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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 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,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.Size of the sectionThe 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 for 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 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 formulas.For flat,rolled shapes,the thickness should 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 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 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 Section,to the computed stresses,rather than to the allowable strengths. 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 endurance 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 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 stronger 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 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 used 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 undesirable 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 benefical.Shot blasting and peening are two such methods.Shot blasting is performed by directing a highvelocity stream of hardened balls or pellets at the surface to be treated.Peening uses a series of hammer blows on the surface.Crankshafts,springs,and other cyclically loaded machine parts can benefit from these methods.Corrosion and Environmental FactorsEndurance strength data are typically measured with the specimen in air.Operating conditions that expose a component to water,salt solutions,or other corrosive environments can significantly reduce the effective endurance strength.Corrosion may cause harmful local surface roughness and may also alter the internal grain structure and chemistry of ehe material.Steels exposed to hydrogen are especially affected adversely.NitridingNitriding is a surface-hardening process for alloy steels in which the material is heated to 950F（514C）in a nitrogen atmosphere,typically ammonia gas,followed by slow cooling.Impvovement of endurance strength of 50% or more can be achieved with nitriding.Wrought versus Cast Materials Metal alloys having similar chemical compositions can be either wrought or cast to produce the final form.Wrought materials are usually rolled or drawn.Wrought materials usually have a higher endurance strength than cast materials of similar composition in regions where no significant stress concentration exits.However,in the vicinity of notches and other discontinuities,the endurance strength of wrought and cast materials is more nearly equal.One possible explanation of this phenomenon is that the cast material is likely to have more isotropic material properties than the wrought material and is less affected by the presence of the stress concentration.To use the more conservative approach,it is recommended that a factor of 0.8 be applied to the basic endurance strength if a cast steel is used.For cast iron,a factor of 0.70 is recommended.Type of StressEndurance strength data are obtained from the rotating beam test that produces completely reversed and repeated normal stresses.The maximum stress is producted at the surface of the specimen,and the stress vanes linearly to zero at the center of the circular cross section.If the actual loading is different from bending,a factor for the type of loading should be applied to the endurance strength.Axial TensionUnder pure tension, all of the material-not just the surfaceis subjected to the maximum stress. A factor of 0.80 is suggested to be the bending endurance strength to reflect this different behavior.Effect of Stress Ratio on Endurance StrengthFigure 5-10 shows the general variation of endurance-strength data for a given material when the stress ratio R varies from -1.0 to +1.0,covering the range of cases including the following: Repeated,reversed stress(Figure 5-3);R=-1.0 Partially reversed fluctuating stress with a tensile mean stress【Figure 5-4(b)】;-1.0 R 0 Repeated,one-direction tensile stress(Figure 5-6);R=0 Fluctuating tensile stress【Figure 5-4(a)】;0 R 1.0 Static stress(Figure 5-1);R=1Note that Figure 5-10 is only an example,and it should not be used to determine actual data points.If such data are desired for a particular material,specific data for that material must be found either experimentally or published literature.The most damaging kind of stress among those listed is the repeated,reversed stress with R=-1.(See Reference 2.page 27.)Recall that the rotating shaft in bending as shown in Figure 5-2 is an example of a load-carrying member subjected to a stress ratio R=-1.Fluctuating stresses with a compressive mean stress as shown in Parts(c) and (d) of Figure 5-4 do not significantly affect the endurance strength of the material because fatigue failures tend to originate in regions of tensile stress.Note that the curves of Figure 5-10 show estimate of the endurance strength, Sn ,as a function of the ultimate tensile strength for steel.These data apply to ideal polished specimens and do not include any of ethe other factors discussed in this section. For example,the curve for R=-1.0(reversed bending)shows that the endurance strength for steel is approximately 0.5 times the ultimate strength(0.50Sn)for large numbers of cycles of loading(approximately 10 or higher).This is a good general estimate for steels. The chart also shows that types of load producing R greater than -1.0 but less than 1.0 have less of an effect on the endurance strength. This illustrates that using data from the reversed bending test is the most conservative.We will not use Figure 5-10 directly for problem in this book because our procedure for estimating the actual endurance strength starts with the use of Figure 5-9 which presents data from reversed bending tests .Therefore,the effect of stress ratio is already included. Section 5-9 of this chapter includes methods of analysis for loading cases in which the fluctuating stress produces a stress ratio different from R=-1.0ReliabilityThe data for endurance strength for steel shown in Figure 5-9 represent average values derived from many tests of specimens having the appropriate ultimate strength and surface conditions.Naturally,there is variation among the data points；that is, half are higher and half are lower than the reported values on the given curve.The curve,then ,represents a reliability of 50%,indicating that half of the parts would fail. Obviously, it is advisable to design for a higher reliability, say.90%,99%,or 99.9%.A factor can be used to estimate a lower endurance strength than can be used for design to produce the higher reliability values. Ideally,a statistical analysis of actual data for the material to be used in the design should be obtained.Reference 8 shows a method for analyzing such data.By making certain assumptions about the form of the distribution of strength data.Reference 8 also reports the values as approximate reliability factors,Cr. We will apply these factors to the average endurance strength.影响耐久性强度的因素耐力强度公布的数据是由特殊的疲劳试验装置测量出的，通常采用抛光试样遭受了反向弯曲载荷的形式,就像图中所描绘的。如果一个机器零件的实际操作状况是不同的。通常是疲劳强度必须从报告值减少。一些降低强度的耐久性的因素下一步讨论。 这个讨论涉及到对仅受正应力的材料拉伸强度的耐久性，也就是拉应力造成的弯曲的行动或轴向张力。疲劳破坏是最可能发生在高强度的压力，而不是压应力区。当涉及到波动扭转剪应力时候需要分别进行讨论。截面的大小 测试样本通常是直径0.30英寸（7.6毫米）。大断面尺寸具有较低的优势，有一个不太有利的应力分布，并有减少性能的均匀性，特别是经过热处理的零件。参考文献1包括一个测定高达10.0英寸直径（250毫米）旋转轴的尺寸因素建议的方法。我们将在这里使用该方法还因为我们立足于扭转弯曲现象的分析。表显示尺寸因素之间的关系被应用到耐久力所占了横截面得大小图显示了表中一些混合的曲线方程 ，从曲线读数应提供可接受的精度 当组件被设计是不是像一个圆轴，须作出判断，以确定特征尺寸的公式中使用。对于平面，轧制钢，应考虑厚度，指出这些方程组采用的近似。表面处理抛光表面的任何偏差都会降低耐久性图中显示的耐力优势粗略估计，相对于最终拉伸强度钢表面的几个实际情况，至关重要的是，受疲劳载荷得到保护，免受划痕，划伤，腐蚀，因为它们大大降低零件的疲劳强度。应力集中突然变化的几何形状，尤其是尖锐凹槽和缺口在高应力集中发生，是有可能发生疲劳破坏的地方。应注意的设计和制造的循环加载零件应力集中系数保持为较低值。我们将利用应力集中因素，从这节中的方法发现，讲的是计算压力，而不是许用强度缺陷；裂缝材料内部缺陷，尤其是在铸件内部缺陷可能是疲劳裂纹的地方开始关键零部件，可通过检查内部缺陷的X射线技术，如果他们没有得到检查，一个高于平均设计因素应该被指定为铸造件，并以较低的耐力强度应使用。温度大多数材料在高温下耐力强度较低。报道中的值是房间的温度。160F(72C)以上的温度会减少其韧性材料强度大部分耐久性。非统一的材料性能不同材料在不同方向上有不同的材料特性是因为进行处理的方式不同，冷轧薄板或棒的产品通常在轧制方向的强于他们在横向方向。疲劳试验过程中很可能已经运行在测试杆以较强的方向，这种材料在强调横向方向可能导致较低的耐力力量。 非均匀性也可能存在，因为不完整的焊缝熔深，夹渣附近，并在部分在焊缝几何形状变化。对热处理材料焊接可能会因为当地的焊缝附近退火材料的强度而改变。有些焊接过程可能产生残余拉应力而降低了材料的有效持久强度。焊后退火或正火通常用来缓解这些压力，但这种方法对基体材料强度的影响必须予以考虑。残余应力疲劳破坏通常开始在相对较高的拉应力的位置。任何制造过程中往往会产生残余应力会降低组件的持久强度已经提到的焊接就是一个可能会产生残余拉应力的过程。研磨和加工，特别是高的材料去除率，也造成不良的残余拉应力。循环加载的关键部件加工领域应以温和的方式或理由。 生产过程的残余压应力可以证明是有益的。喷砂处理和强化就是这样两种方法，喷丸是利用高速丸流的冲击作用清理和强化基体表面的过程。喷丸采用了表面上的一系列冲击，曲轴，弹簧，循环载荷等机械零件可以受益于这些方法腐蚀与环境因素耐力强度数据通常测量空气中的标本。揭露出一个的水，盐溶液，或其他腐蚀性环境工作条件中能显着降低有效持久强度。腐蚀可能导致有害的局部表面粗糙度，也可以改变内部晶粒结构和外置式换热器材料化学性质。钢接触到氢尤其受到不利影响。氮化氮化是一种合金钢表面硬化过程中，被加热的物质在氮气环境中，以950（514）缓慢冷却。氮化后可实现耐力强度提高50以上。锻造与铸造材料 金属合金具有类似的化学成分可以是锻造或铸造生产的最后形式。锻造材料通常轧制或拉伸。锻造材料在没有明显的应力集中通常具有比同类铸铁材料耐久度较高的地区。然而，在缺口和其他间断附近，锻造和铸造材料的强度耐力更接近相等。这种现象的一个可能的解释是，铸造材料很可能拥有比锻造材料更各向同性的材料性能和较少受到应力集中的影响。如果使用更保守的方法，建议了如果使用铸钢，则0.8系数应用到基本的持久强度。如果是铸铁则推荐0.70应力类型耐力强度数据是从旋转梁测试，完全扭转，重复生产的正常压力。开发与生产的最大压力是在试样表面，叶片的应力为零，线性的圆形横截面的中心。如果实际加载是从不同的弯曲加载的，则一个影响装载方式的因素应该应用于耐久力轴向拉力在纯拉力下，所有的材料，不仅仅是表面，正在承受最大的压力。0.8这个值所对应的弯曲疲劳强度因素，以反映此不同的行为应力作用比对耐力的力量图5-10显示了一个给定材料耐力强度数据一般变化时的应力比R变化从-1.0到+1.0，覆盖范围包括下列例子：重复，扭转应力（图5-3）与r=-1.0部分逆转波动与拉伸应力平均应力【图5-4（b）】;-1.0r0重复，单向拉伸应力（图5-6）和r =0拉应力波动5-4（a），0r1.0静态应力（图5-1）与r= 1注意，图5-10只是一个例子，它不应该被用来确定点的实际数据. 如果这些数据是需要一个特定的材料，该材料的具体数据必须无论是实验或出版的文献中发现。最具破坏性的压力，就是重复颠倒与R=- 1的压力。（见参考文献2第27页）回想一下，在弯曲旋转轴，如图5-2所示是一个遭受了应力比R=-1负载的例子平均波动与压缩应力就如部分显示应力（c）和（d）。图5-4没有显着影响材料的疲劳耐力力量，因为失效往往源于拉伸应力区。请注意图的持久强度Sn作为钢的抗拉强度功能，如5-10显示的曲线。这些数据适用于理想的抛光试样，不包括在本节讨论的任何其他因素。例如，曲线的R=-1.0（扭转弯曲）表明，对钢材的强度大约是耐力的极限强度的0.5倍（0.50锡）因装载周期（约10或更高）。这是一个很好的钢材的一般估计。图表还显示，负载类型生产r大于-1.0，但小于1.0有一对耐力强度的影响较少。这说明了使用从扭转弯曲试验数据是最保守的。我们不会直接用图5-10来解决这本书里的问题，因为我们估算准确的耐力强度的程序是由图5-9来开始计算的，此图体现了从逆向弯曲试验中得出的数据。因此应力比的影响已经包括在内。此章5-9的部分包括了分析加载状况的方法，比如出现以下的状况：波动应力产生应力比不同于R=-1.0。可靠性耐久力在图5-9所示的数据代表钢的强度从拥有适当的极限强度和表面状况标本多次试验得出的平均值。当然，有数据点之间的变化，也就是说，有一半是高，一半人更在给定曲线的报告值低。曲线，那么，代表着50的可靠性，表明了零件一半会失败显然，最好是设计一个更高的可靠性，比如90，99，或99.9。一个可以用来估计比设计可用于生产可靠性高值低强度的耐力的因素。理想情况下，应获得对材料的实际数据统计分析中使用的设计。参考文献8显示了用于分析这些数据的方法。通过使对数据分布的力量形成一定的假设。参考文献8还报告近似可靠性因素Cr的值。我们将运用这些因素的平均持久强度。第 17 页 共 17 页
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