Stress Distribution in Tubular Joints

From a stress distribution viewpoint, a co-axial tubular joint is much preferred since this type of joint cannot be subjected to the destructive peel stresses. We can consider a tube which slides inside the second tube such that the adhesive fills the annular gap (Fig. 29). 

We will consider a torsional load on a co-axial joint, with an overlap of 25 nun using a single part hot setting epoxy adhesive. The inner tube external diameter is 25 mm and both tubes are 1.5 mm thick steel. A torsional load of 100 Nm and a glue line of 0.05 mm gives a stress distribution as shown in Fig. 30. 

The stress situation is not acceptable. However, if the glue line thickness is increased from 0.05 mm to 0.25 mm, not only is the stressed joint satisfactory, but it is still satisfactory with a 50% load increase from 100 Nm to 150 Nm (see Fig. 31). 

The graph shows that much of the central area carries no stress and therefore, a reduction in the overlap length could be considered without any significant change in load bearing capacity (see Fig. 32). 

If we change the tubes material from steel to aluminium, in order to cope with the 100 Nm torsional stress, we will need to increase the outer tube to 2 mm thickness 

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