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Creep Rupture Strength

Stress required to cause fracture in a creep test within a specified time. Alternate

term is Stress Rupture Strength.

creep-rupture strength

The stress that will cause fracture in a creep test at a given time in a specifiedconstant environment. Also called stress-rupture strength.

Creep StrengthMaximum Stress required to cause a specified amount of creep in a

specified time. Also used to describe maximum Stress that can be generated in a

material at constant temperature under which creep rate decreases with time. An

alternate term is creep limit.

The concept of fatigue is very simple, when a motion is repeated, the object that is doing the

work becomes weak. For example, when you run, your leg and other muscles of your body

become weak, not always to the point where you can't move them anymore, but there is anoticeable decrease in quality output. This same principle is seen in materials. Fatigue occurs

when a material is subject to alternating stresses, over a long period of time. Examples of

where Fatigue may occur are: springs, turbine blades, airplane wings, bridges and bones.

The Mean stress has the effect that as the mean stress is increased, fatigue life decreases. Thisoccurs because the stress applied is greater.

I mentioned previously that scratches and other imperfections on the surface will cause a

decrease in the life of a material. Therefore making an effort to reduce these imperfections by

reducing sharp corners, eliminating unnecessary drilling and stamping,shot peening, and most ofall careful fabrication and handling of the material.

Another Surface treatment is calledcase hardening, which increases surface hardness and fatigue

life. This is achieved by exposing the component to a carbon-rich atmosphere at high

temperatures. Carbon diffuses into the material filling interstisties and other vacancies in the

material, up to 1 mm in depth.

Exposing a material to high temperatures is another cause of fatigue in materials. Thermalexpansion and contraction will weaken bonds in a material as well as bonds between two

different materials. For example, in space shuttle heat shield tiles, the outer covering of silicon

tetra boride (SiB4) has a different coefficient of thermal expansion than the Carbon-CarbonComposite. Upon re-entry into the earth's atmosphere, this thermal mismatch will cause theprotective covering to weaken, and eventually fail with repeated cycles.

Another environmental affect on a material is chemical attack, or corrosion. Small pits may formon the surface of the material, similar to the effect etching has when trying to find dislocations.
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This chemical attack on a material can be seen in unprotected surface of an automobile, whether

it be by road salt in the winter time or exhaust fumes. This problem can be solved by adding

protective coatings to the material to resist chemical attack.

Factors that affect fatigue-life

Cyclic stress state: Depending on the complexity of the geometry and the loading,

one or more properties of the stress state need to be considered, such as stress

amplitude, mean stress, biaxiality, in-phase or out-of-phase shear stress, and load

sequence,

Geometry: Notches and variation in cross section throughout a part lead to stress

concentrations where fatigue cracks initiate.

Surface quality. Surface roughness cause microscopic stress concentrations that

lower the fatigue strength. Compressive residual stresses can be introduced in the

surface by e.g.shot peeningto increase fatigue life. Such techniques for producing

surface stress are often referred to aspeening, whatever the mechanism used to

produce the stress.Low Plasticity Burnishing,Laser peening, andultrasonic impact

treatmentcan also produce this surface compressive stress and can increase the

fatigue life of the component. This improvement is normally observed only for high-

cycle fatigue.

Material Type: Fatigue life, as well as the behavior during cyclic loading, varies

widely for different materials, e.g. composites and polymers differ markedly frommetals.

Residual stresses: Welding, cutting, casting, and other manufacturing processes

involving heat or deformation can produce high levels of tensileresidual stress,

which decreases the fatigue strength.

Size and distribution of internal defects: Casting defects such asgas

porosity,non-metallic inclusionsand shrinkage voids can significantly reduce fatigue

strength.

Direction of loading: For non-isotropic materials, fatigue strength depends on

the direction of the principal stress.
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Grain size: For most metals, smaller grains yield longer fatigue lives, however, the

presence of surface defects or scratches will have a greater influence than in a

coarse grained alloy.

Environment: Environmental conditions can cause erosion, corrosion, or gas-phaseembrittlement, which all affect fatigue life.Corrosion fatigueis a problem

encountered in many aggressive environments.

Temperature: Extreme high or low temperatures can decrease fatigue strength.

[edit]

Fatigue mechanism

There has always been an aura of mystery regarding why metals, and

materials in general, fail in fatigue. The impression seems to have developedthat a part may function satisfactorily for many, many loadings, but when

metal fatigue occurs, the failure is sudden and catastrophic. As early as

1839, Poncelet (Ref 10.1) in France described this phenomenon as fatigue,

presumably analogous to human fatigue that results when a motion is

repeated successively. Fatigue comes on gradually in human endurance, and

what frequently is overlooked is that fatigue of a metallic part also develops

gradually. Failure is not really sudden but is the end result of progressive

deterioration that eventually produces the failure event. What the process

consists of has been the subject of many studies over some 150 years, andwe now understand reasonably well the nature of the fatigue mechanism. As

with the fatigue process itself, our understanding did not develop suddenly

or smoothly, and not without much controversy along the way.

Slip (ceramics), an aqueous suspension of minerals, and frequentlydeflocculant.

A slip is a suspension in water of clay and/or other materials used in the production

ofceramicware.[1]Deflocculant, such assodium silicate, can be added to the slip to

disperse the raw material particles. This allows a higher solids content to be used, or

allows a fluid slip to be produced with the a minimum of water so that drying shrinkage

is minimised, which is important duringslipcasting[2]. Usually the mixing of slip is

undertaken in ablunger[3]although it can be done using other types of mixers or even

by hand.

Use
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A slip may be made for various purposes in the production and decoration of ceramics.

Slip can be used:

As a means of mixing the constituents of a clay body.

To shape ware byslip casting.

To join of sections of unfired ware, such as handles and spouts.

To adhere figures or other motiffs to unfired ware to form a bas-relief. This technique

is known as sprigging, an example isJasperware[4].

Decoratively when placed onto a wet or leather-hard clay body surface by dipping,

painting or splashing. Such type of ware is often described as slipware. Slipware

may be carved or burnished to change the surface appearance of the ware.

Specialized slip recipes may be applied tobiscuit wareand then refired. Decorative

slips may be a different color than the underlying clay body or offer other decorativequalities. Colored slips are can be used to create pieces ofceramic artby

techniques similar to paint in other media.[citation needed]

creep is the tendency of a solid material to slowly move or deform permanently under

the influence ofstresses. It occurs as a result of long term exposure to high levels of

stress that are below theyield strengthof the material. Creep is more severe in

materials that are subjected toheatfor long periods, and near melting point. Creep

always increases with temperature.In crystallography,crystal twinningrefers to

intergrown crystal forms that display a twin boundary

Crystal twinning occurs when two separate crystals share some of the samecrystallatticepoints in a symmetrical manner. The result is an intergrowth of two separate

crystals in a variety of specific configurations. A twin boundary or composition surface

separates the two crystals.Crystallographersclassify twinned crystals by a number

oftwin laws. These twin laws are specific to thecrystal system. The type of twinning can

be a diagnostic tool in mineral identification.

Advantages and Disadvantages of Powder Metallurgy

Advantages:

1. Elimination or reduction of machining

2. High Production Rates

3. Complex Shapes to be Produced

4. Wide Variations in Compositions are Possible

5. Wide Variation in Properties are Available

6. Scrap is Eliminated or Reduced

Disadvantages:
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1. Inferior Strength Properties

2. Relatively High Die Cost

3. High Material Cost

4. Design Limitations

5. Density Variations Produce Property Variations

6. Health and Safety HazardsPowder Metallurgy the name given to the process by which fine powdered materials are

blended, pressed into a desired shape (compacted), and then heated (sintered) in a controlled

atmosphere to bond the contacting surfaces of the particles and establish the desired

properties.

it is commonly designated as P/M

it readily lends itself to the mass production of small, intricate parts of high precision, often

eliminating the need for additional machining or finishing.

has a little material waste; unusual materials or mixtures can be utilized; and controlled

degrees of porosity or permeability can be produce.

Major areas of application tend to be those for which the P/M process has strong economical

advantage or where the desired properties and characteristics would be difficult to obtain by

any other method.

Other Areas where Powder Metallurgy Products are used extensively:

Household appliances

Recreational equipment

Hand tools

Hardware items

Business machines

Industrial motors Hydraulics

Areas of Rapid Growth :

Aerospace applications

Advanced composites


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Electronic components

Magnetic materials

Metalworking tools

A variety of biomedical and dental applicationsHigh-volume Materials :

Stainless steel

High-strength and high-alloy steels

Aluminum

Aluminum alloys

Iron

Copper

4 Basic Steps of Powder Metallurgy :

1. Powder Manufacture

2. Mixing or Blending

3. Compacting

4. Sintering

Optional Secondary processing often follows to obtain special properties or enhanced

precision.

Important Properties and Characteristics of the metal or material powders that are used:

Chemistry

Purity

Particle size

Size distribution

Particle shape

Surface texture of the particles

Process features of the powder particles that size and shape can varied and depend on :


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Velocity and media of the atomizing jets or the speed of electrode rotation

Starting temperature of the liquid (which affects the time that surface tension can act on the

individual droplets prior to solidification)

Environmental provided for cooling

When cooling is slow (such as in gas atomization) and surface tension is high, spherical shapes

can form before solidification.

Irregular shapes are produced due to more rapid cooling, such as water atomization.

Other methods of Powder Manufacture :

Chemical reduction of particulate compounds (generally crushed oxides or ores)

Electrolytic deposition from solutions of fused salts

Pulverization or grinding of brittle materials (comminution)

Thermal decomposition of hydrides or carbonyls

Precipitation from solution

Condensation of metal vapors

Almost any metal, metal alloy, or nonmetal like ceramic, polymer or wax or graphite lubricant

can be converted into powder form by any of the methods.

Some methods can produce only elemental powder, often of high purity. While others can

produce pre-alloyed particles.

Compactingone of the most critical steps in the P/M process.

Green compactloose powder is compressed and densified into shape, usually at room

temperature.

With the feed bottom punch in its fully raised position, a feed shoe moves up into position over

the die. The feed shoe is an inverted container filled with powder, connected to the powder

supply by a flexible tube. With the feed shoe in position, the bottom punch descends to a presetfill depth, and the shoe retracts, leveling the powder. The upper punch retracts and the bottompunch rises to eject the green compact. As the die shoe advances for the next cycle, its forward

edge clears the compact from the press, and the cycle repeats.

During compacting, the powder particles move primarily in the direction of the applied force.

The opposing force is probably a combination of:

1. Resistance by the bottom punch


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2. Friction between the particles and the die surfaces

P/M Injection Molding-small, complex-shaped components have been fabricated from plastic for many years by means

of injection molding.

-recently developed alternative to conventional powder metallurgy compaction.-while the powdered material does not flow like a fluid: complex shapes can be produced bymixing ultrafine (usually less than 10 um) metal, ceramic, or carbide powder with a

thermoplastic/wax material (up to 50% by volume).

*A water-soluble methylcellulose binder is one attractive alternative to the thermoplastics.

Sintering

The word sinter comes from the Middle High German Sinter, a cognate of English cinder. In the

sintering operation, the pressed- powder compacts are heated in a controlled atmosphere

environment to a temperature below the melting point but high enough to permit the solid-

state diffusion and held for sufficient time to permit bonding of the particles. Most sintering

operations involve three stage and many sintering furnaces employ three corresponding zones.

The first operation, the burn-off or purge, is designed to combust any air, volatize and remove

lubricants or binders that would interfere with good bonding and slowly raise the temperature

of the compacts in a controlled manner. The second or the high- temperature stage is where

the desired solid state diffusion and bonding between the powder particles take place. Finally,

a cooling period is required to lower the temperature of the products while maintaining them

in a controlled atmosphere. These three stages must be conducted in a protective atmosphere.

This is critical since the compacted shapes have residual porosity and internal voids that are

connected to exposed surfaces. Reducing atmospheres, commonly based on hydrogen,

dissociated ammonia, or cracked hydrocarbons, are preferred since they can reduce any oxide

already present on

Secondary operations are be performed to improve:

1. Density

2. Strength

3. Shape

4. Corrosion Resistance

Tolerances

Properties of P/M Products
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Mechanical properties show a strong dependence on product density, with the fracture-limited

properties of toughness, ductility and fatigue life being more sensitive than strength and

hardness.

The voids in the P/M part act as stress concentrators and assist in starting and propagating

fractures.

The yield strength of P/M products made from weaker metals is often equivalent to the samematerial in wrought form.

If higher strength materials are used or the fracture-related tensile strength is specified, the

P/M properties tend to fall below those of wrought equivalents by varying but usually

substantial amounts.

When larger presses or processes such as P/M forging or HIP are employed to produce higher

density, the strength of the P/M products approaches that of the wrought material. With full

density and fine grain size.

With full density and fine grain size. P/M parts often have properties that exceed their wrought

or cast equivalents.

Since mechanical properties of powder metallurgy products are so dependent upon density, it

is important that P/M products be designed and materials selected so that the final properties

will be achieved with the anticipated amount of final porosity.

Physical Properties can also be affected by porosity

Corrosion resistance tends to be reduced due to the presence of entrapment pockets and

fissures.

Electrical, thermal, and magnetic properties all vary with density.

Porosity actually promotes good sound and vibration damping, and many P/M parts are

designed to take advantage of this feature.

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