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