227 7 failure in materials

8/13/2019 227 7 Failure in Materials

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Prof.Dr. Bilgehan gel

Introduction to Materials

VII. Failure of Materials


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Failure

Ductile Fracture: (Fig.1)

The crack propagation involves high amount of energy

absorbtion

The fracture surface is dull and microscopically dimple formationis seen.

Brittle Fracture: (Fig.2)

No or very low energy absorbtion during fracture.

The fracture surface is flat and shiny


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Fig.7.1. Brittle Fracture

3

The transmissionaxe of a 4x4 vehicle

The half of a charpyimpact specimen


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Fig.7.2. Ductile Fracture

4

The half of a charpy

impact specimen

Necking in a

rectangular tensiletest specimen


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5


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Fatigue Failures

Fatigue failures occur in materials subjected to cyclic loading.

Cyclic loading amplitude can be even lower than the yield strength

of the material.

A small crack is nucleated on the tension side and propagates.

Tensile stresses are needed to propagate a crack.


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Fatigue Failures

Micromechanisms of Fatigue:

Crack nucleation can take place due to:

1. Formation of Slip Bands (Crystallographic Slip)

2. Inclusions or hard second phase particles (especially important for

hardened alloys)


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Fatigue Failures

Animation.7.1. Single Amplitude Loading


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Fatigue Failures

Animation.7.2 Cyclic Loading on a Beam


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Factors Affecting Fatigue Life

Surface Irregularities: The Fatigue crack can propagate more easily, if the surface is

irregular.

The stress distribution just at the tip of the crack is high. (Fig.7.3)

A scratch at the surface or a sharp corner act as stressconcentration point. So, even the nominal stress 0may be low, the

stress maxjust at the tip of the crack may be very high.

Also the crack length a and the crack tip radius effect the fatigue

life.

The large cracks (large a) and sharper cracks (smaller ) causes ahigher max. So fatigue crack propagation become easy.


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Fig.7.3. Notch effect


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Tensile Strength: The Fatigue life of a component increase, with an increase in the

Ultimate Tensile Strength (Fig.7.4)

Therefore, high strength materials give higher fatigue strength.

This is due to that the crack initiation by plastic deformation at thesurface becomes more difficult.

However, this relation is lost at very high strength values, because

the surface imperfections become the limiting factor. No further

increase is observed


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Fig.7.4. The relation between UTS and Fatigue Life.

Real data shows a scatter (due to surface imperfections)


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Fig.7.5. Initiation of a fatigue crack after a plastic deformation.


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Surface Hardness:Improves the fatigue life:

Cold Working

Shot peening and cold working are advantageous (E.G. Cold

threading of the bolts)

Case Hardening

Induction hardening, carburizing improves the fatigue life.

Surface Softening

Cladding, decarburization has an adverse effect


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Fig.7.6. Effect of strength on fatigue life.


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As mentioned previously non-metallic inclusion stem fromsteelmaking process.

Steels having lower content of non-metallic inclusions are named as

clean steels.

Clean steels have better fatigue life (Figure.7.6b)


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Fig.7.6b. Effect of non-metallic inclusions on fatigue life.


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Creep

Tendency of a material to deform slowly under stress and hightemperature

Deformation is permanent

For materials, creep is observed at temperatures above 0,5Tm

(Melting Temperature).

The stages of creep can be divided into (Fig.4)

1. Primary Creep

2. Secondary Creep

3. Tertiary Creep


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Creep

Fig.7.7. Stages of a creep failure


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The Mechanism of Creep

Creep starts with dislocation motion in elastic region

Grains slip on each other and form POROSITY at three-point

juctions (Fig.7.8)

These porosities weaken the material and rupture takes place

Creep is one of the events, where COARSE GRAINS are wanted

I t d ti t M t i l


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Creep

Fig.7.8. The grain boundary slip at high temperatures and formation of

porosity.

I t d ti t M t i l


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Design for Creep Resistant Materials

Generally Ni based alloys are used.

Coarse grained materials.

Microalloying to form grain boundary precipitates, which delay the

grain boundary sliding.

Powder metallurgy products, with Al2O

3or Y

2O

5oxide particle

additions.

For very critical parts single crystal superalloys

Introd ction to Materials


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Ductile to Brittle Transition Temperature

A sudden ductility loss is observed below a definite temperature.

This temperature is named as Ductile to Brittle Transition

Temperature (DBTT)

Charpy impact testing is helpful in determining the DBTT

As seen in Fig.7.9, metal starts to behave like a glass, below a

definite temperature. No toughness left.

In polymers, it is named as Glassy Transition Temperature Tg.



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Fig.7.9. The ductile to brittle temperature transformation of steels.



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Ductile to Brittle Transition Temperature

DBTT is an important failure type for BCC

It is seen at cryogenic temperatures

Fig.7.10. The DBTTbehaviour of FCC and

BCC metals.


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Fig.7.11a. The effect of dissolved oxygen on DBTT of steels.



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Fig.7.11b. The effect of sulphur content on DBTT of steels.



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Stress Corrosion Cracking

Cracks form in metal alloys under definite environmental conditions. - All these factors must be present together:

Residual Tensile Stress

An environment

A susceptible material

- Austenitic stainless steels >>>in Cl environment

- High strength steels >> Halide ions, nitric acid

- Copper alloys >> NH3containing solutions



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Stress Corrosion Cracking

Sources of stress: Residual stresses from manufacture (cold deformation, assembly,

welding, solidification (casting))

In service: Residual stresses due to: Overloading, Thermal cycling.

Fig.7.12. A crack formed due to SCC.



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Hydrogen Embrittlement

Source of Hydrogen in steels:

During refining (precipitates upon solidification from supersaturated

concentrations)

Acid cleaning (pickling) prior to coating etc.

Electroplating

Contact with water or other hydrogen-containing liquids or gases



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Hydrogen Embrittlement

The failure types caused by hydrogen: Hydrogen Embrittlement

Hydrogen induced blistering (small bubble-like failures on the metal

surface)

Inner cracks in large sectioned steels.

It is especially a problem for high strength steels.



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Prof Dr Bilgehan gel

Fig.7.13. Intergranular fracture in a steel failed due toHE. (during cadmium plating).

227 7 failure in materials

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