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Mechanical Properties of Materials

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  1. 材料力学性能 Mechanical Properties of Materials Prof: Li Min Tel: 86057927 ( 2#333 ) 13863933539 E-Mail:kd_limin@126.com Department of Material Science and Engineering Mechanical Properties of Materials

  2. Chapter Ⅰ Mechanical property under a unidirectional static tensile force  Force-Elongation Curve and Stress-Strain Curve  Elastic Deformation  Plastic Deformation  Fracture Chapter Ⅰ

  3. §1.1Force-Elongation Curve and Stress-Strain Curve 力-伸长曲线和应力-应变曲线 1. The tensile test ( GB/T228-2002 ) perfect specimen a unidirectional static or slowly tensile force room-temperature and ambient air Chapter Ⅰ

  4. 扎克船用锅炉 Chapter Ⅰ

  5. 莱钢锚链 Chapter Ⅰ

  6. Automated tensile testing machine Localized deformation of the ductile material 2. Force-Elongation curve and Stress-Strain curve Material: annealed (退火)low carbon steel Chapter Ⅰ

  7. Deformation process of polycrystalline materials • Deformation process of industrial material consists of three stages: • Elastic deformation, which is reversible Plastic or permanent deformation Fracture Chapter Ⅰ

  8. Engineering stress-strain curve Aluminum alloy PVC Glass Composite Chapter Ⅰ

  9. e/ % True stress-strain curve of typical metals True stress-strain curve Chapter Ⅰ

  10. Terminology: Engineering stress 工程应力σ Engineering strain 工程应变ε True stress 真实应力S True strain 真实应变e The relation between the true stress-true strain and engineering stress-engineering strain? Chapter Ⅰ

  11. The relation between the true stress-true strain diagram and engineering stress-engineering strain diagram. The curves are identical to the yield point(屈服点). Chapter Ⅰ

  12. Summary: Tensile test Engineering stress-strain True stress-strain True stress and true strain calculation Question: Properties obtained from the tensile test? Chapter Ⅰ

  13. §1.2 Elastic Deformation(弹性变形) 1.Features and physical principle Reversibility(可逆性), monodromy(单值性), small deformation External force Attractive force(引力) Repulsive force(斥力) Fext + Fatt + Frep = 0 Elastic behavior is related to atomic bonding forces. Double atomic model Chapter Ⅰ

  14. §1.2 Elastic Deformation(弹性变形) 2. Hooke’s Law Chapter Ⅰ

  15. §1.2 Elastic Deformation(弹性变形) 2. Hooke’s Law Chapter Ⅰ

  16. 3. Elastic Properties  modulus of elasticity—stiffness of materials 弹性模量—材料的刚度 A stiff component(构件), with a high modulus of elasticity, will show much small changes in dimensions if the applied stress is relative small and cause only elastic deformation. Young’s modulus E Shear modulus G Poisson’s ratio ν Chapter Ⅰ

  17. Modulus of elasticity is considered microstructure insensitive property since the value is dominated strongly by the strength of atomic bonds. Melting point, Debye characteristic temperature 弹性模量是一个对组织不敏感的力学性能指标。合金化、压力加工、热处理、细化晶粒、温度、加载速率等都不能对其产生明显影响。 Chapter Ⅰ

  18. proportional limit and elastic limit 比例极限和弹性极限 Proportional limit(p):the level of stress above which the relationship between stress and strain is not linear Elastic limit(e):the critical stress value needed to initiate plastic deformation “Specified elastic limit 0.01”――the stress value corresponding to 0.01%offset elongation(残余伸长) Chapter Ⅰ

  19. e ε 0 εe • modulus of resilience (弹性比功 ) The elastic energy that a material absorbs during loading and subsequently releases when the load is removed. -在开始塑性变形前单位体积所能吸收的最大弹性变形功; -储备弹性能的能力; Chapter Ⅰ

  20. Discussion: 1、The ability of a spring(弹簧) to perform satisfactorily depends on high/or low modulus of resilience? 2、What materials are selected on the spring? 3、How to change the modulus of resilience? Chapter Ⅰ

  21. 举例: 没有满载,变形却达到最大 汽车弹簧 使用一段时间,弓形↓↓ 分析:产生上述现象的原因?如何改善? 重点:区分弹性与刚度两个概念 Chapter Ⅰ

  22. 哪些材料会出现明显的滞弹性? 4. anelasticity(滞弹性) Time–dependent elastic strain 快速加载或卸载后,随时间延长产生附加弹性应变的现象。 组织不均匀, 滞弹性倾向增大 Chapter Ⅰ

  23.  ε ε 0 0 加载和卸载时的应力应变曲线不重合形成一封闭回线 ------弹性滞后环 elastic hysteresis loop Energy absorbed by materials internal friction 内耗(消振性) Chapter Ⅰ

  24. Engineering examples:  music wire  machine body  turbine blade Chapter Ⅰ

  25.  1 30.1 24.0 4 8.5 0 ε 17.8 2 28.7 3 5. Bauschinger effect(包申格效应) 产生了少量塑性变形(残余应变为1%-4%)的材料,再同向加载则弹性极限(屈服强度)升高;反向加载则弹性极限(屈服强度)降低的现象。 Annealed rolled brass Chapter Ⅰ

  26. X80高强度管线钢的生产工艺流程  在工程应用上,材料的成型加工工艺需考虑包申格效应; 大型输油输气管道管线的UOE工艺 Chapter Ⅰ

  27. 利用包申格效应? 薄板的反向弯曲变形 Chapter Ⅰ

  28. 消除方法: 预先进行较大的塑性变形; 在第二次反向受力前先使其在回复或再结晶 温度下退火; 超弹性 ( hyperelasticity ) 粘弹性 ( viscoelasticity ) Chapter Ⅰ

  29. summary: 1、property index:E、 p、 e、 0.01、ae 2、“elasticity” and “stiffness” 3、engineering application of anelasticity and Bauschinger effect Chapter Ⅰ

  30. §1.3 Plastic deformation(塑性变形) 1. Deformation mode and features  Plastic deformation in single crystalline materials  slip (滑移)  Twinning (孪生) Basic feature of slip and twinning Both slip and twinning are shear processes under shear stresses, i.e. it is a translational movement of one portion of the crystal with respect to the other on a specific plane and along a specific direction. Chapter Ⅰ

  31. slip twinning

  32. Basic distinction between single crystal and polycrystals • A great deal of grains with different orientation • Grain boundary  Basic features of plastic deformation of polycrystal Nonsimultaneous and inhomogenous 不同时性和不均匀性  Interaction and intercoordination 相互制约和相互协调 Chapter Ⅰ

  33. 2. 屈服现象 ( yield phenomenon ) 外力不增加,试样仍能继续伸长,或外力增加到一定数值后突然下降,随后在外力不增加或上下波动的情况下,试样继续伸长变形。 Chapter Ⅰ

  34.  Explanation of yield phenomenon Cottrell atmospheretheory 柯氏气团理论 Dislocation propagation theory 位错增殖理论 Chapter Ⅰ

  35.  yield strength 规定非比例伸长应力(p):试样标距部分的非比例伸长达到规定的原始标距百分比时的应力。 规定总伸长应力(t):试样标距部分的总伸长(弹性+塑性伸长)达到规定的原始标距百分比时的应力。 规定残余伸长应力(r):试样卸除拉伸力后,其标距部分的残余伸长达到规定的原始标距百分比时的应力。 “条件屈服强度0.2”---- 试样残余伸长量为标距长度的 0.2%时的应力。 0.2%-offset yield strength Chapter Ⅰ

  36. Discussion:  Advantage and disadvantage of sharp yield?  Engineering application of yield strength Strength design of component The relationship between yield strength and processing property Chapter Ⅰ

  37. 3. Factors affecting yield strength Dislocation population and motion Grain boundary, composition, second phase External factors  Influence of crystal structure A、lattice obstacle : B、interaction obstacle of dislocation: Chapter Ⅰ

  38. 细晶强化  Grain size (晶粒大小) Grain boundary resistance----Hall—Petch equation: Technological method? Grain refinement strengthening Chapter Ⅰ

  39.  Solute element(溶质元素) Solid solution strengthening (固溶强化) It is caused by increased resistance to dislocation motion. Increasing both atomic size difference and amount of alloying element increases solid-solution strengthening. • 间隙固溶体的强化效果比置换固溶体的大! Interstitial solid solution , substitutional solid solution Chapter Ⅰ

  40. Deforming particles Nondeforming particles 切过机制 绕过机制  Second phase particle(第二相粒子) Dispersion or precipitation strengthening Chapter Ⅰ

  41. Ni合金中位错绕过Ni3Al相的电镜照片 Chapter Ⅰ

  42.  External factors  temperature  strain speed  stress state Chapter Ⅰ

  43. 10μm 10μm 10μm 10μm (a) (b) ) (c) c) (d) d) (a)鲱骨状共晶组织; (b)鱼骨状共晶组织; (c)粒状共晶组织; (d)蜂窝状共晶组织; Chapter Ⅰ

  44. (b) (a) (a) (c) Chapter Ⅰ

  45. Cr V Ni Si Ti Nb Fe-Cr-V-Ti-Ni-Nb-Si-B-C系表面冶金涂层时效组织的成分面扫描

  46. 4. Work hardening or strain hardening 应变硬化(形变强化、加工硬化) 当外力超过屈服强度后,塑性变形并不是象屈服平台那样继续流变下去,而需要不断地增加外力才能继续进行。 自行车链条(16Mn钢板): 原始厚度3.5mm 150HB b=520MPa 五次冷轧后1.2mm 275HB b>1000MPa Applications: Cold drawing wire冷拔高强度钢丝and cold coiling冷卷弹簧; 坦克和拖拉机的履带(track)、破碎机的颚板(jaw plate) 以及铁路的道岔(switch); Chapter Ⅰ

  47. S-e曲线:流变曲线(均匀塑变阶段) Hollomon 关系式 硬化系数 应变硬化指数 Chapter Ⅰ

  48. e n的变化范围  Strain-hardening exponent 应变硬化指数 n――反映材料抵抗继续塑性变形的能力,反映了形变强化的趋势,表征材料应变硬化的性能指标。 n值愈大,材料对继续塑性变形的抗力愈高。 完全的弹性体 n的测定: 无应变硬化 Chapter Ⅰ

  49.  Engineering significance of strain hardening  Advantages: Strength, hardness, wear-resistance↑ Reliability↑ Necessary condition of cold-working (forming) Ductility ↓and machinability ↑  Disadvantage: Continuous forming becomes difficult. Chapter Ⅰ

  50. 喷丸处理后的截齿 Chapter Ⅰ