FOR REFERENCE ONLY Unit 23 Assignment 4

8/8/2019 FOR REFERENCE ONLY Unit 23 Assignment 4

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Assignment 4: Diagnose causes of failure of materials

TasksFor a product, of you choice, appropriate to your vocational experiences, describethe component and its function.

The component I have chosen to investigate is the HP Turbine blade. The turbine inan assembly of discs with blades that are attached to the turbine shafts, nozzleguide vanes casings and structures.

The turbine extracts energy from the hot gas stream received from the combustionchamber. In a turbofan this power is used to drive the fan and compressor.

Turbine blades convert the energy stored within the gas into kinetic energy. Likecompressor, the turbine comprises of a rotating disc with blades and static vanes,called nozzle guide vanes. The gas pressure and temperature both fall as passesthrough the turbine.

HP turbine blades and nozzle guide vanes are designed with cooling passages andthermal barrier coatings, to ensure long life while operating at such hightemperatures. Cooling air is taken from the compressor and is fed around thecombustion chamber into the blades to cool the aerofoil.

HP Turbine

IP Turbine

LP Turbine


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Then:Identify potential causes of failure that may occur in service.

Discuss the factors that may influence the service life of the component.

There are many various things that may affect the service life of an enginecompressor blade. While some of these reasons may seem quite general they arecategorised into each part of the components life.

Design Material SelectionToo much detail on the design for the

manufacturing processAerodynamic shapeIncorrect surface modellingThicknessSmall RadiiLack of finite element analysis

Manufacturing Manufacturing Process (i.e. sandcasting opposed to investmentcasting)Machine Malfunction

Inaccurate drawingsSurface ImperfectionsIndentationsInclusionsPoor surface finish (i.e. Corrosion)

Delivery Inadequate PackagingInappropriate transportation

HP turbine blade cooling flows

Blade cooling air


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In Serv

ce Acc

dental Overload

ng

Expos re to extreme heat

Expos re to static mechanical

stressesCreep**

Des

be a d st ate t e typ

a

s

p

eat es t at w

d be

p esent n

a es ass

ated w t

Duct e

acture

Ductile fracture involvesplasticdeformation in the vicinity of an advancingcrack

and is a slow process It isstable, and will notcontinue unless there is an increase inthe level of appliedstress It normally occurs in a trans-granular manner(across the

grains) in metals that have goodductility and toughness Often, a considerable

amount ofplasticdeformation including necking is observed in the failed

component. Thisdeformation occursbefore the final fracture.

Ductile fractures are normallycausedbysimple overloads orby applying too high a

stress to the material, and exhibitcharacteristicsurface features with a significant

portion of the fracture surface having an irregular, fibrous face. They also have asmall shear lip, where the fracture surface is at a 45 angle to the appliedstress.

The shear lip, indicating thatslip occurred, gives the fracture the cup -and-cone

appearance. Simple macroscopic observation of this fracture maybe sufficient toidentify the ductile fracture mode.

Examination of the fracture surface at a high magnification using a scanningelectron microscope (SEM) reveals a dimpledsurface. Figure 3 on the left hand

side shows that under a normal tensile stress, these dimples are usuallyround orequiaxed(having the same dimensions in all directions) while figure 3 on the right

handside shows ifshearstress hasbeen dominant, the d imples are oval-shaped or

elongated, with the ovalspointing towards the origin of fracture.

Microscopic images ofductile fracture x 1000.


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Britt e

racture.

In brittle fractures, cracksspreadveryrapidly, with little or no plastic flow, and are

so unstable thatcrackpropagation occurs without further increase in appliedstress.They occur in high strength metals, in metals with poorductility and toughness, and

in ceramics.

Even metals that are normallyductile may fail in a brittle manner at low

temperatures, in thicksections, at high strain rates(such as impact), or when flawsplay an importantrole. Brittle fractures are frequently observed when impactrather

than overloadcauses failure.

Brittle fracture can be identifiedby observing the features on the failedsurface.

Normally, the fracture surface is flat andperpendicular to the appliedstress in a

tensile test. If a failure occursbycleavage, each fracturedgrain is flat anddifferen tlyoriented, giving a shiny, crystalline appearance to the fracture surface

Initiation of a crack normally occurs atsmall flaws which cause a concentration of

stress. Normally, the crackpropagates most easily alongspecificcrystallographicplanesbycleavage. However, in some cases, the crack may take an inter-granular

(along the grain boundaries)path, particularly when segregation or inclusionsweaken the grain boundaries(Figure below). It hasbeen identified that a crack may

propagate at a speed approaching the speed ofsound in the material.

Microscopic image of intergranularbrittle fracture x 1000

FatigueFracture.

Fatigue is a form of failure that occurs in materialssubjected to fluctuatingstresses for example, solderjoints under temperature cycling. Under these circumstances,

it ispossible for failure to occur at a stress level considerably lower than the tensile

oryieldstrength for a static load.


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The term fatigue is usedbecause this type of failure normally occurs after a l engthyperiod ofrepeatedstresscycling. It is the single largestcause of failure

(approximately 90%) of metallic materials, andpolymers andceramics(other than

glasses) are also susceptible to this type of failure. Although failure isslow in

coming, catastrophic fatigue failures occurverysuddenly, and without warning.

Fatigue failure isbrittle-like in nature even in normallyductile metals in that

there isvery little, if any, grossplasticdeformation associated with failure. Theprocess occursby the initiation andpropagation ofcracks, and the fracture surface

is usuallyperpendicular to the direction of an appliedstress.

A majorproblem with fatigue is that it isdominatedbydesign. Whilst it ispossible to

assess the inherent fatigue resistance of a material, the effects ofstress-raiserssuchassurface irregularities andchanges in cross-section, as well as the crucial area of

jointing(solderjoints!)can be a majorproblem.

Failure by fatigue is the result ofprocesses ofcrack nuc leation andgrowth, or, in the

case ofcomponents which maycontain a crack introducedduring manufacture, the

result ofcrackgrowth onlybrought aboutby the application ofcyclical stresses. Theappearance of a fatigue fracture surface isdistinctive an dconsists of two portions, a

smooth portion, often possessingconchoidal, or mussel shell, markingsshowing the

progress of the fatigue crack up to the moment of final rupture, and the final fast

fracture zone.

At higher magnifications, using a scanning electron microscope, fatigue striations

can be observedbelow. Each striation is thought to represent the advance distance

of the crack frontduring a single loadcycle.

SEM image of fatigue striations x 1000

An importantpointregarding fatigue f ailure is thatbeach marksdo not occur on the

region over which the final rapid failure occurs. Thisregion will exhibit eitherductile

orbrittle failure evidence ofplasticdeformation beingpresent forductile, and

absent forbrittle failure.

C

rrosive attack.

Corrosion fatigue damage occurs more rapidly than wouldbe expected from the

individual effects of fatigue orcorrosion. In general, different environments have


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different effects on the service life of a given material. Similarly, the corrosionfatigue behaviour ofdifferent materials is usuallydifferent in the same environment.

The behaviour established for a given material/environmentsystem or for a given

set of testconditionscannotbe applied indiscriminately to othersystems or

conditions.

The corrosion fatigue behaviour of metallic materials has hadconsiderable attention

from researchers over the years, throughout the world. Most engineering materialsare, to a greater or lesser extent, susceptible to corrosion, in the form of either

general or localizedcorrosion. The reduction in fatigue lifetime ofcomponents as a

result ofpresence of an aggressive environment is becomingverycommon. Thisemanates from the fact thatcorrosion fatigue isresponsible for manyservice failures

in a wide variety of industries , including aircraftdesign. This has led to an important

consideration to be encounteredbydesign engineer s forsaferdesign.

Several studies have examinedpit initiation andg rowth behaviourduring the

corrosion fatigue process. It is well established thatcorros ion pit initiation andgrowin the earlystages of the corrosion fatigue process. Corrosion fatigue cracksstart to

grow from these corrosion pits andcause the final failure of the metallicstructures .In the studies of the fatigue andcorrosion fatigue, crack initiation mechanisms

aimed to identify the preferential sites forcrack initiation and microstructural

particularities and/orpeculiarities associated to these sites. These sites are possiblyresponsible forpremature fatigue orcorrosion fatigue crack initiation andcontribute

to reducing the fatigue life of the alloys. It hasbeen identified that the main stages

ofdamage leading to environment-assisted fatigue failure from defect-free surfacesinclude: breakdown of the surface passive film, pitdevelopment andgrowth,

transition from pitting to cracking, crackgrowth andcrackcoalescence. The

environment acts on the material through the surface, producing un iform orlocalizedchemical attackbydiffusive mass transfer. All alloys used in engineering

developsurface passive films as a result ofsurface oxidation duringprocessing. Thedegree ofprotection given by a surface film depends on the diffusion rate s of

various environmental constituents through the film and on the stability of the

coating itself against environmental attack. In carbon steel with little or any alloyingadditionsshows weakpassive behaviour and isconsidered active when immersed in

environments asbenign as water. In these alloys, corrosion occursveryquickly

following the immersion in aqueous environments. Addi tions ofsufficient alloying

elementssuch as Cr, Cu and Ni improve the corrosion resistance through theformation of a tightly adhered mixed oxide film on the surface of the alloy. This

increases the pittingresistance and more aggressive environments are required tobreakdown the oxide film.

The pits formations are a majorconsideration for engineeringcomponents with high

integritysurface finish. If a residual or applied mechanical stresses occur togetherwith an aggressive environment, the earlydev elopment ofpits andsubsequent

crackscan play a majorrole in the total lifetime of a component. Various factors areimportant to corrosion fatigue behaviour. The effect ofdecreasing frequency on pit

nucleation andgrowth can be observed in Figure below.


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Corrosion pits are typicallysmaller than a millimetre in depth andserve as micronotches with locally elevates the stress level. Furthermore, the pH level of thecorrosive environmental inside the pitcan be more acidic than that in the bulk,

causingpossible acceleration in the rate of fatigue crackgrowth. Once that thestage ofdevelopment andgrowth ofpits and the initiation of a crack from a pit

happened, the subsequentstage in the accumulation ofdamage under environment-

assisted fatigue following the stage of the transition from a pit to a crack. Figure

below shows the transition from a pit to a crack.

4. Eva uate t e et ods t at could beused to provide anestimateof t e

lifeof t ecomponent.

A simple method of estimating a components life it to carry out fatigue tests on it.

Bysetting up a testrig that will replicate the typical forces the component will sufferunder normal in service use. The turbine blade wouldbe attached the testrig. Thenyou would have actuatorscreating forces up anddown on the blade, usingstrain


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gauges to measure the forces applied. The actuators wouldbe set to cycle throughloadcasesdeterminedby the operator.

The aim of the rig testing is to establish a set ofpoints along an S-N Curve. By

setting the maximum force to say 400MPa, and using a frequency that wouldbe

typical to the componentyou can determine how manycycles it takesbefore

failure. Thiscan sometimesbe a very longprocess and often a high frequency i sused to complete the testdata in less time. This would then be repeated for

different forces, possibly 350, 300, 250, 200 MPa etc; until a goodcurve can beproduced, and extrapolateddown until maximum cycles at low forcescan be

observed.

This isdone because it would take far too long to actually test with these

conditions, it would take too long. Also after a certain point the curve or line

becomesrelativelyconstant.

5. Suggest suitablemet ! ods forimproving t ! e servicelifeof t ! ecomponent.

Suitable methods for improving the service life of the component wouldbe toanalysis the testing that isdone on a previousproduct and look at areas thatcould

be improved and modified to create a superiorproduct. Other ways wouldbe to

ensure that the points mentioned in the list forquestion 1 are all considered in thelife of the blade. Materialscould also be changed and new alloys with specific

propertiescouldbe used to increase the life of the component.

To reduce the effect ofcorrosion addition ofsufficient alloying elementssuch as Cr,Cu and Ni improve the corrosion resistance through the formation of a tightly

adhered mixed oxide film on the surface of the alloy.

Bibliographyhttp://www.ami.ac.uk/courses/topics/0124_seom/index.html

http://www.iasmirt.org/iasmirt-2/SMiRT19/S19_FinalPapers/G05_2.pdf

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FOR REFERENCE ONLY Unit 23 Assignment 4