影响耐久性强度的因素机械设计及制造专业外文翻译中英文对照

1、本科生毕业论文(设计)外文翻译FACTORS AFFECTING THE ENDURANCE STRENGTHPublished data for en dura nee stre ngth are determ ined by special fatigue testi ng devices,which typically use a polished speeime n subjected to a reversed bending load,similar that sketched in figure .If the actual operati ng con diti ons of

2、a part in a machi ne are differe nt.a nd they usually are,the faigue stre ngth must be reduced from the reported value.Some of the factors that decrease the en dura nee stre ngth are discussed n ext.This discussi on relates to the en dura nee stre ngth for materials subjected only to tensile normal

3、stresses,thatis,tensile stressesresulting from bending actions or axial tension.Casesinvolving fluctuating torsional shear stressesare discussed separately.Fatigue failures are most likely to occur in regions of high tensile stress rather tha n compressive stress.Size of the sectionThe test specimen

4、 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 uni formity of properties,particularly with heat-treated parts.Refere nce 1 in cludes a suggested method of determing the size factor for rotating shafts up

5、to 10.0 inches(250 mm)in diameter. We will use that method here also becausewe are basing our analysis on the reversed bending phenomenon.Table shows the suggested relati on ships for determ ing a size factor,to be applied to the en dura nce strength to account for the size of the cross section .Fig

6、ure shows plots of the equati ons from Table,with some ble nding of the curves,Read in gfrom 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,rol

7、led shapes,the thickness should be used.Noted that the use of these equati ons is approximate.Surface FinishAny deviati on from a polished surface reduces en dura nce stre ngth.Figure shows rough estimates for the endurance strengths,comparedwith the ultimate tensile strength of steels for several p

8、ractical surface conditions,It is critical that parts subjected to fatigue loading be protected from nicks,scratches,and corrosion because they drastically reduce fatigue stre ngth.Stress ConcentrationsSudde n cha nges in geometry especially sharp grooves and no tches where high stress concentration

9、s occur,are likely places for fatigue failures to occur.Care should be take n in the desig n and manu facture of cyclically loaded parts to keep stress concen trati on factors to a low value.We will apply the stress concen trati on factors ,as found from the methods of Secti on ,to the computed stre

10、sses,rather tha n to the allowable stre ngths.FlawsIn ternal 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 in spected, a higher-tha n-average desig n factor shou

11、ld be specified for cast parts,a nd a lower en dura nee stre ngth should be used.TemperatureMost materials have a lower endurances strength at high temperatures.The reported values are for room temperatures.Operation above 160F(72 ) will reduce the endurance strength of most ductile materials.Nonuni

12、form Material PropertiesMany materials have differe nt material properties in differe nt directi ons because of the manner in which the material was processed.Rolled sheet or bar products are typically stron ger in the directi on of rolli ng tha n they are in the tran sverse 第3页共19页本科生毕业论文（设计）direct

13、i on.Fatigue tests are likely have been run on test bars oriented in the stronger directio n.Stressi ng of such material in the tran sverse directi on may result in lower en dura nee stre ngth.Nonuniform properties are also likely to exist in the vic inity of welds because of in complete weld pen et

14、rati on, slag in clusi on s,a nd variati ons 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 ann eali ng n ear the weld.Some welding processes may result in th production of residual tensile stresses that decrease

15、 the effective en dura nee stre ngth 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 con sidered.第5页共19页FIGURE 5-8 Size factor06o1,0246 JJShafi diameter (in)050!00150200250Sh

16、afi diameteT (mm)TABLE 5-1 Suggested size factorsU.S. Customary UnnsSI UnitsSiz亡 rangeFor D io jiKhe?Siie rangeFor D in mmD040in0.40 D 2,0 in2 0 D-09D 10 mm10 50 mu50 /J FIGURE 5-9Endurance strength sn versus tensile strength for wrought steel for various surface conditionsTensile firrnglh, MPa60080

17、0I 0001 300Tensile $urn窘th, Ksi本科生毕业论文(设计)Residual StressesFatigue failures typically initiate at locations of relatively high tensile stress.Grinding and machining,especially with high material removal rates,also cause un desirable residual ten sile stress.Weldi ng has already bee n men ti oned as

18、a process that may produce residual ten sile 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.Processesthat produce residual co

19、mpressive stresses can prove to be ben efical.Shot blasti ng 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,andother cycl

20、ically loaded machi ne parts can ben efit from these methods.Corrosion and Environmental FactorsEndurance strength data are typically measured with the specimen in air.Operati ng con diti ons that expose a comp onent to water,salt solutio ns,or other corrosive environments can significantly reduce t

21、he effective endurance stre ngth.Corrosi on may cause harmful local surface rough ness and may also alter the internal grain structure and chemistry of ehe material.Steels exposed to hydroge n are especially affected adversely.NitridingNitriding is a surface-hardening process for alloy steels in whi

22、ch the material is heated to 950F (14C) in a nitrogen atmosphere,typically ammonia gas,followed by slow cooli ngm pvoveme nt of en dura nee stre ngth of 50% or more can be achieved with n itridi ng.Wrought versus Cast MaterialsMetal alloys hav ing similar chemical compositi ons can be either wrought

23、 or cast to produce the final form.Wrought materials are usually rolled or draw n. Wrought materials usually have a higher endurance strength than cast materials of similar compositi on in regi ons where no sig ni fica nt stress concen trati on exits.However,i n the vici nity of no tches and other d

24、isc on ti nu ities,the en dura nce stre ngth 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 tha n the wrought material and is less affected by the prese nceof the stress con

25、cen trati on.To use the more con servative approach,it is recomme nded 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 recomme nded.Type of StressEn dura nce stre ngth data are obta ined from the rotati ng beam test that produ

26、ces completely reversed and repeated normal stresses.The maximum stress is producted at the surface of the specime n,and the stress vanes lin early to zero at the center of the circular cross section.lf the actual loading is different from bending,a factor for the type of loadi ng should be applied

27、to the en dura nee stre ngth.Axial TensionUn der pure tension, all of the material-no t just the surface is subjected to the maximum stress. A factor of 0.80 is suggested to be the bending en dura nee stre ngth to reflect this differe nt behavior.Effect of Stress Ratio on Endurance StrengthFigure 5-

28、10 shows the gen eral variati on of en dura nce-stre ngth data for a give n material when the stress ratio R varies from -1.0 to +1.0,covering the range of cases in clud ing the follow ing: Repeated,reversed stress(Figure 5-3);R=-1.0 Partially reversed fluctuating stress with a tensile mean stress【F

29、igure5-4(b)】 ;-1.0 R 0第9页共19页本科生毕业论文（设计）Static(U” Fluctuatingtenik stressrOne dirtcticmParti allyreversedReversedIf)4 |(P IO6 W7 0 INumber of cycles to failure0090和7035a40302()ln-Bi 4a 一ufuajk总 吕5 ultlFs-Es Repeated, on e-directi on ten sile stress(Figure 5-6);R=0 Fluctuating tensile stress Figure 5

30、-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.lf such data are desired for a particular material,specific data for that material must be found either experimentally or published literature.The most damag

31、i ng 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 show n in Figure 5-2 is an example of a load-carry ing member subjected to a stress ratio R=-1.Fluctuat ing stresses with a compressive mea n stress

32、 as show n in Parts(c) and (d) of Figure 5-4 do not significantly affect the endurance strength of the material第9页共19页本科生毕业论文（设计）because fatigue failures tend to originate in regions of tensile stress.Note that the curves of Figure 5-10 show estimate of the en dura nee stre ngth, Sn ,as a funetion o

33、f the ultimate tensile strength for steel.These data apply to ideal polished speeime ns and do not in elude any of ethe other factors discussed in this seetion. For example,the curve for R=-1.0(reversed bending)shows that the en dura nee stre ngth for steel is approximately 0.5 times the ultimate st

34、rength(0.50 Sn)for large numbers of cycles of loading(approximately 10 or higher).This is a good gen eral estimate for steels. The chart also shows that types of load produc ing R greater tha n -1.0 but less tha n 1.0 have less of an effect on the en dura nee stre ngth. This illustrates that using d

35、ata from the reversed bending test is the most eon servative.We will not use Figure 5-10 directly for problem in this book because our procedure for estimati ng the actual en dura nee stre ngth starts with the use of Figure 5-9 which prese nts data from reversed bending tests .Therefore,the effect o

36、f stress ratio is already in eluded. Seeti on 5-9 of this chapter in eludes methods of an alysis for loading casesin which the fluctuating stress produces a stress ratio different from R=-1.0ReliabilityThe data for en dura ncestre ngth for steel show n in Figure 5-9 represe nt average values derived

37、 from many tests of speeimens 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 ,representsa reliability of 50%,indicating that half

38、 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 en dura nee strength than can be used for design to produce the higher reliability values. Ideally,a statistical an alysis of actual data for the mat

39、erial to be used in the desig n should be obtained.Referenee 8 shows a method for analyzing such data.By making certain assumptions about the form of the distribution of strength data.Refere nee 8 also reports the values as approximate reliability factors,Cr. We will apply these factors to the avera

40、ge en dura nee stre ngth.第13页共19页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

41、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-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

42、，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160 F(72 C)以上的温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。 度会减少其韧性材料强度大部分耐久

43、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。本科生毕业论文(设计)FIGURE 弋 SizeLIfactor).0.Q.

44、8.7 o.oo. U号爲一困s尺寸246810Shafi diameter (in)轴的直泾50100150200250Shaft diameter (mm)TABLE 5-1 Suggested size factors 建仪的大小尺寸 芙国单位LS, Customary Units尺寸范围Size ranee直径I央寸For D w inches尺寸范围Sice range直径1盍术For O in mmD 0 40 in0J0 D2.0 tn2.0 D KKO inG= 10G = (D/O3)-006*D 10 mm10 D 50 mm50 /? 250 mmCT = Lt)C； =

45、 (D/7,矿叭C = LB5D-1119HGURE 5-9Enduriincc strength sn versus tensile strengthioo拉伸强度Tensile 5ln:nglhh MPla60080I QOO1 200for wrQUghc steel for various surface conditions的 下 况sn 面強I各与银耐F 仝一MuaJU8010012014016018020022040200I 400W0003 划一loo0Tensile strength, Kst第17页共19页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

46、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

47、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 F( 514 C)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

48、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= 1ix一匚 3J 社】色-ns lunluxFs+外应力注意，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