Page 1
7/31/2019 10 Fracture Mechanisms Notes
http://slidepdf.com/reader/full/10-fracture-mechanisms-notes 1/32
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.
Page 2
7/31/2019 10 Fracture Mechanisms Notes
http://slidepdf.com/reader/full/10-fracture-mechanisms-notes 2/32
Macroscopic
Fracture Modes
Page 3
7/31/2019 10 Fracture Mechanisms Notes
http://slidepdf.com/reader/full/10-fracture-mechanisms-notes 3/32
Page 4
7/31/2019 10 Fracture Mechanisms Notes
http://slidepdf.com/reader/full/10-fracture-mechanisms-notes 4/32
Initiation & Propagation
Initiation Propagation
Page 5
7/31/2019 10 Fracture Mechanisms Notes
http://slidepdf.com/reader/full/10-fracture-mechanisms-notes 5/32
I. Transgranular
Fracture
II. Intergranular
Fracture
Page 6
7/31/2019 10 Fracture Mechanisms Notes
http://slidepdf.com/reader/full/10-fracture-mechanisms-notes 6/32
Cleavage Mechanism
I. Brittle transgranularfracture
A. BCC and HCP metals
only
Page 7
7/31/2019 10 Fracture Mechanisms Notes
http://slidepdf.com/reader/full/10-fracture-mechanisms-notes 7/32
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
Page 8
7/31/2019 10 Fracture Mechanisms Notes
http://slidepdf.com/reader/full/10-fracture-mechanisms-notes 8/32
Page 9
7/31/2019 10 Fracture Mechanisms Notes
http://slidepdf.com/reader/full/10-fracture-mechanisms-notes 9/32
Page 10
7/31/2019 10 Fracture Mechanisms Notes
http://slidepdf.com/reader/full/10-fracture-mechanisms-notes 10/32
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
Page 11
7/31/2019 10 Fracture Mechanisms Notes
http://slidepdf.com/reader/full/10-fracture-mechanisms-notes 11/32
Small, Spherical, Bonded Inclusions
Page 12
7/31/2019 10 Fracture Mechanisms Notes
http://slidepdf.com/reader/full/10-fracture-mechanisms-notes 12/32
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.
Page 13
7/31/2019 10 Fracture Mechanisms Notes
http://slidepdf.com/reader/full/10-fracture-mechanisms-notes 13/32
Cleavage Mechanism
D. Propagation1. More difficult to cross grain
boundaries
2. Fine, spherical 2nd phase particles
resist propagation
Page 14
7/31/2019 10 Fracture Mechanisms Notes
http://slidepdf.com/reader/full/10-fracture-mechanisms-notes 14/32
Intergranular Mechanism
II. Brittle Fracture occurring in all metals:
A. Less energy absorbed than in cleavage
because yield is not necessary
Page 15
7/31/2019 10 Fracture Mechanisms Notes
http://slidepdf.com/reader/full/10-fracture-mechanisms-notes 15/32
I. Transgranular
Fracture
II. Intergranular
Fracture
Page 16
7/31/2019 10 Fracture Mechanisms Notes
http://slidepdf.com/reader/full/10-fracture-mechanisms-notes 16/32
Page 17
7/31/2019 10 Fracture Mechanisms Notes
http://slidepdf.com/reader/full/10-fracture-mechanisms-notes 17/32
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
Page 18
7/31/2019 10 Fracture Mechanisms Notes
http://slidepdf.com/reader/full/10-fracture-mechanisms-notes 18/32
Ductile Mechanism
III. Ductile Fracture in allmetals
A. Void nucleation, growth,
coalescence phenomenon, surfaceis dimpled.
Page 19
7/31/2019 10 Fracture Mechanisms Notes
http://slidepdf.com/reader/full/10-fracture-mechanisms-notes 19/32
Page 20
7/31/2019 10 Fracture Mechanisms Notes
http://slidepdf.com/reader/full/10-fracture-mechanisms-notes 20/32
Page 21
7/31/2019 10 Fracture Mechanisms Notes
http://slidepdf.com/reader/full/10-fracture-mechanisms-notes 21/32
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
Page 22
7/31/2019 10 Fracture Mechanisms Notes
http://slidepdf.com/reader/full/10-fracture-mechanisms-notes 22/32
Mechanisms of Inclusion Failure
Page 23
7/31/2019 10 Fracture Mechanisms Notes
http://slidepdf.com/reader/full/10-fracture-mechanisms-notes 23/32
Small, Spherical, Bonded Inclusions
Page 24
7/31/2019 10 Fracture Mechanisms Notes
http://slidepdf.com/reader/full/10-fracture-mechanisms-notes 24/32
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
Page 25
7/31/2019 10 Fracture Mechanisms Notes
http://slidepdf.com/reader/full/10-fracture-mechanisms-notes 25/32
Fracture Modes
Page 26
7/31/2019 10 Fracture Mechanisms Notes
http://slidepdf.com/reader/full/10-fracture-mechanisms-notes 26/32
Dimple Formation: Mode I
Page 27
7/31/2019 10 Fracture Mechanisms Notes
http://slidepdf.com/reader/full/10-fracture-mechanisms-notes 27/32
Dimple Formation: Mode II
Page 28
7/31/2019 10 Fracture Mechanisms Notes
http://slidepdf.com/reader/full/10-fracture-mechanisms-notes 28/32
Failure Stresses
1. Shear stresses produce plastic
deformation
Ductile Failure: Shear Stress Driven/
Preceded By Yield
Page 29
7/31/2019 10 Fracture Mechanisms Notes
http://slidepdf.com/reader/full/10-fracture-mechanisms-notes 29/32
Plane of Max Shear
Stress
Page 30
7/31/2019 10 Fracture Mechanisms Notes
http://slidepdf.com/reader/full/10-fracture-mechanisms-notes 30/32
Failure Stresses
1. Shear stresses produceplastic deformation
Ductile Failure: Shear StressDriven
2. Normal stresses tend toopen up cavities, cracks
Brittle Failure : Normal Stress
Driven
Page 31
7/31/2019 10 Fracture Mechanisms Notes
http://slidepdf.com/reader/full/10-fracture-mechanisms-notes 31/32
What Kind of Fracture?
Page 32
7/31/2019 10 Fracture Mechanisms Notes
http://slidepdf.com/reader/full/10-fracture-mechanisms-notes 32/32
What Kind of Fracture?