Strengthening and Stiffening

The biological energy necessary to resist wind damage would otherwise be spent on growth and reproduction. Plants that withstand the pressure of wind are often stunted and without many offspring. 

Human bone strength has been found to be related directly to intermittent applied forces, similar to (not surprisingly ) forces generated during walking or running. Osteoporosis (weakening of the bones due to the resorption of bone material) in the elderly can cause bones to become brittle and break easily. Exercise is one way to reduce the effects of reduced bone density. 

The integrity of certain types of bacteria depends on cell walls that they construct. The antibiotic penicillin was effective because it interfered with this construction process (Palumbi, 2001). From the Law of Laplace (Section 2.9.4)  

2(wall tensile stress)(wall thickness) (wall radius) 

It can be seen that a bacterium can resist higher pressures for thicker walls, smaller size, or walls that are made of stronger materials. Bacteria not round shaped have a radius of infinity, and cannot theoretically resist any pressure, so one reason that bacteria are shaped as spheres and cylinders is that they must be in order not to implode or explode. Cylindrically shaped bacteria must have some curvature to the parts of their walls that are straightest, and the walls must be thicker at those parts. The use of stronger materials for their walls is presumably not an option because bacteria utilizing weaker wall materials would have been eliminated from the gene pool long ago. 

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The best combination of properties of polyethylene-based ionomers, such as stiffness, strength, transparency, and toughness, are realized at partial degrees of conversion of about 40-50% [13]. The initial increase in properties is a result of the presence of ionic interactions, which strengthen and stiffen the polymer. There is, however, some loss of crystallinity as a result of the presence of the ionic groups. When the loss of crystallin-... 

Mathematically the critical fiber length necessary for effective strengthening and stiffening can be described as follows ... 

For an amorphous polymer that is drawn at an elevated temperature, the oriented molecular structure is retained only when the material is quickly cooled to the ambient temperature this procedure gives rise to the strengthening and stiffening effects described in the previous paragraph. However, if, after stretching, the polymer is held at the temperature of drawing, molecular chains relax and assume random conformations characteristic of the predeformed state as a consequence, drawing wOl have no effect on the mechanical characteristics of the material. 

This year s U.S. production of thermoplastics, thermosets, and synthetic rubber is expected to be 29 billion pounds. About 80% of this is based on only a few common monomers. To improve performance, the polymer industry rarely changes to a new, probably more expensive polymer, but instead it shifts from mere homopolymers to copolymers, polyblends, or composites. These three types of multicomponent polymer systems are closely inter-related. They are intended to toughen brittle polymers with elastomers, to reinforce rubbers with active fillers, or to strengthen or stiffen plastics with fibers or minerals. 

One other mechanism appears to operate when fractioning paper and boxboard. The application of water to the surface will raise the coefficient of friction by lifting fibers out of the surface of the paper. If only water is applied, these fibers tend to go back into the surface if another box or paper bag is passed over its surface. However, if these fibers are coated with colloidal silica particles from the application, they are stiffened and strengthened and tend to remain upright creating a rougher surface. 

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) ... 

The word "composites" has a modern ring. But using the high strength of fibres to stiffen and strengthen a cheap matrix material is probably older than the wheel. The Processional Way in ancient Babylon, one of the lesser wonders of the ancient world, was made of bitumen reinforced with plaited straw. Straw and horse hair have been used to reinforce mud bricks (improving their fracture toughness) for at least 5000 years. Paper is a composite so is concrete both were known to the Romans. And almost all natural materials which must bear load - wood, bone, muscle - are composites. 

In vacuum distillation columns, the plate-support rings will act as stiffening rings and strengthen the vessel see Example 13.2. 

The employees returned on Sunday to complete the job. They decided to strengthen this poorly-constructed box by welding a 2-inch (5 cm) angle iron stiffener around the middle of the ill-designed box. After the angle iron improvement was completed, the mechanic closed the valve and moved away. It was a good thing he did move away rapidly, because the box quickly exploded. 

Many linear polymers are too flexible to be of use in making everyday objects because they lack the strength, the rigidity, or the elasticity for the job. Linear polymers can be stiffened and strengthened by bonds between the chains. This process is known as cross-linking and we will look now at some ways in which this can be achieved. 

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