1.0Introduction So far we have studied mechanical properties of
metals at room temperature and we assumed rightly so that they are
independent of time. If we apply constant elastic stress on a metal
specimen at room temperature, the elastic deformation is calculated
as;e= /Where E is the elastic modulus, is the applied stress, and e
is the elastic stress. Since the elastic modulus is constant, the
elastic strain is a function only of the stress. If we repeat the
same test for a metal at a high temperature the metal will
immediately deform elastically and then continue to deform at a
constant slow rate for a period of time before it increases rapidly
until fracture. The time dependent deformation under constant load
at high temperatures is called creep and the resulting strain is a
function of the applied stress, temperature, and time. The
temperature at which a material starts to creep depends on its
melting point. It is found that creep in metals starts when the
temperature is > 0.3 to 0.4 Tm (the melting temperature in
Kelvin). Most metals have high melting points and hence they start
to creep only at temperatures much above room temperature. This is
the reason why creep is less familiar phenomena than elastic or
plastic deformation.1.1Ceramic MaterialCeramic
materialsareinorganic,non-metallicmaterials made from compounds of
a metal and a non-metal. Ceramic materials may becrystallineor
partly crystalline. They are formed by the action of heat and
subsequent cooling.Claywas one of the earliest materials used to
produceceramics, aspottery, but many different ceramic materials
are now used in domestic, industrial and building products. Ceramic
materials tend to be strong, stiff, brittle, chemically inert, and
non-conductors of heat and electricity, but their properties vary
widely.1.2Type of Ceramic MaterialAceramicmaterial may be defined
as any inorganic crystalline material, compounded of a metal and a
non-metal. It is solid and inert. Ceramic materials are brittle,
hard, and strong in compression, weak in shearing and tension. They
withstand chemical erosion that occurs in an acidic or caustic
environment. In many cases withstanding erosion from the acid and
bases applied to it. Ceramics generally can withstand very high
temperatures such as temperatures that range from 1,000 C to 1,600
C. Exceptions include inorganic materials that do not have oxygen
such assilicon carbide. Glass by definition is not a ceramic
because it is an amorphous solid or non-crystalline. However, glass
involves several steps of the ceramic process and its mechanical
properties behave similarly to ceramic materials.1.2.1Crystalline
CeramicCrystalline ceramic materials are not amenable to a great
range of processing. Methods for dealing with them tend to fall
into one of two categories either to makes the ceramic in the
desired shape, by reaction in situ or by "forming" powders into the
desired shape, and thensinteringto form a solid body.Ceramic
forming techniquesinclude shaping by hand known as throwing,slip
casting,tape casting, injection molding, dry pressing, and other
variations.1.2.2Non-Crystalline CeramicNon-crystalline ceramics are
being glasses and tend to be formed from melts. The glass is shaped
when either fully molten, by casting, or when in a state of
toffee-like viscosity, by methods such as blowing to a mold. If
later heat-treatments cause this glass to become partly
crystalline, the resulting material is known as
aglass-ceramic.1.3Properties of CeramicThe physical properties of
any ceramic substance are a direct result of its crystalline
structure and chemical composition.Solid state chemistryreveals the
fundamental connection between microstructure and properties such
as localized density variations, grain size distribution, type of
porosity and second-phase content, which can all be correlated with
ceramic properties such as mechanical stress strength by the
Hall-Petch equation,hardness,toughness,dielectric constant, and
theopticalproperties exhibited by transparent materials.Physical
properties of chemical compounds which provide evidence of chemical
composition include odor, color, volume, density, melting point,
boiling point, heat capacity, physical form at room temperature,
hardness, porosity, and index of refraction.
1.3.1Mechanical PropertiesCeramic materials are
usuallyionicorcovalentbonded materials, and can be crystalline or
amorphous. A material held together by either type of bond will
tend tofracturebefore anyplastic deformationtakes place, which
results in poortoughnessin these materials. Additionally, because
these materials tend to be porous, theporesand other microscopic
imperfections act asstress concentrators, decreasing the toughness
further, and reducing thetensile strength. These combine to
givecatastrophic failures, as opposed to the normally much more
gentlefailure modesof metals.These materials do showplastic
deformation. However, due to the rigid structure of the crystalline
materials, there are very few available slip
systemsfordislocationsto move, and so they deform very slowly. With
the non-crystalline (glassy) materials,viscousflow is the dominant
source of plastic deformation, and is also very slow. It is
therefore neglected in many applications of ceramic
materials.1.3.2Electrical PropertiesSome ceramics
aresemiconductors. Most of these aretransition metal oxidesthat are
II-VI semiconductors, such aszinc oxide. While there are prospects
of mass-producing blueLEDsfrom zinc oxide, ceramicists are most
interested in the electrical properties that showgrain
boundaryeffects. The best demonstration of their ability can be
found inelectrical substations, where they are employed to protect
the infrastructure fromlightningstrikes. They have rapid response,
are low maintenance, and do not appreciably degrade from use,
making them virtually ideal devices for this application.Under some
conditions, such as extremely low temperature, some ceramics
exhibithigh temperature superconductivity. The exact reason for
this is not known, but there are two major families of
superconducting ceramics.1.3.3Optical PropertiesOptically
transparent materialsfocus on the response of a material to
incoming light waves of a range of wavelengths.Frequency selective
optical filterscan be utilized to alter or enhance the brightness
and contrast of a digital image. Guided light wave transmission via
frequency selective waveguidesinvolves the emerging field of
fiberopticsand the ability of certain glassy compositions as
atransmission mediumfor a range of frequencies simultaneously with
little or nointerferencebetween competingwavelengthsor frequencies.
Thisresonant modeofenergyanddata transmissionvia
electromagneticwave propagation, though low powered, is virtually
lossless. Optical waveguides are used as components inintegrated
optical circuitsor as the transmission medium in local and long
hauloptical communicationsystems. Also of value to the emerging
materials scientist is the sensitivity of materials to radiation in
the thermalinfrared(IR) portion of the electromagnetic spectrum.
This heat-seeking ability is responsible for such diverse optical
phenomena asNight-visionand IRluminescence.2.0Introduction to
CreepCreep is the permanent elongation of a component under a
static load maintained for a period of time. This phenomenon occurs
in metals and certain nonmetallic materials, such as
thermoplastics, rubbers and ceramic, and it can occur at any
temperature. Advanced engineering ceramics have a number of
material properties that have made them one of the most important
classes of engineering materials. Ceramics have an extremely high
elastic modulus, maintain consistent performance at elevated
temperatures, and have great resistance to wear and corrosion,
which has contributed to their widespread use as bearing surfaces,
heat resistance, and insulation applications. The ability for
ceramics to perform at high temperature has made them the go to
material for high end automobile brake rotors and pads, space
re-entry vehicle heat shields, and ball bearings in high speed and
high temperature applications. The use of fiber reinforcement with
a ceramic matrix provides an increase in tensile strength and
fracture resistance, making ceramics a viable material for
structural applications. The system shown is designed to produce
controlled temperatures to 3100F and the ability to perform creep
and modulus of rupture tests on ceramic materials.2.1Creep
TestingCreep testing aims to investigate plastic deformation of a
material when subjected to a constant load or stress at a high
temperature. High temperature allows metal to deform more easily
since atoms can move more readily. Generally, metals creep at a
temperature above approximately 0.4 Tm (Tm is the absolute
temperature of the metal). Therefore, low melting point metals will
creep at lower temperature in comparison to high melting point
metals. Hence, greater movement of dislocations or slips can
happen. New slip systems and grain-boundary movement are also
possible at higher temperatures. Therefore, engineering alloys
utilized at high temperatures is susceptible to creep as well as
recrystallization and grain coarsening. In the case of age-hardened
metals, over-ageing is feasible, which results in reduced hardness
and strength due to the coarsening of the second phase
precipitates. 2.2Creep-Testing MachineAcreep-testing
machinemeasures the tendency of a material after being subjected to
high levels of stress such as high temperatures, to change its form
in relation to time or known as creep of an object. It is a device
that measures the alteration of a material after it has been put
through different forms of stress.
Figure 1.0: Creep testing configuration showing specimen fitted
in the testing machine coupled with a high temperature
furnace.Creep machines are important to see how much strain or load
an object can handle under pressure, so engineers and researchers
are able to determine what materials to use. The device generates a
creep time-dependent curve by calculating the steady rate of creep
in reference to the time it takes for the material to change. Creep
machines are primarily used by engineers to determine the stability
of a material and its behavior when it is put through ordinary
stresses.2.3Standard of Creep Testing usedThe ASTM and ISO have
developed standard test methods to aid in the proper testing of the
wide variety ceramic materials. These tests address the various
applications of ceramic materials and environments in which they
will be used. Popular standards for testing ceramic materials at
high temperatures are: ASTM C1291 for tensile creep of monolithic
ceramics ASTM C1337 for tensile creep of continuous fiber
reinforced ceramics ASTM C1359 for rectangular shaped continuous
fiber reinforced ceramics ASTM C1366 for monolithic ceramics ISO
22215 for tensile creep of monolithic and particulate reinforced
ceramics.2.4Creep Testing Material Creep specimens made from
ceramic
Micrometer or vernia caliper
Permanent pen
Creep Testing Machine
Hot and cold bags
Thermometer
2.5Creep Testing Procedure1. Remove any load from the arm of
creep machine.
2. Measure and record the specimen dimensions for the
calculation of stress and strain from the creep test.
3. Fit a specimen on a creep test machine as shown in Figure 2.1
with a dial gauge positioned in a mid-range of the specimen gauge
length for the calculation of specimen extension.
4. Hung the weights of known values at the end of the sample to
determine the applied stress. Specimen extension will be read on
the dial gauge and time is recorded using stopped watch.
5. Repeat the tests at the same load used above but at different
temperature
2.6Stage of CreepCreep is dependent on time so the curve that
the machine generates is a time vs. strain graph. The slope of a
creep curve is the creep rate d/dt. The trend of the curve is an
upward slope. The graphs are important to learn the trends of the
alloys or materials used and by the production of the creep-time
graph; it is easier to determine the better material for a specific
application.
Figure 2.0: Schematic illustration of a typical creep curve.
From the graph in Figure 2.0, we are able to determine the
temperature and interval in which an object will be disturbed once
exposed to the load. Some materials have a very small secondary
creep state and may go straight from the primary creep to the
tertiary creep state. This is dependent on the properties of the
material that is being test on. This is important to note because
going straight to the tertiary state causes the material to break
faster from its form.Nevertheless, each metal creeps at different
rate and thus require different time to finish the test, ranging
from minutes, hours, days, weeks or months. According to the
typical creep curve in figure 2.0, it should be noticed that the
creep curve can be divided into three main stages; primary,
secondary and tertiary creeps. Each stage of creep behavior is
influenced from both work hardening and annealing mechanisms
occurring at the same time. However, work hardening and annealing
will take place at different rates depending on response of metals
to applied tensile force with time. The creep rate therefore
changes accordingly. There are three stages of creep:1.Primary
Creep: The primary creep or transient creep exhibits a decreasing
creep rate with time as shown in figure 2.0. A very sharp increase
in the initial stage is observed with the original strain, o,
taking place before the creep rate starts to decrease. The creep
rate then diminishes until reaching the secondary creep region as
detailed in figure 2.0. This diminished creep rate in the primary
creep region accounts from work hardening mechanism of the metal.
Multiplication and interaction of dislocations rule out the
annealing effect at this stage, resulting in increasing the creep
resistance of the metal. 2.Secondary Creep/Steady State Creep:
Beyond the primary stage, the creep rate is reaching a steady state
where the creep rate is said to be relatively constant with time
and gives the minimum creep rate of all the three regions. This
minimum creep rate is used to represent the creep rate of the metal
being tested at particular test temperature and load. The constant
creep rate is due to balancing of strain hardening and annealing
(recovery) processes according to the applied stress and
temperature. The amount of dislocations being generated by work
hardening is equal to the number of dislocations being annealed
out.
3.Tertiary Creep: The tertiary creep region gives a rapid creep
rate approaching failure. This is due to the formation of necking.
Load bearing capability decreases due to the simultaneous reduction
in the cross-sectional area of the specimen, which is related to
local stress acting on this area. Furthermore, tertiary creep is
associated with microstructural alterations due to increasing
temperature such as coarsening of precipitate phases,
recrystallization and diffusion of phases. These mechanisms
effectively increase the tertiary creep rate, and eventually lead
to fracture under creep.
Figure 3.0: Effects of stress levels on the shape of creep
curves at constant temperature.However, factors influencing the
shape of the creep curve depend on the levels of the stress and
temperatures involved. If the temperature is remained constant, the
creep curves will shift upward and to the left with increasing
applied stresses as shown in figure 3.0. Similarly, if the creep
test is carried out at various temperatures but at a constant
stress level, the creep rate will increase with increasing
temperatures. A linear graph denotes that the material under stress
is gradually deforming and this would be harder to track at what
level of stress an object can handle. This would also mean that the
material would not have distinct stages, which would make object's
breaking point would be less predictable.3.0ConclusionAs
conclusion, the creep test has the objective of precisely measuring
the rate at which secondary or steady state creep occurs.
Increasing the stress or temperature has the effect of increasing
the slope of the line if the amount of deformation in a given time
increases. The results are presented as the amount of strain
(deformation), generally expressed as a percentage, produced by
applying a specified load for a specified time and temperature.
From the creep testing, the designer can calculate how the
component will change in shape during service and hence to specify
its design creep life. 4.0 References 4.1Book1. Kingery, W. D.
(1960). Introduction to ceramics.2. Callister, W. D., &
Rethwisch, D. G. (2007).Materials science and engineering: an
introduction(Vol. 7, pp. 665-715). New York: Wiley.3. Cannon, W.
R., & Langdon, T. G. (1983). Creep of ceramics.Journal of
Materials Science,18(1), 1-50.4. Carroll, D. F., & Wiederhorn,
S. M. (1989). Creep testing of ceramics. InA Collection of Papers
Presented at the 13th Annual Conference on Composites and Advanced
Ceramic Materials, Part 2 of 2: Ceramic Engineering and Science
Proceedings, Volume 10, Issue 9/10(pp. 1244-1244). John Wiley &
Sons, Inc..4.2Website1.
http://en.wikipedia.org/wiki/Creep_(deformation)2.
http://en.wikipedia.org/wiki/Ceramic_materials3.
http://ceramics.org/learn-about-ceramics/structure-and-properties-of-ceramics4.
http://www.testresources.net/application/modulus-of-rupture-and-creep-test-equipment-for-ceramics-at-1700c-3100f10