材料科学与工程基础：第3章 力学性能1

1、Ch.3 Mechanical Properties （力学性能）3.1 Introduction (简介)3.2 Concepts of Stress and Strain (应力应变的概念)3.3 Elastic Deformation (弹性变形)3.4 Mechanical Behavior-Metals (金属的力学行为)3.5 Hardness (硬度)3.1 IntroductionHardness tester & hardness scalesmechanical property of materialsstress and strainElastic deformatio

2、nModulusViscoelasticitypermanent deformationStrengthFracture 材料的力学状态 mechanical states of matrials1.金属的力学状态A 晶态结构，B 较高的弹性模量 和强度，C 受力开始为弹性 形变，接着一段 塑性形变，然后 断裂， 总变形能很大，D 具有较高的熔点。某些金属合金A 呈非晶态合金，B 具有很高的硬度和强度，C 延伸率很低而并不脆。D 温度升高到玻璃化转变温度以上，粘度明显降低， 发生晶化而失去非晶态结构。2. 无机非金属的力学状态A 玻璃相熔点低，热稳定性差，强度低。B 气相（气孔）的存在导致陶瓷

3、的弹性模量和机械 强度降低。C 陶瓷材料也存在玻璃化转变温度Tg。D 绝大多数无机材料在弹性变形后立即发生脆性断 裂，总弹性应变能很小。陶瓷材料的力学特征 高模量 高硬度 高强度 低延伸率3. 聚合物的力学状态（1） 非晶态聚合物 的三种力学状态 玻璃态 高弹态 粘流态（2） 结晶聚合物的力学状态T m 、 T f A 结晶聚合物常存 在一定的非晶部分，也有玻璃化转变。 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 ，则熔融之后即转变成粘流态， 玻璃化温度(Tg)是非晶态塑料使用的上限温度 是橡胶使用的下限温度 熔点(Tm)是结晶聚合物使

4、用的上限温度 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 (机翼) is constructed and the steel in a

5、n automobile axle (车轴). 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 (压缩), 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 (强度)Hardness (硬度)Ductility (塑性)Stiffness (刚度)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 (应力) and Strain (应变) 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 (延长) 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 (收缩) 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（单轴地） 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、夹、钳）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（伸长计、张力计） 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 （同时地）measure the instantaneous applied load (瞬时外加负载), and to the resulting elongations (发生长度). 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