大叶大学机械与自动化工程学系.ppt

2.1 Introduction Material Crystalline A material is made up of a number of small units called crystals or grains. Noncrystalline Commonly used engineering materials 2.2 Material Property Definitions Mechanical properties of material What are the mechanical properties? How to get the mechanical properties? The factor considered in the selection of a material in design Functional Strength Stiffness Durability Machinability Workability Malleability Hardness Economic Initial cost Maintenance cost Replacement cost 2.3 Static Strength Stress-Strain Diagrams for Ductile Materials Conventional or engineering stress-strain diagram (OABCDE) True stress-strain diagram (OABCF) Yield Strength Elastic range (OA) Proportional limit (Sp) Yield point (yield strength) (Sy) Perfectly plastic (BC) Strain Hardening (Cold working) Range CD An increase in stress is required for a continued increase in strain Unloading at point g in region CD Retain to the point h along line gh, and line gh // line OA Permanent set (Oh) Reloading at point h Stress-strain curve (hgDE) New yield point (g) The ductility is reduced after unloading and then reloading process Ultimate Tensile Strength (Su) Definition of ductility of a material Offset Yield Strength 0.2% offset method Determine the yield strength for certain materials which don’t show a distinctive yield point. Stress-Strain Diagram for Brittle Materials No well-defined linear region. No difference between the ultimate strength and the fracture strength. The strain at the rupture is much smaller. Rupture occurs with no noticeable prior change in the rate of elongation. The fracture is associated with the tensile stresses. Stress-Strain Diagrams in Compression For most ductile materials, the yield strength is about the same in tension and compression (Even material). For brittle material, the stresses in compression are much greater than in tension (Uneven material). 2.4 Hooke’s Law and Modulus of Elasticity 2.5 Generalized Hooke’s Law Dilat