Process Technology - Cracking

8/3/2019 Process Technology - Cracking

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Manea Stefan & YiWei Liu


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A fracture is the (local) separation of an object ormaterial into two, or more, pieces under the actionof stress.

The word fracture is often applied to bones ofliving creatures (that is, a bone fracture), or tocrystals or crystalline materials, such as gemstonesor metal. Sometimes, in crystalline materials,individual crystals fracture without the body

actually separating into two or more pieces.Depending on the substance which is fractured, afracture reduces strength (most substances) orinhibits transmission of light (optical crystals).


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Fracture strength

Stress vs. strain curve typical of aluminum1. Ultimate tensile strength2. Yield strength3. Proportional limit stress4. Fracture5. Offset strain (typically 0.2%)


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Fracture strength, also known as breakingstrength, is the stress at which a specimen failsvia fracture.[1] This is usually determined for a

given specimen by a tensile test, which chartsthe stress-strain curve (see image). The finalrecorded point is the fracture strength.


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Ductile materials have a fracture strengthlower than the ultimate tensile strength (UTS),whereas in brittle materials the fracturestrength is equivalent to the UTS.[1] If a ductilematerial reaches its ultimate tensile strength ina load-controlled situation,[Note 1] it willcontinue to deform, with no additional loadapplication, until it ruptures. However, if the

loading is displacement-controlled,[Note 2] thedeformation of the material may relieve theload, preventing rupture.


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If the stress-strain curve is plotted in terms oftrue stress and true strain the curve will alwaysslope upwards and never reverse, as true stress

is corrected for the decrease in cross-sectionalarea. The true stress on the material at the timeof rupture is known as the breaking strength.This is the maximum stress on the true stress-

strain curve, given by point 3 on curve B.


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Types

Brittle fracture

In brittle fracture, no apparent plastic deformation

takes place before fracture. In brittle crystallinematerials, fracture can occur by cleavage as theresult of tensile stress acting normal tocrystallographic planes with low bonding

(cleavage planes). In amorphous solids, bycontrast, the lack of a crystalline structure resultsin a conchoidal fracture, with cracks proceedingnormal to the applied tension.


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Ductile fracture

In ductile fracture, extensive plasticdeformation (necking) takes place before

fracture. The terms rupture or ductile rupturedescribe the ultimate failure of tough ductilematerials loaded in tension. Rather thancracking, the material "pulls apart," generally

leaving a rough surface. In this case there isslow propagation and an absorption of a largeamount energy before fracture.


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Many ductile metals, especially materials withhigh purity, can sustain very large deformationof 50100% or more strain before fracture

under favorable loading condition andenvironmental condition. The strain at whichthe fracture happens is controlled by the purityof the materials. At room temperature, pure

iron can undergo deformation up to 100%strain before breaking, while cast iron or high-carbon steels can barely sustain 3% of strain.


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Because ductile rupture involves a high degreeof plastic deformation, the fracture behavior ofa propagating crack as modeled above changes

fundamentally. Some of the energy from stressconcentrations at the crack tips is dissipated byplastic deformation before the crack actuallypropagates.


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The basic steps are: void formation, voidcoalescence (also known as crack formation),crack propagation, and failure, often resulting

in a cup-and-cone shaped failure surface.


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References

^ a b Degarmo, E. Paul; Black, J T.; Kohser,Ronald A. (2003), Materials and Processes in

Manufacturing (9th ed.), Wiley, p. 32, ISBN 0-471-65653-4.


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Process Technology - Cracking

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