Machinery's Handbook, 31st Edition
Fatigue 211 number of cycles to failure. This domain has commonly been called “low-cycle fatigue” or cyclic strain-controlled fatigue. The transition from low- to high-cycle fatigue behavior occurs in the range from approx imately 10,000 to 100,000 cycles. Many define low-cycle fatigue as failure that occurs in 50,000 cycles or less. Thermal Fatigue: Cyclic temperature changes in a machine part will produce cyclic stresses and strains if natural thermal expansions and contractions are either wholly or partially constrained. These cyclic strains produce fatigue failure just as if they were pro- duced by external mechanical loading. When strain cycling is produced by a fluctuating temperature field, the failure process is termed “thermal fatigue.” While thermal fatigue and mechanical fatigue phenomena are very similar, and can be mathematically expressed by the same types of equations, the use of mechanical fatigue results to predict thermal fatigue performance must be done with care. For equal values of plastic strain range, the number of cycles to failure is usually up to 2.5 times lower for thermally cycled than for mechanically cycled samples. Corrosion Fatigue: Corrosion fatigue is a failure mode where cyclic stresses and a corro sion-producing environment combine to initiate and propagate cracks in fewer stress cycles and at lower stress amplitudes than would be required in a more inert environment. The corrosion process forms pits and surface discontinuities that act as stress raisers to accelerate fatigue cracking. Cyclic loads may also cause cracking and flaking of the cor- rosion layer, baring fresh metal to the corrosive environment. Each process accelerates the other, making the cumulative result more serious. Surface or Contact Fatigue: Surface fatigue failure is usually associated with rolling surfaces in contact, and results in pitting, cracking, and spalling of the contacting surfaces from cyclic Hertz contact stresses that cause the maximum values of cyclic shear stresses to be slightly below the surface. The cyclic subsurface shear stresses generate cracks that propagate to the contacting surface, dislodging particles in the process. Combined Creep and Fatigue: In this failure mode, all of the conditions for both creep failure and fatigue failure exist simultaneously. Each process influences the other in pro ducing failure, but this interaction is not well understood. Factors of Safety.— There is always a risk that the working stress to which a member is subjected will exceed the strength of its material. The purpose of a factor of safety is to minimize this risk. Factors of safety can be incorporated into design calculations in many ways. For most calculations the following equation is used: (1) where f s is the factor of safety, S m is the strength of the material in pounds per square inch, and s w is the allowable working stress, also in pounds per square inch. Since the factor of safety is greater than 1, the allowable working stress will be less than the strength of the material. In general, S m is based on yield strength for ductile materials, ultimate strength for brit- tle materials, and fatigue strength for parts subjected to cyclic stressing. Most strength values are obtained by testing standard specimens at 68 ° F in normal atmospheres. If, however, the character of the stress or environment differs significantly from that used in obtaining standard strength data, then special data must be obtained. If special data are not available, standard data must be suitably modified. General recommendations for values of factors of safety f s are given in the following list. f s Application 1.3–1.5 For use with highly reliable materials where loading and environmental conditions are not severe, and where weight is an important consideration. 1.5–2 For applications using reliable materials where loading and environmental conditions are not severe. s w S m f s = ---
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