Fatigue strength,

The fatigue strength is defined as that stress level at which the test specimen will sustain N cycles prior to failure. The data are generated on a machine that runs at 1800 cycles per minute. This test is of value to material manufacturers in determining consistency of their product (Chapter 2). 

This review concerns the long-term behavior of plastics when exposed to conditions that include continuous stresses, environment, excessive heat, abrasion, and continuous contact with liquids. This subject has been reviewed in Chapter 2 (LONG-TERM LOAD BEHAVIOR) but since it is a very important subject the review is continuing. Tests such as those outlined by ASTM D 2990 that describe in detail the specimen preparations and testing procedure are intended to produce consistency in observations and records by various manufacturers, so that they can be correlated to provide meaningful information to product designers. 

The procedure under this heading is intended as a recommendation for Uniformity 

The requirements for consistent results are outlined in detail as far as accuracy of time interval, of readings, etc., in the procedure. Each report of test results should indicate the exact grade of material and its supplier, the specimen s method of manufacture, its original dimensions, type of test (tension, compression, or flexure), temperature of test, stress level, and interval of readings. 

When a load is initially applied to a specimen, there is an instantaneous strain or elongation. Subsequent to this, there is the time-dependent part of the strain (creep), which results from the continuation of the constant stress at a constant temperature. In terms of design, creep means changing dimensions and deterioration of product strength when the product is subjected to a steady load over a prolonged period of time. 

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Fatigue improvement Fatigue resistance Fatigue strength... 

A more important effect of prestressiag is its effect on the mean stress at the bore of the cylinder when an internal pressure is appHed. It may be seen from Figure 6 that when an initially stress-free cylinder is subjected to an internal pressure, the shear stress at the bore of the cylinder increases from O to A. On the other hand, when a prestressed cylinder of the same dimensions is subjected to the same internal pressure, the shear stress at the bore changes from C to E. Although the range of shear stress is the same ia the two cases (distance OA = CE), the mean shear stress ia the prestressed cylinder, represented by point G, is smaller than that for the initially stress-free cylinder represented by point H. This reduction in the mean shear stress increases the fatigue strength of components subjected to repeated internal pressure. 

Fig. 17. Effect of mean shear stress on the fatigue strength of EN25 for life of 10 cycles (92). To convert MPa to psi, multiply by 145.

Lead—copper alloys are the primary material used in the continuous extmsion of cable coverings for the electrical power cable industry in the United States. Other alloys, containing tin and arsenic as well as copper, have also been developed for cable sheathing in the United States to provide higher fatigue strength. 

Elastohydrodynamic Lubrication (EHL). Lubrication needs in many machines ate minimized by carrying the load on concentrated contacts in ball and toUet beatings, gear teeth, cams, and some friction drives. With the load concentrated on a small elastically deformed area, these EHL contacts ate commonly characterized by a very thin separating hydrodynamic oil film which supports local stresses that tax the fatigue strength of the strongest steels. 

Fig. 9. Effect on fatigue strength of the plastic deformation of a carburized steel surface by shot peening (B) as compared to nitriding (A) and heat treating...

Symbol Nominal composition, % Thermal conductivity, W/(m-K) Electrical resistivity, fiO-cm Yield bend fatigue strength, MPa ... 

Load bend fatigue strength of alloys capable of withstanding 4—5 cycles before failure in 0—90—0 degree cycles, which is above the three-cycles-to-failure minimum in MIL-S l D-883 values pertain to a 0.25-mm thick strip that has been sheared to 0.45-mm width. 

A significant aspect of hip joint biomechanics is that the stmctural components are not normally subjected to constant loads. Rather, this joint is subject to unique compressive, torsion, tensile, and shear stress, sometimes simultaneously. Maximum loading occurs when the heel strikes down and the toe pushes off in walking. When an implant is in place its abiUty to withstand this repetitive loading is called its fatigue strength. If an implant is placed properly, its load is shared in an anatomically correct fashion with the bone. 

Biomaterials. Just as stem designs have evolved in an effort to develop an optimal combination of specifications, so have the types of metals and alloys employed in the constmction of total joint implants. Pure metals are usually too soft to be used in prosthesis. Therefore, alloys which exhibit improved characteristics of fatigue strength, tensile strength, ductihty, modulus of elasticity, hardness, resistance to corrosion, and biocompatibiUty are used. 

The geometry of the bowl parts is important, and for intermittently discharging centrifuges, fatigue strength must be considered. In general the tangential stress in a bowl wall is CJ. ... 

Table 2. Fatigue Strength of Typical Bearing Metals...

For high, fatigue strength in automotive bearings, a very thin layer of babbitt is desirable so that much of the load is taken on a stronger backing material. Relative improvement in fatigue resistance was found to be as follows with tin babbitt (14) ... 

Ball- and roller-bearing materials ate commonly selected to provide a minimum Rockwell hardness of 58—60 R at load-carrying contacts (37,38). Below this level, fatigue strength drops so rapidly as to seriously impair the utiUty of a material for rolling beatings which involve contact stresses in the 700—2800 MPa... 

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