Creep and fatigue

The long-term response of materials to load in either a steady or fluctuating form is considered. The role of the reinforcement in resisting the load is considered as well as the role of the resin in protecting the reinforcement and transferring the load to the fibres. 

In stage I the component is initially stressed and some damage is introduced as may be detected by a drop in stiffness. 

Stage II takes place over a much longer time period in which there is only a slow change in strain and stiffness with time ( steady state ). 

Stage III is characterized by an ever-increasing amount of damage, which could lead to failure. This can usually be detected by an increasing drop in stiffness. If the stress is very low then this stage may only be reached after a very long time if at all (fatigue or creep limit). 

What constitutes failure (section 1.4) will be dictated by the design, but a 10% drop in stiffness is becoming accepted in fatigue. For creep, a strain limit is usually imposed by the design. 

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The material in use as of the mid-1990s in these components is HDPE, a linear polymer which is tough, resiUent, ductile, wear resistant, and has low friction (see Olefin polymers, polyethylene). Polymers are prone to both creep and fatigue (stress) cracking. Moreover, HDPE has a modulus of elasticity that is only one-tenth that of the bone, thus it increases the level of stress transmitted to the cement, thereby increasing the potential for cement mantle failure. When the acetabular HDPE cup is backed by metal, it stiffens the HDPE cup. This results in function similar to that of natural subchondral bone. Metal backing has become standard on acetabular cups. 

In this chapter the various approaches to the fracture of plastics are described and specific causes such as impact loading, creep and fatigue are described in detail. 

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. 

Many plastic products seen in everyday life are not required to undergo sophisticated design analysis because they are not required to withstand extreme loading conditions such as creep and fatigue loads. Examples include containers cups toys boxes housings for computers, radios, televisions and the like and nonstructural or secondary structural products of various kinds in buildings, aircraft, appliances, and electronic devices. These type products require reviewing... 

In many cases, a product fails when the material begins to yield plastically. In a few cases, one may tolerate a small dimensional change and permit a static load that exceeds the yield strength. Actual fracture at the ultimate strength of the material would then constitute failure. The criterion for failure may be based on normal or shear stress in either case. Impact, creep and fatigue failures are the most common mode of failures. Other modes of failure include excessive elastic deflection or buckling. The actual failure mechanism may be quite complicated each failure theory is only an attempt to explain the failure mechanism for a given class of materials. In each case a safety factor is employed to eliminate failure. 

Acetal This crystalline plastic is strong, stiff, and has exceptional resistance to abrasion, heat, chemicals, creep and fatigue. With a low coefficient of surface friction, it is especially useful for mechanical products such as gears, pawls, latches, cams, cranks, plumbing parts, etc. It is chrome platable. 

Ides operates the Prospector materials database with a search facility covering 38,000 plastic products from 400 global manufacturers (www.ides.com). Among the durability properties listed are creep and fatigue. 

CNF is an industrially produced derivative of carbon formed by the decomposition and graphitization of rich organic carbon polymers (Fig. 14.3). The most common precursor is polyacrylonitrile (PAN), as it yields high tensile and compressive strength fibers that have high resistance to corrosion, creep and fatigue. For these reasons, the fibers are widely used in the automotive and aerospace industries [1], Carbon fiber is an important ingredient of carbon composite materials, which are used in fuel cell construction, particularly in gas-diffusion layers where the fibers are woven to form a type of carbon cloth. 

Kaufman, J.G. Properties of Aluminum Alloys Tensile, Creep, and Fatigue Data at High and Low Temperatures, ASM International, Materials Park, OH, 1999. Kaufman, J.G. Introduction to Aluminum Alloys and Tempers, ASM International, Materials Park, OH. 2000. 

QE22A 2% Pr 0.7% Zr 2.57c Ag 40 78 549 Castings for aerospace uses, (up io 260° C) Superior [ensile slrength plus excellent creep and fatigue strength... 

Pure lead has low creep and fatigue resistance, but its physical properties can be improved by the addition of small amounts of silver, copper, antimony, or tellurium. Lead-clad equipment is in common use in many chemical plants. The excellent corrosion-resistance properties of lead are caused by the formation of protective surface coatings. If the coating is one of the highly insoluble lead salts, such as sulfate, carbonate, or phosphate, good corrosion resistance is obtained. Little protection is offered, however, if the coating is a soluble salt, such as nitrate, acetate, or chloride. As a result, lead shows good resistance to sufuric acid and phosphoric acid, but it is susceptible to attack by acetic acid and nitric acid. 

In hard tissue replacement, polyurethane filled with hydroxyapatite is used. The composite must have high strength and stiffness and excellent creep and fatigue resistance. The material in the study met these requirements when hydroxyapatite was pretreated with hexamethylene diisocyanate. The treatment improved the adhesion between the matrix and the filler. ... 

Good creep and fatigue resistance Low water sorption High resistance to chemicals (most acidic or basic aqueous media)... 

Improving the long-term service fife and performance of polymer blends (thermal aging/ embrittlement resistance, creep and fatigue resistance, weatherabUity, etc.). 

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