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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 Deform2. Plastically Deform3. Fracture
The factors which control which mode acts include:
1. Microstructural features and defects2. Temperature3. Strain rate4. Amount of energy applied5. Stress state (amount of material constraint)
Fracture vs Flow Curve
Source: G. Dieter, Mechanical Metallurgy, 3rd Edition, McGraw-Hill, 1986
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
Ludwik Theory Diagram
Mechanisms of Fracture
• 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
How does fracture manifest itself? Two broad categories:
Ductile vs Brittle Failure
Very Ductile
ModeratelyDuctile BrittleFracture
behavior:
Large Moderate%AR or %EL Small• Ductile fracture is usually desirable!
Adapted from Fig. 8.1, Callister 7e.
• Classification:
Ductile: warning before
fracture
Brittle: No
warning
• Evolution to failure:
• Resulting fracture surfaces (steel)
50 mm
particlesserve as voidnucleationsites.
50 mm
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.)
100 mmFracture surface of tire cord wire loaded in tension. Courtesy of F. Roehrig, CC Technologies, Dublin, OH. Used with permission.
Moderately Ductile Failure
necking
void nucleation
void growth and linkage
shearing at surface fracture
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
Adapted from Fig. 8.3, Callister 7e.
cup-and-cone fracture brittle fracture
Ductile Fracture
Source: G. Dieter, Mechanical Metallurgy, 3rd Edition, McGraw-Hill, 1986
Manifests differently for different microstructures
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
• 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
Cylindrical Tensile Specimen
Source: Reed-Hill, Abbaschian, Physical Metallurgy Principles, 3rd Edition, PWS Publishing Company, 1994.
Intergranular Fracture
Brittle FailureArrows indicate pt at which failure originated
Adapted from Fig. 8.5(a), Callister 7e.
• Intergranular(between grains)
• Intragranular (within grains)
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.)
316 S. Steel (metal)
Reprinted w/ permission from "Metals Handbook", 9th ed, Fig. 650, p. 357.
Copyright 1985, ASM International, Materials
Park, OH. (Micrograph by D.R. Diercks, Argonne
National Lab.)
304 S. Steel (metal)Reprinted w/permission from "Metals Handbook", 9th ed, Fig. 633, p. 650. Copyright 1985, ASM International, Materials Park, OH. (Micrograph by J.R. Keiser and A.R. Olsen, Oak Ridge 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.
3 mm
4 mm160 mm
1 mm(Orig. source: K. Friedrick, Fracture 1977, Vol. 3, ICF4, Waterloo, CA, 1977, p. 1119.)
Brittle Fracture Surfaces