Fracture, Fatigue and Creep

Bessy johny

Asst. Prof.

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Fracture

• WHY STUDY Failure?

• Breaking two or pieces- external load

• Two steps in the process of fracture:

– Crack initiation – Crack Propagation

Fracture

Brittle

Ductile

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• Different types of fracture

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Ductile fracture a) Necking

b) microcracks formation

c) Crack formation

d) Crack propagation

e) fracture

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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 is called cleavage

• This type of fracture is called cleavage fracture

• This type of fracture are generally found in BCC and HCP, but not FCC

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Transgranular and Intergranular Fracture

• Crack propagation across grain boundaries is known as Intragranular/transgranular

• While propagation along grain boundaries is termed Intergranular

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Ductile – Brittle Transition

• Ductile materials fracture abruptly and with little plastic

deformation

• Crack propagation takes precedence over plastic deformation

• Ductile – Brittle transition occurs when

1. Temperature is lowered

2. Rate of straining increased

3. Notch or stress raiser is introduced

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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.

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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

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• It is assumed that the crack is elliptical in shape and is oriented with major axis perpendicular to the applied stress

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• Maximum stress at the crack tip

𝜎𝑚=2𝜎𝑜𝑐

𝜌

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• Increase in surface energy is required to generate extra surface

area

• Source of this increased surface energy is the elastic energy

which is released as the crack spreads

• Griffith criterion -A crack will propagate when the decrease

in elastic strain energy is at least equal to the energy required to

create the new crack surface

• The change in surface energy due to the change in crack length

must be just equal to the change in elastic strain energy.

•𝑑𝑈𝐸

𝑑𝑐=𝑑𝑈𝑠

𝑑𝑐

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Protection against fracture

• Introducing compressive stresses

• Polishing surfaces

• Avoiding sharp corners

• Improving purity of the materials

• Grain refinement

• Avoid precipitation of second phase

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Fracture mechanics

• It is the discipline concerned with the behavior of materials containing cracks or small flaws.

• Fracture toughness measures the ability of the material containing a flaw to withstand an applied load.

• Stress intensity factor 𝐾 = 𝑓𝜎 𝜋𝑐

Unit is MPa 𝑚

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Significance

• Used to design and select materials considering the inevitable presence of flaws

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Mode of fracture

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Appearance of typical fatigue fracture surface

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CREEP

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• 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 of load is negligible. Rate at which material deformed determines deformation characteristics

• At temp above 0.4 Tm permanent deformation is a function of time too. This behaviour is CREEP.

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• Materials are often placed under steady loads for longer 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).

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Creep Test • To 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.

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Creep curves

• Shows the relationship between creep strain vs time at a particular temperatures.

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Mechanism of creep

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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 with columnar grains

• Single grained materials

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Factors affecting creep

1. Thermal stability and melting point

2. Grain size and shape

3. Precipitation hardening

4. Dispersion hardening

5. Cold working or work hardening

6. Formation of substitutional solid solution

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Structural changes during creep

1. Deformation by slip

2. Sub-grain formation

3. Grain boundary sliding

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Superplasticity

• The unusual ability of some metals and alloys to elongate

uniformly thousands of percent at elevated temperatures, much

like hot polymers or glasses.

• Most superplastic alloys are of eutectic or eutectoid

compositions.

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conditions

• The material must possess very fine grain size

• It must be highly strain rate sensitive

• A high loading temperature, greater than 50% of the melting

temperature of the metal

• A low controlled strain rate

• Presence of second phase is also preferred

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Applications

• Widely made use of in metal forming processes like

thermoforming, blow forming, vacuum forming, deep drawing

etc. for the production of large complex shaped products.

• Deep or complex shapes can be made as one piece, single

operation pressings, rather than multistep conventional

pressings or multi piece assemblies.

• The elevated temperatures required to promote superplasticity

also reduce the flow stress of the material and thereby the

force requirements.

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