材料科学与工程导论双语课件教学课件 ppt 作者 陈克正 王玮 刘春延 等编Chapter 4.ppt

Thank You! * * * * * * * * * * * * * * * * * * GEOMETRY, LOAD, MATERIAL Elliptical flaw: Elliptical flaw: ● Condition for crack propagation: ● Values of K for some standard loads geometries: Stress Intensity Factor: Fracture Toughness: EFFECT OF TEMPERATURE Ductile-to-brittle transition temperature FCC metals (e.g. Cu, Ni) ● Increasing temperature. ● Ductile-to-brittle transition temperature (DBTT). BCC metals (e.g., iron at T ? 914 ?C) polymers High strength materials (?y ?E/150) More Ductile Brittle FATIGUE Tension on bottom Compression on top Specimen Counter Flex coupling ● Fatigue = failure due to cyclic stress. ●Stress varies with time. ●Key points: Fatigue... -can cause part failure, even though ?max??critical. -causes ~90% of mechanical component failures! FATIGUE DESIGN PARAMETERS ● Fatigue endurance limit, Sfat: ● For some materials, the fatigue limit is zero! FATIGUE MECHANISM ● Crack grows incrementally: ● Failed rotating shaft: --crack grew even though Kmax?KC --Crack grows faster if ● ?? increases ● Crack gets longer ● Loading freq. increases. Increase in crack length per loading cycle: Crack origin typ. 1 to 6 IMPROVING FATIGUE LIFE Compressive stress at surface C-rich gas --Method 1: carburizing --Method 2: shot peening 1. Impose a compressive surface stress (to suppress surface cracks from growing) 2. Remove stress concentrators CREEP ●Occurs at elevated temperature, T ? 0.4 Tmelt ●Deformation changes with time. CREEP MECHANISMS Dislocation creep Diffusion creep 1. Dislocation trapped by obstacle 2. Vacancies replace atoms on half plane (dislocation climb) 3. Dislocation glides past obstacle Atoms diffuse to stressed surfaces obstacle STEADY STATE CREEP ● Strain rate increases for larger T, ? ● Most of component life spent here. ● Strain rate is constant at a given T, ? -- strain hardening is balanced by, recovery 4.4 Phase Diagrams and Phase Transformation 4.4.1 Phase Diagrams ● When we combine two elements