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INTRODUCTION1.1 High Temperature Materials:-
A metal oralloyor composite material which serves above about
1000F (540C).More specifically, the materials which operate at such
temperatures consist principally of some stainless steels,
superalloys, refractory metals, and certaincomposite materials. The
giant class of alloys called steels usually sees service below
1000F. The most demanding applications for high-temperature
materials are found in aircraft jet engines, industrial gas
turbines, and nuclear reactors. However, many furnaces, ductings,
and electronic and lighting devices operate at such high
temperatures.
In order to perform successfully and economically at high
temperatures, a material must have at least two essential
characteristics: it must be strong, since increasing temperature
tends to reduce strength, and it must have resistance to its
environment, sinceoxidationandcorrosionattack also increase with
temperature. High-temperature materials, always vital, have
acquired an even greater importance because of developingcrisesin
providing society with sufficient energy. The machinery which
produces electricity or some other form of power from a heat source
operates according to the basic Carnot cycle law, where the
efficiency of the device depends on the difference between its
highest operating temperature and its lowest temperature. Thus, the
greater this difference, the more efficient is the device a result
giving great impetus to create materials that operate at very high
temperatures.1.2 What is High Temperature?A definition of high
temperature can be confusing. One often used definition in
materials science and technology is that it is a temperature equal
to, or greater than, about two-thirds of the melting point of a
solid. Another definition, attributed to Leo Brewer, is that high
temperatures are those at which extrapolations of a materials
properties, kinetics, and chemical behavior from near ambient
temperatures are no longer valid. For example, chemical reactions
not favorable at room temperature may become important at high
temperatures thermodynamic properties rather than kinetics tend to
determine the high temperature reactivity of a material.
Vaporization processes and species become increasingly important at
high temperatures. Unusual compounds and vapor species, which do
not conform to the familiar oxidation states of the elements, may
form. For example, in the vaporization of Al2O3, common high
temperature gas species can include Al2O, AlO, and AlO2.The
complexity of the vapor phase also increases with temperature; BeO
vapor species include not only the elemental vapors, but at high
temperatures, also significant (BeO)n species, with n = 1 to 6.
While the vaporization of BeO in air to the elements is suppressed
by the oxygen, the (BeO)n vapor pressures are independent of air,
and can produce much larger active corrosion rates than those
calculated using only the elemental gas species. With increasing
temperatures, ordered defect structures become disordered, and
solid solution ranges increase significantly. For example,
stoichiometric solids such as MgAl2O4 may develop significant
composition ranges at high temperatures. Physical properties of
materials that correlate with the above high temperature chemical
behavior is also unpredictable from extrapolations of low
temperature properties. Examples of High Temperature Activities and
Materials High temperature materials provide the basis for a wide
variety of technology areas, including energy, electronic, photonic
and chemical applications. While some applications involve the use
of these materials at high temperatures, others require materials
processed at high temperatures for room temperature uses. In
electrochemistry, the interaction of these materials with each
other, the atmosphere, and the movement of electrons are of high
importance. The high value of a cross-cutting technology such as
high temperature materials to a wide variety of technical arenas is
reflected by the number of science and engineering disciplines
involved in the study of processing and properties of these
materials, including ceramic science, chemistry, chemical
engineering, electrical engineering, mechanical engineering,
metallurgy, and physics. The diversity of interests ranges from
experimental observations to predicting behavior, from scientific
principles to engineering design, from atomic scale models to
performance while in use. Materials are selected on the basis of
service requirements, notably strength, so corrosion resistance
(stability) may not be the primary design consideration. Assemblies
need to be strong and resilient to the unique loads and stresses
imparted on them, which can include signicant temperature changes
and thermal gradients for many high-temperature applications. In
making a choice, it is necessary to know what materials are
available and to what extent they are suited to the specic
application. The decision is quite involved and the choice is
signicantly affected by the environment and the intended use, be it
a reactor vessel, tubes, supports, shields, springs, or others.
Some problems may occur because of distortion and cracking caused
by thermal expansion/contraction; typically, a high-temperature
alloy might change 4 in. /ft from ambient to 1,000C (1,832F). The
user or designer needs to properly understand that the environment
indicates the materials selection process at all stages of the
process or application. For example, an alloy that performs well at
the service temperature may corrode because of aqueous (dew point)
corrosion at lower temperatures during off-load periods, or through
some lack of design detail or poor maintenance procedures that
introduce local air draughts that cool the system (e.g., at access
doors, inspection ports, etc.).To provide as optimum performance as
possible, it is necessary for a supplier to be aware of the
application, and for the user to be aware of the general range of
available materials. Otherwise, severe problems can result. For
example, a catastrophic failure occurred within weeks for an
ignitor, made with Type 304 stainless steel (UNS S30400, iron, 19%
Cr, 9% Ni, 0.08% C).Type 304 stainless steel would have been
suitable for clean oxidizing conditions to about 900C (1,650F) in
continuous service, or 845C (1,550F) in intermittent (temperature
cycling) service. The failure occurred because of overheating with
contributions from suldation (hot corrosion). The true cause of
failure was a material mix-up, because Type 304 was not specied but
was inadvertently used Mechanical limits of materials in
considering traditional alloys, it is important for the designer
and user to be fully aware of the mechanical limits of a material.
For example, the ASME Pressure Vessel Codes advise that the maximum
allowable stress shall not exceed whichever is the lowest of (i)
100% of the average stress to produce a creep rate of 0.01% in
1,000 h (ii) 67% of the average stress to cause rupture after
100,000 h and (iii) 80% of the minimum stress to cause rupture
after 100,000 h. These recommendations may be better appreciated by
extracting typical data for Type 304 intended for use in a pressure
vessel up to 815C (1,500F). Based upon ASME tables,for a load of 17
MPa (2.5 ksi) at 760C (1,400F), the expected design life would be
24 yr at 788C (1,450F), the life falls to 7 yr and at 815C
(1,500F), it is only 2.2 yr.Thus a short-term temperature excursion
can have a signicant effect on equipment life. Also to be noted is
that a small increase in loading, for example, from 2.5 to 3 ksi at
760C (1,400F), can markedly reduce the life expectancy, here, from
24 to 9 yr. Overheating is the most common cause of
high-temperature corrosion failure, but the temperature inuence on
mechanical properties is of equal or even more signicance in that
many failures occur because of creep deformation (creep voids) and
thermal fatigue. Overheating can arise for various reasons,
including an unexpected accumulation of tenacious deposits that can
foul tubes in a heat exchanger.1.3. Engineering Application of High
Temperature Materials:
Applications of High temperature materials are categorized
below:
Aircraft gas turbines: disks, combustion chambers, bolts,
casings, shafts, exhaust systems, cases, blades, vanes, burner
cans, afterburners, thrust reversers. Steam turbine power plants:
bolts, blades, stack gas re-heaters. Reciprocating engines:
turbochargers exhaust valves, hot plugs, valve seat Inserts. Metal
processing: hot-work tools and dies, casting dies. Medical
applications: dentistry uses, prosthetic devices. Space vehicles:
aerodynamically heated skins, rocket engine parts. Heat-treating
equipment: trays, fixtures, conveyor belts, baskets, fans, furnace
mufflers. Nuclear power systems: control rod drive mechanisms,
valve stems, springs, ducting. Chemical and petrochemical
industries: bolts, fans, valves, reaction vessels, piping, pumps.
Pollution control equipment: scrubbers. Metals processing mills:
ovens, afterburners, exhaust fans. Coal gasification and
liquefaction systems: heat exchangers, re-heaters, piping.
Fig. 1.3.1 Effective use of advanced High Temperature materials
to cut CO2 emissions from jet engines and land based gas
turbines.1.4. CARBON-CARBON COMPOSITESCarbon is a unique element
that can exhibit different properties in different forms. Some
forms of carbon are extremely hard, like diamond, while some forms
are extremely soft and ductile. Thus, in addition to its well
defined allotropic forms (diamond and graphite), carbon can take
any number of quasi-crystalline forms ranging from amorphous or
glassy carbon to highly crystalline graphite. The latest form of
carbon (C60), discovered recently, is called Fullerene. Fullerene
is the roundest of all round molecules, more like a soccer ball,
and has properties, like high strength, low density, ferromagnetic
properties, Superconductivity and is an excellent semiconductor.
For a long time, carbon has been known for its high temperature
properties and is widely used in heating elements. But its
application in structures was limited because of its failure even
at low strains, thermal shock sensitivity, anisotropy and
processing difficulties for large and complex shapes. The advent of
carbon-carbon (CC) composites changed the scene drastically.1.4.1
Unique Features Of CC Composite:Carbon-Carbon Composites are a new
class of engineering materials that are the best among all high
temperature materials because they are thermally stable and do not
melt up to 3000C, have high thermal conductivity and low thermal
expansion (thus having high resistance to thermal shock) and retain
their mechanical strength to the end. Also, these composites
maintain good frictional properties over the entire temperature
range with low wear. They have high fracture toughness and do not
fracture in a brittle manner like conventional ceramics.
1.4.2 CC composites, their nature and position within the family
of carbon materials:Carbon-carbon composites, or more precisely
carbon fiber reinforced carbon composites, consist of synthetic
pure elemental carbon. Carbon, as a solid, is the unique solid
substance that can be made to exhibit the broadest variety of
different, even controversial, structures and properties. Some
carbons can be extremely strong, hard and stiff, while other forms
can be soft and ductile. Many carbons are highly porous, exhibit a
large surface area, some others are impervious to liquids and
gasses. These variations are not caused by alloying additions as
they might be in metals. Instead, they result from structural
effects, such as the number of defects, geometry and amount of
carbon phases with modified extent of crystalline order. Monolithic
carbons can be extremely brittle. However, carbon fiber rein-forced
carbons can exhibit a high fracture toughness and
pseudo-plasticity. In this sense, carbon-carbon composites can be
compared to the fiber reinforced plastics, which are tailored
materials, exhibiting properties designed to fit the need of the
user. Carbon-carbon composites can be classified in the
intermediate range of conventional synthetic graphite materials,
i.e., the polygranular electrode materials or pyrolytic graphic,
and carbon fiber reinforced polymers, the most likely candidates
for future lightweight aerospace materials. Because carbon-carbon
composites can be described as being intermediary materials, they
exhibit properties taken from each extreme materials grouping, with
fabrication technology being derived from each grouping as
well.1.4.3 The analogy of carbon-carbon composites with
conventional graphite materials:As conventional graphite materials,
carbon-carbon composites, are high temperature materials par
excellence. The preconditions for a material to be
Called a high temperature material are the following.
i. Thermal stability as a solid
ii. High resistance against thermal shock due to high thermal
conductivity and low thermal expansion behavior
iii. High strength and stiffness in high temperature
applications.
These demands are well met by conventional graphite materials.
Described as the best of the refractory materials, synthetic
carbons possess a thermo stability guaranteed up to 3000 K,
exhibiting only one major disadvantage; that is, a sensitivity to
high temperature oxidation.1.4.4 Mechanical and Thermal properties
of the CC composites:Thermal expansion: The carbon fiber itself has
nearly zero thermal expansion in the fiber direction. and
approximately 10.10-6/k perpendicular to the fiber axis. This
system works like a single crystal, with partial compensation of
the expansion perpendicular to the carbon layers. Carbon fiber
bundles with rod-like prepregs, can compensate for the thermal
expansion of the single fibers by a pore structure within the
matrix which combines these monofilaments. Fig. 1.4.4.1 shows an
experimental result with four-dimensional reinforced carbon carbon
composites. The thickness of the rod-like prepreg will also
influence the bulk behavior. If the fixed position of the
reinforcing yarn bundles and the porous matrix between them in
multidirectional reinforcement is considered. one can imagine that
it is possible to arrange tailored thermal bulk expansion behavior
through the selected position of the fiber sticks. A similar
technique is also applied to carbon fiber reinforced polymers in
advanced composites (CFRPs).
Fig. 1.4.4.1 Thermal expansion behavior of different rod-like
prepregs.Influence on fracture behavior:
Additional advantageous behavior of carbon-carbon composites,
which is caused by the pore structure, is found in their fracture
behavior.Three-dimensional reinforcement gives fractured surfaces
pullout, and thus a pseudoplastic fracture behavior. The formation
of stepwise cracks is shown in a stress-strain diagram, presented
in Fig. 1.4.4.2. A surprisingly good fracture toughness can be
achieved in 3-D reinforced composites as shown in Fig. 1.4.4.3
Quantitative Information on fracture behavior by means of the
stress intensity factor is shown in Fig. It is essential to
consider the fatigue behavior of carbon-carbon composites in any
discussion of their physical properties.
Fig. 1.4.4.2 Pseudoplastic fracture behavior of CFRC, especially
if 3D reinforced.
Fig. 1.4.4.3 Fracture behavior by means of the stress intensity
factor.1.5 About Gas Turbine:
Of the various means of producing mechanical power the turbine
is in many respects the most satisfactory. The absence of
reciprocating and rubbing members means that balancing problems are
few, that the lubricating oil consumption is exceptionally low, and
that reliability can be high. The inherent advantages of the
turbine were first realized using water as the working fluid, and
hydro-electric power is still a significant contributor to the
world's energy resources. Around the turn of the twentieth century
the steam turbine began its career and, quite apart from its wide
use as a marine power plant, it has become the most important prime
mover for electricity generation. Steam turbine plant producing
well over 1000 MW of shaft power with an efficiency of 40 per cent
is now being used. In spite of its successful development, the
steam turbine does have an inherent disadvantage. It is that the
production of high-pressure high-temperature steam involves the
installation of bulky and expensive steam generating equipment,
whether it be a conventional boiler or nuclear reactor. The
significant feature is that the hot gases produced in the boiler
furnace or reactor core never reach the turbine; they are merely
used indirectly to produce an intermediate fluid, namely steam. A
much more compact power plant results when the water to steam step
is eliminated and the hot gases themselves are used to drive the
turbine. Serious development of the gas turbine began not long
before the Second World War with shaft power in mind, but attention
was soon transferred to the turbojet engine for aircraft
propulsion. The gas turbine began to compete successfully in other
fields only in the mid nineteen fifties, but since then it has made
a progressively greater impact in an increasing variety of
applications.
In order to produce an expansion through a turbine a pressure
ratio must be provided and the first necessary step in the cycle of
a gas turbine plant must therefore be compression of the working
fluid. If after compression the working fluids were to be expanded
directly in the turbine, and there were no losses in either
component, the power developed by the turbine would just equal that
absorbed by the compressor. Thus if the two were coupled together
the combination would do no more than turn itself round. But the
power developed by the turbine can be increased by the addition of
energy to raise the temperature of the working fluid prior to
expansion. When the working fluid is air a very suitable means of
doing this is by combustion of fuel in the air which has been
compressed. Expansion of the hot working fluid then produces a
greater power output from the turbine, so that it is able to
provide a useful output in addition to driving the compressor. This
represents the gas turbine or internal-combustion turbine in its
simplest form. The three main components are a compressor,
combustion chamber and turbine, connected together as shown
diagrammatically in Fig. 1.5.1. In practice, losses occur in both
the compressor and turbine which increase the power absorbed by the
compressor and decrease the power output of the turbine.
A certain addition to the energy of the working fluid, and hence
a certain fuel supply, will therefore be required before the one
component can drive the other. This fuel produces no useful power,
so that the component losses contribute to a lowering of the
efficiency of the machine. Further addition of fuel will result in
a useful power output, although for a given flow of air there is a
limit to the rate at which fuel can be supplied and therefore to
the net power output. The maximum fuel/air ratio that may be used
is governed by the working temperature of the highly stressed
turbine blades, which temperature must not be allowed to exceed a
certain critical value. This value depends upon the creep strength
of the materials used in the construction of the turbine and the
working life required. These then are the two main factors
affecting the performance of gas turbines: component efficiencies
and turbine working temperature. The higher they can be made, the
better the all-round performance of the plant. It was, in fact, low
efficiencies and poor turbine materials which brought about the
failure of a number of early attempts to construct a gas turbine
engine. For example, in 1904 two French engineers, Armengaud and
Lemale, built a unit which did little more than turn itself over:
the compressor efficiency was probably no more than 60 per cent and
the maximum gas temperature that could be used was about 740 K.The
overall efficiency of the gas turbine cycle depends also upon the
pressure ratio of the compressor. The difficulty of obtaining a
sufficiently high pressure ratio with adequate compressor
efficiency was not resolved until the science of aerodynamics could
be applied to the problem. The development of the gas turbine has
gone hand in hand with the development of this science, and that of
metallurgy, with the result that it is now possible to find
advanced engines using pressure ratios of up to 35:1, component
efficiencies of 85-90 per cent, and turbine inlet temperatures
exceeding 1650 K.
Fig.1.5.1 Simple gas turbine system
In the earliest days of the gas turbine, two possible systems of
combustion were proposed: one at constant pressure, the other at
constant volume. Theoretically, the thermal efficiency of the
constant volume cycle is higher than that of the constant pressure
cycle, but the mechanical difficulties are very much greater. With
heat addition at constant volume, valves are necessary to isolate
the combustion chamber from the compressor and turbine. Combustion
is therefore intermittent, which impairs the smooth running of the
machine. It is difficult to design a turbine to operate efficiently
under such conditions and, although several fairly successful
attempts were made in Germany during the period 1908-1930 to
construct gas turbines operating on this system, the development of
the constant volume type has been discontinued. In the constant
pressure gas turbine, combustion is a continuous process in which
valves are unnecessary and it was soon accepted that the constant
pressure cycle had the greater possibilities for future
development. It is important to realize that in the gas turbine the
process of compression, combustion and expansion do not occur in a
single component as they do in a reciprocating engine. They occur
in components which are separate in the sense that they can be
designed, tested and developed individually, and these components
can be linked together to form a gas turbine unit in a variety of
ways. The possible number of components is not limited to the three
already mentioned. Other compressors and turbines can be added,
with intercoolers between the compressors, and reheat combustion
chambers between the turbines. A heat exchanger which uses some of
the energy in the turbine exhaust gas to preheat the air entering
the combustion chamber may also be introduced. These refinements
may be used to increase the power output and efficiency of the
plant at the expense of added complexity, weight and cost. The way
in which these components are linked together not only affects the
maximum overall thermal efficiency, but also the variation of
efficiency with power output and of output torque with rotational
speed. One arrangement may be suitable for driving an alternator
under varying load at constant speed, while another may be more
suitable for driving a ship's propeller where the power varies as
the cube of the speed. Apart from variations of the simple cycle
obtained by the addition of these other components, consideration
must be given to two systems distinguished by the use of open and
closed cycles. In the much more common open cycle gas turbine which
we have been considering up to this point, fresh atmospheric air is
drawn into the circuit continuously and energy is added by the
combustion of fuel in the working fluid itself. In this case the
products of combustion are expanded through the turbine and
exhausted to atmosphere. In the alternative closed cycle shown in
Fig.1.5.2 the same working fluid, be it air or some other gas, is
repeatedly circulated through the machine. Clearly in this type of
plant the fuel cannot be burnt in the working fluid and the
necessary energy must be added in a heater or 'gas-boiler' wherein
the fuel is burnt in a separate air stream supplied by an auxiliary
fan. The closed cycle is more akin to that of steam turbine plant
in that the combustion gases do not themselves pass through the
turbine. In the gas turbine the 'condenser' takes the form of a
precooler for cooling of the gas before it re-enters the
compressor.
Fig.1.5.2 Simple Closed Cycle
1.6. Failure Modes in Gas Turbine Blading:
Blade failures have plagued designers and operators since the
inception of turbomachinery. Turbine Blades are subjected to
significant rotational and gas bending stresses at extremely high
temperature, as well as severe thermo mechanical loading cycles as
a consequence of normal start-up and shutdown operation and
unexpected trips. The most difficult and challenging point is the
one located at the turbine inlet because there are several
difficulties associated to it like extreme temperature, high
pressure, high rotational speed,vibration,small circulation area
and so on. These effects produced in the blades are shown on the
Table . Table 1.6.1 Severity of the different surface related
problems for gas turbine applications.
(Effects)
(Applications)Oxidation Hot corrosionInterdiffusionThermal
Fatigue
AircraftSevereModerateSevereSevere
Land-based Power GeneratorModerateSevereModerateLight
Marine EnginesModerateSevereLightModerate
In order to overcome those barriers, gas turbine blades are made
using advanced materials and modern alloys (super alloys) that
contains up to ten significant alloying elements, but its
microstructure is very simple; consisted of rectangular blocks of
stone stacked in a regular array with narrow bands of cement to
hold them together. This material (cement) has been changed because
in the past,intermetallic form of titanium was used in it, but now
a days, it has been replaced by titanium.The change gave improved
high temperature strength and also improved oxidation resistance.In
the past, the low-cycle fatigue (LCF) test and the MansonCoffin
equation were widely used to evaluate the reliability of the
substrates of gas turbine blades. However, the LCF test, which only
can simulate fatigue conditions under high isothermal temperatures,
cannot model actual operating conditions. For that reason,
thermo-mechanical fatigue (TMF) tests, which can simulate both
mechanical fatigue and thermal fatigue simultaneously, are
preferred. TMF tests are the most appropriate for simulating actual
combined loading conditions during service. In particular, the
leading edge (LE) of the airfoil in a blade is under out-of-phase
(OP) thermo-mechanical fatigue. Therefore, it is very important to
evaluate the TMF characteristics [16]. In this study, LCF and TMF
tests for the life prediction of Ni-base superalloy were carried
out using the furnace. In particular, in the thermo-mechanical
fatigue tests, both IP (in-phase) and OP (out of-phase) TMF tests
were conducted.
Predominant failure mechanisms and commonly affected components
are:
Low cycle fatigue-compressor and turbine blades and disks.
High cycle fatigue-compressor and turbine blades, disks,
compressor stator vanes.
Thermal fatigue-nozzles, combustor components.
Environmental attack (oxidation, sulphidation, hot corrosion,
standby corrosion)-hot section blades and stators, transition
pieces, and combustors.
Creep damage-hot section nozzles and blades.
Erosion and wear.
Impact overload damage (due to foreign object damage (FOD),
domestic object damage (DOD) or clash/clang of compressor blades
due to surge).
Thermal aging.
Fig1.6.1.Pictorial overview of turbine blades section. Table
1.6.2FAILURE MODES AND CAUSES OF DIFFERENT PARTS OF A TURBINE
SECTION. ComponentFailure ModeCause
Turbine Rotor BladesLow cycle and High
cyclefatigue,creep,corrosion,sulphidation, erosionCentrifugal and
temperature stress, vibratory stress, environmental, fuel problems,
excessive temperature spreads, cooling problems
Turbine Stator BladesCreep rupture corrosion,sulphidation,
bowing,fatigue,thermal fatigueCooling problems, Improper
temperature profile
Turbine Rotor DiscCreep-rupture, low cycle fatigueImproper wheel
space cooling, Thermal stresses
A. Fatigue
Fatigue accounts for a significant number of turbine and
compressor blade failures and is promoted by repeated application
of fluctuating stresses. Stress levels are typically much lower
than the tensile stress of the material. Common causes of vibration
in compressor blades include stator passing frequency wakes,
rotating stall, surge, choke, inlet distortion, and blade flutter.
In the turbine section, airfoils have to function not only in a
severe vibratory environment, but also under hostile conditions of
high temperature, corrosion, creep, and thermomechanical
fatigue.Ewins (1976) provides a detailed treatment of blade
vibration.
B.Low Cycle Fatigue
Low cycle fatigue (LCF) occurs as a result of turbine start/stop
cycles and is predominant in the bores and bolt hole areas of
compressor and turbine disks that operate under centrifugal
stresses. It is typically a problem associated with machines that
have been in operation for several years. In this situation, minute
flaws grow into cracks that, upon attaining critical size, rupture.
Cracks also develop in the nozzle sections. To some extent, this is
to be expected under normal operation and cycling service.
C.Thermomechanical Fatigue
Thermomechanical fatigue (TMF) is associated with thermal
stresses, e.g., differential expansion of hot section components
during startup and shutdown, and is particularly severe during
rapid starts and full load emergency trips. The stress levels
induced may initiate cracks, if they exceed the material yield
stress. Temperature variations as high as 360F (200C) per minute
are often experienced in hot section blading. This is the reason
why full load trips are so detrimental in terms of life reduction,
consuming as much as 200 equivalent hours per trip.
LITERATURE REVIEW2.1 Dian-sen Li, et.Al.- High temperature
compression properties and failure mechanism of 3D needle-punched
carbon/carbon composites-Elsevier, 31 October 2014.This paper
reports the high temperature compression properties and failure
mechanism of 3D needle punched C/C composites. The results show
that the stressstrain curves show non-linear and plasticity failure
feature after 600 0C. The compression properties decrease
significantly with increasing the temperature due to material
oxidation. The composite exhibits shear fracture at the angle of
450 the major damage patterns are the tearing of 900 oriented
fibers and shear failure of 00 oriented fibers on the shear
surface. After 6000C, the local and plastic failure feature becomes
more obvious.2.2 Ruiying Luo, et. Al.- Thermo physical properties
of carbon/carbon composites and physical mechanism of thermal
expansion and thermal conductivity-Elsevier, 14 August 2004.Five
different carbon/carbon composites (C/C) have been prepared and
their thermo physical properties studied. These were three needled
carbon felts impregnated with pyrocarbons (PyC) of different
microstructures, chopped fibers/resin carbon +PyC, and carbon
cloth/PyC. The results show that the XY direction thermal expansion
coefficient (CTE) is negative in the range 0100 0C with values
ranging from -0.29 to -0.8510 6 /K. In the range 0900 0C, their CTE
is also very low, and the CTE vs. T curves have almost the same
slope. In the same temperature range composites prepared using
chopped fibers show the smallest CTE values and those using the
felts show the highest. The microstructure of the PyC has no
obvious effect on the CTE for composites with the same preform
architecture. Their expansion is mainly caused by atomic vibration,
pore shrinkage and volatilization of water. However, the PyC
structure has a large effect on thermal conductivity (TC) with
rough laminar PyC giving the highest value and isotropic PyC giving
the lowest. All five composites have a high TC, and values in the
XY direction (25.6174 W/mK) are much larger than in the Z direction
(3.550 W/mK). Heat transmission in these composites is by phonon
interaction and is related to the perform and PyC structures.2.3
Wenfei Luo, et.al.- Effects of different loading methods on thermal
expansion behaviors of 2D cross-ply carbon/carboncompositesfrom850
1C to2300 1C-Elsevier, 27 March 2014.
2D cross-ply cc composites were prepared by isothermal chemical
vapor infiltration process. Thermal expansion behavior of 2D cc
composites and effects of static loading and fatigue loading
conditions on coefficient of thermal of the composites from 850 0C
to 2300 0C. In static test cases, different damage levels were
produced inside samples by displacement controlled three-point
bending moment. Afterwards, the thermal expansion behaviors were
investigated. It showed that CTE of the C/C composites after static
loading and fatigue loading had a decrease compared with CTE of the
as-prepared C/C composites. After static loading, there was a
larger decrease in CTE for the C/C composites with more severe
damage levels. Moreover, the influence of damage on CTE reached to
the greatest at around 15500 C.2.4 Xiaoling Liao, et.al.- Effects
of tensile fatigue loads on flexural behavior of 3D braided C/C
composites- Elsevier, 06 November 2007.3D braided C/C composites
have been prepared by an isothermal chemical vapor infiltration
(CVI) process, and the evolution of flexural behavior perpendicular
to the fiber direction after tensile fatigue loads was examined.
The results show that the flexural property of C/C composites was
enhanced with the increase in fatigue cycles, and the fracture mode
also evolved from brittle of original samples to pseudo-plastic of
fatigued samples. It is suggested that the weakened interface and
reduced residual thermal stresses by fatigue loads play important
roles in enhancing the property of C/C composites.
2.5 Yanhui Chu,et.al.- Thermal fatigue behavior of C/C
composites modified by SiCMoSi2CrSi2 coating- Elsevier,26 May
2011.Carbon/carbon (C/C) composites were modified by SiCMoSi2CrSi2
multiphase coating by pack cementation, and their thermal fatigue
behavior under thermal cycling in Ar and air environments was
investigated. The modified C/C composites were characterized by
scanning electron microscopy and X-ray diffraction. Results of
tests show that, after 20-time thermal cycles between 1773 K and
room temperature in Ar environment, the flexural strength of
modified C/C samples decreased lightly and the percentage of
remaining strength was 94.92%.
2.6 Jing Cheng, et.al.- Internal friction behavior of
unidirectional carbon/carbon composites after different fatigue
cycles- Elsevier,15 February 2014.Internal friction behavior was
utilized as an indirect metric to study structural change in
carbon/carbon composites after fatigue tests. In this work, two
kinds of unidirectional carbon/carbon composites with different
densities were prepared by isothermal chemical vapor infiltration.
The internal friction behavior of the composites after different
fatigue cycles was studied. After the initial 104 fatigue cycles,
since the matrix began to break and shed, frictional damping that
happened between the fiber and matrix interfaces increased and the
bulk internal friction increased rapidly. Between 104 and 5105
fatigue cycles.2.7 A. Ozturk- The influence of cyclic fatigue
damage on the fracture toughness of carbon-carbon composites-
Elsevier,21 February 1996.
The influence of cyclic loads on the fracture toughness of a
tightly woven carbon-carbon composite was investigated as a
function of stress levels. Results of fracture toughness tests were
correlated with microstructural examination using scanning electron
microscopy (SEM). Values for the stress intensity factor, Krc, were
determined using the ASTM single-edge notched bend test. Results
were discussed in terms of the effects of applied cyclic stress
levels and the relationship of the load-displacement curves. The
fracture toughness of the composite remained unaffected when the
maximum tensile load in the fatigue cycle was up to 80% of the
static tensile strength.2.8 Torsten Windhorst, et.al,-
Carbon-carbon composites: a summary of recent developments and
applications- Elsevier, 21 April 1997.
Carbon Fibre Reinforced Carbon (CFRC), or Carbon-carbon, is a
unique composite material consisting of carbon fibres embedded in a
carbonaceous matrix. Originally developed for aerospace
applications, its low density, high thermal conductivity and
excellent mechanical properties at elevated temperatures make it an
ideal material for aircraft brakes, rocket nozzles and re-entry
nose tips. It withstands temperatures in excess of 2000C without
major deformation. The properties are very much dependent on the
manufacturing methods used for production. Although the general
production technology is known, the combination of processes to
achieve specially tailored properties remains the expertise of
particular manufacturers.2.9 G. Rohini Devi, et.al.- Carbon-Carbon
Composites -An Overview- Defence Science Journal, Vol 43, No 4,
October 1993.Carbon-carbon composites are a new class of
engineering materials that are ceramic in nature but exhibit
brittle to pseudo plastic behaviotir. Carbon-carbon is a unique
all-carbon composite with carbon fiber embeded in carbon matrix and
is known as an inverse composite. Due to their excellent
thermo-structural properties, carbon-carbon composites are used in
specialized application like re-entry nose-tips, leading edges,
rocket nozzles, and aircraft brake discs apart from.several
industrial and biomedical applications. The multidirectional
carbon-carbon product technology is versatile and offers design
flexibility. This paper describes the multidirectional preform and
carbon-carbon process technology and research and development
activities within the country .Carbon-carbon product experience at
DRDL has also been discussed. Development of carbon-carbon brake
discs process technology using the liquid impregnation process is
described- Further the test results on material characterization,
thermal, mechanical and tribological properties are presented.2.10
Ying-bo Fei,et.al.- Influence of heat treatment temperature on
microstructure and thermal expansion properties of 2D carbon/carbon
composites- Elsevier ,21 October 2013.
The influence of heat treatment from 1900 to 2650 0C on
microstructure and thermal expansion properties of thermal gradient
chemical vapor infiltration (TCVI)-infiltrated 2D carbon/carbon
composites was investigated. The structure evolution was
characterized by polarized light microscopy, X-ray diffractometer,
Raman spectroscopy and thermal expansion behavior was studied by
thermal dilatometer. The results revealed that with the increasing
heat treatment temperature, successive micro structural changes in
carbon matrix and fibers occurred, and cracks and pores in the
composites increased. These resulted in a 35.6% decrease of CTE in
Z direction and a 13.9% decrease in XY direction in the composites
heat-treated at 2650 0C compared with as-deposited composites. The
CTE mainly depended on the thermal expansion of matrix in Z
direction while it relied on that of the fibers in XY
direction.
PROBLEM IDENTIFICATION3.1. The High-temperature Materials
Problem
The strength of metals decreases with increasing temperature.
Since the mobility of atoms increases rapidly with temperature, it
can be appreciated that diffusion-controlled processes can have a
very significant effect on high temperature mechanical properties.
High temperature will also result in greater mobility of
dislocations by the mechanism of climb. The equilibrium
concentration of vacancies likewise increases with temperature. New
deformation mechanisms may come into play at elevated temperatures.
In some metals the slip system changes, or additional slip systems
are introduced with increasing temperature. Deformation at grain
boundaries becomes an added possibility in the high-temperature
deformation of metals. Another important factor to consider is the
effect of prolonged exposure at elevated temperature on the
metallurgical stability of metals and alloys. For example,
cold-worked metals will recrystallize and undergo grain coarsening,
while age-hardening alloys may overage and lose strength as the
second-phase particles coarsen. Another important consideration is
the interaction of the metal with its environment at high
temperature. Catastrophic oxidation and intergranular penetration
of oxide must be avoided.
Thus, it should be apparent that the successful use of metals at
high temperatures involves a number of problems. Greatly
accelerated alloy-development programs have produced a number of
materials with improved high-temperature properties, but the
ever-increasing demands of modern technology require materials with
even better high-temperature strength and oxidation resistance. For
a long time the principal high-temperature applications were
associated with steam power plants, oil refineries, and chemical
plants. The operating temperature in equipment such as boilers,
steam turbines, and cracking units seldom exceeded 1000F. With the
introduction of the gas-turbine engine, requirements developed for
materials to operate in critically stressed parts, like turbine
Applications to Materials Testing buckets, at temperatures around
1500F. The design of more powerful engines has pushed this limit to
around 1800F. Rocket engines and ballistic-missile nose cones
present much greater problems, which can be met only by the most
ingenious use of the available high-temperature materials and the
development of still better ones. There is no question that the
available materials of construction limit rapid advancement in
high-temperature technology.
An important characteristic of high-temperature strength is that
it must always be considered with respect to some time scale. The
tensile properties of most engineering metals at room temperature
are independent of time, for practical purposes. It makes little
difference in the results if the loading rate of a tension test is
such that it requires 2 hr or 2 min to complete the test. Further,
in room-temperature tests the elastic behavior of the material is
of little practical consequence. However, at elevated temperature
the strength becomes very dependent on both strain rate and time of
exposure. A number of metals under these conditions behave in many
respects like viscoelastic materials. A metal subjected to a
constant tensile load at an elevated temperature will creep and
undergo a time-dependent increase in length.
The tests which are used to measure elevated-temperature
strength must be selected on the basis of the time scale of the
service which the material must withstand. Thus, an
elevated-temperature tension test can provide useful information
about the high-temperature performance of a short-lived item, such
as a rocket engine or missile nose cone, but it will give only the
most meagre information about the high-temperature performance of a
steam pipeline which is required to withstand 100,000 hr of
elevated-temperature service. Therefore, special tests are required
to evaluate the performance of materials in different kinds of
high-temperature service. Test measures the dimensional changes
which occur from elevated-temperature exposure, it also measures
the effect of temperature on the long-time load-bearing
characteristics.
METHODOLOGY
Preliminary Analysis
COMPRESSOR/PUMP
TURBINE
STEAM
HYDRAULIC
GAS
CASING
BEARING
SHAFT
DISC
BLADE
EFFICIENCY OPTIMIZATION
EFFICIENCY OPTIMIZATION
STRENGTH
STRENGTH
WEIGHT
WEIGHT
HIGH TEMP.
PROPERTY MATERIAL
HIGH TEMP.
PROPERTY MATERIAL
INCLUSIVE PROPERTY
MATERIAL
SUPERALLOYS
COMPOSITE
MATERIALS
CARBON-
CARBON
COMPOSITE
CAUSES OF
FAILURE
CAUSES OF
FAILURE
LCF
HCF
THERMAL
FATIGUE
CREEP
EROSION
TEST
ANALYSIS
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