Fundamentals of Material Science

CHAPTER 9

Fracture, Fatigue and Creep

Fundamentals of Material Science Dr. Gamal Abdou

Fracture

WHY STUDY Failure?

Breaking two or pieces- external

•

• load

• Two steps in the process of fracture:––

Crack

Crack

initiation

Propagation

Brittle

Fracture

Ductile

Brittle vs. Ductile Fracture

• Ductile materials - extensive plastic deformation

energy absorption (“toughness”) before fracture

and

• Brittle materials - little plastic deformation and low

energy absorption before fracture

3

• Different types of fracture

Ductile fracture

a)

b)

c)

d)

e)

Necking

microcracks formation

Crack formation

Crack propagation

fracture

Brittle Fracture

•

Exhibits little or no plastic deformation and low energy absorption

before failure.

Crack propagation spontaneous and rapid

Occurs perpendicular to the direction of the applied stress,

forming an almost flat fracture surface.

Crack propagation corresponding to Successive and repeated

•

•

•

breaking of atomic bonds along specific crystallographic planes

called cleavage

is

•

•

This type of fracture

This type of fracture

FCC

is called cleavage fracture

are generally found in BCC and HCP, but not

Transgranular and Intergranular

Fracture

Crack propagation across grain boundaries is known as

transgranular

•

• While propagation

Intergranular

along grain boundaries is termed

Ductile – Brittle Transition

Ductile materials fracture abruptly and with little plastic

deformation

Crack propagation takes precedence over plastic deformation

•

•

•

1.

2.

3.

Ductile – Brittle transition occurs when

Temperature is lowered

Rate of straining increased

Notch or stress raiser is introduced

Ductile-Brittle Transition Temp

The temperature at which the stress to propagate a crack бf is equal to

the stress to move dislocations бy .

When бy < бf material is ductile

When бy > бf material is brittle

This transition is commonly observed in materials having BCC and

HCP structures.

For ceramic materials, the transition takes place at elevated

temperatures.

For polymers the transition occurs over a narrow range, below room

temp.

•

•

•

•

•

•

Griffith theory of fracture

Measured fracture strength of most brittle materials are

significantly lower than theoretical strength- what is the

reason?

• Stress concentration

• Brittle materials contains a population of fine cracks which

produce a stress concentration

• Stress amplification is assumed to be at the crack tip

• Magnitude of this amplification depends on the crack

orientation and geometry

• It is assumed that the crack is elliptical in shape and is

oriented with major axis perpendicular to the applied stress

Protection against fracture

Introducing compressive stresses

Polishing surfaces

Avoiding sharp corners

•

•

•

•

•

•

Improving purity of the

Grain refinement

materials

Avoid precipitation of second phase

Mode of fracture

Fatigue: CyclicStresses (I)

Random

stress

fluctuations

Periodic and

asymmetrical

about zero

stress

Periodic and

symmetrical

about zero

stress

Fatigue: Crack ini t iat ion and propagat ion ( I I )

• Crack initiation at the sites o f stress concentration

(microcracks. scratches. indents. interior corners.

dislocation

important.

slip steps. etc.) . Quality o f surface is

•

:;.....

Crack propagation

( T

Stag�e I: initial

along

slow

crystalpropagation

planes with high resolved

shear stress. Involves just a

few grains, and has flat Stage II

fracture surface

> Stage II: f aster propa gation

perpendicular to the applied

stress. Crack grows by

andrepetitive

sharpening

blunting

process

fracture

at crack

surface.tip. Rough o

• Crack eventually reaches critical dimension and29propagates very rapidly

Appearance of typical fatigue

surfacefracture

• Permanent deformation of materials on the application

of a load can be either plastic deformation or creep.

• The permanent deformation at temperature below

0.4 Tm is called PLASTIC DEFORMATION.

Amount of deformation occurring after the application

load is negligible. Rate at which material deformed

determines deformation characteristics

• of

• At temp above 0.4 Tm permanent deformation

function of time too. This behaviour is CREEP.

is a

CREEP

• Materials are often placed under steady loads forlonger periods of time

Without increase in load materials undergoes

deformation

Creep is predominant at higher temperature, ie. An elevated temperature effect.

Creep is a time-dependent and permanent

deformation of materials when subjected to a

constant load at a high temperature over a longer

periods. (> 0.4 Tm).

•

•

•

Creep TestTo determine the continuing change in the deformation of

materials at elevated temperatures

•

• Four variables measured during a creep test are stress, strain,

temperature and time.

Creep curves

Shows the relationship between creep• strain

vs time at a particular temperatures.

Mechanism of creep

Creep resistant materials

Materials of high melting point like

refractories, superalloys, ceramics etc.

Alloys with solutes of lower diffusivity

Coarse grained materials

•

•

•

• Directionally solidified alloys

grains

Single grained materials

with columnar

•

Factors affecting creep

1.

2.

3.

4.

5.

6.

Thermal stability and melting

Grain size and shape

Precipitation hardening

Dispersion hardening

point

Cold working

Formation of

or work hardening

substitutional solid solution

Structural changes

Deformation by slip

Sub-grain formation

during creep

1.

2.

3. Grain boundary sliding

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