Fatigue reinforcements

Nylon, an aUphatic polyamide, was introduced as a commercial tire cord in 1947 and grew in usage to - 5.4 billion kg/yr (- 2 billion Ib/yr) in the 1990s (10,11). Nylon-reinforced tires use nylon-6 poljmier (polycaprolactam) fibers as well as nylon-6,6 (poly(hexamethylenediainine adipamide)) fibers. Nylon tire cords are characterized by extremely good fatigue resistance in compression and good adhesion to most mbber compounds with simple RFL adhesives. 

High impact strength, increased hardness, lower thermal expansion, and high fatigue strength are also important properties required of denture-base materials. To address these deficiencies, alternatives to the traditional PMMA dentures have been sought. These include the use of other base polymers and reinforced designed denture systems. 

Applications. Initial appHcations have been largely in military and aerospace areas. These include hydrauHc seals for military aircraft and fuel seals and diaphragms for both military and civiHan aircraft. Shock mounts for EZ are used on aircraft engines. Large fabric-reinforced boot seals are used in the air intake system on the M-1 tank. The material s useful temperature range, fuel and fatigue resistance, and fire resistance were determining factors in this appHcation. 

The general fatigue behaviour which is observed in glass fibre reinforced plastics is illustrated in Fig. 3.32. In most grp materials, debonding occurs... 

Fig. 3.32 Typical fatigue behaviour of glass reinforced polyester...

There is no general rule as to whether or not glass reinforcement enhances the fatigue behaviour of the base material. In some cases the matrix exhibits longer fatigue endurances than the reinforced material whereas in other cases the converse is true. In most cases the fatigue endurance of grp is reduced by the presence of moisture. 

In this book no prior knowledge of plastics is assumed. Chapter 1 provides a brief introduction to the structure of plastics and it provides an insight to the way in which their unique structure affects their performance. There is a resume of the main types of plastics which are available. Chapter 2 deals with the mechanical properties of unreinforced and reinforced plastics under the general heading of deformation. The time dependent behaviour of the materials is introduced and simple design procedures are illustrated. Chapter 3 continues the discussion on properties but concentrates on fracture as caused by creep, fatigue and impact. The concepts of fracture mechanics are also introduced for reinforced and unreinforced plastics. 

Pipes, particularly small-bore pipes, often have failed by fatigue because their support was insufficient and tlie pipes were free to vibrate. On other occasions, pipes failed because their support was too rigid and tliey were not fi ee to expand. Supports for pipes usually are constructed on site, and often it is not apparent until start-up tliat tlie supports are inadequate. Even at tliis point, tlie necessary reinforcement may not be done because it is not considered a priority at start-up. 

Plates and pipe inside these vessels must be of heavy design and welded (not bolted) to the vessel itself. Otherwise, vibration forces within the vessels and piping can create fatigue cracks and equipment feilure. Figure 13-26. Even if applicable vessel codes do not require such strength reinforcement, it is advisable for the designer to insist on these extra rugged details. 

During the last half-century, aeronautics technology has soared, with plastics playing a major role. Lightweight durable plastics and reinforced plastics (RPs) save on fuel while standing up to forms of stress like creep and fatigue, in different environments. 

Examples of fatigue curves for unreinforced (top) and reinforced (bottom) plastics are shown in Fig. 2-44. The values for stress amplitude and the number of load cycles to failure are plotted on a diagram with logarithmically divided abscissa and English or metrically divided ordinates. 

Reinforced plastic In common with metals and unreinforced plastics, RPs also is susceptible to fatigue. However, they provide high performance when compared to un-... 

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