fracture & strength of materials

8/4/2019 Fracture & Strength of Materials

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MECHANICAL PROPERTIES -FRACTURE


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What must you be able to do?

describe the different types of fracture thatdifferent materials undergo.

explain how deformations occur in metals.


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Cleaving a crystal


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BRITTLE FRACTURE

No plastic deformation occurs before fracture.The material does not stretch at all.

Fracture occurs along CLEAVAGE PLANES.

Fracture will occur when

a stress is applied at 90o

to the cleavage plane.


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DUCTILE FRACTURE

A large amount of plastic deformation

occurs before fracture.

Metals, which are

malleable and ductile,undergo ductile fracture when they reachtheir limit of deformation.


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CREEP A TYPE OF DEFORMATIONTHAT LEADS TO FAILURE

The distorted images viewed through thepane are due to variations in thickness in the

pane.


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Creep is the tendency of a material todeform or move permanently to relieve

stress.

Glass panes are thicker at the bottom than atthe top due to creep.

The glass has flowed downwards under itsown weight in order to relieve the stress of

supporting itself.A chocolate bar will deform under its ownweight on a hot day.


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Creep deformation can be found in any

material and is dependent on temperatureand time.

Materials with low melting points creep at

room temperature.

Creep can eventually result in failure ofthe material.


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The filament of a light bulb is made up ofvery thin tungsten wire.

Sagging of the coil increases with time due tocreep deformation caused by the weight of

the filament.Eventually adjacent coils touch each other,resulting in overheating and the filament

breaks.


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Failure in metals dislocations in crystals

Metals that are solids have a crystalstructure. This means that the atoms arearranged in a particular pattern that isrepeated throughout the piece of metal.

When the metal cools and solidifies, theatoms form tiny crystals, called grains or

crystallites.The grains are bonded together atboundaries called grain boundaries.


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Some metals have a

crystal structure whileothers do not and theirstructure is referred to as

amorphous.

Each dot is an atom.Atoms are arranged

in a regular pattern

grain

grain boundary


12/19Metal crystal grain

vacancies

Vacancies occur in crystals where atoms aremissing. These imperfections result in the

atoms shifting when a much smaller forcethan usual is applied.


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A SLIP PLANE forms resulting from theimperfections.

The slip plane is an imaginary line ofweakness that divides the crystal, or part ofthe crystal, into two regions.

Imperfections can occur in any directionthroughout the crystal and so slip planes

occur in all directions throughout the crystal.

The addition of impurities to a metal can

counteract the effects of the imperfections.


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Adding carbon to iron results in a muchstronger and harder crystal structure.

Tin added to copper forms bronze. Thisdiscovery lead to the development of humancivilization during the Bronze Age.

Iron and carbon atoms in

a cast iron crystal.


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The addition of impurities these impurities

counteract the effects of the imperfections.

vacancy

Added impurity


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TYPES of DISLOCATION:

Screw dislocation

Edge dislocation:

A layer of atoms endsabruptly.

Screw dislocation:

An extra layer of atoms isadded to an existing layeras a flap.


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The following points regarding dislocationsare important:

A. the metals atoms are held together bymetallic bonds. The bonds surrounding thedislocations are relatively weak, so these arethe first to break in plastic deformation.


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B. When a crystal is under stress, thedislocations can be forced to move through

the crystal, being passed on from layer tolayer.

dislocation Horizontal arrows showapplied stress

Dislocation moves in the crystalby breaking/making a line ofmetal metal bonds


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C. Grain boundaries form a barrier for themovement of the dislocation when stress is

applied.

fracture & strength of materials

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