaded immovable joints. They are usually made as flat or hollow tubular seals fastened along the packed hole outline (lids, hatches, car and instrument bodies, doorway and window openings, etc). It should be emphasized that the introduction of Cl into sealing compositions is strongly advisable since this is found in the narrow gaps and clearances where the corrosion process intensity is the highest. 

Circular Ranged Uniform p Fastened to shell f-mi varies with sheH and oint stiffness from 0.33 to 0.3 knuckle radius, f... 

Where temperature variations are encountered in the service of an item containing dissimilar materials, adhesives perform another useful function. Flexible adhesives are able to accommodate differences in the thermal expansion coefficients of the adherends and therefore prevent damage that might occur if stiff fastening systems were used. 

P(2) In addition, the assumed distribution of internal forces shall be realistic with regard to the relative stiffnesses of the joint members and/or the adhesive/fasteners. 

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. 

Stiffness constant of the fastener installation Normalised radial stress k for basic load case i Normalised tangential stress k( for basic load case i Normalised shear stress ks for basic load case i Radial distance from e.g. to fastener i... 

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). 

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. 

As previously discussed, the applied load is unevenly distributed between the fasteners, depending on the stiffness properties (elasticity and geometry) of the members being joined. This is not taken into account by the presented analytical approach. The analytical results must therefore be corrected. 

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