Joint design flexible materials

When bonding elastic material, forces on the elastomer during cure should be carefully controlled, as too much pressure will cause residual stresses at the bond interface. Stress concentrations may also be minimized in rubber-to-metal joints by elimination of sharp comers and by the use of metal adherends sufficiently thick to prevent peel stresses that may arise with thinner-gauge metals. As with all joint designs, polymeric joints should avoid peel stresses. Figure 7.16 illustrates methods of bonding flexible substrates so that the adhesive will be stressed in its strongest direction. ... 

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. 

Reference [20] notes that any such system should be easy to learn and use, accurate, easy to update, flexible, well based and linked to existing design methods and tools. It should recognise innovation, include diverse environments and be acceptable to all partners in the construction process. The authors point out that the clear statement of assumptions enables them to limit the number of factors. Apart from the environment and the material, joints, contact with other materials and local movement all provide sites for degradation. Protective layers help provided that they remain intact, but degradation proceeds rapidly once they are penetrated. 

The designer should note that if this type of rigid membrane is used, it will be because no flexible membrane can accept the exposure to the anticipated chemical environment. Therefore, there is no flexible expansion joint material that can accept this exposure either. Consequently, any vessel that must be lined with such a membrane should be so designed that no expansion joints are required. For such design, see the section on design elsewhere in this volume. 

We will assume in the following that thermal expansion must be accommodated by designing sufficiently flexible pipe configurations expansion joints are usually not permissible because of reliability concerns, deposition of process materials in the pockets of the joints, and the difficulty of catering for the end-thrusts created. 

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