Assembly stress

Plastic products are often constrained from freely expanding or contracting by rigidly attaching them to another structure made of a material (plastic, metal, etc.) with a lower coefficient of linear thermal expansion. When such composite structures are heated, the plastic component is placed in a state of compression and may buckle, etc. When such composite structures are cooled, the plastic component is placed in a state of tension, which may cause the material to yield or crack. The precise level of stress in the plastic depends on the relative compliance of the component to which it is attached, and on assembly stress. 

Tourriere, H., Chebli, K., Zekri, L., Courselaud, B., Blanchard, J. M., Bertrand, E., and Tazi, J. (2003). The RasGAP-associated endoribonuclease G3BP assembles stress granules. J. Cell Biol. 160, 823—831. 

Mechanical stresses exist in the MEA as a combination of built-in stresses, assembly stresses, and operation stresses. The cell/stack fabrication and assembly process result in the built-in and the assembly stresses. During operation, stresses can be induced by the change of water content in the membrane. There are possibly other sources of stresses due to various off design conditions. 

Nau-BS, Design for Optimum Gasket Assembly Stress and Eeak Rate Control in Bolted Gasketed Joints,... 

Once such method of controlling the surface undulations is through stress-guided self-assembly. Stress-driven self-assembly is a promising route to either induce ripples in a desired pattern [12] or guide undulation growth such that a specific corrugation is produced [4]. 

Figure 11-131. Examples of bolted assemblies, stress problems, and suggested solutions.

Repeated assembly operations are not recommended. Also, a minimum torque should be used to keep the screw assembly stress within the design limit of the material. The amount of torque which can be placed on the screw depends on the cross-sectional area of the boss and/or the total number of threads. Since sufficient threads can generally be provided to take resultant loads, applied torque becomes dependent on the boss cross section . 

The stresses occurring in plastics parts include residual stress induced during the cooling phase, mechanical operative stresses, or assembly stresses during application of the plastics part. 

Mechanical fasteners are screws and bolts, self-tapping screws, or inserts. They may be reusable and can be moulded in metal or plastic. Plastic fasteners are of low weight and are corrosion and impact resistant. The high stress produced near fasteners can make parts more susceptible to chemical and thermal attack. The design stress, residual stress, and assembly stress in a component should not exceed the maximum allowable stress for the plastic. To reduce the stress concentration, inserts with smooth rounded surfaces can be used, which produce less stress than knurled inserts. 

Variations in tolerances and accumulations of tolerances can result in mismatched parts and cause high assembly stresses. Dissimilar materials will also cause additional stresses, if reactions to the assembly environment result in unequal size changes. 

Guard against assembly stresses. High stresses can be avoided in assembly by carefully attendirig to tdra-ances on interference fits of bushings and inserts and by not forcing poorly fitted parts. 

---