Restraint, mechanical

Whenever the expansion or contraction that would occur normally as a result of heating or cooling an object is prevented, thermal stresses are developed. The stress is always caused by some form of mechanical restraint. 

A polymeric matrix is strengthened or stiffened by a particulate second phase in a very complex manner. The particles appear to restrict the mobility and deformability of the matrix by introducing a mechanical restraint, the degree of restraint depending on the particulate spacing and on the properties of the particle and matrix. In the simplest possible case, two bounds have been predicted for the composite elastic modulus (Broutman and Krock, 1967, Chapters 1 and 16 Lange, 1974 see also Section 2.6.4 of this book) ... 

Mechanical restraints are either internal or external. External restraint occurs when an object or component is... 

Figure 2.4 Peel or cleavage forces may be suppressed by the use of local mechanical restraints, such as rivets, bolts or spot welds.

The slip-fit bond technique recommended above may offer several bonuses such as a reduction in the tendency for bushes to be forced out of place, the fact that bearings will no longer need mechanical restraints and the possibility of using unground bearings. Finally, the costly process of component matching may be entirely eliminated. 

Mechanical restraints are usually not appropriate in practical working joints, which should therefore be designed so that, wherever possible, both elastic and inelastic deformation of the adherend load the adhesive in compression. 

Rotaxanes and catenanes are unusual supramolecular assemblies held together by mechanical restraint. A rotaxane consists of a macrocycle threaded onto a linear molecule in a similar manner to the mounting of a wheel on an axle. Accordingly, the name rotaxane is derived from Latin for wheel and axle, rota and axis, respectively. A catenane consists of macrocycles joined as links in a chain... 

The stmcture of a chemical system is the result of restrictions on the possible positions of its atomic nuclei and on the distributions of its electron density in space, due to potentials or other quantum mechanical restraints acting among the nuclei and the electrons the more restrictions there are, the more complex is the structure. The structure of a chemical system is described by a set of nuclear coordinates (jc, y, z), together... 

The topics of thermal expansion and thermal stresses are discussed in Sections 19.3 and 19.5.) Thermal stresses do not arise if this mechanical restraint is absent. Therefore, one obvious way to prevent this type of fatigue is to eliminate, or at least reduce, the restraint source, thus allowing imhindered dimensional changes with temperatme variations, or to choose materials with appropriate physical properties. 

HNBR—Highly Saturated Nitrile is moderately priced and is available in acrylonitrile levels similar to NBR. It has excellent strength and is generally used with a mechanical restraint or reduced extrusion gap. It has good general chemical and very good temperature compatibility, up to 175°C or 200°C. HNBR is unusual in that it does not lose hardness and modulus as the temperature increases to nearly the same extent as other elastomers. 

Avoid abrupt changes of section at the ends or edges of joints where peel stresses are likely to be significant. Where possible avoid vulnerable points by redesign or addition of loose pieces to cover or reinforce such regions. Where this is not practicable consider strengthening the region by use of rivets or other mechanical restraints. 

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