1、本科生毕业论文（设计） 第 1 页 共 17 页 外文翻译 FACTORS AFFECTING THE ENDURANCE STRENGTH Published 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 conditio
2、ns of a part in 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 stres
3、ses,that is,tensile 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. 本科生毕业论文（设计） 第 2 页 共 17 页 Size of the
4、section The test specimen is usually 0.30 inch (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 f
5、or rotating shafts up to 10.0 inches(250 mm)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
6、cross section .Figure shows plots of the equations 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 formu
7、las.For flat,rolled shapes,the thickness should be used.Noted that the use of these equations is approximate. Surface Finish Any deviation from a polished surface reduces endurance strength.Figure shows rough estimates for the endurance strengths,compared with the ultimate tensile strength of steels
8、 for several practical surface conditions,It is critical that parts subjected to fatigue loading be protected from nicks,scratches,and corrosion because they drastically reduce fatigue strength. 本科生毕业论文（设计） 第 3 页 共 17 页 Stress Concentrations Sudden changes in geometry ,especially sharp grooves and n
9、otches where high stress concentrations occur,are likely places for fatigue 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
10、 Section,to the computed stresses,rather than to the allowable strengths. Flaws Internal 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-a
11、verage design factor should be specified for cast parts,and a lower endurance strength should be used. Temperature Most 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 Properties Many materials have different 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
