影響耐久性強度的因素外文翻譯

1、<p><b>　　外文翻譯</b></p><p>　　FACTORS AFFECTING THE ENDURANCE STRENGTH</p><p>　　Published data for endurance strength are determined by special fatigue testing devices,which typica

2、lly 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 re

3、duced from the reported value.Some of the factors that decrease the endurance strength are discussed next.</p><p>　　This discussion relates to the endurance strength for materials subjected only to tensile n

4、ormal 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 o

5、f high tensile stress rather than compressive stress.</p><p>　　Size of the section</p><p>　　The test specimen is usually 0.30 inch (7.6 mm)in diameter.Larger section sizes exhibit lower strength

6、s,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

7、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 det</p><p>　　When the component

8、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

9、is approximate.</p><p>　　Surface Finish</p><p>　　Any deviation from a polished surface reduces endurance strength.Figure shows rough estimates for the endurance strengths,compared with the ultim

10、ate 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 stren

11、gth. </p><p>　　Stress Concentrations</p><p>　　Sudden changes in geometry ,especially sharp grooves and notches where high stress concentrations occur,are likely places for fatigue failures to oc

12、cur.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,t

13、o the computed stresses,rather than to the allowable strengths. </p><p><b>　　Flaws</b></p><p>　　Internal flaws of the material,especially likely in cast parts,are places in which fat

14、igue 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

15、 should be used.</p><p>　　Temperature</p><p>　　Most materials have a lower endurances strength at high temperatures.The reported values are for room temperatures.Operation above 160°F(72

16、76;C) will reduce the endurance strength of most ductile materials.</p><p>　　Nonuniform Material Properties</p><p>　　Many materials have different material properties in different directions bec

17、ause 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.</p><p>　　Fatigue tests are li

18、kely 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.</p><p>　　Nonuniform properties are also likel

19、y 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.</p><p>　　Also.welding of heat-treated materials may alter th

20、e strength of the material because of local annealing near the weld.</p><p>　　Some welding processes may result in th production of residual tensile stresses that decrease the effective endurance strength of

21、 the material.</p><p>　　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.</p>&l

22、t;p>　　Residual Stresses </p><p>　　Fatigue failures typically initiate at locations of relatively high tensile stress.Grinding and machining,especially with high material removal rates,also cause undesirab

23、le residual tensile stress.Welding has already been mentioned as a process that may produce residual tensile stress.</p><p>　　Any manufacturing process that tends to produce residual stress will decrease the

24、 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 bl

25、asting 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 o</p><p>

26、　　Corrosion and Environmental Factors</p><p>　　Endurance strength data are typically measured with the specimen in air.Operating conditions that expose a component to water,salt solutions,or other corrosive

27、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 h

28、ydrogen are especially affected adversely.</p><p><b>　　Nitriding</b></p><p>　　Nitriding is a surface-hardening process for alloy steels in which the material is heated to 950°F（

29、514°C）in a nitrogen atmosphere,typically ammonia gas,followed by slow cooling.Impvovement of endurance strength of 50% or more can be achieved with nitriding.</p><p>　　Wrought versus Cast Materials <

30、/p><p>　　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

31、 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 mate

32、rials is more nearly equal.One possible explanation of this phenomenon is that </p><p>　　To use the more conservative approach,it is recommended that a factor of 0.8 be applied to the basic endurance strengt

33、h if a cast steel is used.For cast iron,a factor of 0.70 is recommended.</p><p>　　Type of Stress</p><p>　　Endurance strength data are obtained from the rotating beam test that produces completel

34、y 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

35、from bending,a factor for the type of loading should be applied to the endurance strength.</p><p>　　Axial Tension</p><p>　　Under pure tension, all of the material--not just the surface—is subjec

36、ted to the maximum stress. A factor of 0.80 is suggested to be the bending endurance strength to reflect this different behavior.</p><p>　　Effect of Stress Ratio on Endurance Strength</p><p>　　F

37、igure 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:</p><p>　　Repeated,

38、reversed stress(Figure 5-3);R=-1.0</p><p>　　Partially reversed fluctuating stress with a tensile mean stress【Figure 5-4(b)】;-1.0 < R < 0</p><p>　　Repeated,one-direction tensile stress(Figu

39、re 5-6);R=0</p><p>　　Fluctuating tensile stress【Figure 5-4(a)】;0 < R < 1.0</p><p>　　Static stress(Figure 5-1);R=1</p><p>　　Note that Figure 5-10 is only an example,and it shou

40、ld 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.</p><p>　　The mos

41、t 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 su

42、bjected to a stress ratio R=-1.</p><p>　　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 beca

43、use fatigue failures tend to originate in regions of tensile stress.</p><p>　　Note that the curves of Figure 5-10 show estimate of the endurance strength, Sn ,as a function of the ultimate tensile strength f

44、or 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

45、is approximately 0.5 times the ultimate strength(0.50×Sn)for large numbers of cycles of loading(approximately 10 or higher).This is a good general esti</p><p>　　We will not use Figure 5-10 directly for

46、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 i

47、ncluded. 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=-1.0</p><p>　　Reliability</p><p>　　Th

48、e 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

49、 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 advi

50、sable to design for a higher reliability, say.90%,99%,or 9</p><p>　　影響耐久性強度的因素</p><p>　　耐力強度公布的數(shù)據(jù)是由特殊的疲勞試驗裝置測量出的，通常采用拋光試樣遭受了反向彎曲載荷的形式,就像圖中所描繪的。如果一個機器零件的實際操作狀況是不同的。通常是疲勞強度必須從報告值減少。一些降低強度的耐久性的因素下一

51、步討論。</p><p>　　這個討論涉及到對僅受正應(yīng)力的材料拉伸強度的耐久性，也就是拉應(yīng)力造成的彎曲的行動或軸向張力。疲勞破壞是最可能發(fā)生在高強度的壓力，而不是壓應(yīng)力區(qū)。當(dāng)涉及到波動扭轉(zhuǎn)剪應(yīng)力時候需要分別進行討論。</p><p><b>　　截面的大小</b></p><p>　　測試樣本通常是直徑0.30英寸（7.6毫米）。大斷面尺寸具有

52、較低的優(yōu)勢，有一個不太有利的應(yīng)力分布，并有減少性能的均勻性，特別是經(jīng)過熱處理的零件。參考文獻1包括一個測定高達10.0英寸直徑（250毫米）旋轉(zhuǎn)軸的尺寸因素建議的方法。我們將在這里使用該方法還因為我們立足于扭轉(zhuǎn)彎曲現(xiàn)象的分析。表顯示尺寸因素之間的關(guān)系被應(yīng)用到耐久力所占了橫截面得大小圖顯示了表中一些混合的曲線方程 ，從曲線讀數(shù)應(yīng)提供可接受的精度</p><p>　　當(dāng)組件被設(shè)計是不是像一個圓軸，須作出判斷，以確定特

53、征尺寸的公式中使用。對于平面，軋制鋼，應(yīng)考慮厚度，指出這些方程組采用的近似。</p><p><b>　　表面處理</b></p><p>　　拋光表面的任何偏差都會降低耐久性圖中顯示的耐力優(yōu)勢粗略估計，相對于最終拉伸強度鋼表面的幾個實際情況，至關(guān)重要的是，受疲勞載荷得到保護，免受劃痕，劃傷，腐蝕，因為它們大大降低零件的疲勞強度。</p><p&g

54、t;<b>　　應(yīng)力集中</b></p><p>　　突然變化的幾何形狀，尤其是尖銳凹槽和缺口在高應(yīng)力集中發(fā)生，是有可能發(fā)生疲勞破壞的地方。應(yīng)注意的設(shè)計和制造的循環(huán)加載零件應(yīng)力集中系數(shù)保持為較低值。我們將利用應(yīng)力集中因素，從這節(jié)中的方法發(fā)現(xiàn)，講的是計算壓力，而不是許用強度</p><p><b>　　缺陷；裂縫</b></p>&l

55、t;p>　　材料內(nèi)部缺陷，尤其是在鑄件內(nèi)部缺陷可能是疲勞裂紋的地方開始關(guān)鍵零部件，可通過檢查內(nèi)部缺陷的X射線技術(shù)，如果他們沒有得到檢查，一個高于平均設(shè)計因素應(yīng)該被指定為鑄造件，并以較低的耐力強度應(yīng)使用。</p><p><b>　　溫度</b></p><p>　　大多數(shù)材料在高溫下耐力強度較低。報道中的值是房間的溫度。160°F(72°C)

56、以上的溫度會減少其韌性材料強度大部分耐久性。</p><p><b>　　非統(tǒng)一的材料性能</b></p><p>　　不同材料在不同方向上有不同的材料特性是因為進行處理的方式不同，冷軋薄板或棒的產(chǎn)品通常在軋制方向的強于他們在橫向方向。疲勞試驗過程中很可能已經(jīng)運行在測試桿以較強的方向，這種材料在強調(diào)橫向方向可能導(dǎo)致較低的耐力力量。</p><p&g

57、t;　　非均勻性也可能存在，因為不完整的焊縫熔深，夾渣附近，并在部分在焊縫幾何形狀變化。對熱處理材料焊接可能會因為當(dāng)?shù)氐暮缚p附近退火材料的強度而改變。有些焊接過程可能產(chǎn)生殘余拉應(yīng)力而降低了材料的有效持久強度。焊后退火或正火通常用來緩解這些壓力，但這種方法對基體材料強度的影響必須予以考慮。</p><p><b>　　殘余應(yīng)力</b></p><p>　　疲勞破壞通常開

58、始在相對較高的拉應(yīng)力的位置。任何制造過程中往往會產(chǎn)生殘余應(yīng)力會降低組件的持久強度已經(jīng)提到的焊接就是一個可能會產(chǎn)生殘余拉應(yīng)力的過程。研磨和加工，特別是高的材料去除率，也造成不良的殘余拉應(yīng)力。循環(huán)加載的關(guān)鍵部件加工領(lǐng)域應(yīng)以溫和的方式或理由。</p><p>　　生產(chǎn)過程的殘余壓應(yīng)力可以證明是有益的。噴砂處理和強化就是這樣兩種方法，噴丸是利用高速丸流的沖擊作用清理和強化基體表面的過程。噴丸采用了表面上的一系列沖擊，曲軸

59、，彈簧，循環(huán)載荷等機械零件可以受益于這些方法</p><p><b>　　腐蝕與環(huán)境因素</b></p><p>　　耐力強度數(shù)據(jù)通常測量空氣中的標(biāo)本。揭露出一個的水，鹽溶液，或其他腐蝕性環(huán)境工作條件中能顯著降低有效持久強度。腐蝕可能導(dǎo)致有害的局部表面粗糙度，也可以改變內(nèi)部晶粒結(jié)構(gòu)和外置式換熱器材料化學(xué)性質(zhì)。</p><p>　　鋼接觸到氫尤其

60、受到不利影響。</p><p><b>　　氮化</b></p><p>　　氮化是一種合金鋼表面硬化過程中，被加熱的物質(zhì)在氮氣環(huán)境中，以950℉（514℃）緩慢冷卻。氮化后可實現(xiàn)耐力強度提高50％以上。</p><p><b>　　鍛造與鑄造材料</b></p><p>　　金屬合金具有類似的化學(xué)

61、成分可以是鍛造或鑄造生產(chǎn)的最后形式。鍛造材料通常軋制或拉伸。鍛造材料在沒有明顯的應(yīng)力集中通常具有比同類鑄鐵材料耐久度較高的地區(qū)。然而，在缺口和其他間斷附近，鍛造和鑄造材料的強度耐力更接近相等。這種現(xiàn)象的一個可能的解釋是，鑄造材料很可能擁有比鍛造材料更各向同性的材料性能和較少受到應(yīng)力集中的影響。</p><p>　　如果使用更保守的方法，建議了如果使用鑄鋼，則0.8系數(shù)應(yīng)用到基本的持久強度。如果是鑄鐵則推薦0.70

62、</p><p><b>　　應(yīng)力類型</b></p><p>　　耐力強度數(shù)據(jù)是從旋轉(zhuǎn)梁測試，完全扭轉(zhuǎn)，重復(fù)生產(chǎn)的正常壓力。開發(fā)與生產(chǎn)的最大壓力是在試樣表面，葉片的應(yīng)力為零，線性的圓形橫截面的中心。如果實際加載是從不同的彎曲加載的，則一個影響裝載方式的因素應(yīng)該應(yīng)用于耐久力</p><p><b>　　軸向拉力</b>&l

63、t;/p><p>　　在純拉力下，所有的材料，不僅僅是表面，正在承受最大的壓力。0.8這個值所對應(yīng)的彎曲疲勞強度因素，以反映此不同的行為</p><p>　　應(yīng)力作用比對耐力的力量</p><p>　　圖5-10顯示了一個給定材料耐力強度數(shù)據(jù)一般變化時的應(yīng)力比R變化從-1.0到+1.0，覆蓋范圍包括下列例子：</p><p>　　■重復(fù)，扭轉(zhuǎn)應(yīng)力

64、（圖5-3）與r=-1.0</p><p>　　■部分逆轉(zhuǎn)波動與拉伸應(yīng)力平均應(yīng)力【圖5-4（b）】;-1.0<r<0</p><p>　　■重復(fù)，單向拉伸應(yīng)力（圖5-6）和r =0■拉應(yīng)力波動5-4（a），0<r<1.0■靜態(tài)應(yīng)力（圖5-1）與r= 1</p><p>　　注意，圖5-10只是一個例子，它不應(yīng)該被用來確定點的實際數(shù)據(jù). 如

65、果這些數(shù)據(jù)是需要一個特定的材料，該材料的具體數(shù)據(jù)必須無論是實驗或出版的文獻中發(fā)現(xiàn)。</p><p>　　最具破壞性的壓力，就是重復(fù)顛倒與R=- 1的壓力。（見參考文獻2第27頁）回想一下，在彎曲旋轉(zhuǎn)軸，如圖5-2所示是一個遭受了應(yīng)力比R=-1負載的例子</p><p>　　平均波動與壓縮應(yīng)力就如部分顯示應(yīng)力（c）和（d）。圖5-4沒有顯著影響材料的疲勞耐力力量，因為失效往往源于拉伸應(yīng)力區(qū)。

66、</p><p>　　請注意圖的持久強度Sn作為鋼的抗拉強度功能，如5-10顯示的曲線。這些數(shù)據(jù)適用于理想的拋光試樣，不包括在本節(jié)討論的任何其他因素。例如，曲線的R=-1.0（扭轉(zhuǎn)彎曲）表明，對鋼材的強度大約是耐力的極限強度的0.5倍（0.50×錫）因裝載周期（約10或更高）。這是一個很好的鋼材的一般估計。圖表還顯示，負載類型生產(chǎn)r大于-1.0，但小于1.0有一對耐力強度的影響較少。這說明了使用從扭轉(zhuǎn)彎

67、曲試驗數(shù)據(jù)是最保守的。</p><p>　　我們不會直接用圖5-10來解決這本書里的問題，因為我們估算準(zhǔn)確的耐力強度的程序是由圖5-9來開始計算的，此圖體現(xiàn)了從逆向彎曲試驗中得出的數(shù)據(jù)。因此應(yīng)力比的影響已經(jīng)包括在內(nèi)。此章5-9的部分包括了分析加載狀況的方法，比如出現(xiàn)以下的狀況：波動應(yīng)力產(chǎn)生應(yīng)力比不同于R=-1.0。</p><p><b>　　可靠性</b></

68、p><p>　　耐久力在圖5-9所示的數(shù)據(jù)代表鋼的強度從擁有適當(dāng)?shù)臉O限強度和表面狀況標(biāo)本多次試驗得出的平均值。當(dāng)然，有數(shù)據(jù)點之間的變化，也就是說，有一半是高，一半人更在給定曲線的報告值低。曲線，那么，代表著50％的可靠性，表明了零件一半會失敗顯然，最好是設(shè)計一個更高的可靠性，比如90％，99％，或99.9％。一個可以用來估計比設(shè)計可用于生產(chǎn)可靠性高值低強度的耐力的因素。理想情況下，應(yīng)獲得對材料的實際數(shù)據(jù)統(tǒng)計分析中使用