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Author Topic: note for section A  (Read 588 times)

July 21, 2010, 05:01:08 PM
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work hardening

Almost all ductile materials become stronger when they are deformed plastically. As a result its strength increases. This increase in strength is called strain hardening. it provides opportunities for the engineer to modify and control the properties of ductile materials, particularly metals.

Work hardening is also known as strain hardening. It is the strengthening of a metal by plastic deformation. This strengthening occurs because of dislocation movements within the crystal structure of the material.

Any material with a reasonably high melting point such as metals and alloys can be strengthened in this fashion. Alloys not amenable to heat treatment, including low-carbon still, are often work hardened. Some materials cannot be work hardened at normal ambient temperatures. Their names are given below:
•   Indium
•   Pure Copper
•   Aluminum

Examples of desirable and undesirable work hardening:

Generally speaking work hardening may be desirable or undesirable depending on the context.

•   An example of undesirable work hardening is during machining when early passes of a cutter inadvertently work harden the work piece surface, causing damage to the cutter during the later passes.

•   An example of desirable work hardening is that which occurs in metalworking processes that intentionally induce plastic deformation to exact a shape change. these processes are known as cold working or cold forming processes.

Disadvantage Of Work Hardening:

Although work hardening or strain hardening increases both the yield strength and the ultimate strength, it reduces the ductility. Part of the deformation occurred during cold working of the material, before the test bar was made and the gage marks established. Thus, less ductility is observed during testing. Cold working increases the yield strength more than it does the ultimate strength.

. Ductile Fracture

Ductile fracture is a mode of fracture that is attended by extensive gross plastic deformation.
The failure of many ductile materials can be attributed to cup and cone fracture. This form of ductile fracture occurs in stages that initiate after necking begins. the stages of ductile fracture are given below:

•   Small micro voids form in the interior of the material.
•   Deformation continues and the micro voids enlarge to form a crack.
•   The crack continues to grow and it spreads laterally towards the edges of the specimen.
•   Finally, crack propagation is rapid along a surface that makes about 45 degree angle with the tensile stress axis. The new fracture surface has a very irregular appearance. the final shearing of the specimen produces a cup type shape on one fracture surface and a cone shape on the adjacent connecting fracture surface, hence the name cup and cone fracture.


Basic Factors Of Brittle Fracture:

Three basic factors contribute to a brittle cleavage type of fracture. They are –

•   a tri-axial state of stress
•   a low temperature
•   a high strain rate or rapid rate of loading

The above mentioned factors do not have to be present at the same time to produce brittle fracture. a tri-axial state of stress, such as exists at a notch, and low temperature are responsible for most service failures of the brittle type. However, since these effects are accentuated at a high rat of loading, many types of impact tests have been used to determine the susceptibility of materials to brittle behavior. Steels which have identical properties when tested in tension or torsion at slow strain rates can show pronounced differences in their tendency of brittle fracture when tested in a notched impact test.
•   ductile fracture results from the application of an excessive force to a metal that has the ability of the material to flow or deform, which may or may not lead to fracture, depending upon the magnitude of the force applied.
•   The property of ductility is somewhat related to the property of toughness, although the latter is usually measured in the presence of a notch or other stress concentration. The chirpy v notch impact test is commonly used as a measure of toughness. However, the ability to absorb energy and deform plastically prior to fracture is characteristic of both ductility and toughness.
•   the property of ductility permits metal parts to be formed without fracture and to be adjusted in shape. a simple example is familiar to all persons who wear metal eyeglass.

Characteristics of Ductile Fracture:

1.   There is considerable gross permanent or plastic deformation in the region of ductile fracture. In many cases, this may be present only in the final rupture region of a fracture that may have originated with a fatigue or brittle   fracture.
2.   The surface of a ductile fracture is not necessarily related to the direction of the principal tensile stress, as it is in a brittle fracture.
3.   The characteristic appearance of the surface of a ductile fracture is dull and fibrous. this is caused by deformation on the fracture surface, which will be discussed in the section on the micro structural aspects of ductile fracture.

« Last Edit: January 01, 1970, 04:00:00 AM by Guest »


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