Fracture
• This is BIG topic
• Underlines all of Failure Analysis – One of the
big fields that metallurgists/ material scientists
get involved in
• There are several fields that are specific to
fracture including:
– Fracture mechanics – calculation of fracture behavior
using very high level math (imaginary calculus)
– Fractography – study of the morphology of fracture
surfaces
• We are going to do another drive by on this topic
3 Possible Reponses…
Remember we discussed that there were 3 possible responses
when stress is applied to a material
The material can:
1. Elastically Deform
2. Plastically Deform
3. Fracture
The factors which control which mode acts include:
1.
2.
3.
4.
5.
Microstructural features and defects
Temperature
Strain rate
Amount of energy applied
Stress state (amount of material constraint)
Fracture vs Flow Curve
Ludwik Theory Diagram
Plastic flow is terminated by fracture
when strain hardening, triaxial stress, or
high strain rate inhibit plastic deformation
and applied stress is higher than fracture
stress
Source: G. Dieter, Mechanical Metallurgy, 3rd Edition, McGraw-Hill, 1986
Mechanisms of Fracture
How does fracture manifest itself? Two broad categories:
• Ductile fracture
– Occurs after significant plastic deformation
• Brittle fracture
– Little or no plastic deformation
– Catastrophic failure
– Typically unstable crack propagation
– Cracks can propagate at the speed of
sound in the material
Ductile vs Brittle Failure
• Classification:
Fracture
behavior:
Very
Ductile
Moderately
Ductile
Brittle
Large
Moderate
Small
Adapted from Fig. 8.1,
Callister 7e.
%AR or %EL
• Ductile
fracture is usually
desirable!
Ductile:
warning before
fracture
Brittle:
No
warning
Moderately Ductile Failure
• Evolution to failure:
necking
s
• Resulting
fracture
surfaces
void
nucleation
void growth
and linkage
shearing
at surface
fracture
50
50mm
mm
(steel)
100 mm
particles
serve as void
nucleation
sites.
From V.J. Colangelo and F.A. Heiser,
Analysis of Metallurgical Failures (2nd
ed.), Fig. 11.28, p. 294, John Wiley and
Sons, Inc., 1987. (Orig. source: P.
Thornton, J. Mater. Sci., Vol. 6, 1971, pp.
347-56.)
Fracture surface of tire cord wire
loaded in tension. Courtesy of F.
Roehrig, CC Technologies, Dublin,
OH. Used with permission.
Void Sheet Mechanism
Source: Reed-Hill, Abbaschian, Physical Metallurgy Principles, 3rd Edition, PWS Publishing Company, 1994.
Ductile Fracture of Tensile Specimen
Ductile vs. Brittle Failure
cup-and-cone fracture
Adapted from Fig. 8.3, Callister 7e.
brittle fracture
Ductile Fracture
Manifests differently for different microstructures
Source: G. Dieter, Mechanical Metallurgy, 3rd Edition, McGraw-Hill, 1986
Brittle Fracture
Typically 2 Types:
1. Transgranular Cleavage
2. Intergranular Fracture
Brittle Fracture: Cleavage
Brittle Transgranular Cleavage
Effect of State of Stress
• Cleavage crack
nucleation and
propagation are favored
by high tensile stresses
• Slip requires shear
stress
• Large tensile stresses
and restricted shear –
favors cleavage
• Stress state is important
consideration
Source: Reed-Hill, Abbaschian, Physical Metallurgy Principles, 3rd Edition, PWS Publishing Company, 1994.
Notches Produce Tri-axial Stress State
Cylindrical Tensile Specimen
• When loaded in tension
reduced cross-section at
notch will be the first to yield
• As elongates in vertical
direction – wants to shrink in
horizontal plan
• This motion is resisted by
metal above and below
which has not yet yielded
• Creates triaxial stress state
Source: Reed-Hill, Abbaschian, Physical Metallurgy Principles, 3rd Edition, PWS Publishing Company, 1994.
Intergranular Fracture
Brittle Failure
Arrows indicate pt at which failure originated
Adapted from Fig. 8.5(a), Callister 7e.
Brittle Fracture Surfaces
• Intragranular
• Intergranular
(between grains)
4 mm
304 S. Steel
(metal)
(within grains)
316 S. Steel
(metal)
Reprinted w/permission
from "Metals Handbook",
Reprinted w/ permission
9th ed, Fig. 633, p. 650.
from "Metals Handbook",
Copyright 1985, ASM
9th ed, Fig. 650, p. 357.
International, Materials
Copyright 1985, ASM
Park, OH. (Micrograph by
International, Materials
J.R. Keiser and A.R.
Park, OH. (Micrograph by
Olsen, Oak Ridge
D.R. Diercks, Argonne
National Lab.)
National Lab.)
Polypropylene
(polymer)
Reprinted w/ permission
from R.W. Hertzberg,
"Defor-mation and
Fracture Mechanics of
Engineering Materials",
(4th ed.) Fig. 7.35(d), p.
303, John Wiley and
Sons, Inc., 1996.
1 mm
(Orig. source: K. Friedrick, Fracture 1977, Vol.
3, ICF4, Waterloo, CA, 1977, p. 1119.)
160 mm
Al Oxide
(ceramic)
Reprinted w/ permission
from "Failure Analysis of
Brittle Materials", p. 78.
Copyright 1990, The
American Ceramic
Society, Westerville, OH.
(Micrograph by R.M.
Gruver and H. Kirchner.)
3 mm