Fatigue of compositesComposites Solutions Ltd Company

Ebrahim Farmand ashtiani

Azadeh Khoushabi

Nov2009

Composite course report

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Introduction.........................................................................................................................3

Damage mechanisms involved in Fatigue of composites...................................................4

Effect of fiber failure strain…………………………………………….………..5

Effect of fiber stiffness………………………………………………...….…….6

Effect of matrix ductility………………………………………….……...……..6

Cross-ply laminates……………………………………………………..….....…7

Angle-ply laminates………………………………………………….…..…...…8

Woven fabric composites…………………………………………….…..….…..9

Wind turbine rotor blades……………………………………………………...........…10

Motivation…………………………………………………………….......…..10

Machine design……………………………………………………..….….......10

Service condition……………………………………………………..……….10

Material requirements……………………………………….………….…..…11

Composite design……………………… ……………………………….…...12

Summary and conclusions.................................................................................14

Helicopter rotor blades……………………………………………………………..…...15

Motivation…………………………………………………………………..…15

Machine design………………………………………………………….…..…15

Service condition…………………………………………………… …..……15

Material requirements……………………………………………….…….…...16

Composite design…………………………………………………………..….16

Process................................................................................................................17

Summary and conclusions..................................................................................18

Bibliography…………………………………………………………………………………………………………..…...19

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Introduction

In the last few decades, increasing demand for high strength-to-weight ratio materials for construction industry and advanced applications like aerospace engineering has led to the increasing utilization of fibre-reinforced polymeric composites. In the majority of engineering applications vibration and other fluctuating loads are mostly inevitable; in the other hand, the mechanical fatigue is considered to be the most common type of failure of structural components; thus for an appropriate and efficient material selection, dynamic properties of materials especially fatigue life, strength and mechanisms should be considered.Fatigue of FRP composites is a more complicated phenomena (comparing to the metals fatigue) due to the fact that Different damage mechanism and degradation can contribute to the fatigue failure of FRP composites e.g. fibre fracture, matrix cracking, fibre buckling and their interactions. Lots of parameters can affect fatigue behaviour of FRP composites including Type of fibre reinforcement, type of resin, fibre volume fraction, and interfacial strength. As well as fatigue usual parameters like loads magnitude and frequency, specimen shape and R ratio. However lots of research has been done in order to control damage initiation and damage evolution in The aim of this report is to review design and selection of polymer matrix composites in fatigue performance point of view so first different mechanisms behind fatigue of these materials will be discussed and then two situations where fatigue of this type of composites is a critical issue will be considered:Wind turbine blades and helicopter rotor blades, which correspond to a different fatigue conditions. (fig 1)

Fig1.different fatigue conditions

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Damage mechanisms involved in Fatigue of composites

Fig.2 shows the fatigue life diagram of a unidirectional composite loaded in parallel cyclic tension, the horizontal axis of the diagram is the logarithm of the number of load cycles to failure. The vertical axis is the maximum strain reached in the first load cycle.

According to this fig three regions are shown: Region 1: scatter band connected with fiber failure and is centered about the composite strain to failure, εc

Region 2: sloping scatter band reaches region 1 at certain low number of cycles and approaching a fatigue limit, εm, at a large number of cycles. Region 3: region below fatigue limit, no failure happens at any number of cycles which considered for most applications.

Fig.2 fatigue life diagram of a unidirectional composite loaded in parallel cyclic tension

At region 1 a certain fibers’ number of specimen which is exposed to the first-cycle maximum strain are likely to have failed at different sites without critically weakening any special part (fig.3a) ,at the second –cycle more fibers, mostly at the neighborhood of the first failed fibers, are likely to fail (fig.3b).at the following if matrix could flow ,the redistribution of stress resist the failure but a composite with stiff, brittle fibers will allow unstable growth of a small matrix crack formed by a core of fiber failures (fig.3c)

a b c

Fig.3 the fiber failure mechanism in region 1

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At region 2 ,the first-cycle strain is not enough to cause wide-spread fiber failure, by following cycle, matrix fatigue cracks are developed and grow by failing and also by debonding of fibers in their way ,these matrix cracks are bridged by fibers (fig.4a) .the failure happens from a n enough large crack that would grow unstably at the applied load.

a b

Fig.4 a.the fiber-bridged matrix cracking mechanism at region 2b.the matrix cracking mechanism at region 3

As shown in Figure 2, The lower end of this region band reaches εm which is the fatigue limit of the matrix material under strain-controlled fatigue.

At region 3, the strains are lower than the matrix fatigue limit, so the cracks can not grow anymore, the strain is unable to cause significant numbers of fibers to fail, and therefore there are small opening displacements of matrix cracks that can not lead to failure. (fig.4b)Base on what mentioned above, the effect of Fiber failure strain, Fiber stiffness and matrix ductility on the fatigue life diagram are discussed below:

1. Effect of fiber failure strain:

The range of strains in which fatigue failure will happen is εc -εm, because the composite with same matrix have the same εm, so the fatigue behavior of a composite can vary significantly as a function of the fiber failure strain.In the composite with εc < εm , such as unidirectional ultrahigh modulus graphite-epoxy composite, only the region 1 existed at the fatigue life diagram(fig.5).although there is not a fatigue failure in this situation when the tension load is parallel to the fiber, by the loading angle greater than a few degrees fatigue failure will happen because of shear stress between fibers andmatrix which cause interfacial damage.

2. Effect of fiber stiffness:

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Fig.5 fatigue life diagram of unidirectional ultrahigh modulus graphite-epoxy composite

Fiber-bridged matrix cracking is main mechanism in the second region thus the crack opening displacement is controlled by fibers; stiff fiber can control the crack growth. So the crack growth will be a function of fiber stiffness. On the other hand the fatigue life is reversely a function of crack growth. So the more stiff fiber the more fatigue life. On the other hand, as the fatigue life is increased by stiff fibers, the fatigue limit also increases.The effect of fiber stiffness on the fatigue life time is shown at the fig.6

Fig.6 Effect of fiber stiffness and matrix ductility on

3. Effect of matrix ductility:

If the matrix flows the crack tip will be rendered blunt, this blunting deformation should be added to the elastic crack opening displacement, resulting in higher strain on the bridging fibers, therefore crack growth rates will increase. The effect of matrix ductility is shown in fig.6

In the real case loading condition is more complicated than tension-tension loading parallel to fibers, thus at the following the tension-tension loading inclined to fibers is discussed.In this case the fiber-matrix interface is subjected to a normal stress and a shear stress. These two stresses combined would produce inter facial cracks and mixed-mode crack growth. Failure will happen when the cracks growths become unstable. At this case fiber failure is not necessary to damage the specimen, damage will happen by separation of specimen in two parts. In this case region 1 of the fatigue life diagram will disappear, the scatter band of region 2 starts at lower strain and slopes less with increasing value of the loading angle (fig.7). The fatigue limit also decreases with increasing θ angle due to increasing normal stress on the interface. (Fig.8)

Fig.7 Fatigue life diagram of unidirectional composite loaded in cyclic tension inclined to the fiber direction

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Fig.8 Variation of the fatigue limit with the inclination angle θ

In most of the structural components laminate lay-ups are used to sustain multiaxial loading, to understand the fatigue life of these composites we shall discuss on angle-ply, cross-ply.

Angle-ply laminates:When this lay-up loaded in one of the two symmetry directions of (+ - θn)s, a ply in the laminate is exposed to normal and shear stresses. The normal stress is much less damaging than shear stresses, the effect of these shear stresses is as same as inclined loading of a unidirectional composite, but the fatigue damage process former is more elaborate than in the latter, which would result in a higher fatigue resistance in the angle-ply laminates.(Fig.9)

Fig.9 (a) mixed-mode cracking in inclined loading of a unidirectional composite(b) Damage in an angle ply laminate

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At the inclined loading of a unidirectional failure happens when a single crack it reaches a critical length by fatigue and grows unstably. But at the angle-ply laminates more several ply cracks are produce along fibers and by continuing the load cycling, the ply cracks grow into the interlaminar surface causing local delamination. The delimitation’s growth causes the separation and failure.Figure10.compares the fatigue limit variation with the angle θ in the two cases for a glass-epoxy composite. The improvement of εm due to the ply constraint effect in angle ply laminates occur for angles smaller than 60 G.

Fig.10Variation of the fatigue limit of unidirectional fiber glass epoxy loading inclined (broken line) and angle ply laminate vs. θ

.Cross-ply laminates:A study of the fatigue of graphite-epoxy cross-ply laminates was done and damage process is described as a three-stage process. (fig.11)

Fig.11 The stiffness reduction with fatigue cycles in a cross ply laminate showing stages I, II, and III of the fatigue process

Stage I: Transverse crack forms with increasing density until a saturation state is reached ,the corresponding crack density increaseStage I I: Axial splitting in the longitudinal plies happens and lead to local delamination mostly at intersection of the longitudinal and transverse cracks happen.Stage I I I: The delamination are found to grow and coalesce, forming strip-like longitudinal regions that fail by fiber breakage.

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Fig.12 shows the fatigue life diagram of the constraint of the longitudinal plies effect The location of the scatter-band in the fatigue life diagram, as the same way as the fiber stiffness affects region 2 in unidirectional composites. Thus thick, stiff plies will push the scatter-band to the right while thin, compliant plies will not produce this advantage.

Fig.12 Fatigue life diagram of a cross-ply laminate of graphite-epoxy

Woven fabric composites:Damage mechanism in woven fabric composites mainly consists of matrix cracks within the fiber bundles lying normal to the loading direction. These cracks are all over the matrix, by continuing the load cycling some of the cracks initiate local disbands so delamination happens that is different separation of plies in a straight-fiber laminate.The comparative study showed the similarity of type and progression of damage mechanism in cross-ply laminate and eight-harness satin weave composite. The difference is the regularity of the delamination in the woven composite.fig.13 shows a comparison of the normalized Young's modulus reduction of the two composites. Two curves are nearly same until about 50% of the composite fatigue lives beyond which the woven fabric composite shows greater modulus reduction, presumably due to the delamination at interlaced regions.

Fig.13 longitudinal Young’s modulus vs. fatigue cycles of a woven fabric composite and a cross-ply laminate of graphite-epoxy

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Wind turbine rotor blades

1. Motivation:

Due to the interest in Renewable energy resources, Wind energy is a fast growing market worldwide, for higher efficiency, the size of the today’s commercially erected wind turbines increased rapidly, Thus, wind rotor blades should be designed which can bear the condition. Fatigue is one of the critical problems which should be considered by designers.

2. Machine design:

Size: 11 -120 m diameter Weight: max 18 tons per rotor

Design of rotor blades: Slender structures with the aspect ratio varies between 15 and 30

Fig.14 Cross section of a wind rotor blade

3. Service condition:

Life time: about 20-30 years with more than 108-1010 load cycles Rpm: 10-20 Environmental conditions: wind turbine blades are subjected to a hostile environment throughout their service lifetimes such as: UV radiation, temperature fluctuations ,rain, ice, humidity, thunderstorms, lighting, hailstones, erosion from sand particles, and extreme dryness in desert environments Fatigue Loads: The periodic nature of the forces which causes fatigue of the blade is due to the wind speed variations versus height and time and also the circular movement of the blades that change their load condition periodically.Each blade is exposed to a flap-wise and an edge-wise bending fatigue load which are both static and dynamic and also centrifugal forces; these longitudinal tensile loads can be neglected because of the low rotational speed of the blades (Fig.15). Flap-wise bending happens as a react of the blade to the normal loads which lift the aerodynamic profile exposed to the wind, the blade is also exposed to gravity

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which cause the Edge-wise bending, this bending cause fatigue because the blade bends once at the right-hand and after rotation of 180 , ̊ at the left hand which is a completely different direction.

Fig.15 Wind rotor blade forcesShear webs convey the shear stress to the spar caps, because of this role, the fiber direction for this part should be oriented at ±45 G, and it is necessary to improve fatigue resistant of shear webs.The shell of the blade should be resistant to torsion, thus it obtains a ±45 G dominated fiber lay-up. The bending spar bear Flapwise loads thus these loads do not cause fatigue at the shell is not under the fatigue. But the spar is not stiff enough to bear the lag wise loads and shell must tolerate the main part of edge wise load.

4. Material requirements:

According to the service condition mentioned above, following properties are required: 1. High material stiffness is needed to maintain optimal aerodynamic

performance2. Low density is needed to reduce gravity forces3. long-fatigue life is needed to reduce material degradation

Evolution of materials used in wind turbine blade:Wind turbine blades have been made from metals, wood and composites. At the following table the advantages and disadvantages are indicated, which shows why composites have become the blade material of choice:

Material Advantages Disadvantages

Metal Low cost manufactured with a high degree of reliability

High weight Relatively low fatigue resistance

Wood

relatively high strength-to-weight ratio good stiffness high resilience yield

an inherent problem with moisture stability problem of joining parts for large blades difficult to obtain in reproducible and high quality

Composite

High strength High stiffness -

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high fatigue resistance

5. Composite design:

Fibers Type of the fiber: Glass fibers/carbon fibersComparing glass and carbon fiber, carbon fiber has better mechanical and fatigue behavior so carbon fibers could be considered as an alternative to the glass fibers (fig.16).

Fig. 16 effect of glass/carbon fibers on the fatigue behavior

However, technological problems and the prices of carbon fibers (about 8 times higher than glass fibers) are still hampering reasons for a wider application.

Fiber content:

The S_N behavior of composite materials at a constant R value is typically fitted below equation:σ /σ0=1-b log (N)

Where b is fatigue sensitivity coefficient and σ/σ0 is maximum normalized stress and N is cycles to failure. As it can be concluded from the equation, lower value of b corresponds to steeper fatigue curve which is associated with higher fatigue resistance. Fig.17 shows the effect of fiber content on the fatigue behavior of a family group of composites. The variation from good to poor materials is a direct result of the change in volume fraction from 31% for the good to 54% for the poor materials.

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Fig..17Extremes of normalized S_N fatigue data for a single family of fiber glass laminates with 72% 0 G plies and28 % ±45 G plies, R=0.1

Fiber architecture:

Typically the fabrics used in wind turbine application have either stitched or woven strand structures. Stitched and woven triax laminates have lower fatigue properties, this is because of local stress concentration (approximately 2.5) generated at the crossing of layers. This large stress concentration is a consequence of tying varies layers of the laminate together in order to hold the fibers very close to each other.

The influence of fiber content also in combination with the architecture of the fabric is illustrated in fig18 the main difference between AA and DD laminate is that the 0 ̊ and ±45 ̊ layers in the AA material are stitched together.

Fig.18 Fatigue sensitivity coefficient for various fiber contents in fiber glass stitched and Unstitched laminates

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Since unstitched laminates can have higher fiber content manufactures who use processes such as RIM (resin infusion molding) should pay attention to fatigue evaluation of this material.

Matrix In turbine design polyester, vinyl ester and epoxy matrix are mostly used, fig19 illustrates the effect of these resins on the fatigue behavior of a fiber glass laminate:

Fig.19 Effect of matrix material on tensile fatigue in a fiberglass laminate with 0 Gand ±45 G plies

According to the theoretical discussion, matrix with lower ductility behaves better at the fatigue.

Environmental effect on the matrix property degradation should be also considered, the matrix materials used in turbine blades absorb moisture from the humidity in the surrounding air and/or from water lying on their surfaces. As the moisture content increases, the glass transition temperature Tg of the matrix is depressed and the matrix will swell, eventually microcracks may form in the matrix, these changes may destroy the structural integrity of the laminate .these effects are significant in compression and shear where the matrix is the primary load carrier.The effect of absorbed moisture on the physical properties of a typical epoxy and polyester laminate system has been studied and it was shown that the static compressive strength was reduced approximately 30% in both cases.Good bonding with the fibers, Low shrinkage, Low viscosity (at room or moderate temperature) to improve wet ability and decreasing the process time, are other general properties required for selecting an appropriate matrix.

Summary and conclusions : High fiber contents may lead to a steeper slope of the fatigue curves: according to fig… 31%fiber content has the better fatigue resistance than others.

According to the fig… the carbon fiber is a better choice but because of its high cost and this fact that a shear web may be more fatigue-critical than a spar cap using a combination of glass fiber and carbon fiber in a hybrid construction is suggested in a way that carbon fiber used for spar scap and the other part made by fiber glass. It is better to use unstitched fibers according to fig …

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Epoxy can be chosen as a matrix because of its smaller shrinkage and more rigidity, however due to the fact that epoxy is more expensive than polyester and vinylester , Epoxy can be replaced by vinylester which also has appropriate fatigue resistance. The RIM/RTM can be the processing methods.

Helicopter rotor blades

1. Motivation:

In the middle of 80s a world-wide survey of serious aircraft accidents involving fatigue fracture indicated that the accidents cover civil and, to a limited extent, military aircraft a total of 1885 accidents since 1927 were identified as having fatigue fracture (specially in rotor blades area) as a related cause, and these accidents resulted in 2240 deaths. Until now the drive for developing composites for aircrafts rotor blades has been icreasing fatigue resistace in addition to reducig streangth to weight ratio.

2. Machine design:

Long and slender with a high aspect ratio and airfoils shape Due to the mentioned geometry Rotor aeroelastic analysis typically consider the blade to be a 1-D beam structure. A box-beam is an idealization of typical helicopter blade structure and is typically the main load carrying member in the rotor blade

Fig.20 cellular structure of helicopter rotor blade profile

3. Service condition:

Load cycles: the lades should tolerate about 108 cycles in its life time

Life time: 1,000 and up to about 2,000 hours of flight. At a modest cruise speed of 65 mph, this equates to between 65,000 and 130,000 miles of flight.

Rmp: very depended on the helicopter class mostly in the range of 500-16000

Fatigue Loads: The helicopter rotor is exposed to a highly dynamic and unsteady aerodynamic environment leading to severe vibratory loads on the rotor systems which can induce damage in the composite rotor blade.Flap (out of plain bending) and lag (in plain bending) deflections, twist deflection at the tip are fatigue stresses which should be considered.Helicopter rotors are subject to 1/rev loads as the primary source of fatigue. This fatigue mechanism is due to the time-varying velocity experienced by the blade section in forward flight, which leads to time-varying aerodynamic forces.

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4. Material requirements: 1. Higher stiffness2. Higher fatigue resistance3. Low weight 4. Corrosion resistance5. Electromagnetic transparence

Evolution of materials used in wind turbine blade:in the 1960s most helicopters had rotor blades made from metallic materials (such as aluminium)nowadays blades last about 20 times more than before the principal reason for this is the much better fatigue performance of composite materials (comparing to metals), here used in an application where designers must allow for tens of millions of fatigue load cycles.

5. Composite design: Fibers Type of the fiber: The number of fibres in use today has escalated, with many ceramic and polymer fibres such as aramids, silicon carbide, quartz, polyethylene, alumina, etc. also being available today. Carbon fibre continues, however, to dominate the aerospace composites market, followed by glass fibres, often used for its radar transparency for radome applications.At present, the compressive properties of carbon-fibre composites are a factor limiting their wider usage in aerospace applications. In fatigue loading, carbon composites are much more sensitive to compression loading than to tension. Ultimately, the worst fatigue case for carbon composites is fully reversed fatigue or tension–compression loading. Aramid fibres can be considered as an alternative option, which is because of their high stiffness and good fatigue resistance.Taking into account all of these factors, hybrid structures seem appropriate in this case.

Matrix For unidirectional materials under tensile loading, the fibres carry virtually all the Load, the tensile fatigue behaviour might be expected to depend solely on the fibres, and since carbon fibres are not sensitive to fatigue loading, good fatigue behaviour should result.However, experimental evidence has shown that the slopes of the S–N curves are determined principally by the strain in the matrix figure.21. Illustrate peak tensile strain vs. cycles to failure

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(S–N plots) for a range of unidirectional carbon-fibre-reinforced materials.

Fig.21 peak tensile strain vs. cycles to failure for a range of unidirectional carbon-fibre-reinforced materials

The reason is this that matrix cracking is important while studying the various modes of damage, because the first failure mode observed in laminated material is matrix cracking in transverse plies. As applied load increases, two distinct types of delamination called edge delamination and load delamination (or transverse tip delamination) develop. These matrix dominated failure modes can be determined by the strength of laminate since they can cause fiber breakage in primary load bearing plies.

Process   :

The method used to lay-up the composite blades begin with the dry assembly of carbon fiber and glass fiber pre-forms. A special binder under the reinforcement layer lightly bonds the section together when heat and pressure reapplied. This facilitates the assembly of very accurate pre-formed pieces which can be stored for unlimited periods at room temperature. The pre-forms are assembled into appropriate mould halves. Which are brought together and polyurethane foam core is introduced. A special membrane layer at foam interface prevents the dry reinforcement being impregnated with foam and also provides a good interface bond. The foam-filled blade pre-form is then removed from the mould and the root fittings are assembled together with the lightning conductor braid and leading edge reinforcement layers using a binder and hot iron.The complete assembly is replaced in the blade mould for resin injection; to reduce viscosity during injection the mould is heated. The resin input feed matches the natural soak rate of the composite material. When resin appears at the mould outlet, it is clear that the injection process is complete. After curing, the blade moulding is removed and de-flashed.Afterwards the bonding of inner sleeve and blade cuff, and polyurethane spraying are carried out.The final product is illustrated in the figure.22

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Fig.22

6.Summary and conclusions :

It is decided that the main loads should be carried by two carbon fiber spars from the tip to the blade shank where their leading and trailing edges converge until they are joined and enter the machined metal root components as a cylinder. At the hub end of the cylinder the layers of carbon fiber are opened out by the insertion of glass fiber wedge.

Epoxy -Carbon fibre spars, polyurethane foam core covered with tough polyurethane are used in this system.

Aramid fibres can be considered as an alternative option, which is because of their high stiffness and good fatigue resistance.

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