fracture mechanic dr.behzad heidar shenas. course outline -an overview on the materials...
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Fracture Mechanic
Dr.Behzad Heidar shenas
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Course Outline- An overview on the materials characteristics:1. Types of crystal structures2. Defects3. Stress-Strain diagram evaluation
- Destructive examination
- Concept of Fracture- Fracture mechanic- Fatigue failure- Scc fatigue corrosion- Creep- Design and failure analysis - NDT* Book: Deformation and Fracture Mechanics of Engineering Materials, Richard W.Hertzberg,
Fifth Edition
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Assessment
- Attendance: 10%- Quiz (2): 15%- Midterm: 30%- Project: 10%- Final: 35%
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Engineering Materials
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The Structure of Metals
Figure 1.1 An outline of the topics described in Chapter 1.
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Crystal Structure of Metals
• Body-centered cubic (BCC) - alpha iron, chromium, molybdenum, tantalum, tungsten, and vanadium.
• Face-centered cubic (FCC) - gamma iron, aluminum, copper, nickel, lead, silver, gold and platinum.
• Hexagonal close-packed - beryllium, cadmium, cobalt, magnesium, alpha titanium, zinc and zirconium.
Common crystal structures for metals:
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Body-Centered Cubic Crystal Structure
• Example: Iron (Fe)• Good strength
• Moderate ductivity
The body-centered cubic (bcc) crystal structure: (a) hard-ball model; (b) unit cell; and (c) single crystal with many unit cells
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Face-Centered Cubic Crystal Structure
• Example: Aluminum (Al)
• Moderate strength
• Good ductivity
The face-centered cubic (fcc) crystal structure: (a) hard-ball model; (b) unit cell; and (c) single crystal with many unit cells
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Hexagonal Close-Packed Crystal Structure
• Example: Beryllium, Zinc
• Low strength
• Low ductivity
The hexagonal close-packed (hcp) crystal structure: (a) unit cell; and (b) single crystal with many unit cells
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Solidification
(a) Nucleation of crystals at random sites in the molten metal; note that the crystallographic orientation of each site is different.
(b) and (c) Growth of crystals as solidification continues.
(d) Solidified metal, showing individual grains and grain boundaries; note the different angles at which neighboring grains meet each other.
Schematic illustration of the stages during solidification of molten metal; each small square represents a unit cell.
Grain boundary
Volume imperfection
voids inclusions
Plane imperfection
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Solidification of Molten Metal
Schematic illustration of the stages during solidification of molten metal; each small square represents a unit cell. (a) Nucleation of crystals at random sites in the molten metal; note that the crystallographic orientation of each site is different. (b) and (c) Growth of crystals as solidification continues. (d) Solidified metal, showing individual grains and grain boundaries; note the different angles at which neighboring grains meet each other.
Schematic illustration of the stages during solidification of molten metal; each small square represents a unit cell. (a) Nucleation of crystals at random sites in the molten metal; note that the crystallographic orientation of each site is different. (b) and (c) Growth of crystals as solidification continues. (d) Solidified metal, showing individual grains and grain boundaries; note the different angles at which neighboring grains meet each other.
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Imperfections
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Defects in a Single-Crystal Lattice(point imperfections)
Schematic illustration of types of defects in a single-crystal lattice: self-interstitial, vacancy, interstitial, and substitutional.
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Deformation Mechanisms
Cubic metals readily deform by plastic shear or slip.
Sliping: one plane of atoms slides over the next adjacent plane.
Shear deformation also occurs when compression or tension forces are applies
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Permanent Deformation
Figure 1.5 Permanent deformation (also called plastic deformation) of a single crystal subjected to a shear stress: (a) structure before
deformation; and (b) permanent deformation by slip. The b/a ratio influences the magnitude of the shear stress required to cause slip.
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Mechanism of slip
The strength of metals = shear modulus/6
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Slip by Dislocation
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Movement of Dislocation(line imperfection)
Movement of an edge dislocation across the crystal lattice under a shear stress. Dislocations help explain why the actual strength of metals in much lower than that predicted by theory.
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If we place a shear stress along the horizontal direction, the dislocation can be moved with shearing displacement within the crystal.
The slip mechanism requires energy, E IGb2
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Plastic deformation of compounds
In Metals : each atom is surrounded by similar atoms.
In compounds: two or more atom types are there, deformation
brings like atoms together and separates a fraction on unlike
atoms
High Energy is required (resistance to shear)
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Plastic deformation of compounds
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