assignment 2 material tia

7/26/2019 Assignment 2 Material Tia

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1.0 INTRODUCTION

Engineers are responsible in choosing the suitable material for a specic

product. The product must not only full the requirement demanded by the

customer but also have the safety features which coexist with its function. In

design, materials selection can be a complex, iterative process that solves a

particular set of engineering objectives for a given component. aterials

selection and design are also closely related to the objectives of failure analysis

and prevention. !y predicting the behaviour of the material upon any changes,

the engineers are able to choose the suitable material. The level of performance

of components in service depends on several factors such as inherent properties

of materials, load or stress system, environment and maintenance. This is an

important aspect as proven by "mithers #apra, a material experts claim that

$%& of all failures encountered are in some part due to a poor material choice

and specication according to their failure diagnostics statistics '(). The failure in

engineering material component not only caused from the poor material

selection but also manufacturing defects, exceeding design limits and

overloading and even inadequate maintenance. !efore manufacturing the

product, the material needs to be tested to determine the quality of a materialsince this may be one aspect of process control in production plant, to determine

such properties as strength, hardness, and ductility, to chec* for +aws within a

material or in a nished component and to assess the li*ely performance of the

material in a particular service condition. These material testing such as tensile

test and compression test will provide information about the material as well as

its performance capabilities.

lthough many methods of testing on the materials are done, the

materials may also still fail in service, sometimes with disastrous results whichendanger life of many. In order to avoid such disasters occurring, the designer

avoids using materials continuously at their maximum allowable stress. This is

done by employing a factor of safety in the design where the design stress does

not exceed %- & of the yield stress. nce the material is deformed, the failures

of the material occur where the material cannot behave according to its original

form.

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Figure 1.0 Fractography surface of railway rim crackig. !"#

".0 T$%&' OF F(I)UR& IN *(T&RI()

/ailure of material is caused when the material is deformed due to the

load or stress system, environment, maintenance or its inherent properties of

materials as mentioned earlier. The failure of material can be explained the

stress0strain graph below1

Figure ".1 'tress +strai Cur,e !-#

ost of materials possess an elastic limit as shown above. 2hen stress is

applied, the material will strain in an elastic manner up to a certain point.!eyond this point the strain developed is no longer directly proportional to the

applied stress, and also, the strain developed is no longer fully recoverable. 3pon

unloading of the stress elastic strain is recovered but the material will be left in a

state of permanent, or plastic, strain. 4owever the mechanism of plastic

behaviour is not the same for all classes of materials and it is necessary to

consider the various materials groups separately. etals, in general, are

characteri5ed by possessing high elastic modulus values, and also the ability to

be strained m a plastic manner. "ome metals will begin to deform plastically at

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very low values of stress and will yield to a very considerable extent before

fracture occurs. The failure of material varies according to several factors. There

are many type of failure of material which includes foreign object damage 7/89,

fatigue failure, corrosion, creep failure and many more. '$) 8espite from that the

most common failures are creep failure, fatigue failure and corrosion of the

material.

:reep failure happens usually occurs at elevated temperatures since slip

in the lattice structure is easier at this temperature. Even in designing the steam

and gas turbines the material for the rotor and stator turbine blade are carefully

chosen to minimi5e the creep failure. It would be catastrophic if the rapidly

rotating blades of the rotor touched the stator blades due to dimensional change

through creep. :reep can be easily dened as gradual extension of a material

under a constant applied load. :reep is an important factor when designing

metals when they are required to wor* continuously at high temperatures. The

phenomena of the creep failure are shown below1

Figure "." Creep Cur,e !#

The ;rimary :reep or the (ststage creep are fairly rapid rate but slows

down as wor* hardening 7strain hardening9 sets in and the strain rate decreases.

The extension due to creep is additional to the instantaneous elongation of

material to be expected when any tensile load is applied. 8uring the secondary

creep period, the increase in strain is approximately proportional to time. t this

stage the strain rate is constant and at its lowest value. 8uring the tertiary creep

period of creep, the strain rate increases rapidly, nec*ing occurs and the test

piece fails. Thus the initial stress, which was within the elastic range and did notproduce early failure, did eventually result in failure after some period of time.


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ore than =% & of failures in engineering components are attributed to

fatigue failure, thus *nowledge of fatigue to be implemented when designing the

product is very important '>). /atigue failure happens when the material

subjected to a stress which is alternately applied and removed a very large

number of times, or which varies between two limiting values, will fracture at a

very much lower value of stress than in a normal tensile test. The fatigue crac*

which ultimately causes fatigue failure usually starts at a point of stress

concentration. The detail on the fatigue failure phenomena will be discussed in

the next section.

:orrosion is another type of common failure in metals. :hoosing the right

material is important in design consideration. Thee environment aspects needs

to be included since the availability of oxygen in the environment may initiate

oxidation which than leads to corrosion. In order to avoid corrosion to ta*e place,

an anti0rust agent should be applied periodically. The corrosion problem can be

overcome by using a composite material.

-.0 /O F(TIU& F(I)UR& INITI(T&D FRO* T/& %OINT OF %/$'IC()

2&/(3IOUR

/atigue failure can occur at loads considerably lower than tensile or yield

strengths of material under a static load. /atigue failure is brittle0li*e relativelylittle plastic deformation even in normally ductile materials which is sudden and

catastrophic. /atigue failure proceeds in three distinct stages1 crac* initiation in

the areas of stress concentration, incremental crac* propagation and nal

catastrophic failure. :yclic stresses characteri5ed by maximum, minimum and

mean stress, the range of stress, the stress amplitude, and the stress ratio.

!elow shows the cyclic stress curve which leads to fatigue failure. /atigue failure

may happen below the yield strength with condition of cyclic loading. "tress are

applied as below where the stress maximum is applied and unloading of the

stress is done until the stress minimum point. The loading process continue by

applying the same amount of stress and the pattern of the loading0unloading

continue which then leads to fatigue failure.

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Figure -.1 Cyclic 'tress cur,e !4#

The material having fatigue failure underwent the repeated stress cycle in

which maximum and minimum stresses are asymmetrical relative to the 5ero

stress level? mean stress m, range of stress r , and stress amplitude a are

indicated. "0@ 7"tress0@umber of cycles to failure9 curve denes number of

cycles0to0failure for given cyclic stress. /or frequencies A 6--45, metals are

insensitive to frequency? fatigue life in polymers isfrequency dependent. !elow

shows the "0@ behavior curve.

Figure -." '+N cur,e !5#

ccording to the graph above , the higher the magnitude of the stress, the

smaller the number of cycles the material is capable to sustain before failure.:ertain materials have a fatigue limit or endurance limit which represents a

stress level below which the material does not fail and can be cycled innitely. If

the applied stress level is below the endurance limit of the material, the

structure is said to have an innite life. any non0ferrous metals and alloys, such

as aluminum, magnesium, and copper alloys, do not exhibit well0dened

endurance limits. These materials instead display a continuously decreasing "0@

response, similar to :urve ! in /igure


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these materials is sometimes dened as the stress that causes failure at (x(-Cor

%x(-Cloading cycles. 'C)

/atigue will ultimately occur regardless of the magnitude of the stress. /or

these materials, the fatigue response is specied as fatigue strength, which is

dened as the stress level at which failure will occur for some specied number

of cycles. There are two type of cyclic fatigue namely the high0cyclic fatigue and

low0cyclic fatigue. 4igh0cyclic fatigue74:/9 is for low stress levels where

deformations are totally elastic, longer lives result. This requires large numbers

of cycles are required to produce fatigue failure. This type of failure is associated

with fatigue lives greater than about (-$to (-=cycles. In contrast, the low0cyclic

fatigue 7D:/9 is associated with relatively high loads that produce not only elastic

strain but also some plastic strain during each cycle. :onsequently, fatigue lives

are relatively short occurs at less than about (-$ to (-% cycles. /igure below

shows clear diBerence between the two behaviors.

Figure -.- '+N cur,e with )CF a6 /CF. !4#

7.0 /O F(TIU& F(I)UR& INITI(T&D FRO* T/& %OINT OF

*ICRO'TRUCTUR&

/ailure of material usually found when the material already deformed as it

can clearly see by na*ed eyes. 4owever the fatigue failure process can be

observe from the microstructure view. The fatigue failure process can be divided

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into three phases. In the rst phase namely crac* initiation phase. s the crac*

initiated a fatigue crac* propagates along high shear stress planes 7$% degrees9

. This is *nown as stage I or the short crac* growth propagation stage. The crac*

propagates until it is caused to decelerate by a microstructural barrier such as a

grain boundary, inclusions, or pearlitic 5ones, which cannot accommodate the

initial crac* growth direction. Therefore, grain renement is capable of increasing

fatigue strength of the material by the insertion of a large quantity of

microstructural barriers, i.e. grain boundaries, which have to be overcome in the

stage I of propagation. "urface mechanical treatments such as shot peening and

surface rolling, contribute to the increase in the number of microstructural

barriers per unit length due to the +attening of the grains.

In the crac* propagation phase, the stress intensity factor increases as a

consequence of crac* growth or higher applied loads, slips start to develop in

diBerent planes close to the crac* tip, initiating this crac* propagation stage.

"ome of the crac* join together and begin to propagate through the material in a

direction that is perpendicular to the maximum tensile stress. Eventually, the

growth of one or a few crac* of the larger crac*s will loading, the growth of the

dominate crac* or crac*s will continue until the remaining un0crac*ed section of

the component can no longer support the load. t this point, the fracture

toughness is exceeded and the remaining cross0section of the material

experiences rapid fracture. /inally, stage III is related to unstable crac* growth

as max approaches I:. t this stage, crac* growth is controlled by static modes of

failure and is very sensitive to the microstructure, load ratio, and stress state

7plane stress or plane strain loading9. acroscopically, the fatigue fracture

surface can be divided into two distinct regions. The rst region corresponds to

the stable fatigue crac* growth and presents a smooth appearance due to the

friction between the crac* wa*e face. :oncentric mar*s *nown as Fbeach mar*sG

can be seen on the fatigue fracture surface, as a result of successive arrests or a

decrease in the rate of fatigue crac* growth due to a temporary load drop, or due

to an overload that introduces a compressive residual stress eld ahead of the

crac* tip. Therapid overload fracture is the nal stage which is the failure where

the material is permanently deformed. The process can be clearly seen from the

gure below.

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Figure 7.1 'tages i Fatigue Failure !8#

The eBect of fatigue failure also can be seen by na*ed eyes. !y examining

the fracture side of the failure, two distinct regions can be felt. ne being smooth

or burnished as a result of the material. The microstructure are as below 1

Figure 7." The surface of a fatigue fracture !8#

!eachmar*s, or clamshell mar*s, may be seen in fatigue failures of materials

that are used for a period of time, allowed to rest for an equivalent time period

and the loaded again as in factory usage. "triations are thought to be steps in

crac* propagation, were the distance depends on the stress range. !eachmar*s

may contain thousands of striations as shown below1

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Figure 7.- 2eachmark as the result of fatigue failure !8#

.0 R&CO**&ND(TION' (ND DI'CU''ION ON 'O*& *&('UR&' TO

%R&3&NT F(TIU& F(I)UR&

The fatigue failure ends the life of the product. 4ow the lifespan of the

material can be prolong by ta*en measures that may prevent the fatigue failure

of the material. !efore manufacturing the product prevention can be made

during the design process by eliminate or reduce stress raisers by streamlining

the part or component, avoid sharp surface tears resulting from punching,

stamping, shearing, or other processes, prevent the development of surface

discontinuities during processing and reduce or eliminate tensile residual

stresses caused by manufacturing. ther than that , secondary process ofstrengthening mechanism can be apply after it is manufactured can also be done

to avoid the failure of the material. "trengthening mechanism alters the

properties of the material. Its ability to withstand load not only restricted below

the yield strength. The load can be applied above the yield strength as shown

below1

Figure .1 The rage of strai har6eig a9ility o the stress strai cur,e !10#

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ne of the strengthening mechanism is the wor* hardening in which

dislocations interact with each other by generating stress elds in the material.

The interaction between the stress elds of dislocations can impede dislocation

motion by repulsive or attractive interactions. dditionally, if two dislocations

cross, dislocation line entanglement occurs, causing the formation of a jog which

opposes dislocation motion. These entanglements and jogs act as pinning points,

which oppose dislocation motion. Increasing the dislocation density increases the

yield strength which results in a higher shear stress required to move the

dislocations. "econdly is alloying or solid solution hardening. "olute atoms of one

element are added to another, resulting in either substitutional or interstitial

point defects in the crystal. The solute atoms cause lattice distortions thatimpede dislocation motion, increasing the yield stress of the material. "olute

atoms have stress elds around them which can interact with those of

dislocations. The presence of solute atoms imparts compressive or tensile

stresses to the lattice, depending on solute si5e, which interfere with nearby

dislocations, causing the solute atoms to act as potential barriers to dislocation

propagation. Increasing the concentration of the solute atoms will increase the

yield strength of a material.

;recipitation hardening is another method in which second phase

precipitates act as pinning points in a similar manner to solutes, though the

particles are not necessarily single atoms. 8islocation in the atomic arrangement

will overcome the slip from happening. lthough these measures can be ta*en to

reduce the fatigue failure, upon high loading and repeated loading may also

cause failure when the load exceeds %-& of the yield strength.

:.0 DI'CU''ION (ND CONC)UDIN R&*(R;'

/ailure on material is a common problem encounter in the industries.

easures were ta*en before the manufacturing process by choosing the right

material that suits the function as well as able to have long lifespan that may.

This is important as it avoid endangering the society which uses these materials.

4owever choosing the suitable material alone does not ensure to prolong the life

span of the material. ne of the common failures is fatigue failure which result

from the repetitive loading on the material despite the load is below the yield

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strength. The cyclic loading causes crac* initiation on the material followed by

the crac* propagation. 2hen ample amount of stress concentration and

intensity occur at the crac* , the failure of the material occur. The failure is

permanent where it can no longer turn to its original form. The fatigue failure can

be reduce by strengthening mechanism such as wor* hardening and

precipitation hardening. These activities help to increase the strength of the

material.

4.0 R&F&R&NC&'

'() Faterial "election,G 'nline). vailable1

http1www.smithersrapra.comexpert0supportfailure0preventionmaterial0

selection. 'ccessed = June 6-(%).

'6) Ku, L.M. Diu and 4.:., F/ailure odes and aterials ;erformance of #ailway

2heels,GJournal of Materials Engineering and Performance, vol. Nol H 7@o %9,

p. %C-O%C$, ct 6---.

' June 6-(%).

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'(-

)

F8eformation 7Engineering9,G 'nline). vailable1

http1en.wi*ipedia.orgwi*i8eformationP7engineering9. 'ccessed > June

6-(%).

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assignment 2 material tia

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