3 Blade Structure

8/8/2019 3 Blade Structure

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WE Handbook- 3- Structural Design

Wind Turbine Blade Structural Engineering

As we saw in the discussion on aerodynamics the blade design process starts with abest guess compromise between aerodynamic and structural e ciency. The choiceo materials and manu acturing process will also have an infuence on how thin (henceaerodynamically ideal) the blade can be built, and at what cost. There ore, the structural

engineering process has a critical role in bringing all the disciplines o design andmanu acture together and producing the optimum solution in terms o per ormanceand cost.

The blade as a structural beam

The li t orce on the blade, which drives the turbine round, is distributed along the bladeapproximately in proportion to the local radius, i.e. there is more li t orce close to thetip than there is near the hub. The reasons why this is so, and why it is desirable, are

explained in the preceding chapter.

The li t orce tends to make the blade bend. I we look at a section o the blade at somepoint along its length, all the li t orces outboard o that point will have a cumulativee ect on the tendency to bend, with those urthest away having the greatest e ect asthey have the greatest leverage. The e ect is called bending moment. The bendingmoment is greatest at the root o the blade: at this point there is more blade outboard(contributing to bending moment) than at any other point along the blade. At the tip thebending moment drops to zero.

Bending moment against radius in a large turbine blade


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So it is intuitive that the blade must be thickest, i.e. strongest, at the root and can taperin thickness towards the tip where the bending moment is less. As it happens, thatsuits the aerodynamics too: the blade needs a thinner section at the tip where dragis most critical and the local chord (width) o the blade is small. Also or turbines thatrely on stall or power regulation in strong winds, a thin section stalls more easily so isbene cial at the tip. Closer to the root the chord is wider, but to avoid making it very

wide (hence expensive) the blade needs to be thicker to generate enough li t given thelower wind speed close to the hub (thicker aero oils can generate a greater maximumli t be ore they stall).

Un ortunately the thickness needed to make the blade sti and strong enough isgreater than that required or aerodynamic e ciency, so a compromise must be oundbetween structural weight (= cost) and loss o aerodynamic e ciency.

Internal beam structure

I the blade was solid rather than hollow, the required thickness at each point along theblade would simply be determined by the bending moment at that point. But consideringhow the material in this hypothetical solid blade is working, as the blade bendsdownwind, the material on the upwind ace o the blade stretches, carrying tension,and the material on the downwind ace compresses. The material mid-way betweenthe two aces, i.e. in the middle o the blade, is neither in tension nor compression i.e.it does not do much work. So to reduce the cost o the blade it makes sense to takeout some o that material in the middle, and make the blade hollow.

In the extreme case you would be le t with two strips o material, one on the upwind

ace and one on the downwind ace. This would not work or two reasons: shearstrength and aerodynamics. The aerodynamics is obvious: there must be a continuousshell to give the aerodynamic shape. Shear strength is less obvious but is most easilyvisualised by thinking about what would happen to the two strips o material i theywere not joined by anything: they would slide relative to each other and act like twoseparate, very thin, blades. They would lose all the bending strength that we aretrying to maintain. So to work properly they must be structurally joined together;this connection is called a shear web. The classic embodiment o this concept is thesteel I-beam.

Blade with spar caps& shear web

Steel I-Beam Blade with box spar


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WE Handbook- 3- Structural Design 3

The key thing to understand about structural beams is that the material carrying bendingloads (the spar caps) should be separated as ar apart as possible but joined together bya shear web. The wind turbine blade works in much the same way as the steel I-beamexcept that there are shells around the outside that orm the aerodynamic shape. Theshells contribute some bending strength but the majority comes rom the spar caps,equivalent to the fanges o the I-beam. There are two common ways to achieve the

shear web connection: either the spar caps are built as part o the shell and a separateshear web is bonded between them, or the shear webs and spar caps are built togetheras a box spar and glued into the shell.

Laminate orientation

Modern wind turbine blades are made rom bre-rein orced plastics (FRP) owing tothe materials superior strength-to-weight ratio compared to wood and metals. FRP isparticularly suited to long, slender structures like wind turbine blades because most o

the stresses are in one direction and the bres can be aligned to suit. It is airly evidentthat most o the bres in the spar caps should be oriented along the blade, since thatis the direction o the bending loads (tension on the upwind side and compression onthe downwind side).

It is perhaps less obvious that the bres in the shear webs should be laid diagonally, sothat they meet the spar caps at 45 degrees in either direction.

Consider a ramework made o three bars, pin-jointed at their ends as shown in thediagram below.

I a load is applied to the ramework, it will de orm to a rhombus shape (shearing).

The key thing to note here is that the bars are still the same length but the (imaginary)diagonal lines are not; one is now shorter and the other longer. So the best way tosti en the ramework would be to add diagonal bars.

Shearing and rein orcement o a simple rame


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The ramework could be extended by adding more bars to orm a longer beam. Eachsection would need a pair o diagonals to give it shear sti ness. Both diagonals areneeded (rather than just one); one set works in tension and the other in compression.These orces balance out transversely to keep the spar caps a constant distance apart.Longitudinally their cumulative e ect is to prevent the spar caps rom sliding (shearing)relative to one another.

To make the spar caps and shear webs, two di erent abric styles are typically used:

unidirectionals are the best way to achieve the high axial bre content o the spar caps,while 45 stitched biaxial abric is used or the shear webs. The spar caps usuallyincorporate some biaxial to trans er load between the unidirectional bres, either byinterleaving the shear web plies (in the case o a blade with a box spar) or by includingextra plies (in the case o a blade with a separate shear web). In the latter case, careneeds to be taken to ensure su cient overlap o the shear web with the spar cap totrans er all the load through the relatively weak adhesive that bonds them together.

Optimum geometry

The ability to vary the strength o the blade without changing the outside shape (bymaking the spar caps thicker or thinner) gives us some reedom to optimise the design

or minimum cost. I the blade is made thinner, it may per orm better aerodynamicallybut thicker spar caps will be needed, making the blade more expensive. [The shear webmust also be stronger but it will be narrower too, so the total amount o shear materialis similar]. Optimum geometry is arrived at iteratively by considering turbine design,loads, structural design, and manu acturing costs.

Extended ramework with shear rein orcement


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Fatigue

It is well known that structures loaded repeatedly tend to ail at a lower load thanexpected. For metal structures the expression metal atigue is well known andunderstood and is easily demonstrated by bending a spoon back and orth until it breaks.The same e ect occurs in other materials, to a greater or lesser extent, including FRP.Wood is relatively good at resisting atigue, which is why it has been a popular choice

or smaller wind turbine blades, but woods lower strength-to-weight ratio compared toFRP precludes its use in larger blades.

Wind turbine blades are subjected to varying li t orce due to wind gusts, wind shear(blades see more wind at the top o their rotation than at the bottom), up-draught andyaw error (i the turbine isnt pointed straight into the wind, the blades angle o attack,hence li t orce, will vary as they rotate), and turbulence. The li ting orce acts in thefapwise direction (like a birds wing). The blades are also loaded in the edgewisedirection, by their own weight, which causes reversing bending moments as the blades

rotate rom one side o the turbine to the other.

Fatigue can be accounted or at a simple level by designing or a lower allowable stressin the material. Fatigue testing o small samples o material by loading and unloadingthem thousands or millions o times allows a relationship to be plotted between thestress and the number o cycles to ailure. The result is called an S-N curve:

S-N curve or a typical wind turbine blade unidirectional material

So or 0 million cycles, which corresponds roughly to 0 years use o a wind turbine,the allowable stress might be around 30% o the ultimate (once-only) strength. Thisdepends heavily on the material: carbon bres in epoxy prepreg resin may withstand

Fatigue Cycles (Log N)

SPX 8080 / EGL 1600 / 32%. FVF=54%. 24 Micron OCF R25HR=0.1 Frequency=4Hz

Remaining static strength = 97%


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50% o their ultimate load or 0 million cycles, whereas a lower quality glass brelaminate with vinylester resin might take less than 0% o its ultimate load.

Simply designing or a lower maximum stress is ne as a rst check but i atigue isdriving the design, it will not provide enough certainty. Overcoming the uncertainty byusing a larger sa ety actor (i.e. designing so the structure only sees very low stresses)

will then result in signi cant extra material cost (see sa ety actors section).

More sophisticated design approaches look at the anticipated variation o load over time(the load spectrum) and use statistical methods to calculate an equivalent number o loadcycles at each o several di erent stress values. Usually this is done using an algorithmcalled rainfow counting. The damage caused by each set o load cycles can then becalculated by seeing how much o the blades li etime has been used up on the S-N curveat each value o stress. For instance i that stress would normally cause ailure a ter 000cycles and the load spectrum predicts 00 cycles at that stress, 0% has been used up.I we make the assumption that the damage at each stress level can be added togetherto get a cumulative damage then we can see how much o the blades total atigueli e will be used up when it has been subjected to that load spectrum (this is calledPalmgren-Miners law, a ter the engineers who proposed it). So there might be 0%used up by loads at one stress level, 35% used up by more cycles at a lower load, andso on; i the total exceeds 00% the blade will ail be ore its design li e is reached.

Blade shells

Whilst the primary unction o the shells is to provide the aerodynamic shape, they

also play a structural role in sti ening and strengthening the spar, particularly to resisttorsion (twisting) loads. Just as or bending, a bigger section resists torsion better thana small one, and the blade shells are o course signi cantly larger in cross-sectionalarea than the box spar so are very use ul in this respect. Structures loaded in torsionexperience pure shear loading, so just like the spar shear webs, the shells have a highproportion o bres running diagonally.

The shells also have bres running along the length o the blade. To an extent thisassists the spar caps in fapwise bending but the main reason is to give the blade more

strength in edgewise bending as structural beams work best with the load-carryingmaterial separated as ar apart as possible (like the spar caps in fapwise bending).Looking at the cross-section below, the shells are signi cantly wider than the spar soare the most e cient way to provide edgewise bending sti ness.

Edgewise bending is primarily due to the blades own weight. Since the centre o gravityo the blade is not very ar outboard, by ar the biggest edgewise bending moment is atthe root. What this means is that, whilst the shells can support the edgewise bendingstresses or most o the length o the blade, o ten some extra rein orcement o thetrailing edge is needed near the root.


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I the blade shells were made just o FRP the thickness required to make them strongenough would be very small, only a ew millimetres. However, since there can be ametre or more o shell between the spar cap and the trailing edge, the shell would betoo fexible i it was that thin. The fexibility would be a problem both or keeping theaerodynamic shape and to resist buckling (the tendency o fexible structures to defectsideways under compressive loading).

To make it thicker with more FRP would add considerable weight (and cost) so insteadthe shells are made as a sandwich construction o FRP skins either side o a lowdensity core, usually rigid oam or balsa wood. This works along the same lines as theI-beam or box spar, with the core carrying shear loads and the skins providing bendingsti ness. The oam core can be omitted where the spar caps support the shell, indeedit is advantageous to do this as it allows the spar caps to be better separated to carrythe bending loads, which as we have already shown, is bene cial.

Root design

The blade root, as mentioned in the previous chapter on aerodynamics, generallyneeds to be circular in section to connect to the circular pitch bearing in the hub.Since the blade needs to be removable or maintenance, this is invariably a boltedconnection. I the spar was metal, it could have a fange welded to it but the sameapproach would be ine cient in FRP as the resin would have to support high stressesaround the fange corner.

Instead most blade roots consist o a thick tube o solid laminate with studs or T-boltsscrewed in or set into it in adhesive. The load can be trans erred into the studs over

their whole length, which suits the limited load capacity o the adhesive or threadedhole.

Section through blade near root showing TE tapes and cored/uncored area o shells


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Somehow in the manu acture o the blade, the I-section or box section spar must be joinedto the cylindrical laminate at the root. For a box spar it is possible to laminate the spar on amandrel that blended smoothly rom circular at the root to rectangular urther out, so that thethick root laminate could be interleaved seamlessly with the box spar. In practice this canlengthen the time taken to make the spar, owing to the large number o plies that must belaid down to make the root. The high thickness also poses some manu acturing issues with

respect to high exothermic temperatures that can occur in thick laminate sections, and thehigh quality o laminate that is required in this critical area o the structure. As a consequencemany manu acturers make the root as a separate component and bond it into the structure.Naturally i the adhesive ailed, the consequences would be terminal: in short, the jointrequires care ul design and a strong, atigue-resistant adhesive. In particular, great care mustbe taken to avoid a concentration o stress in the adhesive at either end o the joint.

Sti ness

So ar we have looked at the strength o the blade, but it needs to be sti enough too.(Strength and sti ness have distinctly di erent meanings in engineering which can bevisualised with a simple example: a rubber bungee is easy to stretch but hard to break,so is strong but not sti ; an eggshell by comparison is sti but not strong).

I the blade is not sti enough it might su er one o two problems, both due to thetower. I the tower is round, as most are, the wind orms a turbulent wake downwind oit. A turbine placed downwind o the tower would su er big variations in li t orce as theblades passed through the wake, leading to reduced power and high atigue loading.So most designs put the turbine upwind o the tower. However this means that as the

blades bend, they get closer to the tower, so must be made su ciently sti not to hit it.Designers typically aim or a minimum clearance between the blade tips and the towero 30% o the unloaded clearance under the worst-case loading.

Blades bending close to tower Image: Vestas


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It might seem that a simple solution would be to increase the initial tip clearance, eitherby putting the rotor urther upwind o the tower, tilting it out at the bottom, pre-bendingthe blades or coning (angling all the blades upwind relative to the fat rotor plane). All othese approaches are adopted to an extent but each has a cost associated with it, eitherin aerodynamic losses or greater manu acturing cost ( or instance a greater overhang othe rotor rom the tower means the nacelle yaw bearing must be stronger).

The other problem is that the blades still experience a pressure pulse rom the airfowing around the tower even i they are upwind o it. This not only contributesto the atigue loading (though less so than i the blades were downwind) but moresigni cantly can cause the blades to resonate. Resonance can be likened to pushing achild on a swing: i you push at the same requency as the swing moves, the amplitudeo the motion gets greater and greater; pushing out o sync with the swing reducesthe motion. Resonance caused by tower passing greatly increases the vibration o theblade, hence the atigue stresses on the blade, hub and bearings. So the blade mustbe sti and light enough to keep the natural requency o vibration well above the tower

passing requency. Un ortunately adding material to sti en the blade increases theblade weight, and consequently it may be necessary to thicken the shape o the bladeto address natural requency issues.

Methods to increase static tip clearance


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Since the blades are joined together at the hub, vibrations o one blade a ect the otherstoo. So their natural requencies must not coincide with the requency at which theyor their neighbours pass the tower (called the P and 3P requencies). For example, ione blade o a three-blade turbine passes the tower every second, the natural period ooscillation o the blades must be neither one second nor three seconds. For turbinesthat operate at variable speed (hence a range o blade passing requencies) this

requirement becomes even harder to satis y.

Fibre types

The most popular bres to use in FRP are glass and carbon. Carbon bres are abouttwice as strong as glass and three times sti er. Their extra sti ness also allowsthe surrounding resin to withstand atigue better by reducing the strain in the resin.Un ortunately carbon bres are much more expensive, so they tend to be used onlywhere their properties are essential or the per ormance o the blade.

In general this means carbon is used only on some o the largest turbines (over about0m diameter), and even then only on the spar caps. The reason why larger blades need

carbon more than smaller ones is that it is much harder to achieve su cient sti ness ona long blade without adding excessive weight. The extra weight not only means greatercost o material but also lower natural requency (so the tower passing requency canbe a problem) and higher atigue loading due to edgewise bending (which it will beremembered is caused by the weight o the blade fexing it one way then the other as itrotates). For these reasons the extra cost per kilogramme o carbon can be economicallyjusti ed i it allows the global weight o a blade to be reduced signi cantly.

Indeed the weight o the blade theoretically increases with the cube o its length,while the power output only increases with the square o the length. I we accept theapproximation that cost o manu acture is proportional to weight, this means that itbecomes hard to justi y making turbines larger unless the savings on cost per kilowattdue to having less towers, less generators etc can o set the additional blade cost. Toget past this hurdle it is necessary to make blades lighter, which either means makingthem thicker (and there ore less aerodynamically e cient) or using carbon.

The structural design process

Flapwise bending, due to the li t orces, is usually the most dominant loading and isknown most accurately at the outset, so the design starts by considering the spar capand shear web laminates needed to withstand it. As we stated at the beginning o thischapter, there is a trade-o between aerodynamic e ciency (thin blades) and structurale ciency (thick blades), both o which have a strong e ect on the cost o electricitygenerated. The design process there ore requires the optimum thickness distributionto be ound, by nding the e ect o varying thickness on both the power output andthe structural weight.


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There is then an iterative loop to be ollowed in the structural design because theedgewise atigue loading depends on the blades mass. Until the blade is designed, itsmass is not known so must be assumed. Once the mass is known more accurately theblade laminates may need to be re-designed to account or the new loading (derived

rom the mass). At this stage atigue may be dealt with by simply lowering the allowablestress o the material. Design codes such as Germanisher Lloyds provide conservative

de ault knock-down actors on strength to be used i atigue data is unavailable, typicallyabout 5 or glass bre and 3 or carbon at 0 million cycles.

With the basic bending parameters (fapwise and edgewise) o the blade determined,the shells can be checked or torsional sti ness and resistance to buckling. The rootcan be designed to carry the bolt loads rom the blade to the hub and the adhesive jointanalysed to keep stresses in the adhesive down to an acceptable level.

Up to this point in the design, the analysis will usually have been done using aspreadsheet or other simple analytical method, which gives only an approximate answerbut allows rapid iterations o the design towards the optimum. For the nal analysis,a more accurate answer can be provided by Finite Element Analysis (FEA). This usesa three-dimensional virtual model o the blade which is broken up into thousands osquare elements, like tiles. Because the computer can model accurately the behaviouro each element and knows how the elements are joined together, it can simulate thestructural response o the whole blade to any load scenario.

Gurits Blade Design Tool enables the shape to be matched with the mathematical approximation


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I atigue has been identi ed as potentially critical in any areas, it can be considered inmore detail once the design is close to nalised. The stress at the worst locations canbe analysed under the anticipated loading spectrum to calculate the cumulative damageover the li etime o the blade, and check that this will not cause premature ailure.

The fowchart overlea shows the structural design process and the interaction o theaerodynamic design with the static and dynamic load calculations:

Output rom FEA showing areas near the root that are closest to ailure


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Sa ety Factors

As or other engineered structures, the structural design o a wind turbine blade relieson sa ety actors to account or the unknowns: inaccuracies o the assumptions madein analysis, variations o geometry, loading and material behaviour in essence all thereal world e ects. At its simplest, a sa ety actor multiplies up the design load orreduces the design stress to achieve su ciently low probability o ailure.

Wind turbine design codes use the (more precise) method o partial sa ety actors, thatis to say the overall sa ety actor consists o several partial actors multiplied together.Each o the partial actors accounts or one e ect, e.g. uncertainty o loading, ageingo the material, manu acturing variations or material type. By using partial actors,the di erences between ( or instance) one manu acturing method or another can betaken into account more accurately than with a simple global actor. A urther actormay be added or the consequence o ailure; thus i the integrity o a particular partis essential or the turbines survival, that parts actor will be higher to reduce the

probability o ailure o the whole machine.

Testing

To achieve an acceptable (in) requency o ailures, the partial sa ety actors on materialstrength would have to be very large i no testing was done. The cheapest and simplesttests per ormed are on small samples (coupons) o the material, to establish thematerial properties, both the ultimate strength and the resistance to atigue (the S-Ncurve). From these coupon tests, statistical calculations can be used to determine thelowest likely strength o the material used in the real structure (called the characteristicvalue o strength). Several repeats o the same test are needed, on coupons taken

rom di erent batches o material, because o the natural variability o materials. Bytaking up to twenty test values or each material property, the characteristic values canbe calculated with reasonable con dence. Normally the characteristic value is chosenso that 5% o material will be at least that strong, with 5% con dence.

The partial sa ety actors are then applied to the characteristic value to give a designallowable value o strength. So the process is:

Coupon testing mean & standard deviation characteristic value partial actors design allowable

Note that in composites the strength is normally given in terms o strain (the maximumelongation that the material can withstand) rather than stress (the maximum orceper unit area). This is because laminated composites have varying stress through thethickness o the material, depending on bre orientation, which makes calculation o the

ailure load complicated. The strain however is constant or varies linearly, so analysisusing strains is more straight orward.

A ter the design is completed, a prototype is built and tested to simulate the real bladein service. One blade will be tested to the once-only ultimate fapwise load and anotherloaded repeatedly to check the atigue strength.


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Since the design li e o a blade is usually 0 years, a atigue test over the same timescalewould be prohibitively lengthy. Instead the S-N curves obtained or the basic materialsare used to calculate an ampli ed loading that could be expected to cause equivalent

atigue damage to the ull design li e a ter ewer load cycles. By doing this, and byloading the blade aster, the test can be accelerated to take only a ew months.

Summary o structural design

The structural design process must take into account the aerodynamic shape, materialproperties under extreme and atigue loading, and the manu acturing method.

Spar: Primarily the blade is loaded in bending, due to the aerodynamic li t orces(fapwise) and to a lesser extent the blades own weight (edgewise). To resistbending, unidirectional bres running along the length o the blade are placed as

ar apart as possible in the fapwise direction. These are called spar caps and tobe e ective they must be joined by a shear web comprised o diagonal bres.

Shells: The aerodynamic shape is ormed by shells which are sti ened by using a sandwichconstruction. Thin skins, usually o glass rein orced plastic, are placed either sideo a light weight oam core. The resulting sandwich construction is sti enough toresist bending due to aerodynamic pressures and buckling. With diagonal bres inthe laminate, the shell provides the blade with the necessary torsional sti ness.

Root: Where the blade is bolted to the rotor hub, the spar must be circular and thelaminate is locally thickened. For ease o manu acture, this part is o ten madeseparately and bonded to the spar and shells as a secondary operation.

Blade Testing or Design Validation and Certifcation


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Strength and Sti ness: The blade must be both strong enough not to break and stienough not to strike the tower. It must also be sti and light enough so thatits natural requency o vibration does not coincide with the requency at whichthe blades pass the tower, or resonance will occur, ampli ying the vibrationsuntil tower strike or atigue ailure occurs.

Fatigue: With a typical 0 year design li e, the blades will fex about ten million times.This weakens the material ( atigue) so or normal operating loads, the blademust be designed to a lower stress than or extreme, one-o loading situations(e.g. a 50-year gust). Fatigue testing o small coupons o material is usedto establish an S-N curve that shows how many cycles the material canwithstand at a given stress.

Fibre type: Glass bre rein orced laminates o er good strength to weight ratio. Carbonbre is more expensive but much sti er and stronger, so tends to be used or

the spar caps o longer blades.

Structural design process:

n Material properties are generated by coupon testing and reduced by partial sa etyactors appropriate to the material and manu acturing method.

n On provision o the preliminary aerodynamic pro le fapwise loads due toaerodynamic li t are used to calculate the preliminary laminate design, hence con rm

that the design can be made to work structurally within the chosen aerodynamicshape.

n Given the preliminary laminates, the mass o the blade can be used to estimateedgewise atigue loading.

n Blade shells are checked or buckling resistance and torsional sti ness.

n Manu acturing processes and material selection are de ned with the implicationson weight and cost.

n Further loading/laminate iterations converge on a nal design which is then checked

by Finite Element Analysis or sti ness, buckling stability and strength includingatigue.

n Finally a prototype blade is built and tested, both or extreme fapwise loading andatigue loading to validate the design

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