Composites: Part B xxx (2013) xxx–xxx

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Composites: Part B

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Fatigue behavior of filament wound E-glass/epoxy composite tubesdamaged by low velocity impact

1359-8368/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.compositesb.2013.06.039

⇑ Corresponding author. Tel.: +90 3322231931; fax: +90 3322410635.E-mail addresses: muyaner@selcuk.edu.tr (M. Uyaner), memduhkara@selcuk.e-

du.tr (M. Kara), aykutsahinn@hotmail.com (A. S�ahin).

Please cite this article in press as: Uyaner M et al. Fatigue behavior of filament wound E-glass/epoxy composite tubes damaged by low velocityComposites: Part B (2013), http://dx.doi.org/10.1016/j.compositesb.2013.06.039

Mesut Uyaner a,⇑, Memduh Kara b, Aykut S�ahin c

a Selcuk University, Engineering and Arch. Faculty, Dept. of Material and Metall. Eng., 42075 Konya, Turkeyb Selcuk University, Kadinhani Post Secondary Vocational School, Kadinhani, 42800 Konya, Turkeyc The Graduate School of Natural and Applied Science of Selçuk University, Department of Mechanical Engineering, 42003 Konya, Turkey

a r t i c l e i n f o a b s t r a c t

Article history:Received 7 January 2013Received in revised form 31 May 2013Accepted 20 June 2013Available online xxxx

Keywords:A. Polymer–matrix composites (PMCs)B. FatigueB. Impact behaviorD. Mechanical testingS–N diagram

In this paper, filament wound glass fiber reinforced plastic (GRP) tubes were studied. Impact tests wererealized at 5 J and 10 J energy levels. Force–time and force–displacement curves were obtained and dam-age zones were also examined. The burst strengths of tubes were found in accordance with ASTM-D 1599before and after impact. For fatigue tests, PLC controlled hydraulic inner-pressure fatigue test rig was setup. Damaged and non-damaged tubes were subjected to the fatigue test in accordance with ASTM-D2992. The samples were exposed to five different stress levels; 30%, 35%, 40%, 50%, 60% of the burststrength. Number of cycles occurred up to the final failure was recorded and S–N diagrams were plotted.The burst strength and the fatigue life of the damaged GRP pipes were decreased as the impact energyincreased. Also the decrease in the fatigue life was greater than that of the burst strength.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Glass/epoxy composite pipes are commonly used in defense,aerospace and aviation industry due to their high strength toweight ratio. Because of this property, it is strategically importantto optimize the manufacturing and mechanical properties of theseadvanced composites. At the design stage, it is equally important toknow the material mechanical properties under repeated loading(fatigue) conditions and failure behavior a priori. There are a signif-icant number of experimental and numerical studies in the openliterature dealing with glass–epoxy composites. Generally themain interest in these studies are the effects of the mechanicalproperties, fiber winding angles and surface defects on the fatiguelife of the component subjected to repeated loadings. Ellyin andMartens [1] experimentally investigated the leakage behaviorand biaxial fatigue behavior of a filament-wound multi-directionalglass-fiber/epoxy pipe subjected to various hoop and axial load-ings. Their procedure for determining the long-term strength of acomposite pipe was based on ASTM Standard D2992. They con-ducted the fatigue tests by cycling the internal hydrostatic pres-sure at a rate of 25 cycles per minute over the full pressurerange (i.e. R = 0.05). Tarakçıoglu et al. [2] studied the effect of sur-face cracks on the strength of glass/epoxy filament wound pipes

both theoretically and experimentally. They obtained fracturetoughness values for composites having several winding anglesexperimentally. The test samples are selected from filamentwound GRP composites which are mainly used for bazooka launch-ers. Rebiere et al. [3] showed the influence of transverse and longi-tudinal cracks on the stress field distribution and on the stiffnessloss and Poisson ratio reduction of a cross-ply laminate subjectedto fatigue tests. Many researchers showed that as the load and cy-cles to failure increased, the longitudinal cracks increase [4,5].Yuanjian and Isaac investigated the fatigue of impacted hemp fibermat reinforced polyester in tensile–tensile mode [6]. Generally, fil-ament wound composites are tested under uniaxial fatigue condi-tions [7,8]. According to Ferry et al. [9] fatigue resistance of acomposite material is based on the resistance against failure ofthe matrix. According to O’brien and Reisfinder [10], rigidity de-creases as failure starts to develop. Hwang and Han [11] reportedthat the decrease in the secant modulus is determined by failuredeveloping locally. It is shown that if the frequency at which thetest were conducted is low, material is subjected to creep ratherthan fatigue since a low frequency means extended period of load-ing [12]. Richard and Perreux [13] investigated fiber reinforcedcomposites with a reliability method for optimizing the resistanceto the failure. Wang et al. [14] studied the failure behavior of fila-ment wound composites (GRP) pipes under internal pressure. Thefatigue behavior of surface-notched glass/epoxy composite pipeswas investigated experimentally by Günaydın et al. [15]. In theirstudy, surface-notched composite pipes were repaired using

impact.

2 M. Uyaner et al. / Composites: Part B xxx (2013) xxx–xxx

two- to seven-layer glass/epoxy composite patches and compositepipes were subjected to cyclic internal pressure to investigate theeffectiveness of the repair.

Damage of the GRP pipes during the manufacturing process,assembly and maintenance significantly affects the fatigue life ofthe component. There is a recent study by Sari et al. [16] aboutthe fatigue life of (±55�)3 GRP closed-ended composite pipes whichis somewhat similar to our study. However the main concern ofthis paper is the burst strength and fatigue life of filament woundE-glass/epoxy tubes. The major difference is that all the tests wereperformed in open ended condition in this study. The projectilemass, the strike velocity and the specimen characteristics are alsodifferent from the Ref. [16]. Note also that they did not conform toany standards in performing the monotonic and dynamic tests.Therefore the results of this study and the Ref. [16] cannot be com-pared directly.

2. Materials and method

2.1. Material properties

The tubes that were used in the experiments of this study arecomposed of 8 layers of ±55� glass–epoxy filament wound compos-ite tubes. Mechanical properties of the fiber and the resin were gi-ven in Table 1 and the geometry of the composite tubes was shownin Fig. 1. The tubes are placed in the vee specimen holder as shownin Fig. 2 and subjected to low velocity impact tests.

The fatigue tests for five different stress levels (30%, 35%, 40%,50%, 60% of the burst strength) were separately repeated threetimes for each undamaged and damaged specimens (0, 5 and10 J). That is, 15 tests were totally performed.

2.2. Mechanical tests

The test machine used in this study has the ability to record thedamage from initiation to failure for various impact energies. Theimpactor was 6.35 kg in weight and a semi-spherical geometry inshape, having a tip radius of 24 mm (Fig. 2). The low velocity im-pact tests were carried out at 5 and 10 J of impact energies.

In order to determine the monotonic burst pressure values,internal monotonic pressure tests were performed according tothe standard, ASTM D1599 [17]. In the experiments conductedaccording to this standard, the final damage (explosion) shouldhappen after 60–70 s. In all of the experiments conducted in thisstudy, the mentioned rule has been complied. The maximum pres-sure value reached was recorded as the burst strength of the corre-sponding specimens.

The fatigue tests were conducted according to ASTM D2992[18]. The composite pipe was placed inside a mica cabin after theapparatus that provides the internal pressure is fixed into it(Fig. 3). After setting up the connection of the composite tube withthe hydraulic power unit, the fatigue tests were done using the PLCcontrolled fatigue testing instrument with internal pressure. Firstthe maximum and the minimum value of the internal pressure tobe used in the fatigue tests were entered to the PLC unit. Five dif-ferent values of the maximum pressure were applied correspond-ing to the 30%, 35%, 40%, 50% and 60% of the monotonic burstpressure of the experimental specimens. The force ratio of

Table 1Mechanical properties of the fiber and the resin.

E (GPa) rTS (MPa) q (g/cm3) et (%)

E-glass 73 2400 2.6 1.5–2Epoxy resin 3.4 50–60 1.2 4–5

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R = rmin/rmax = 0.05 and the frequency of f = 0.416 Hz (25 cyclesper minute) were used in the fatigue tests. The hydraulic systemwas designed such that it could be possible to test three specimenssimultaneously.

The open-ended internal pressure test apparatus used in bothmonotonic and dynamic testing is shown in Fig. 4.

3. Results and discussions

3.1. The results of the low velocity impact tests

The tests were carried out at 5 and 10 J of impact energy levels.In order to obtain these energy levels, the height at which theimpactor should be released from was calculated theoretically.The impactor mass was 6.35 kg and the potential energy of theimpactor was:Y¼ m � g � h

It was found that the height should be 80.26 mm for impact energyof 5 J and 160.5 mm for impact energy of 10 J. The specimens wereimpacted at the mid-point using a semi-circular shaped impactor.After the first impact, the impactor was not let to bounce back toprevent consecutive damage. The contact force was measured usingthe force sensor from the beginning to the end of the impact and itis plotted in the contact force–time graphs. Contact force versus dis-placement and the absorbed energy versus time graphs were ob-tained by performing the kinetic analysis of the impact [19].

The contact force–time histories at different energy levels forGRP tubes are shown in Fig. 5. It can be seen that as the impact en-ergy increases the peak crush force increases. It was also found thatas the impact energy increases, the contact time also increases.Note that there is much more oscillations in the curve belongingto 10 J which is indicative of more damage occurrence at this en-ergy level.

Fig. 6 shows the contact force–displacement curves at differentenergy levels for the GRP pipe. The area under the force–displace-ment curves gives the corresponding absorbed energy by the tubes.As the contact force increased the maximum displacement valuealso increased. The change in the slope of the curves indicatesthe damage (delamination and matrix cracking).

3.2. Damage assessment

After impact, high-resolution photographs of the damaged re-gions of the test specimens were taken using a simple backlightingmethod [19] which consists of inserting a strong light source insidethe tube. GRP specimens were prepared by cutting coupons fromthe damage regions, examined under an optical microscope using8� magnification and transverse cross-sectional images were ex-tracted. The damaged samples were subjected to ignition testASTM D2584 standards [20] and the fibers were examined.

When the damaged regions of the samples were examined, itcan be clearly seen that as the impact energy is increased, the dam-aged regions expand. The center of the damaged region is the re-gion where the impactor penetrates the specimens. Penetrationof the impactor causes local crushing of the matrix. The perimeterof the damaged region is the region where the delaminations takesplace in all or most of the layers [19].

The damage zones are shown in Fig. 7a and b for low velocityimpacted samples at 5 J and 10 J impact energy levels respectively.

In Fig. 8a and b, the transverse cross-sectional images obtainedby using the optical microscope are given. Through the thicknesscross-sectional views of the entire damaged regions could not beviewed fully at one shot. Therefore three images were taken for

t wound E-glass/epoxy composite tubes damaged by low velocity impact.9

L = 300 mm d= 72 mm t = 3mm D= 78 mm n = 8 plies

Fig. 1. Geometry of the tube.

Fig. 2. Low velocity impact test rig.

M. Uyaner et al. / Composites: Part B xxx (2013) xxx–xxx 3

each damaged region and these images are combined side by sideto obtain one full image of the damaged regions.

When the cross-sectional views are examined it can be seen thedamage is consisting of the myriad radial matrix cracks and delam-inations through the tube thickness. In the layered compositematerials, the bending rigidity of the layers differs significantlydue to different fiber orientation between the interlayers. The mostimportant cause of the delamination is the difference in the bend-ing rigidity between the layers and the shear stresses due to bend-ing. In the production of the tubes with filament winding, ±55�double layer was not processed in one step; rather, it was com-pleted in several steps. Composite tubes having (±55�)4 windingorientation angle was manufactured in four steps. In each step asingle package of double layer was manufactured. In high energyimpact tests (10 J) in addition to the delaminations between thesepackages, extra delaminations occurred in the ±55� layers insidethe package. In thick layered composites matrix cracking occursin the first layer struck by the impactor. As indicated by Abrate[21], the damage in thick layered composites progresses in theshape of a top-down view of a pine tree. This was also observedin the high level of impact energy tests. Even though very few ra-dial cracks were observed in the layers of 5 J impacts, in 10 J impacttests the amount of radial cracks were much greater than that ofthe 5 J impacts. These cracks significantly affect the fatigue life ofthe specimens. In the ignition test of the impacted tubes, it is ob-served that there were no fiber failures in the damage zone.

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3.3. Open-ended monotonic tests

Non-impacted and impacted tubes at 5 J and 10 J are subjectedto monotonic internal pressure tests and the burst strengths weredetermined. The maximum pressure value is recorded as the burststrength of the corresponding specimen subjected to explosiontests conducted according to ASTM D1599 [17] standards. Theburst strength of the non-impacted and impacted tubes at 5 Jand 10 J were found to be 457.5 MPa, 450 MPa and 442.5 MParespectively. These results show that increasing impact damagedue to the increasing value of the impact energy decreases theburst strength in the tubes. Since as the impact energy increasesthe delamination and the radial cracks in the layers increase.

3.4. Fatigue damage analysis

Tests were conducted in order to determine the fatigue life ofthe GRP tubes having ±55� winding angle non-damaged and dam-aged, with damage at 5 J and 10 J of impact energy level. Internalpressure was applied to the tube with a load ratio of R = 0.05 andat 30%, 35%, 40%, 50% and 60% of the monotonic burst pressures.The fatigue life graphs based on the final failure are presented inFigs. 9, 11 and 13 for the 30%, 35%, 40%, 50% and 60% of the mono-tonic burst pressures, respectively. In these figures, the maximumapplied hoop stress (S) is plotted versus the number of cycles to fi-nal failure (Nf). Also represented in the graphs is the least squaresregression line of the data.

The fatigue life of the specimens was decreased by the lowvelocity impact damages. This decrease in life was much more pro-nounced than the decrease in the burst strength. It was found that,if the fatigue lifetime data of impact damaged specimens were nor-malized against the post-impact residual tensile strength, then alldata points lay close to a common S–N curve. This implies thatresidual fatigue lifetimes of damaged specimens could be predictedfrom the knowledge of their residual strength and the S–N curvefor undamaged material [6]. This parallelism between the residualstrength and fatigue life is not seen in our study. The reason is thatthese composite tubes are mainly designed for withstanding theinternal pressure of the fluid carried by the tube instead of loadbearing capability. Damages seen in the tubes are sweating, leakingand explosion [18] and early development of these damages re-duces the fatigue life.

The most important factors affecting the fatigue life are delam-inations occurring in the samples and cracks in the matrix. Thesedamages allowed the pressurized oil to reach the surface at low fa-tigue cycles. Since the damage at 10 J impact energy is much se-vere, the fatigue life of the samples significantly decreased.

3.4.1. Non-impacted GRP tubeIn the fatigue analysis of non-damaged GRP tubes without dam-

age, it was observed that the fibers were aligned in the tangentialdirection. As a consequence of this the length of the tube got short-ened, however its diameter was increased. The fibers were stressed

t wound E-glass/epoxy composite tubes damaged by low velocity impact.9

0

500

1000

1500

2000

2500

3000

0 2 4 6 8 10

Time [ms]

Con

tact

forc

e [N

]

5 J10 J

Fig. 5. Contact force–time histories at different energy levels for GRP compositetubes.

0

500

1000

1500

2000

2500

3000

0 1 2 3 4 5 6Displacement [mm]

Con

tact

forc

e [N

]

5 J10 J

Fig. 6. Contact force–displacement curves at different energy levels.

Cooling unit

Oil tank

PLC control

unit

Pressure safety valve

Pressure transducer

Valve

Fig. 3. PLC controlled fatigue testing instrument.

GRP PipeSteel Seal

Fig. 4. Open-ended internal pressure test apparatus.

4 M. Uyaner et al. / Composites: Part B xxx (2013) xxx–xxx

back and forth in the tangential direction, shear and compressivestresses developed at the points where the fibers intersect.

Due to these stresses, delaminations and debonding wereobserved. These damages that appear as whitening propagated in

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fiber directions and consequently matrix cracking was observedwith increased cycles. When the matrix cracked, the first leak fromthe specimen was observed as a droplet. As the cycles increased,the leak became in the shape of a water jet, and after a few cyclesthe specimen got exploded. In Fig. 9, fatigue stress versus cycles to

t wound E-glass/epoxy composite tubes damaged by low velocity impact.9

Center of the impact

Damage zone

(a) (b)Fig. 7. Damage zones for low velocity impact tests at (a) 5 J and (b) 10 J.

(a)

(b)

Delamination

Radial matrix cracks

Center of the impact

t

Fig. 8. Sectional views of (a) 5 J and (b), 10 J (Magnification: 8�).

050

100150200250300350400450500

1 10 100 1000 10000 100000 1000000

Number of cycles to final failure, Nf

App

lied

max

hoo

p st

ress

, S [M

Pa]

Monotonic burst point

Fig. 9. S–N curve for non-impacted GRP tube.

M. Uyaner et al. / Composites: Part B xxx (2013) xxx–xxx 5

final failure (S–N) curve for the fatigue experiments of non-im-pacted GRP tubes is given. At higher stresses (about 60% of burststrength) the diameter of the specimens got increased and thelength of the specimens got decreased much more than that atlower stresses. Because of this the compressive and the shear stres-ses increase. This in turn decreases the fatigue life of the samples.As the stress levels decreased, the fatigue failures were observed athigher cycles. Namely, final failures for the 30%, 35%, 40%, 50% and60% of monotonic burst pressure were realized at 582,300,134,037, 49,270, 5514 and 754 number of cycles, respectively.

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In Fig. 10, damage zones at 30%, 35%, 40%, 50% and 60% of mono-tonic burst pressure levels for fatigue test specimens of the non-impacted GRP tubes are shown. As the magnitude of the cyclic fa-tigue stress level is increased, the damage zone is increased. At allof the stress levels, the final failure was occurred in the form of aviolent explosion.

3.4.2.5. J impacted GRP tubeDelaminations and radial matrix cracks were observed at the

layers close to the inner surface of the specimens that are impactedat 5 J impact energy (see Fig. 8). The impact damage affected thefatigue behavior of the GRP tubes. During the fatigue test, compres-sive and shear stresses developed due to the internal pressure in-creased the delaminations and radial matrix cracks. In thefollowing cycles, debonding of layers at happened the impact dam-aged zones and fiber matrix debonding occurred. The pressurizedoil leaked from the matrix cracks to the surface of the tube andthe first leakage appeared to be in the form of a droplet. After awhile, the leak at the impact damaged zone intensified and becamean oil jet due to the growth of matrix cracks and enlargement ofthe delaminations and finally exploded from the impact damagedzone. The damage in the 5 J impact damaged samples happenedat low cycles and at the impact damaged zone compared to thenon-impacted samples. In Fig. 11, the S–N curve for the fatigueexperiments of impacted GRP composite tubes at 5 J of impact en-ergy is given. As the stress levels decreased, the final damage oc-

t wound E-glass/epoxy composite tubes damaged by low velocity impact.9

Fig. 10. Damaged zones at (a) 30%, (b) 35%, (c) 40%, (d) 50%, and (e) 60% monotonic burst pressure levels for fatigue test specimens of the non-impacted GRP tubes.

050

100150200250300350400450500

1 10 100 1000 10000 100000 1000000

Number of cycles to final failure, Nf

App

lied

max

hoo

p st

ress

, S [M

Pa]

Monotonic burst point

Fig. 11. S–N curve for impacted GRP tubes at 5 J of impact energy level.

050

100150200250300350400450500

1 10 100 1000 10000 100000

Number of cycles to final failure, Nf

App

lied

max

hoo

p st

ress

, S [M

Pa]

Monotonic burst point

Fig. 13. S–N curve for impacted GRP tubes at 10 J of impact energy level.

6 M. Uyaner et al. / Composites: Part B xxx (2013) xxx–xxx

curred at higher cycles as in the case of non-impacted samples.Namely, final failures for the 30%, 35%, 40%, 50% and 60% of mono-tonic burst pressure occurred at 113,285, 33,422, 22,344, 3242 and183 number of cycles, respectively.

In Fig. 12, the photographs of the damaged zones at variousmonotonic burst pressure levels for fatigue test specimens of theimpacted GRP composite tubes at 5 J of impact energy are given.As the stress levels increase, there is a greater final damage re-sulted in the specimens. In all of the samples the final damagewas in the form of a loud explosion. Again the cycles to final dam-age were less than that of the non-impacted specimens.

3.4.3.10. J impacted GRP tubeThere is a big delamination region in 10 J impact damage spec-

imens. Also, radial matrix cracks formed in the most of layers(Fig. 8). The fatigue behavior of the 10 J impacted samples is similarto 5 J impacted specimens. Final failure in 10 J impacted specimens

Fig. 12. Damaged zones at (a) 30%, (b) 35%, (c) 40%, (d) 50%, (e) 60% monotonic burst presof impact energy.

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happens at low cycles due to big delamination regions and exces-sive radial matrix cracks. Fig. 13 shows the S–N curve for the fati-gue experiments of impacted GRP composite tubes at 10 J ofimpact energy level. Again as in the non-impacted and impactedat 5 J samples, as the stress level decreased, final failure occurredat high cycles. Namely, final failures for the 30%, 35%, 40%, 50%and 60% of monotonic burst pressure were realized at 43,308,8210, 1953, 322 and 77 number of cycles, respectively.

The photographs of the damaged zones for fatigue test speci-mens of the GRP tubes impacted at 10 J of impact energy are givenin Fig. 14. In the fatigue tests at low levels of burst pressure, the fi-nal damage was not in the form of an explosion. After a certainnumber of cycles, impacted zone splitted and final damage oc-curred as an intensive oil jet. The final failure of the fatigue testsat high level of burst pressure happened in the form of laudexplosion.

sure levels for fatigue test specimens of the non-impacted GRP composite tubes at 5 J

t wound E-glass/epoxy composite tubes damaged by low velocity impact.9

Fig. 14. Damaged zones at (a) 30%, (b) 35%, (c) 40%, (d) 50%, and (e) 60% monotonic burst pressure levels for fatigue test specimens of the non-impacted GRP tubes at 10 J ofimpact energy.

M. Uyaner et al. / Composites: Part B xxx (2013) xxx–xxx 7

4. Conclusions

The fatigue behavior of the GRP composite tubes in non-im-pacted, impacted at 5 and 10 J of impact energy configurationsaccording to ASTM D 2992 (R = 0.05 and 25 cycles per minute) un-der internal pressure is given below:

� In the low velocity impact tests, as the impact energy increasedthe maximum contact force, displacement and the damage inthe GRP composite tube increased.� The result shows that for low velocity impact tests, when the

impact energy increased, the damage increased. For low impactenergy, matrix cracking and delaminations are observed in theinner layers, but for high energy impact damage are observedin all the plies through the thickness. In the non-damaged spec-imen, damage appears anywhere. In the other damage appearon the impacts.� In the specimens that are damaged by 5 J impact energy, matrix

cracks were observed on the damaged surface and delaminationsin the cross-section and radial matrix cracks on the layers close tothe inner surface. However, matrix cracks occurred both on thedamaged surface and on the inner surface of the specimens thatare damaged by 10 J impact energy. Delaminations occurred inbetween all the layers and radial matrix cracks were formed.� In the low velocity impact damaged tubes, burst strength of the

tubes decreased as the impact energy increased.� Low velocity impact damage reduced the fatigue life of the sam-

ples. This reduction was more as compared to the reduction inburst strength.� Fatigue failure of the impacted and non-impacted tubes was

observed at low cycles in high stress levels and high cycles inlow stress levels.� In the non-impacted specimens, the fatigue damage started in

any place on the surface of the specimens; however damagestarted at the contact location in the impacted samples andresulted in failure at the same place.� In the non-impacted and impacted specimens, at 5 J of impact

energy level, the final failure happened in the form of a violentexplosion. However in the samples damaged at 10 J of impactenergy, final failure was due to leakage because of the speci-mens cannot withstand the internal pressure.� One of the developments of this study could be systematic tests

with different levels of energy impacts. These results could bedirectly used in the used of tube under pressure.

Acknowledgments

This study was carried out as a Master of Science thesis by Ay-kut S�AH_IN in the Graduate School of Natural and Applied Science at

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the University of Selcuk, Konya, Turkey. This work was also sup-ported by Selcuk University Scientific Research Projects underGrant Numbers 08401118 and 10201141.

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