Composites: Part B 43 (2012) 1468–1474

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Composites: Part B

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Tensile behavior of epoxy based FRP composites under extreme service conditions

Fabio Nardone a,⇑, Marco Di Ludovico a, Francisco J. De Caso y Basalo b, Andrea Prota a, Antonio Nanni a,b

a Department of Structural Engineering (DIST), University of Naples ‘‘Federico II’’, Via Claudio 21, Naples, P.O. Box I-80125, Italyb Department of Civil, Architectural, and Environmental Engineering, University of Miami, 105 McArthur Engineering Bldg., Coral Gables, FL 33146-0630, United States

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

Article history:Received 30 April 2011Received in revised form 22 August 2011Accepted 22 August 2011Available online 7 September 2011

Keywords:A. LaminatesB. Environmental degradationC. High-temperature properties

1359-8368/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.compositesb.2011.08.042

⇑ Corresponding author.E-mail address: fabio.nardone@unina.it (F. Nardon

A well established technique for strengthening reinforced concrete (RC) members is based on the use ofexternally bonded (EB) FRP composites. Nevertheless, limited knowledge is available on the mechanicalproperties of FRP composites at extreme service environments. The performance of structural membersstrengthened with EB-FRP laminates exposed to extreme service temperatures or freeze–thaw cycles ismainly associated to either the bond between the FRP laminate and concrete substrate, or the mechanicalproperties of the laminates. This paper focuses on the latter aspects and presents results on a series oftensile tests of glass and carbon FRP (GFRP and CFRP) coupon specimens exposed to temperatures rangingbetween �15 and +70 �C, or after freeze–thaw cycles, including FRP specimens with different number ofplies. The experimental results for GFRP specimens indicate a low influence of ply number on FRPmechanical properties, and a minor reduction of axial tensile strength and strain with increasing the tem-perature. Results for CFRP specimens subjected to extreme service temperatures reflect a significantreduction of mechanical properties, while freeze–thaw cycles do not significantly influence the mechan-ical performance.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Externally bonded fiber reinforced polymer (EB-FRP) laminatesare effective and well established solutions to repair and strength-en reinforced concrete (RC) structural members. This evidence hasbeen translated into design guidelines and codes of practice, forexample by ACI in the USA [1] and by CSA in Canada [2]. However,an obstacle hindering the full acceptance of FRP systems in civilengineering applications is the limited knowledge of such systemsunder extreme service conditions.

EB-FRP materials may be exposed during their service life toharsh environmental conditions including freezing and thawing,extreme temperatures, solar radiation and aggressive chemical at-tack. Seasonal and daily temperature variations in continental cli-mates have the potential to cause numerous freeze–thaw cycles,resulting in differential thermal expansion between the FRP andthe concrete substrate [3]. Additionally, absorption of water andsubsequent freeze–thaw cycles may influence the material behav-ior through microstructural modifications [4]. Besides freeze–thawcycles, thermal effects may change the stiffness of the compositeconstituents and may induce thermal stresses resulting from thedifferent thermal expansion coefficients [4]. The performance atextreme service temperatures of structural members strengthened

ll rights reserved.

e).

with EB-FRP laminates is associated with the FRP-substrate bondas well as the mechanical properties of the laminates. The formerhas been recently addressed in the literature [5,6], and is outsidethe scope of this paper. For the latter, it is known that commer-cially available fibers used as composite reinforcement can retaintheir strength at extreme service temperatures. Specifically, carbonfibers can be used in the range of �50 to +500 �C, while glass fiberscan be used from �60 to +450 �C, and basalt fibers from �200 to+700 �C. Conversely, most polymeric resins used for FRP matricesare susceptible to temperatures higher than the glass transitiontemperature, Tg, defined as the midpoint temperature range overwhich the resin changes from a glassy to a viscoelastic state [1].This change in state degrades the mechanical and bond propertiesof the laminates [7]. Extensive studies carried out on FRP strength-ened RC beams and slabs during exposure to fire have pointed outthat thermal protection is required to preserve the FRP-concretebond [8–10]. On the other hand, prior research has shown thatexposure to subzero temperatures can result in matrix hardening,matrix micro-cracking and fiber–matrix bond degradation; whilefreeze–thaw cycles in the presence of salt can result in accelerateddegradation due to the formation and expansion of salt deposits, inaddition to effects of moisture-induced swelling and drying [11].Furthermore, it has been found that freeze–thaw cycles under bothdry and wet conditions typically have a detrimental effect on thestrength and to a lesser degree on the stiffness of carbon andE-glass composites [12]. To date, the mechanical performance ofFRP laminates remains uncertain due to the limited experimental

Nomenclature

ffu ultimate tensile strength (MPa)A angled splitting failure typeAf net cross-sectional area of fibers sheet (mm2)CV coefficient of variance (%)Echord tensile chord modulus (MPa)P axial load acting on FRP specimen (kN)Pf,th theoretical axial load at failure (kN)Pmax maximum load at failure (kN)

RH relative moisture (%)S longitudinal splitting failure typeT temperature value (�C)Tg FRP glass transition temperatureefu measured ultimate FRP tensile strain (mm/mm)e0fu computed ultimate FRP tensile strains ffu/Echord (mm/

mm)

F. Nardone et al. / Composites: Part B 43 (2012) 1468–1474 1469

available data. This paper provides experimental results aimed atinvestigating how different environmental conditions may influ-ence FRP mechanical properties in terms of FRP tensile chord mod-ulus, ultimate tensile stress and strain. To this end, an array ofsixty-three coupon tests were carried out on epoxy–based lami-nates subjected to a combined action of axial tension and temper-ature in an environmental chamber at controlled conditions, orafter exposure to freeze–thaw cycles.

2. Research significance and objectives

Durability of FRP has been defined by the ACI 440 committee asthe ability to resist cracking, oxidation, chemical degradation, deb-onding, wear, fatigue and the effects of damage from foreign ob-jects for a specified period of time under appropriate loadconditions and under service exposure conditions [13]. Effects oftemperature and moisture, in addition to creep-rupture and fati-gue, are also identified as critical durability concerns [14]. Normalenvironmental conditions are not considered detrimental to FRPcomposites as long as they are properly fabricated, installed, andTg is higher than the service temperature of the structure. However,it has not been proven that extreme service temperatures deterio-rate the mechanical properties of FRP laminates. This paper is acontribution to fill this knowledge gap.

3. Experimental program

Tensile tests to failure were performed on FRP laminates madefrom glass (GFRP) and carbon (CFRP) fiber sheets while subjectedto the following three different conditioning regimens:

� GFRP specimens with one- two- and three-ply, tested underconstant relative humidity (RH) of 0 or 30%, and at tempera-tures ranging from �15 to +36 �C.� One-ply CFRP specimens tested at +20 and +70 �C, respectively,

and at a constant RH of 65%.� One-ply CFRP specimens tested at room conditions (T = +20 �C;

RH = 65%) after exposure to freeze–thaw cycles for 30, 80 and210 cycles.

The first regimen condition for GFRP samples was selectedbased on the range of temperatures and moisture levels found incontinental regions or industrial environments; while the secondregimen was intentionally chosen at a nominal Tg. According toACI 400.2R-08 [1], the Tg ranges from +60 to +80 �C and the servicetemperature should not exceed Tg �15 �C. The last regimen wasselected based on previous durability tests on FRP materials withsimilar number of cycles [3].

The specimen geometry was chosen as per ASTM D3039 [15],where the minimum requirements of specimen length, width andgrip length were followed. Specimens are identified throughout

this paper and summarized in Table 1, using the V-W-X-Y-Z label,where:

� V denotes the FRP material: G for Glass, and C for Carbon;� W denotes the number of plies, namely: one, two, or three; P1,

P2, or P3 respectively;� X denotes the test environment temperature;� Y denotes the loading type: LL for Linear Load increase, SL for

Step Load increase, and FTn refers to linear load increase forspecimens previously exposed to n the number of freeze–thawcycles; and� Z denotes the specimen repetition: A, B or C.

Specimens were prepared via a wet lay-up technique in accor-dance to ASTM D3039 [15] and ACI 440.3R-04 [16], using a horizon-tal two-part sandwich panel mold with a non-stick inner surface of500 mm in width and 360 mm in length (fiber direction). Each FRPpanel was released after 72 h from the mold and individual couponswere cut by means of a water-lubricated diamond saw to the finaldimensions illustrated in Fig. 1. The final step in the preparation pro-cess involved bonding aluminum tabs using a commercially avail-able high-strength two-component epoxy resin. Tabs were pre-stamped to create small dents on the surface to improve themechanical interlock between the tabs and the sanded surface ofthe FRP laminate. The specimens were manufactured by construc-tion industry workforce and this allows assuming that the fiber vol-ume fractions could be in the range from 25% to 40% [17].

3.1. FRP specimens

3.1.1. GlassThirty-six GFRP specimens were manufactured with one, two,

and three plies using a uni-directional sheet with a unit weightof 600 g/m2 made of glass fibers with a tensile strength of3400 MPa, modulus of elasticity of 77 GPa, and ultimate elongationof 4.4%. Coupons were impregnated with a two-part epoxy resinwith ultimate tensile strength of 50 MPa, elongation of 2.5%, mod-ulus of elasticity of 3 GPa, and glass transition temperature of 70 �Cas per manufacture’s specifications.

The specimen dimensions were established at 250 mm inlength and 25 mm width, including 55 mm for the grip at bothends (Fig. 1a). Direct tensile tests for GFRP coupon specimens wereperformed inside a controlled environmental chamber at four dif-ferent temperatures averaging �15, +2, +19 and +36 �C; wherethe relative humidity for tests at �15 and +2 �C was set at 0%,and 30% for tests at +19 and +36 �C. The different RH levels wereselected to mimic real environmental conditions at the chosentemperature values. Three specimens were tested for each combi-nation of temperature and number of plies, as reported in Table 1.Specimens were allowed to condition for a period of thirty minutesprior to loading and temperature and moisture levels were main-tained throughout the duration of the test. The axial load was ap-plied linearly, increasing from the zero until specimen failure.

Table 1Test matrix.

Specimen number Specimen ID Fiber Number of plies Temperature (�C) Moisture (%) Type of test (cycles)

#1 G-P1-T15-LL-A Glass 1 �15 0 Linear load increase#2 G-P1-T15-LL-B Glass 1 �15 0 Linear load increase#3 G-P1-T15-LL-C Glass 1 �15 0 Linear load increase#4 G-P1-T2-LL-A Glass 1 +2 0 Linear load increase#5 G-P1-T2-LL-B Glass 1 +2 0 Linear load increase#6 G-P1-T2-LL-C Glass 1 +2 0 Linear load increase#7 G-P1-T19-LL-A Glass 1 +19 30 Linear load increase#8 G-P1-T19-LL-B Glass 1 +19 30 Linear load increase#9 G-P1-T19-LL-C Glass 1 +19 30 Linear load increase#10 G-P1-T36-LL-A Glass 1 +36 30 Linear load increase#11 G-P1-T36-LL-B Glass 1 +36 30 Linear load increase#12 G-P1-T36-LL-C Glass 1 +36 30 Linear load increase#13 G-P2-T15-LL-A Glass 2 �15 0 Linear load increase#14 G-P2-T15-LL-B Glass 2 �15 0 Linear load increase#15 G-P2-T15-LL-C Glass 2 �15 0 Linear load increase#16 G-P2-T2-LL-A Glass 2 +2 0 Linear load increase#17 G-P2-T2-LL-B Glass 2 +2 0 Linear load increase#18 G-P2-T2-LL-C Glass 2 +2 0 Linear load increase#19 G-P2-T19-LL-A Glass 2 +19 30 Linear load increase#20 G-P2-T19-LL-B Glass 2 +19 30 Linear load increase#21 G-P2-T19-LL-C Glass 2 +19 30 Linear load increase#22 G-P2-T36-LL-A Glass 2 +36 30 Linear load increase#23 G-P2-T36-LL-B Glass 2 +36 30 Linear load increase#24 G-P2-T36-LL-C Glass 2 +36 30 Linear load increase#25 G-P3-T15-LL-A Glass 3 �15 0 Linear load increase#26 G-P3-T15-LL-B Glass 3 �15 0 Linear load increase#27 G-P3-T15-LL-C Glass 3 �15 0 Linear load increase#28 G-P3-T2-LL-A Glass 3 +2 0 Linear load increase#29 G-P3-T2-LL-B Glass 3 +2 0 Linear load increase#30 G-P3-T2-LL-C Glass 3 +2 0 Linear load increase#31 G-P3-T19-LL-A Glass 3 +19 30 Linear load increase#32 G-P3-T19-LL-B Glass 3 +19 30 Linear load increase#33 G-P3-T19-LL-C Glass 3 +19 30 Linear load increase#34 G-P3-T36-LL-A Glass 3 +36 30 Linear load increase#35 G-P3-T36-LL-B Glass 3 +36 30 Linear load increase#36 G-P3-T36-LL-C Glass 3 +36 30 Linear load increase#37 C-P1-TR-SL-A Carbon 1 +20 65 Step load increase#38 C-P1-TR-SL-B Carbon 1 +20 65 Step load increase#39 C-P1-TR-SL-C Carbon 1 +20 65 Step load increase#40 C-P1-TR-SL-D Carbon 1 +20 65 Step load increase#41 C-P1-TR-SL-E Carbon 1 +20 65 Step load increase#42 C-P1-TR-SL-F Carbon 1 +20 65 Step load increase#43 C-P1-TR-SL-G Carbon 1 +20 65 Step load increase#44 C-P1-TR-SL-H Carbon 1 +20 65 Step load increase#45 C-P1-T70-SL-A Carbon 1 +70 65 Step load increase#46 C-P1-T70-SL-B Carbon 1 +70 65 Step load increase#47 C-P1-T70-SL-C Carbon 1 +70 65 Step load increase#48 C-P1-T70-SL-D Carbon 1 +70 65 Step load increase#49 C-P1-TR-FT30-A Carbon 1 +20 65 Freeze–Thaw [30]#50 C-P1-TR-FT30-B Carbon 1 +20 65 Freeze–Thaw [30]#51 C-P1-TR-FT30-C Carbon 1 +20 65 Freeze–Thaw [30]#52 C-P1-TR-FT30-D Carbon 1 +20 65 Freeze–Thaw [30]#53 C-P1-TR-FT30-E Carbon 1 +20 65 Freeze–Thaw [30]#54 C-P1-TR-FT80-A Carbon 1 +20 65 Freeze–Thaw [80]#55 C-P1-TR-FT80-B Carbon 1 +20 65 Freeze–Thaw [80]#56 C-P1-TR-FT80-C Carbon 1 +20 65 Freeze–Thaw [80]#57 C-P1-TR-FT80-D Carbon 1 +20 65 Freeze–Thaw [80]#58 C-P1-TR-FT210-A Carbon 1 +20 65 Freeze–Thaw [210]#59 C-P1-TR-FT210-B Carbon 1 +20 65 Freeze–Thaw [210]#60 C-P1-TR-FT210-C Carbon 1 +20 65 Freeze–Thaw [210]#61 C-P1-TR-FT210-D Carbon 1 +20 65 Freeze–Thaw [210]#62 C-P1-TR-FT210-E Carbon 1 +20 65 Freeze–Thaw [210]#63 C-P1-TR-FT210-F Carbon 1 +20 65 Freeze–Thaw [210]

1470 F. Nardone et al. / Composites: Part B 43 (2012) 1468–1474

3.1.2. CarbonTwenty-seven CFRP specimens were manufactured with one-

ply of reinforcement using sheets with a unit weight of 600 g/m2,made of carbon fibers with a tensile strength of 4830 MPa, modu-lus of elasticity of 230 GPa and ultimate elongation of 2%. Speci-mens were impregnated by a two-part epoxy resin with a tensilestrength of 40 MPa, and elongation of 1.8%, as per manufacture’sspecifications. This resin was characterized by dynamic mechanical

analysis (DMA) performed on tension-clamped samples varyingthe temperature at a rate of 3 �C/min, from �50 to +250 �C, todetermine its Tg. The tests were carried out at a frequency of1 Hz and 5 lm strain amplitude. DMA showed a Tg equal to+52 �C, computed as the temperature corresponding to the peakof the loss tangent (tand).

The CFRP specimen dimensions were established at 360 mm inlength and 25 mm width, including 80 mm for the grip at both

55 140 55

t250

25

1.5

(a)

80 200 80

t

360

25

1.5

(b)Fig. 1. Specimen dimension (mm): (a) GFRP; (b) CFRP.

(a) (b)Fig. 3. Test setup for tensile test (a); instrumentation (b).

F. Nardone et al. / Composites: Part B 43 (2012) 1468–1474 1471

ends (Fig. 1b). The specimen geometry followed ASTMD3039 [15],where the overall length was increased compared to GFRP speci-mens to accommodate longer grips. The uniaxial tensile tests wereconducted in three steps as reflected in Fig. 2a: (i) specimen pre-loaded at 30% of the expected failure load, Pf,th (P = 0.3 Pf,th) so asto simulate service load conditions; (ii) one hour conditioning timein the environmental chamber with pre-load; (iii) load increase upto the specimen failure.

The freeze–thaw cycle duration was equivalent to 24 h and thecycle details are reflected in Fig. 2b. In particular, the first part ofthe cycle consisted in exposure to a temperature equal to �5 �Cand RH of 0% for a period of 6 h, followed by a temperature of+40 �C and RH of 80% for a period of 15 h; where the time elapsedbetween each stage was 1.5 h. Specimens exposed to the freeze–thaw conditioning were then subjected to a direct tensile test witha linearly increasing load up to failure. Five specimens were testedafter exposure to 30 cycles, four specimens after 80 cycles, and sixspecimens after 210 cycles (Table 1).

3.2. Test setup and instrumentation

All tests were performed according to ASTM D3039 [15] and ACI440.3R-04 [16] at a displacement rate of 0.033 mm/s, using a sys-tem composed of an environmental chamber provided by thermo-regulatory unit, two insulated pipes (capable of sustainingtemperatures ranging from �60 to +120 �C), and a thermostatictest room hosting a servo-hydraulic MTS 810 universal testmachine with a maximum stroke of ±75 mm and capacity of500 kN (Fig. 3a). In particular, the thermoregulatory unit(AT250TC) allows achieving a temperature range of �75 to+180 �C with a moisture rate between 0% and 95%. Mechanicalwedge-type grips were used to engage the specimens by applying

0

25

50

75

100

125

0 0.3 0.6 0.9 1.2 1.5 1.8

Temperature [°C]

RH [%]

T [°

C]

Time [hours]

1 hour at fixed environment

P=30%Pf,th →Pf,th

(a)Fig. 2. Temperature and relative moisture exposure pro

a uniform pressure of approximately 25 MPa directly on the alumi-num tabs. The load was measured via an internal 50 kN capacityload cell, while strains were recorded using two uniaxial foil straingages protected with a plastic insulating coating and attached tothe specimen symmetrically at the mid-span and mid-width loca-tion of each specimen (Fig. 3b). All data acquisition was set to allowtemperature compensation by using a Wheatstone bridge.

4. Experimental results and discussion

All results for the tensile tests are presented in Table 2, whichinclude the peak axial load at failure, Pmax; the ultimate tensilestrength, ffu; tensile chord modulus, Echord; and the measured andcomputed ultimate tensile strain, efu and e0fu, respectively. Themean value and coefficient of variation (CV) are also provided. Notethat ffu, was calculated by dividing Pmax by the net cross-sectionalarea of the fiber sheet, Af, and the Echord was computed accordingto ASTM D3039 [15] in the longitudinal strain range starting andending at 1000 and 3000 le, respectively. The experimental strainvalues were based on the average of the two strain gages readings.The measured ultimate tensile strains fluctuated marginally whencompared to the computed values; this is expected for two rea-sons: (i) the reliability of a strain gauge near 20,000 le is reducedand (ii) it is difficult to obtain the strain measurement at the in-stance of failure. For these reasons, the computed ultimate strainis used for discussion in this paper.

Additionally, the failure type according to ASTM D3039 [15] isalso reported in the last column of Table 2, where ‘‘A’’ and ‘‘S’’ refer

0

10

20

30

40

0 6 12 18 24Time [hours]

T [°

C]

RH=0%

RH=80%

(b)file for CFRP specimens (a); Freeze–thaw cycle (b).

Table 2Test results.

Specimen number Pmax (KN) ffu (MPa) Echord (GPa) Measured strain efu Computed strain e0fu Failure type

Exp. Mean CV (%) Exp. Mean CV (%) ffu/Echord Mean CV (%)

#1 12.3 1899 1747 9.4 86 91 4.5 1.90 2.21 1.93 13.5 A + S#2 10.2 1572 93 n/a 1.68 S#3 11.5 1769 93 1.94 1.90 S#4 10.6 1642 1681 4.0 81 86 9.0 1.75 2.02 1.97 4.8 S#5 10.6 1642 81 1.75 2.02 S#6 11.4 1758 94 1.91 1.86 A + S#7 11.5 1767 1714 5.9 87 87 4.7 1.87 2.03 1.97 2.6 A#8 11.5 1777 91 1.95 1.96 S#9 10.4 1597 83 1.96 1.93 A#10 11.0 1697 1656 2.2 88 82 8.7 1.93 1.92 2.03 7.1 A#11 10.7 1643 84 1.79 1.97 A#12 10.6 1629 74 1.81 2.19 A + S#13 23.4 1805 1712 11.0 90 92 2.0 1.82 2.00 1.86 12.0 A#14 23.8 1835 93 1.88 1.96 A + S#15 19.4 1496 94 1.69 1.60 A + S#16 23.7 1828 1591 21.1 90 88 4.7 1.78 2.02 1.81 16.5 S#17 20.6 1592 89 n/a 1.78 S#18 17.5 1354 85 1.69 1.60 A#19 19.7 1521 1689 9.1 93 93 5.1 1.81 1.63 1.81 9.2 S#20 23.6 1823 98 1.43 1.86 A + S#21 22.3 1724 88 1.80 1.95 S#22 23.3 1797 1647 10.1 90 85 5.6 1.93 2.00 1.95 6.9 S#23 19.0 1469 82 1.84 1.79 A#24 21.7 1674 82 1.91 2.05 S#25 32.3 1663 1807 7.2 88 90 1.9 1.41 1.88 2.01 6.7 S#26 37.2 1913 89 1.61 2.15 A + S#27 35.9 1845 92 1.45 2.01 S#28 33.7 1735 1778 2.5 87 88 2.3 1.43 1.99 2.01 1.1 S#29 34.5 1774 87 1.70 2.03 A + S#30 35.5 1824 91 1.65 2.01 A + S#31 31.4 1616 1726 5.6 90 91 1.2 1.31 1.79 1.90 5.2 S#32 34.9 1797 92 1.49 1.96 A + S#33 34.3 1766 90 1.38 1.96 S#34 28.5 1467 1531 6.3% 82 84 2.3 1.39 1.79 1.82 4.8 S#35 28.8 1483 85 1.46 1.75 S#36 31.9 1642 86 1.45 1.92 S#37 23.0 2585 2890 10.7 228 220 3.5 1.05 1.13 1.31 10.1 S#38 25.6 2879 219 1.19 1.31 S#39 23.4 2638 227 1.07 1.16 S#40 23.3 2626 214 1.13 1.23 S#41 30.9 3480 230 1.40 1.51 S#42 27.3 3073 222 1.26 1.38 S#43 24.4 2748 208 1.26 1.32 S#44 27.4 3089 215 1.29 1.44 S#45 19.4 2179 2056 6.1 246 219 11.9 0.60 0.89 0.95 14.3 A + S#46 18.2 2048 207 0.70 0.99 A + S#47 16.8 1886 235 0.70 0.80 A + S#48 18.8 2112 189 0.56 1.12 A + S#49 20.3 2282 2753 14.6 214 211 7.2 1.02 1.07 1.32 21.2 A#50 28.7 3234 187 1.41 1.73 A#51 26.1 2941 208 1.26 1.41 A#52 21.2 2392 227 0.98 1.05 A#53 25.9 2917 220 1.23 1.33 A#54 27.9 3139 3076 4.5 230 225 7.7 1.28 1.36 1.38 9.8 S#55 27.4 3091 199 1.22 1.55 A + S#56 25.6 2879 235 1.17 1.23 S#57 28.4 3196 235 1.57 1.36 A + S#58 22.4 2518 2643 12.0 224 232 10.6 1.05 1.12 1.15 14.7 S#59 24.5 2758 219 1.17 1.26 A + S#60 22.7 2559 251 1.03 1.02 A#61 24.1 2715 196 1.27 1.39 S#62 27.8 3135 265 1.15 1.18 A + S#63 19.3 2172 237 0.89 0.92 A

Note: A and S refer to angled or longitudinal splitting, respectively.

1472 F. Nardone et al. / Composites: Part B 43 (2012) 1468–1474

to angled and longitudinal splitting, respectively (Fig. 4). The GFRPoutcomes are summarized in Table 3.

4.1. GFRP specimens

The CV for the measured ffu ranged from 2.2% to 21.1% (for thespecimens G-P1-T36-LL and G-P2-T2-LL, respectively), and for

the Echord from 1.2% to 9.0% (for the specimens G-P3-T19-LL andG-P1-T2-LL, respectively) which are typical CV magnitude levelsfor testing of material properties [18]. This statistical parameteralone cannot differentiate for possible statistical variances acrossthe measurements recorded for specimens of different reinforce-ment ratios, and number of tests. Single-factor analysis of variance(ANOVA) tests were performed, assuming a normal distribution

GFRP CFRP GFRP CFRP

(a) (b)Fig. 4. Representative tensile failure experienced by tensile specimens: A type (a);S type (b).

Table 3Test results summary for GFRP specimens.

Numberof plies

Mean value ofmechanicalproperty

T = �15 �C T = 2 �C T = 19 �C T = 36 �C

1 ffu (MPa) 1747 1681 1714 16562 1712 1591 1689 16473 1807 1778 1726 1531

1 Echord (GPa) 91 86 87 822 92 88 93 853 90 88 91 84

1 efu (%) 1.93 1.97 1.97 2.032 1.86 1.81 1.81 1.953 2.01 2.01 1.90 1.82

F. Nardone et al. / Composites: Part B 43 (2012) 1468–1474 1473

with a confidence level of 95% (a = 0.05) [19], to test for the reli-ability of the ffu and Echord results provided herein, the outcomesare reported in Tables 4 and 5, respectively, and are discussed inthe next sections.

4.1.1. Number of pliesThe influence of the number of plies in terms of ffu was negligi-

ble for the different temperatures, as the ultimate axial tensileforce carried by specimens increased linearly with increasing num-ber of plies (Table 2), indicating that no significant relative slip be-tween plies occurred during testing. This was confirmed with theANOVA test summarized in Table 4 where the null hypothesis isaccepted, stating there are no significant statistical differences be-tween the three groups of ffu measurements (one-, two-, and three-ply specimens) given that the F-value did not exceed the F-criticalvalue at a probability of 78%, 35%, 93% and 38% for temperaturevalues of �15, +2, +19 and +36 �C, respectively. In particular, thedifference in ffu between one and three-ply specimens was 7.6%for tensile tests at a temperature of +36 �C. This difference droppedto 0.7% for specimens tested at 19 �C, as summarized in the resultsreflected in Table 3.

Table 4ANOVA single factor test for ffu results.

T Groups Count Average Variance Source of variat

�15 �C 1-ply 3 1747 27,106 Between groups2-ply 3 1712 35,217 Within groups3-ply 3 1807 16,708 Total

2 �C 1-ply 3 1681 4485 Between groups2-ply 3 1591 56,169 Within groups3-ply 3 1778 1990 Total

19 �C 1-ply 3 1714 10,233 Between groups2-ply 3 1689 23,702 Within groups3-ply 1726 9370 Total

36 �C 1-ply 3 1656 1289 Between groups2-ply 3 1647 27,456 Within groups3-ply 3 1531 9360 Total

Similar results were obtained for Echord as function of ply num-ber, where the ANOVA test summarized in Table 5 yielded an F-va-lue not exceeding the F-critical value at a probability of 52%, 75%,19% and 82% for temperature values of �15, +2, +19 and +36 �C,respectively. Hence no significant statistical differences werefound for the three groups of Echord data values (one-, two-, andthree-ply specimens).

4.1.2. Temperature effectTemperature had a detrimental effect on the tensile strength,

where with increasing temperature from �15 to +36 �C, ffu de-crease to 5.2%, 3.8% and 15.3% for specimens with one-, two- andthree-plies, respectively (Table 3). The experimental results con-firm previous results [7], and can be explained by the fact thatthe epoxy matrix is softened modifying the mechanical propertiesof the composite due to the temperature increase, resulting in analtered load path through the resin to the fibers. The temperatureeffect on the tensile strength appeared to be more significant forspecimen with three plies. The tensile chord modulus reductionwas comparable for specimens with different number of plies. Inparticular, Echord drop was equal to 9.5%, 8.5% and 6.1%, for speci-mens with one-, two- and three-plies, respectively (Table 3).

The influence of temperature increase from �15 to +36 �C onthe average computed ultimate tensile strain, e0fu had an oppositeeffect, increasing approximately 5% for specimens with one- andtwo-ply. However, in case of three-ply specimens the e0fu decreasedabout 10%, as reflected from the rapid decrease in ultimate tensilestress when compared to the change in tensile chord modulus (Ta-ble 3).

4.2. CFRP specimens

The CV attained for ffu equals 10.7% and 6.1% at +20 and +70 �C,respectively; for Echord equals 3.5% and 11.9% at +20 and +70 �C,

ion SS df MS F F-value (%) F-crit

13,867 2 6933 0.26 78 5.14158,063 6 26,344171,930 852,110 2 26,055 1.25 35 5.14125,290 6 20,882177,400 82122 2 1061 0.07 93 5.1486,612 6 14,43588,734 829,342 2 14,671 1.15 38 5.1476,212 6 12,702105,554 8

Table 5ANOVA single factor test for Echord results.

T Groups Count Average Variance Source of variation SS df MS F F-value (%) F-crit

�15 �C 1-ply 3 91 16 Between groups 11 2 6 0.73 52 5.142-ply 3 92 4 Within groups 46 6 83-ply 3 90 3 Total 57 8

2 �C 1-ply 3 86 59 Between groups 14 2 7 0.30 75 5.142-ply 3 88 10 Within groups 145 6 243-ply 3 88 4 Total 159 8

19 �C 1-ply 3 87 16 Between groups 60 2 30 2.26 19 5.142-ply 3 93 23 Within groups 80 6 133-ply 91 1 Total 140 8

36 �C 1-ply 3 82 50 Between groups 10 2 5 0.20 82 5.142-ply 3 85 22 Within groups 153 6 263-ply 3 84 4 Total 163 8

1474 F. Nardone et al. / Composites: Part B 43 (2012) 1468–1474

respectively; and e0fu equals 10.1% and 14.3% at +20 and +70 �C,respectively. The CV values for the CFRP laminates exposed tofreeze–thaw cycles varied between 4.5% to 14.6%, 7.2% to 10.6%,and 9.8% to 21.2% for average ffu, Echord and e0fu, respectively. Theseare typical CV magnitude levels for material characterization of FRPspecimens [18–20].

4.2.1. Temperature effectExposure of CFRP to elevated temperature results in severe deg-

radation of mechanical properties. A decrease of 28.8 and 27.7% forthe average ffu, and e0fu, respectively, were recorded for CFRP spec-imens when tested at 20 and 70 �C at a relative humidity of 65%.The reduction in ultimate axial strength and strain is due to the ef-fect of the temperature above Tg on the epoxy matrix. No signifi-cant changes in stiffness were recorded as reflected in Table 2.

4.2.2. Freeze–thaw cycles effectThe values for ffu did not significantly change after exposing

CFRP laminates to 30 or 80 freeze–thaw cycles, when comparedto the room condition (T = +20 �C and RH = 65%). Increasing thenumber of cycles to 210 resulted in a slight reduction of tensilestrength (8.6%). The same trend was observed by the tensile strain,with a maximum reduction equal to 12.5% after 210 cycles. No rel-evant variation was reflected by the tensile chord modulus afterfreeze–thaw cycles exposure (maximum variation equal to 5.2%).

5. Conclusions

The results of sixty-three tensile tests on GFRP or CFRP couponsare presented in the paper in order to investigate on the effects ofextreme service temperature or freeze–thaw cycles on FRP lami-nates mechanical properties. Based on the experimental outcomes,the GFRP coupon strength, chord modulus and ultimate strain fordifferent number of plies (i.e. one, two and three) experienced nosignificant changes when tested under direct tension for tempera-tures ranging from �15 to +36 �C. Further, experiments showedthat strength and elastic modulus slightly decreased when thetemperature was raised from �15 to +36 �C; while the ultimatestrain remained almost constant.

A significant decrease of the tensile strength and ultimatestrain, approximately equal to 30%, was recorded in CFRP lami-nates tested at a temperature of +70 �C and RH of 65% when com-pared to similar laminates tested at +20 �C and RH 65%; nosignificant change in the stiffness was measured.

Freeze–thaw cycles ranging from 30 to 80 had a negligible influ-ence to the mechanical properties of CFRP laminates, while 210 cy-cles resulted in slight reductions in tensile strength and ultimatestrain equivalent to 9% and 13%, respectively. Therefore futureFRP durability tests ought to expose specimens to more than210 cycles.

Acknowledgments

The authors acknowledge the financial support provided by theNSF Industry/University Cooperative Research Center ‘‘CICI’’ and itsindustry members, and Rete dei Laboratori Universitari di Ingegne-ria Sismica – ReLUIS for the research program funded by theDepartment of Civil Protection – Executive Project 2010–2013.

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