第5章材料的力學(xué)性能

1、11材料科學(xué)基礎(chǔ)材料科學(xué)基礎(chǔ)Chapter 5 材料的力學(xué)性能22Objectives of Chapter 5o Introduce the basic concepts associated with mechanical properties of materials.o Evaluate factors that affect the mechanical properties of materials.o Review some of the basic testing procedures that engineers use to evaluate many of these p

2、roperties.33Chapter Outline o5.1 材料承受靜載荷時的力學(xué)性能o5.2 材料承受沖擊載荷時的力學(xué)性能o5.3 材料的疲勞o5.4 材料的斷裂韌性o5.5 材料的磨損性能o5.6 材料的蠕變性能44Technological SignificanceFigure The materials used in sports equipment must be lightweight, stiff, tough, and impact resistant. Figure Aircraft, such as the one shown here, makes use of

3、aluminum alloys and carbon-fiber-reinforced composites. 55Terminology for Mechanical Propertiesq Stress - Force or load per unit area of cross-section over which the force or load is acting.q Strain - Elongation change in dimension per unit length.q Youngs modulus - The slope of the linear part of t

4、he stress-strain curve in the elastic region, same as modulus of elasticity.q Shear modulus (G) - The slope of the linear part of the shear stress-shear strain curve.q Viscosity ( ) - Measure of resistance to flow, defined as the ratio of shear stress to shear strain rate (units Poise or Pa-s).q Thi

5、xotropic behavior - Materials that show shear thinning and also an apparent viscosity that at a constant rate of shear decreases with time.66Figure (a) Tensile, compressive, shear and bending stresses. (b) Illustration showing how Youngs modulus is defined for elastic material. (c) For nonlinear mat

6、erials, we use the slope of a tangent as a variable quantity that replaces the Youngs modulus constant77Section 5.1 材料承受靜載荷時的力學(xué)性能材料承受靜載荷時的力學(xué)性能5.1.1 材料的拉伸曲線材料的拉伸曲線oLoad - The force applied to a material during testing.oStrain gage or Extensometer - A device used for measuring change in length and hen

7、ce strain.oGlass temperature (Tg ) - A temperature below which an otherwise ductile material behaves as if it is brittle.oEngineering stress - The applied load, or force, divided by the original cross-sectional area of the material.oEngineering strain - The amount that a material deforms per unit le

8、ngth in a tensile test.885.1.1 材料的拉伸曲線材料的拉伸曲線o單向靜拉伸試驗(yàn)是廣泛應(yīng)用的材料性能檢測方法。o負(fù)荷一伸長曲線-材料的拉伸曲線。o整個拉伸過程中的變形可分為彈性變形彈性變形、屈服變形屈服變形、均勻塑性變均勻塑性變形形及不均勻塑性變形不均勻塑性變形四個階段。o應(yīng)力應(yīng)變曲線(工程應(yīng)力應(yīng)變曲線)o真實(shí)應(yīng)力應(yīng)變曲線99Figure A unidirectional force is applied to a specimen in the tensile test by means of the moveable crosshead. The cross-he

9、ad movement can be performed using screws or a hydraulic mechanism1010Figure Tensile stress-strain curves for different materials. Note that these are qualitative1111Figure The stress-strain curve for an aluminum alloy12121313True Stress and True StrainoTrue stress The load divided by the actual cro

10、ss-sectional area of the specimen at that load.oTrue strain The strain calculated using actual and not original dimensions, given by t ln(l/l0).Figure The relation between the true stress-true strain diagram and engineering stress-engineering strain diagram. The curves are identical to the yield poi

11、nt14145.1.2 材料的變形及其性能指標(biāo)材料的變形及其性能指標(biāo)oElastic limit oTensile strength, NeckingoHookes lawoPoissons ratiooModulus of resilience (Er) oTensile toughnessoDuctility15155.1.2 材料的變形及其性能指標(biāo)材料的變形及其性能指標(biāo)材料變形的實(shí)質(zhì)彈性變形的實(shí)質(zhì)塑性變形的實(shí)質(zhì)超塑性2.材料變形的性能指標(biāo)比例極限p彈性極限e彈性模量E屈服極限s (0.2)抗拉強(qiáng)度b斷裂強(qiáng)度Sk延伸率斷面收縮率1616Figure (a) Determining the

12、0.2% offset yield strength in gray cast ion, and (b) upper and lower yield point behavior in a low-carbon steel1717Figure Typical yield strength values for different engineered materials. (Source: Reprinted from Engineering Materials I, Second Edition, M.F. Ashby and D.R.H. Jones, 1996, Fig. 8-12, p

13、. 85. Copyright Butterworth-Heinemann18181919Figure Range of elastic moduli for different engineered materials. (Source: Reprinted from Engineering Materials I, Second Edition, M.F. Ashby and D.R.H. Jones, 1996, Fig. 3-5, p. 35, Copyright 1996 Butterworth-Heinemann. 20205.1.3 材料的斷裂及其性能指標(biāo)材料的斷裂及其性能指標(biāo)（

14、一）斷裂的類型及斷口特征根據(jù)斷裂前后材料宏觀塑性變形的程度，分為脆性斷裂與韌性斷裂；根據(jù)晶體材料斷裂時裂紋擴(kuò)展的途徑，分為穿晶斷裂和沿晶(晶界)斷裂；根據(jù)微觀斷裂機(jī)理，分為解理斷裂和剪切斷裂等。 材料的斷裂表面稱為斷口。用肉眼、放大鏡或電子顯微鏡等手段對材料斷口進(jìn)行宏觀及微觀的觀察分析，稱為斷口分析。1韌性斷裂與脆性斷裂韌性斷裂韌性斷裂是材料斷裂前產(chǎn)生明顯塑性變形的斷裂過程。韌性斷裂的斷口往往呈暗灰色、纖維狀。塑性較好的金屬材料和高分子材料，室溫下的靜拉伸斷裂具有典型的韌性斷裂特征。脆性斷裂脆性斷裂是材料斷裂前不產(chǎn)生明顯的塑性變形。脆性斷裂的斷口，一般與正應(yīng)力垂直，宏觀上比較齊平光亮，常呈放射

15、狀或結(jié)晶狀。淬火鋼、灰鑄鐵、陶瓷、玻璃等脆性材料的斷口常具有上述特征。實(shí)際上，金屬的脆性斷裂與韌性斷裂并無明顯的界限，脆性斷裂前也會產(chǎn)生微量塑性變形。因此，規(guī)定光滑拉伸試樣的斷面收縮率小于5為脆性斷裂；大于5為韌性斷裂。21215.1.3 材料的斷裂及其性能指標(biāo)材料的斷裂及其性能指標(biāo)2穿晶斷裂與沿晶斷裂根據(jù)材料(包括金屬、陶瓷及結(jié)晶高分子)發(fā)生斷裂時裂紋擴(kuò)展的路徑，分為穿晶斷裂穿晶斷裂和沿晶沿晶(晶界晶界)斷裂斷裂兩種。穿晶斷裂可以是韌性斷裂，也可以是脆性斷裂；而沿晶斷裂則多為脆性斷裂，斷口呈結(jié)晶狀；沿晶斷裂是晶界結(jié)合力較弱的一種表現(xiàn)。例如共價鍵陶瓷晶界較弱，斷裂方式主要是晶界斷裂。離子鍵晶體

16、的斷裂往往以穿晶解理為主。 圖圖56 穿晶斷裂與沿穿晶斷裂與沿晶斷裂示意圖晶斷裂示意圖2222Figure Localized deformation of a ductile material during a tensile test produces a necked region. The micrograph shows necked region in a fractured sample23235.1.3 材料的斷裂及其性能指標(biāo)材料的斷裂及其性能指標(biāo)Microstructural Features of Fracture in Metallic MaterialsoTransgr

17、anular - Meaning across the grains (e.g., a transgranular fracture would be fracture in which cracks would go through the grains).oMicrovoids - Development of small holes in a material.oIntergranular - In between grains or along the grain boundaries.oChevron pattern - A common fracture feature produ

18、ced by separate crack fronts propagating at different levels in the material.2424Figure When a ductile material is pulled in a tensile test, necking begins and voids form starting near the center of the bar by nucleation at grain boundaries or inclusions. As deformation continues a 45 shear lip may

19、form, producing a final cup and cone fracture2525Figure Dimples form during ductile fracture. Equiaxed dimples form in the center, where microvoids grow. Elongated dimples, pointing toward the origin of failure, form on the shear lip2626Figure Scanning electron micrographs of an annealed 1018 steel

20、exhibiting ductile fracture in a tensile test. (a) Equiaxed dimples at the flat center of the cup and cone, and (b) elongated dimples at the shear lip (x 1250)2727Figure Scanning electron micrograph of a brittle fracture surface of a quenched 1010 steel (x 5000). 2828Figure The Chevron pattern in a

21、0.5-in.-diameter quenched 4340 steel. The steel failed in a brittle manner by an impact blow2929Figure The Chevron pattern forms as the crack propagates from the origin at different levels. The pattern points back to the origin30305.1.4 材料的彎曲及其性能指標(biāo)材料的彎曲及其性能指標(biāo)oBend test - Application of a force to th

22、e center of a bar that is supported on each end to determine the resistance of the material to a static or slowly applied load.oFlexural strength or modulus of rupture -The stress required to fracture a specimen in a bend test. oFlexural modulus - The modulus of elasticity calculated from the result

23、s of a bend test, giving the slope of the stress-deflection curve.31315.1.4 材料的彎曲及其性能指標(biāo)材料的彎曲及其性能指標(biāo)1.彎曲試驗(yàn)測定的力學(xué)性能指標(biāo)o彎曲試驗(yàn)在萬能試驗(yàn)機(jī)上進(jìn)行，其試樣分圓柱和方形兩種。加載方式有三點(diǎn)彎曲加載和四點(diǎn)彎曲加載兩種。通過記錄載荷F(或彎矩 )與試樣最大撓度f之間的關(guān)系曲線彎曲圖，來確定材料在彎曲載荷下的力學(xué)性能。o對于脆性材料，可根據(jù)彎曲圖計算抗彎強(qiáng)度抗彎強(qiáng)度式中：Mb 為試樣斷裂時的彎矩 。W為試樣抗彎截面系數(shù)，對于直徑為d0 的圓柱試樣， ，對于寬度為b ，高度為h 的矩形試樣， 。材

24、料的塑性用最大彎曲撓度fmax 表示，fmax 值可由百分表或撓度計直接讀出。此外，從彎曲撓度曲線上還可測算彎曲彈性模量Eb 。bbbM =W330dW=(m )3233bhW=(m )632325.1.4 材料的彎曲及其性能指標(biāo)材料的彎曲及其性能指標(biāo)2. 彎曲試驗(yàn)的特點(diǎn)及應(yīng)用o彎曲加載時受拉一側(cè)的應(yīng)力狀態(tài)基本上與靜拉伸時相同，且不存在拉伸試驗(yàn)時試樣裝卡偏斜對實(shí)驗(yàn)結(jié)果造成的影響。對于難以加工成拉伸試樣的硬脆材料，可用彎曲試驗(yàn)測定斷裂強(qiáng)度，并能顯示出它們的塑性差別。o彎曲試驗(yàn)時，試樣截面上的應(yīng)力分布是表面上應(yīng)力最大，故可靈敏地反映材料的表面缺陷。因此，常用來比較和評定材料表面處理層的質(zhì)量，例如檢

25、驗(yàn)滲碳層的質(zhì)量和性能。o不能使塑性材料斷裂，雖可測定規(guī)定非比例應(yīng)力和彎曲應(yīng)力，但實(shí)際上很少應(yīng)用。o主要用于測定灰鑄鐵、硬質(zhì)合金、陶瓷等材料的抗彎強(qiáng)度?；诣T鐵彎曲試樣一般采用圓柱毛坯試樣，實(shí)驗(yàn)加載速度不大于0.1mm/s。硬質(zhì)合金由于硬度高，難以加工成拉伸試樣，故常用彎曲試驗(yàn)評價其性能和質(zhì)量。陶瓷材料脆性大，測定抗拉強(qiáng)度困難，不能得到精確的結(jié)果，主要以抗彎強(qiáng)度作為評價陶瓷材料性能的指標(biāo)。3333Figure The stress-strain behavior of brittle materials compared with that of more ductile materials343

26、4Figure (a) The bend test often used for measuring the strength of brittle materials, and (b) the deflection obtained by bending3535Figure Stress-deflection curve for Mg0 obtained from a bend test3636Figure (a) Three point and (b) four-point bend test setup373738385.1.5 材料的硬度材料的硬度Hardness of Materia

27、lsoHardness test - Measures the resistance of a material to penetration by a sharp object.oMacrohardness - Overall bulk hardness of materials measured using loads 2 N.oMicrohardness Hardness of materials typically measured using loads less than 2 N using such test as Knoop (HK).oNano-hardness - Hard

28、ness of materials measured at 110 nm length scale using extremely small (100 N) forces.39395.1.5 材料的硬度材料的硬度Hardness of Materials硬度硬度是衡量材料軟硬程度的一種力學(xué)性能，其物理意義是材料表面上不大體積內(nèi)抵抗變形或破裂的能力。硬度試驗(yàn)方法有十幾種，按加載方式不同，可分為壓人法和刻劃法兩大類。布氏硬度布氏硬度、洛氏硬度、維氏硬度和顯微硬度洛氏硬度、維氏硬度和顯微硬度屬于壓人法?？虅澐ò嫌捕群痛斓斗ǖ?。40405.1.5 材料的硬度材料的硬度Hardness of M

29、aterials1.布氏硬度 o布氏硬度是1900年由瑞典工程師J.B.Brinell提出。測量方法是，在負(fù)荷F的作用下，將直徑為D的淬火鋼球壓人試樣表面，保持一定時問后卸除載荷，以試樣壓痕的表面積A去除負(fù)荷F所得的商，作為硬度的計算指標(biāo)，用符號HB表示。壓痕直徑越大，布氏硬度值HB越?。籵布氏硬度值為450650的材料，用硬質(zhì)合金壓頭，用“ HBW”表示；對于布氏硬度值低于450的材料，使用淬火鋼球壓頭，用“HBS ”表示。o布氏硬度值的表示方法，一般記為“數(shù)字+硬度符號(HBS或HBW)+數(shù)字/數(shù)字/數(shù)字”的形式，符號前面的數(shù)字為硬度值，符號后面的數(shù)字依次表示鋼球直徑、載荷大小及載荷保持時

30、間等試驗(yàn)條件。o布氏硬度試驗(yàn)的優(yōu)點(diǎn)是壓痕面積大，試驗(yàn)數(shù)據(jù)穩(wěn)定，重復(fù)性高。其硬度值能反映材料在較大區(qū)域內(nèi)各組成相的平均性能，最適合測定灰鑄鐵、軸承合金等材料的硬度。o操作較為麻煩，對不同的材料需要更換壓頭直徑 和載荷F，壓痕直徑需要測量。因壓痕直徑較大，一般不宜在成品件上直接進(jìn)行檢驗(yàn)。 41415.1.5 材料的硬度材料的硬度Hardness of Materials1. 洛氏硬度o洛氏硬度是1919年由美國人S.P.Rockwell和H.M.Rockwell提出，以壓痕深度作為計量硬度的依據(jù)。o洛氏硬度試驗(yàn)時，采用的壓頭為120的金鋼石圓錐或直徑為1.588mm、3.175 的鋼球。載荷先后分

31、兩次施加。o金屬越硬壓痕深度越小，金屬越軟壓痕深度越深。用常數(shù)k減去壓痕深度h，所得差值作為洛氏硬度的指標(biāo)HR。o硬度值可由表盤上直接讀出。顯然，材料越軟則壓痕 越深；o洛氏硬度試驗(yàn)的優(yōu)點(diǎn)是操作簡便；壓痕面積較小，可檢測成品、小件和薄件；測量范圍大，從很軟的有色金屬到極硬的硬質(zhì)合金；測量迅速，可直接從表盤上讀出硬度值。其缺點(diǎn)是壓痕較小，代表性差；所測硬度值的重復(fù)性差、分散度大；不適于檢測灰鑄鐵、滑動軸承合金及偏析嚴(yán)重的材料。用不同標(biāo)尺測得的硬度值既不能直接進(jìn)行比較，又不能彼此互換。42425.1.5 材料的硬度材料的硬度Hardness of Materials標(biāo)尺標(biāo)尺測量范圍測量范圍初載荷初

32、載荷/N(Kgf)主載荷主載荷N(Kgf)壓頭類型壓頭類型K/mmk/mmHRA608598.1（10）490.3（50）金剛石圓錐體金剛石圓錐體0.20.002HRB2510098.1（10）882.6（90）鋼球鋼球0.260.002HRC206798.1（10）1373（140）金剛石圓錐體金剛石圓錐體0.20.002表表52 洛氏硬度試驗(yàn)條件及應(yīng)用洛氏硬度試驗(yàn)條件及應(yīng)用4343Figure Indentors for the Brinell and Rockwell hardness tests44444545Section 5.2 材料承受沖擊載荷時的力學(xué)性能材料承受沖擊載荷時的力學(xué)

33、性能oImpact test - Measures the ability of a material to absorb the sudden application of a load without breaking.oImpact energy - The energy required to fracture a standard specimen when the load is applied suddenly.oImpact toughness - Energy absorbed by a material, usually notched, during fracture,

34、under the conditions of impact test.oFracture toughness - The resistance of a material to failure in the presence of a flaw.46465.2.1 沖擊彎曲試驗(yàn)沖擊彎曲試驗(yàn)o缺口試樣一次沖擊彎曲試驗(yàn)在擺錘式?jīng)_擊試驗(yàn)機(jī)上進(jìn)行，將試樣水平放置于試驗(yàn)機(jī)支座上，缺口位于沖擊相背方向。沖擊時將質(zhì)量為G 的擺錘舉至高度h0的位置，使其獲得位能Gh0。釋放擺錘沖斷試樣后，擺錘的剩余能量為Ghf ，則擺錘沖斷試樣失去的位能為Gh0-Ghf 。此即為試樣變形和斷裂所吸收的功，稱為沖擊功，以Wk

35、 (Ak)表示，單位為J。o國家標(biāo)準(zhǔn)規(guī)定，沖擊彎曲試驗(yàn)用試樣分為夏比U型缺口試樣和夏比V型缺口試樣，所測得的沖擊功分別記為Whu和Whv。測量陶瓷、鑄鐵或工具鋼等脆性材料的沖擊功時，常采用101055(mm) 的無缺口沖擊試樣。47475.2.2 多次沖擊試驗(yàn)多次沖擊試驗(yàn)o實(shí)踐表明，承受沖擊載荷的機(jī)件多數(shù)是經(jīng)過多次沖擊后斷裂的，其破壞是各次沖擊損傷積累的結(jié)果，根本不同于一次沖擊破壞的過程。o多次沖擊試驗(yàn)后可繪制出沖擊功W一沖斷次數(shù)N曲線o隨沖擊功W的減少，沖斷次數(shù)N增加。圖圖515 多次沖擊曲線多次沖擊曲線48485.2.3 沖擊韌性及其意義沖擊韌性及其意義沖擊韌性沖擊韌性K式中， Wk為沖斷

36、試樣所消耗的沖擊功，MJ ；A0為試樣缺口處橫截面積，m2 oK值越大，表示材料的沖擊韌性越好。沖擊韌性表示材料抵抗沖擊破壞的能力。 同一條件下，同一材料的兩種試樣，其U型缺口試樣的K值顯著大于V型缺口試樣，所以它們的K值不能互相比較。o材料的K值隨溫度的降低而減小。在某一溫度范圍內(nèi)， K值急劇降低，這種現(xiàn)象稱為冷脆。這個溫度范圍稱為冷脆轉(zhuǎn)變溫度范圍。oK值對材料的缺陷，如淬火過熱造成的晶粒粗大、回火脆性、時效不充分、夾雜物形態(tài)、纖維方向等非常敏感，故常用于檢驗(yàn)冶煉、熱加工、熱處理工藝的質(zhì)量。也常用于檢驗(yàn)材料的冷脆性、以確定材料的冷脆轉(zhuǎn)變溫度。 2kk0W =(MJ / m )A4949Fig

37、ure The impact test: (a) The Charpy and Izod tests, and (b) dimensions of typical specimens5050oDuctile to brittle transition temperature (DBTT) - The temperature below which a material behaves in a brittle manner in an impact test.oNotch sensitivity - Measures the effect of a notch, scratch, or oth

38、er imperfection on a materials properties, such as toughness or fatigue life.5151Figure Results from a series of Izod impact tests for a super-tough nylon thermoplastic polymer5252Figure The Charpy V-notch properties for a BCC carbon steel and a FCC stainless steel. The FCC crystal structure typical

39、ly leads top higher absorbed energies and no transition temperature5353Figure The area contained within the true stress-true strain curve is related to the tensile toughness. Although material B has a lower yield strength, it absorbs a greater energy than material A. The energies from these curves m

40、ay not be the same as those obtained from impact test data5454Section 5.3 材料的疲勞材料的疲勞oFatigue is the lowering of strength or failure of a material due to repetitive stress which may be above or below the yield strength.oCreep - A time dependent, permanent deformation at high temperatures, occurring a

41、t constant load or constant stress.oBeach or clamshell marks - Patterns often seen on a component subjected to fatigue.oRotating cantilever beam test - An older test for fatigue testing.oS-N curve (also known as the Whler curve) - A graph showing stress as a function of number of cycles in fatigue.5

42、555Section 5.3 材料的疲勞材料的疲勞o許多機(jī)件承受的是大小及方向不斷變化的交變載荷，例如軸、齒輪、彈簧等。在交變載荷作用下，材料經(jīng)常在遠(yuǎn)低于其屈服強(qiáng)度的載荷下發(fā)生斷裂，這種現(xiàn)象稱為“疲勞”。o疲勞斷裂時，材料沒有明顯的塑性變形，斷裂是突然發(fā)生的，常常造成嚴(yán)重的事故。o按應(yīng)力狀態(tài)，分為彎曲疲勞彎曲疲勞、扭轉(zhuǎn)疲勞扭轉(zhuǎn)疲勞、拉壓疲勞拉壓疲勞、接觸疲勞接觸疲勞和復(fù)復(fù)合疲勞合疲勞；o按應(yīng)力高低和斷裂壽命，分為高周疲勞高周疲勞和低周疲勞低周疲勞。56565.3.1 疲勞曲線疲勞曲線o以max為縱坐標(biāo)，以疲勞斷裂周次N為橫坐標(biāo)繪制的曲線，稱為疲勞曲線。簡寫為-N曲線。實(shí)驗(yàn)表明，金屬材料所受的最

43、大交變應(yīng)力max越大，則斷裂前所能承受的應(yīng)力循環(huán)次數(shù)N越少。當(dāng)應(yīng)力循環(huán)中的最大應(yīng)力max降低到某一數(shù)值，材料可以經(jīng)受無限次應(yīng)力循環(huán)而不發(fā)生疲勞斷裂，-N曲線上出現(xiàn)了趨于水平部分。o不同材料的疲勞曲線形狀不同，大致可分為兩類。一類有水平線，如一般結(jié)構(gòu)鋼和球墨鑄鐵的疲勞曲線，據(jù)此，可標(biāo)定出無限壽命的疲勞強(qiáng)度；另一類無水平線，如有色合金、不銹鋼和高強(qiáng)鋼的疲勞曲線 。5757Figure The stress-number of cycles to failure (S-N) curves for a tool steel and an aluminum alloy58585.3.2 疲勞極限疲勞極限

44、o當(dāng)應(yīng)力低于某一值時，材料經(jīng)無限循環(huán)周次也不發(fā)生斷裂，此值稱為疲勞極限疲勞極限或疲勞強(qiáng)度疲勞強(qiáng)度。疲勞極限是保證機(jī)件疲勞壽命的重要性能指標(biāo)，是評定材料、制訂工藝和疲勞設(shè)計的依據(jù)。光滑試樣的對稱疲勞極限用-1 表示，單位MPa。對于無水平線的疲勞曲線，只能根據(jù)材料的使用要求，確定有限壽命下的疲勞極限。例如，鋼材的循環(huán)基數(shù)為107 ，有色金屬和某些超高強(qiáng)度鋼的循環(huán)基數(shù)為108 。超過這個基數(shù)就認(rèn)為該材料不再發(fā)生疲勞破壞。o常見的對稱循環(huán)載荷有對稱彎曲、對稱扭轉(zhuǎn)、對稱拉壓等，對應(yīng)的疲勞極限分別記為-1 、-1 及 -1p ，其中 -1是最常用的。一般情況下-1 -1p -1 o對于中、低強(qiáng)度鋼， -

45、1 =0.5b 。但抗拉強(qiáng)度較高時，這種線性關(guān)系要改變，因?yàn)閺?qiáng)度較高時，材料的塑性和斷裂韌性降低，裂紋易于形成和擴(kuò)展。59595.3.3 疲勞斷口疲勞斷口o一般來說，典型疲勞斷口由3個特征區(qū)組成，即疲勞裂紋產(chǎn)生區(qū)疲勞裂紋產(chǎn)生區(qū)、疲疲勞裂紋擴(kuò)展區(qū)勞裂紋擴(kuò)展區(qū)和最后斷裂區(qū)最后斷裂區(qū)。o疲勞裂紋萌生的地方，多出現(xiàn)在機(jī)件表面，常和缺口、裂紋、刀痕、蝕坑等缺陷相連。若材料內(nèi)部存在嚴(yán)重冶金缺陷(夾雜、縮孔、偏析、白點(diǎn)等)，也會因局部材料強(qiáng)度降低而在機(jī)件內(nèi)部引發(fā)出疲勞源。o疲勞裂紋產(chǎn)生后，在交變應(yīng)力作用下，繼續(xù)擴(kuò)展長大，這個區(qū)域稱為疲勞裂紋擴(kuò)展區(qū)。其宏觀特征是：斷口較光滑并分布有貝紋線(或海灘花樣)，有時還

46、有裂紋擴(kuò)展臺階。貝紋線是一簇以疲勞源為圓心的平行弧線，凹側(cè)指向疲勞源，凸側(cè)指向裂紋擴(kuò)展方向。近疲勞源區(qū)貝紋線較細(xì)密，表明裂紋擴(kuò)展較慢；遠(yuǎn)離疲勞源區(qū)貝紋線較稀疏、粗糙，表明此段裂紋擴(kuò)展較快。60605.3.3疲勞斷口疲勞斷口o最后斷裂區(qū)，隨著疲勞裂紋的擴(kuò)展，零件的有效截面不斷減小，剩余斷面上的應(yīng)力不斷增加。當(dāng)應(yīng)力超過材料的斷裂強(qiáng)度時，發(fā)生斷裂，形成最后斷裂區(qū)。該區(qū)的斷口比疲勞區(qū)粗糙，宏觀特征如同靜載，隨材料性質(zhì)而變。脆性材料斷口呈結(jié)晶狀；韌性材料斷口為纖維狀，暗灰色，在心部平面應(yīng)變區(qū)呈放射狀或人字紋狀，邊緣平面應(yīng)力區(qū)則有剪切唇區(qū)存在。o疲勞裂紋擴(kuò)展區(qū)與最后斷裂區(qū)所占面積的相對比例，隨應(yīng)力大小和材

47、料的斷裂韌性而變化。所受應(yīng)力小而無大的應(yīng)力集中時，則疲勞裂紋擴(kuò)展區(qū)大；反之，則小。因此，可以根據(jù)疲勞斷口上兩個區(qū)所占的比例，估計所受應(yīng)力高低及應(yīng)力集中程度的大小。o對疲勞斷口的分析是研究疲勞過程、分析疲勞失效原因的一種重要方法。6161Figure Fatigue fracture surface. (a) At low magnifications, the beach mark pattern indicates fatigue as the fracture mechanism. The arrows show the direction of growth of the crack f

48、ront, whose origin is at the bottom of the photograph. (Image (a) is from C.C. Cottell, Fatigue Failures with Special Reference to Fracture Characteristics, Failure Analysis: The British Engine Technical Reports, American Society for Metals, 1981, p. 318.) (b) At very high magnifications, closely sp

49、aced striations formed during fatigue are observed (x 1000)6262Figure Schematic representation of a fatigue fracture surface in a steel shaft, showing the initiation region, the propagation of fatigue crack (with beam markings), and catastrophic rupture when the crack length exceeds a critical value

50、 at the applied stress6363Results of the Fatigue TestoEndurance limit - An older concept that defined a stress below which a material will not fail in a fatigue test.oFatigue life - The number of cycles permitted at a particular stress before a material fails by fatigue.oFatigue strength - The stres

51、s required to cause failure by fatigue in a given number of cycles, such as 500 million cycles.oNotch sensitivity - Measures the effect of a notch, scratch, or other imperfection on a materials properties, such as toughness or fatigue life.oShot peening - A process in which metal spheres are shot at

52、 a component.6464Application of Fatigue TestingFigure Examples of stress cycles. (a) Equal stress in tension and compression, (b) greater tensile stress than compressive stress, and (c) all of the stress is tensile6565Figure Crack growth rate versus stress-intensity factor range for a high-strength

53、steel. For this steel, C = 1.62 1012 and n = 3.2 for the units shown6666Section 5.4 材料的斷裂韌性材料的斷裂韌性oFracture mechanics - The study of a materials ability to withstand stress in the presence of a flaw.oFracture toughness - The resistance of a material to failure in the presence of a flaw.o工程設(shè)計中，一般根據(jù)材料

54、的屈服強(qiáng)度 確定許用應(yīng)力。機(jī)件在許用應(yīng)力下工作，不會發(fā)生塑性變形，更不會發(fā)生斷裂，應(yīng)該是安全的。然而，有些機(jī)件會在很低應(yīng)力的狀態(tài)下，發(fā)生脆性斷裂。這是因?yàn)椋话阌懻摬牧系牧W(xué)性能時，假定材料的內(nèi)部是完整的、連續(xù)的。而實(shí)際材料中不可避免地存在著各種缺陷。例如，夾雜物、氣孔等冶金缺陷和在使用、加工過程中產(chǎn)生的機(jī)械缺陷。這些缺陷破壞了材料的連續(xù)性，成為材料中的裂紋。實(shí)驗(yàn)分析表明，低應(yīng)力脆性斷裂是由材料中裂紋的擴(kuò)展引起的。67675.4.1 斷裂韌性的概念斷裂韌性的概念o斷裂力學(xué)運(yùn)用連續(xù)介質(zhì)力學(xué)的彈塑性理論，考慮了材料的不連續(xù)性，研究材料中裂紋擴(kuò)展的規(guī)律，確定材料抵抗斷裂的力學(xué)性能指標(biāo)斷裂韌性斷裂韌性

55、。斷裂韌性反映了材料抵抗裂紋失穩(wěn)擴(kuò)張的能力。o張開型裂紋(通常稱之為I型裂紋)，其大小可以用應(yīng)力強(qiáng)度因子應(yīng)力強(qiáng)度因子 來描述。o對一個存在裂紋的試樣施加拉伸載荷時，其Y值是一定的。隨應(yīng)力 逐漸增大，或者裂紋長度2a逐漸擴(kuò)展，裂紋尖端的K1增大到某一數(shù)值時，可使裂紋前沿某一區(qū)域的內(nèi)應(yīng)力大到足以使裂紋產(chǎn)生失穩(wěn)擴(kuò)展，即發(fā)生脆斷。這個強(qiáng)度因子的臨界值，稱為材料的斷裂韌性，用K1c 表示。它反映了有裂紋存在時，材料抵抗脆性斷裂的能力。當(dāng) K1 K1c時裂紋失穩(wěn)擴(kuò)展，發(fā)生脆斷；當(dāng) K1 K1c時，裂紋不擴(kuò)展或擴(kuò)展很慢，不發(fā)生快速脆斷；當(dāng) K1= K1c時，裂紋處于臨界狀態(tài)。321K =Y a(MN/mm)

56、68685.4.1 斷裂韌性的概念斷裂韌性的概念oK1是描述裂紋尖端應(yīng)力場大小的力學(xué)參量，它與裂紋類型、物體的形狀、大小以及外加應(yīng)力等參數(shù)有關(guān)，與材料無關(guān)；而斷裂韌性K1c 是評定材料阻止裂紋失穩(wěn)擴(kuò)展能力的力學(xué)性能指標(biāo)，它與裂紋本身的大小、形狀無關(guān)，也和外加應(yīng)力無關(guān)，是材料本身的特性，只和材料的成分、熱處理及加工工藝等有關(guān)。o應(yīng)力強(qiáng)度因子和斷裂韌性的提出，在工程上有重要的意義。例如，知道材料的斷裂韌性 ，再測出構(gòu)件中的最大裂紋長度，就可以計算出裂紋失穩(wěn)擴(kuò)展的臨界載荷，即構(gòu)件所能承受的最大載荷?；蛘呒褐牧系臄嗔秧g性 ，根據(jù)構(gòu)件實(shí)際所受的外加應(yīng)力，確定構(gòu)件中允許存在的最大裂紋長度。所以，斷裂韌性

57、為安全設(shè)計提供了一個重要的力學(xué)性能指標(biāo)，尤其在疲勞、沖擊、高低溫強(qiáng)度、應(yīng)力腐蝕、輻照損傷等強(qiáng)度領(lǐng)域得到了廣泛的應(yīng)用。同時也為發(fā)展新材料、新工藝及合理選材指出了方向。69695.4.2 影響材料斷裂韌性的因素影響材料斷裂韌性的因素o化學(xué)成分對于金屬材料，提高韌性的元素，均提高材料的斷裂韌性。加入細(xì)化晶粒的元素，可使金屬的斷裂韌性提高；強(qiáng)烈固溶強(qiáng)化的合金元素使斷裂韌性降低；形成脆性化合物的元素降低斷裂韌性。o晶粒尺寸晶粒越細(xì)，材料的強(qiáng)度和韌性同時提高，另外，細(xì)化晶粒有助于減輕雜質(zhì)在晶界上的偏析，減少沿晶斷裂，從而提高材料的斷裂韌性。o雜質(zhì)及第二相鋼中的夾雜物以及第二相，如硫化物、氧化物、碳化物等，

58、都是脆性相，這些相的存在，降低鋼的斷裂韌性；脆性相以細(xì)小球狀存在時，對斷裂韌性的有害作用減小。o溫度和加載速度大多數(shù)材料，溫度降低，斷裂韌性降低。對于存在韌脆轉(zhuǎn)變溫度的材料，在韌性溫度區(qū)，材料發(fā)生韌性斷裂，有較高的斷裂韌性；而在韌脆轉(zhuǎn)變溫度以下，材料主要是解理性脆性斷裂，斷裂韌性較低。增加形變速度，與降低溫度有類似的效果，使斷裂韌性下降。7070Figure Schematic drawing of fracture toughness specimens with (a) edge and (b) internal flaws7171Figure The fracture toughness

59、 Kc of a 3000,000psi yield strength steel decreases with increasing thickness, eventually leveling off at the plane strain fracture toughness Klc72727373Figure A scanning electron micrograph showing crack propagation in a PZT ceramic. (Courtesy of Wang and Raj N. Singh, Ferroelectrics, 207, 555575 (

60、1998).)Figure Secondary cracks developed during hardness testing can be used to assess the fracture toughness of brittle materials7474Figure Fracture toughness versus strength of different engineered materials. (Source: Adapted from Mechanical Behavior of Materials, by T.H. Courtney, 2000, p. 434, F