Fracture Mechanic

Dr.Behzad Heidar shenas

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

Assessment

- Attendance: 10%- Quiz (2): 15%- Midterm: 30%- Project: 10%- Final: 35%

Engineering Materials

The Structure of Metals

Figure 1.1 An outline of the topics described in Chapter 1.

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:

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

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

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

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

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.

Imperfections

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.

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

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.

Mechanism of slip

The strength of metals = shear modulus/6

Slip by Dislocation

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.

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

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)

Plastic deformation of compounds