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Objectives1. Identify and describe the three kinds of 

fracture processes.

2. Identify the major surface features

produced by the three processes,

and identify the process which produceda given surface shown in a micrograph.

3. Understand the difference between shear

and normal stress induced failure.

4. Identify the three loading modes for

fracture.

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Macroscopic

Fracture Modes

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Initiation & Propagation

Initiation Propagation

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I. Transgranular

Fracture

II. Intergranular

Fracture

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Cleavage Mechanism

I. Brittle transgranularfracture

A. BCC and HCP metals

only

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Cleavage Mechanism

I. Cleavage (brittle transgranularfracture)

A. BCC and HCP metals only

B. Steps

1. Dislocation formation and

pileup at inclusion or defect

2. Crack initiation

3. Propagation of the crack 

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Cleavage Mechanism

C. Initiation1. Occurs at 2nd phase particles

2. 2nd phase particles resist crack initiation

if stress concentration is small

a. small particles (r < 1 micron)b. spherical particles (rather than

platelike)

c. well bonded to matrix

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Small, Spherical, Bonded Inclusions

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Cleavage Mechanism

C. Initiation1. Occurs at 2nd phase particles

2. 2nd phase particles resist crack initiation

if stress concentration is small

a. small particles (r < 1 micron)b. spherical particles (rather than

platelike)

c. well bonded to matrix for load

transfer3. Brittle constituents at the grain

boundaries are the worst case.

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Cleavage Mechanism

D. Propagation1. More difficult to cross grain

boundaries

2. Fine, spherical 2nd phase particles

resist propagation

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Intergranular Mechanism

II. Brittle Fracture occurring in all metals:

A. Less energy absorbed than in cleavage

because yield is not necessary

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I. Transgranular

Fracture

II. Intergranular

Fracture

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Fracture Mechanisms

II. Intergranular Fracture:

A. Less energy absorbed than in cleavage

because yield is not necessary

B. Caused by degradation of grainboundaries

1. Segregation of tramp impurities

2. Stress corrosion cracking

3. Hydrogen embrittlement

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Ductile Mechanism

III. Ductile Fracture in allmetals

A. Void nucleation, growth,

coalescence phenomenon, surfaceis dimpled.

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Ductile Mechanism

B. Nucleation

1. Cracking of second phase

particles

2. Heterogeneous deformation – 

 causing loss of cohesion of 

matrix/ particle interface

3. Control particle size, shape,

strength, adhesion, and volume

fraction to control nucleation

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Mechanisms of Inclusion Failure

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Small, Spherical, Bonded Inclusions

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Ductile Mechanisms

C. Void growth

1. State of stress determines growth rate

2. Degree of strain hardening is indication

of resistance to void growth

3. Interparticle spacing determines limit of 

growth prior to fracture

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Fracture Modes

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Dimple Formation: Mode I

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Dimple Formation: Mode II

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Failure Stresses

1. Shear stresses produce plastic

deformation

Ductile Failure: Shear Stress Driven/ 

Preceded By Yield

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Plane of Max Shear

Stress

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Failure Stresses

1. Shear stresses produceplastic deformation

Ductile Failure: Shear StressDriven

2. Normal stresses tend toopen up cavities, cracks

Brittle Failure : Normal Stress

Driven

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What Kind of Fracture?

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What Kind of Fracture?