Concentrically loaded joints

Transferred fastener load in x-direction in a concentrically loaded joint... 

Flowever, should the elastic hmit be reached, the lack of an appreciable ductile range in the stress—strain curve for typical FRP laminates is likely to cause failure by direct rupture or tearing, particularly at locations of stress concentration or where free movement of the laminate is prevented, such as at joints or points of connection. In stmctures which rely on flexible dynamic response to attenuate the blast loading, joints should be designed ... 

Figure 5.22 Load displacement curve of the concentrically loaded double-lap joint shown in Figure 5.21.

The connection is a concentrically loaded double lap joint containing two rows of fasteners of equal diameter. The composite plate is fastened to two steel plates. According to section 5.2.2.4 in the EUROCOMP Design Code, the first row of fasteners transfers a greater amount of load than the second row. The first and second fastener rows transfer 0.57Fxand 0.43Fx, respectively. 

Figure 5.27 Finite element model of concentrically loaded double-lap joint (source model).

In a tensile-loaded joint, forces are perpendicular to the plane of the joint. In a shear-loaded joint, forces are parallel to the plane of the joint. In both cases, stresses are distributed uniformly over the entire bonded area. By contrast, cleavage and peel stresses are concentrated on small sections of the bonded joints. Only a fraction of the total bonded area contributes to bond strength. 

The principal advantage of adhesives is in the ability to reduce stress concentration in joints by transferring load via the whole of the bonded area, as opposed to the discrete load points introduced by bolts or spot welds. In addition, the low density of adhesives imparted by their polymeric nature makes their use in joining technology attractive to the designers of weight-efficient structures and their uptake has therefore been widespread within the aerospace industry. 

Using Euler s formula to evaluate the failure load for a 2 m pin jointed condition concentrically loaded gives a factor of safety of about 2-50, but many loads are brought on to scaffold by couplers which have an eccentricity of 55-60 mm so that using Table 2 in the context of loads brought on by couplers is likely to lead to disaster. 

Fig. 28.6. Fatigue data for welded joints in clean air. The class given to a weld depends critically on the weld detail and the loading direction. Classes B and C must be free from cracks and must be ground flush with the surface to remove stress concentrations. These conditions ore rarely met in practice, and most welds used in general construction hove comparatively poor fatigue properties.

Net-tension failures can be avoided or delayed by increased joint flexibility to spread the load transfer over several lines of bolts. Composite materials are generally more brittle than conventional metals, so loads are not easily redistributed around a stress concentration such as a bolt hole. Simultaneously, shear-lag effects caused by discontinuous fibers lead to difficult design problems around bolt holes. A possible solution is to put a relatively ductile composite material such as S-glass-epoxy in a strip of several times the bolt diameter in line with the bolt rows. This approach is called the softening-strip concept, and was addressed in Section 6.4. 

Three basic problems may cause a total joint replacement to fail or to have a limited service life. The first problem, arises, because the elastic modulus of the stem greatly exceeds that of the bone. Flexural loading caused by walking creates local cyclic stress concentrations due to the non-compliance of the stem. These stresses can be intense and even severe enough to cause death of local bone cells. If this... 

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