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1、本科生毕业论文（设计）外文翻译FACTORS AFFECTING THE ENDURANCE STRENGTHPublished data for endurance strength are determined by special fatigue testing devices,which typically use a polished specimen subjected to a reversed bending load,similar that sketched in figure.If the actual operating conditions of a part in
2、a machine are different.and they usually are,the faigue strength must be reduced from the reported value.Some of the factors that decrease the endurance strength are discussed next.This discussion relates to the endurance strength for materials subjected only to tensile normal stresses,that is,tensi
3、le stresses resulting from bending actions or axial tension.Cases involving fluctuating torsional shear stresses are discussed separately.Fatigue failures are most likely to occur in regions of high tensile stress rather than compressive stress.Size of the sectionThe test specimen is usually 0.30 in
4、ch (7.6 mm)in diameter.Larger section sizes exhibit lower strengths,have a less favorable stress distribution,and have less uniformity of properties,particularly with heat-treated parts.Reference 1 includes a suggested method of determing the size factor for rotating shafts up to 10.0 inches(250 mm)
5、in diameter. We will use that method here also because we are basing our analysis on the reversed bending phenomenon.Table shows the suggested relationships for determing a size factor,to be applied to the endurance strength to account for the size of the cross section .Figure shows plots of the equ
6、ations from Table,with some blending of the curves,Reading from the curves should provide acceptable accuracy.When the component being designed is not circular like a shaft,judgment is required to determine the characteristic dimension to use in the formulas.For flat,rolled shapes,the thickness shou
7、ld be used.Noted that the use of these equations is approximate.Surface FinishAny deviation from a polished surface reduces endurance strength.Figure shows rough estimates for the endurance strengths,compared with the ultimate tensile strength of steels for several practical surface conditions,It is
8、 critical that parts subjected to fatigue loading be protected from nicks,scratches,and corrosion because they drastically reduce fatigue strength. Stress ConcentrationsSudden changes in geometry ,especially sharp grooves and notches where high stress concentrations occur,are likely places for fatig
9、ue failures to occur.Care should be taken in the design and manufacture of cyclically loaded parts to keep stress concentration factors to a low value.We will apply the stress concentration factors ,as found from the methods of Section,to the computed stresses,rather than to the allowable strengths.
10、 FlawsInternal flaws of the material,especially likely in cast parts,are places in which fatigue cracks initiate.Critical parts can be inspected by x-ray techniques for internal flaws,If they are not inspected, a higher-than-average design factor should be specified for cast parts,and a lower endura
11、nce strength should be used.TemperatureMost materials have a lower endurances strength at high temperatures.The reported values are for room temperatures.Operation above 160F(72C) will reduce the endurance strength of most ductile materials.Nonuniform Material PropertiesMany materials have different
12、 material properties in different directions because of the manner in which the material was processed.Rolled sheet or bar products are typically stronger in the direction of rolling than they are in the transverse direction.Fatigue tests are likely have been run on test bars oriented in the stronge
13、r direction.Stressing of such material in the transverse direction may result in lower endurance strength.Nonuniform properties are also likely to exist in the vicinity of welds because of incomplete weld penetration,slag inclusions,and variations in the geometry of the part at the weld.Also.welding
14、 of heat-treated materials may alter the strength of the material because of local annealing near the weld.Some welding processes may result in th production of residual tensile stresses that decrease the effective endurance strength of the material.Annealing or normalizing after welding is often us
15、ed to relieve these stresses,but the effect of such treatments on the strength of the base material must be considered.Residual Stresses Fatigue failures typically initiate at locations of relatively high tensile stress.Grinding and machining,especially with high material removal rates,also cause un
16、desirable residual tensile stress.Welding has already been mentioned as a process that may produce residual tensile stress.Any manufacturing process that tends to produce residual stress will decrease the endurance strength of the component. Critical areas of cyclically loaded components should be machined or ground in a gentle fashion.Processes that produce residual compressive stresses can prove to be benefic
