In this article we will discuss about:- 1. Meaning of Fracture in Metals 2. Types of Fractured end in Ductile Failure 3. Creep Curve 4. Improving the Fatigue Life 5. Methods of Protection against Fracture 6. Ductile-Brittle Transition.
Meaning of Fracture in Metals:
Separation of a solid into two or more parts under application of load or stress is called fracture. Depending on the type of load, fracture may be defined by tensile fracture, compressive fracture, shear fracture, fatigue fracture, creep fracture and cleavage fracture etc.
However, these fractures are mainly characterized by either:
1. Ductile fracture, or
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2. Brittle fracture
The process of fracture basically involves the following phenomenon:
(a) Crack initiation, and
(b) Crack propagation
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Ductile fracture occurs after prolonged plastic deformation. The crack initiates from formation of the voids, and propagates slowly. Ductile materials fail showing the character of ductile fracture in normal conditions. However, they may fail as brittle fracture at much lower temperatures and at higher rates of straining. Propagation of crack given by Griffith theory.
i. Cleavage Fracture:
Cleavage is a crystallographic mode of fracture catalysed by shear stresses. In a single crystal showing brittle behaviour, fracture occurs along definite crystallographic planes called cleavage planes. These planes generally have low Miller indices and large interplaner spacing.
The critical stress required to produce cleavage is a function of crystal orientation relative to the stress direction. Cleavage is easily found in BCC and HCP metals at low temperatures. Such metals possess an enhanced ductile behaviour at elevated temperatures. Zinc crystals cleave at room temperature.
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Types of Cleavage Fracture:
i. Integranular cleavage fracture, and
ii. Transgranular cleavage fracture
In interqranular fracture, crack propagates through the grain boundaries while in transgranular case, it propagates along the cleavage planes. Transgranular cleavage is more often observed in those polycrystalline materials that fail in a brittle manner.
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ii. Creep Fracture:
Consider a bar, below figure (a-e), subjected to a steady axial load P at certain temperature T. If this arrangement is kept undisturbed, we shall find the progressive stages of deformation as shown.
Types of Fractured end in Ductile Failure:
Various configuration of fractured ends are found when materials fail in ductile fracture.
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These fractured ends are of following types and are shown in below figure:
i. Cup and cone fracture, crack spread in direction 45° to the tensile axis
ii. Fibrous fracture
iii. Star fracture, and
iv. Granular fracture
The cup and cone fracture is common in plain carbon steels. Percentage of carbon in steel has an impact on the profile of cup and cone.
Mild steel develops deeper cup and cone. With increasing percentage of carbon it becomes shallower. It disappear finally in high carbon steel. Wrought iron shows fibrous fracture ends. Brittle cast iron fails with a plane surface having granular appearance.
Note:
Surface cracks in nature are twice as effective as the internal cracks. Hence surfaces of machine components should be finely finished.
A material fractures at a certain value of applied stress. One of its broken pieces will require more applied stress than the previously applied stress to fracture. It is because the most effective cracks are eliminated in the first fracture.
During tearing of a piece of paper, we fold it and press. Folding and pressing results in creating a scratch that induces higher stress concentration. The paper is then teared apart very easily along the scratch.
A window glass panel is scratched by a mechanical nail to introduce stress concentration. Then the glass breaks very easily in two pieces along the scratch. Fracture resistance of a material increases by making blunt cracks, fillets or notches of some radius.
Griffith Theory:
Griffith has furnished a criterion for the propagation of preexisting crack in brittle material (glass) according to which brittle fracture occurred when the energy release rate during crack growth exceeded the rate than the rate at which energy was required. All metals are not ideally brittle and normally fails with certain amount of plastic deformation.
If the material in which crack is propagating can deform plastically, the crack tip form changes because of plastic strain. A sharp crack tip will be blunted. Also plastic deformation requires time so the amount of plastic deformation that can occur at the crack tip will depend on rate of advancement of the crack. Irwin and Orowan suggested that Griffith criterion can be applied to brittle material undergone plastic deformation before fracture.
Limitations of Griffith Theory:
Some of the limitations of Griffith theory:
i. It cannot explain well the fracture mechanism if the crack in the material is spheroidal.
ii. It is justified for completely brittle materials only.
iii. It does not considered the plastic deformation in the vicinity of the crack.
Creep Curve:
Different Stages of Creep Curves:
Transient or cold creep extends from A to B. It is nonlinear. The rate of creep is initially fast but slows down later-on. This is occurs even at very low temperatures, hence it is called cold creep. This is also known as I stage or primary creep.
Viscous or hot creep is from B to C, and is almost linear. This part of curve occurs at high temperatures, so is also known as hot creep. This is also known as II stage or secondary creep.
Tertiary creep is the last stage before creep fracture. The curve CD rises upward as rate of straining is too fast due to neck formation in the material. This is also known as III stage.
Improving the Fatigue Life:
i. Stress raisers such as sharp corners are avoided so that the stress concentration vanish or diminish considerably.
ii. Materials with finer grain sizes are utilized.
iii. Surface irregularities and cracks are removed by polishing the surfaces.
iv. Compressive stresses are introduced at the surfaces by processes such as shot peening, shot or sand blasting etc.
v. Nitriding and carburizing operations are performed to create strong surface layers.
Methods of Protection against Fracture:
i. By surface treatment e.g., etching and sizing etc. Etching of glass in hydrofluoric acid removes the surface cracks and thus improves the strength of glass.
ii. Introduction of compressive stress on the surface of a material makes the surface cracks ineffective. Plexiglas windscreen of auto-vehicles and tempered glasses require a higher tensile stress to initiate crack propagation as the tensile stress has to overcome the introduced compressive stress.
iii. Tempering process of heat treatment produce compressive stress in the interior part of the silicate glass, and thus the fracture strength, increased by many times.
iv. Chemical strengthening involves ion exchange method through which sodium cations are replaced by potassium anions on the surface of sodium silicate glass.
v. Fine grain control is developed to obtain finer grain sizes in glass and ceramics. The surface cracks are minimized due to fine grains and hence the fracture strength improves.
Ductile-Brittle Transition:
Ductile-brittle transition is a limiting state between ductile and brittle behaviour of a material. This transition between ductile and brittle behaviour is defined by a temperature, called ductile-brittle transition temperature tdb. A ductile material shows brittle nature below this temperature whereas a brittle material possess ductile nature above this temperature.
Conditions Responsible for Brittle Fracture of Ductile Metals:
i. A low or decreasing temperature,
ii. High rate of straining,
iii. Large grain size of material,
iv. High stress concentration,
v. Rough surface conditions, and
vi. Tri-axial stress conditions.
Factors Affecting the Transition Temperature:
i. Fine grained materials possess lower transition temperature than the coarse grained materials.
ii. Transition temperature is raised due to stress concentration such as on sharp notches.
iii. Effect of higher straining rate is to cause increased transition temperature.
iv. Most of the ductile BCC metals behave as brittle materials at low temperatures and at a very high rate of straining, whereas many FCC metals behave as ductile materials at very low temperatures.
v. It is because a higher yield stress σy is required to move dislocations in BCC metals than FCC metals.
vi. This σy increases rapidly when temperature lowers down but this is not the case with the stress required to propagate a crack σf.
vii. Steel structures such as oil rigs, ships and bridges are generally fail in winter than in summer due to ductile-brittle transition effect.
viii. Ductile to brittle transition temperature for metals is around 0.1-0.2 Tm while for ceramics is 0.5-0.7 Tm.
Tm = Melting temperature