材料科學與工程基礎：第3章 力學性能1

1、Ch.3 Mechanical Properties （力學性能）3.1 Introduction (簡介)3.2 Concepts of Stress and Strain (應力應變的概念)3.3 Elastic Deformation (彈性變形)3.4 Mechanical Behavior-Metals (金屬的力學行為)3.5 Hardness (硬度)3.1 IntroductionHardness tester & hardness scalesmechanical property of materialsstress and strainElastic deformatio

2、nModulusViscoelasticitypermanent deformationStrengthFracture 材料的力學狀態(tài) mechanical states of matrials1.金屬的力學狀態(tài)A 晶態(tài)結(jié)構(gòu)，B 較高的彈性模量 和強度，C 受力開始為彈性 形變，接著一段 塑性形變，然后 斷裂， 總變形能很大，D 具有較高的熔點。某些金屬合金A 呈非晶態(tài)合金，B 具有很高的硬度和強度，C 延伸率很低而并不脆。D 溫度升高到玻璃化轉(zhuǎn)變溫度以上，粘度明顯降低， 發(fā)生晶化而失去非晶態(tài)結(jié)構(gòu)。2. 無機非金屬的力學狀態(tài)A 玻璃相熔點低，熱穩(wěn)定性差，強度低。B 氣相（氣孔）的存在導致陶瓷

3、的彈性模量和機械 強度降低。C 陶瓷材料也存在玻璃化轉(zhuǎn)變溫度Tg。D 絕大多數(shù)無機材料在彈性變形后立即發(fā)生脆性斷 裂，總彈性應變能很小。陶瓷材料的力學特征 高模量 高硬度 高強度 低延伸率3. 聚合物的力學狀態(tài)（1） 非晶態(tài)聚合物 的三種力學狀態(tài) 玻璃態(tài) 高彈態(tài) 粘流態(tài)（2） 結(jié)晶聚合物的力學狀態(tài)T m 、 T f A 結(jié)晶聚合物常存 在一定的非晶部分，也有玻璃化轉(zhuǎn)變。 B 在T g 以上模量下降不大 C 在T m 以上模量迅速下降 D 聚合物分子量很大，T m T f ，則熔融之后即轉(zhuǎn)變成粘流態(tài)， 玻璃化溫度(Tg)是非晶態(tài)塑料使用的上限溫度 是橡膠使用的下限溫度 熔點(Tm)是結(jié)晶聚合物使

4、用的上限溫度 The mechanical behavior of a material reflects the relationship between its response of deformation to an applied load or force.3.1 IntroductionExamples of designing materials based on their characteristics: the aluminum alloy from which an airplane wing (機翼) is constructed and the steel in a

5、n automobile axle (車軸). Factors to be considered as design of materials: the nature of the applied load and its duration, as well as the environmental conditions.Examples:It is possible for the load to be tensile (拉伸), compressive (壓縮), or shear (剪切), and its magnitude may be constant with time or i

6、t may fluctuate continuously.Application time may be only a fraction of a second, or it may extend over a period of many years.Service temperature may be an important factor.Mechanical propertiesStrength (強度)Hardness (硬度)Ductility (塑性)Stiffness (剛度)The testing techniques of these mechanical properti

7、es have been standardized in order to have consistency for a variety of parts.3.2 Concepts of Stress (應力) and Strain (應變) Simple Stress-Strain Test: A load is static or changes slowly with time and is applied uniformly over a cross section or surface of a member. (2) It is commonly conducted at room

8、 temperature.Three principal types to be loaded: Schematic illustration of how a tensile load produces an elongation (延長) and positive linear strain. Dashed lines represent the shape before deformation; solid lines, after deformation Schematic illustration of how a compressive load produces contract

9、ion (收縮) and negative linear strain. Left: Schematic representation of shear strain , where = tan; Right: Schematic representation of torsional deformation (i.e. angle of twist ) produce by an applied torque T, which is applied in engineering practice.Tension tests A specimen is deformed, usually to

10、 fracture, with a gradually increasing tensile load that is applied uniaxially（單軸地） along the long axis of a specimen.Standard tensile specimen:Normally, the cross section is circular, but rectangular specimens are also used. During testing, deformation is confined to the narrow center region.The re

11、quirements of a standard tensile specimen: 1. the standard diameter is approximately 12.8 mm, where the reduced section length should be at least four times this diameter; 60 mm is common. 2. Gauge length is used in ductility computations; the standard value is 50 mm.Testing:The specimen is mounted

12、by its ends into the holding grips (柄、夾、鉗）of the testing apparatus, as shown in following figure: Schematic representation of the apparatus used to conduct tensile stress-strain tests. The specimen is elongated by the moving crosshead; load cell and extensometer（伸長計、張力計） measure, respectively, the m

13、agnitude of the applied load and the elongation.Requirements in testing: 1. The tensile testing machine is designed to elongate the specimen at a constant rate, and continuously and simultaneously （同時地）measure the instantaneous applied load (瞬時外加負載), and to the resulting elongations (發(fā)生長度). 2. A str

14、ess-strain test typically takes several minutes to perform and is destructive.Engineering stress (MPa):F: the instantaneous load applied perpendicular to the specimen cross section (N);A0: the original crossectional area before any load is applied (m2).Engineering strain (untiless): l0 : original length before any load is applied, li : the instantaneous length, and l= li-l0.Compression tests A compression test is conducted in a manner similar to the tensile test, except that the force is compressive and the specimen contracts along the direction of the stress.Previous equations a