Mechanical performances of GFRP-steel specimens bonded with different epoxy adhesives, before and after the aging treatments

Accepted Manuscript Mechanical performances of GFRP-steel specimens bonded with different epoxy adhesives, before and after the aging treatments M. Giampaoli, V. Terlizzi, M. Rossi, G. Chiappini, P. Munafò PII: DOI: Reference:

S0263-8223(17)30645-1 http://dx.doi.org/10.1016/j.compstruct.2017.03.020 COST 8338

To appear in:

Composite Structures

Received Date: Revised Date: Accepted Date:

24 February 2017 6 March 2017 7 March 2017

Please cite this article as: Giampaoli, M., Terlizzi, V., Rossi, M., Chiappini, G., Munafò, P., Mechanical performances of GFRP-steel specimens bonded with different epoxy adhesives, before and after the aging treatments, Composite Structures (2017), doi: http://dx.doi.org/10.1016/j.compstruct.2017.03.020

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Mechanical performances of GFRP-steel specimens bonded with different epoxy adhesives, before and

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after the aging treatments.

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M. Giampaoli*, V. Terlizzi, M. Rossi, G. Chiappini, P. Munafò. *Corresponding Author Eng. Margherita Giampaoli, Dipartimento di Ingegneria Civile, Edile e Architettura (DICEA), Università Politecnica delle Marche, Via Brecce Bianche, 60131 Ancona, Italy [email protected] Eng. Vanessa Terlizzi, Dipartimento di Ingegneria Civile, Edile e Architettura (DICEA), Università Politecnica delle Marche, Via Brecce Bianche, 60131 Ancona, Italy [email protected] Dr. Marco Rossi, Assistant Professor Dipartimento di Ingegneria Industriale e Scienze Matematiche, Università Politecnica delle Marche, Via Brecce Bianche, 60131 Ancona, Italy [email protected] Eng. Gianluca Chiappini Dipartimento di Ingegneria Industriale e Scienze Matematiche, Università Politecnica delle Marche, Via Brecce Bianche, 60131 Ancona, Italy [email protected] Prof. Placido Munafò, Full Professor Dipartimento di Ingegneria Civile, Edile e Architettura (DICEA), Università Politecnica delle Marche, Via Brecce Bianche, 60131 Ancona, Italy [email protected]

1

1

Abstract

2

In this paper, the stiffness increase obtained by coupling steel laminates to GFRP pultruded profiles is

3

investigated, three different epoxy adhesives were used and compared. The objective is to verify the

4

applicability of this hybrid system in structural members for curtain walls. Three types of test were used to

5

investigate different properties of the system: shear tests on GFRP-steel single lap joints to verify the

6

compatibility of the bonding system; puncture tests on GFRP-steel squared tubular short profiles to evaluate

7

the response to localized stresses; three-point bending tests on GFRP-steel squared tubular long profiles to

8

study the resistance to flexural loads. The effects of two environmental aging, continuous condensation and

9

UV radiations, were also analyzed. The results demonstrated the compatibility of the bonding system and the

10

stiffness increase of GFRP profiles when the steel reinforcements are applied. The experiments also show a

11

good agreement with analytical models of the tests, confirming the advantageous method of hybridization.

12

With regard to artificial aging, the continuous condensation produces the worst effects on the specimens,

13

while better results were observed after the UV radiations. The results are discussed in details in the paper.

14 15

1. Introduction

16

Nowadays, it is well known that Glass Fibre-Reinforced Polymer (GFRP) pultruded profiles present several

17

favorable properties, such as high specific strength, light weight, low electrical and thermal conductivity and

18

non-corrodibility, rapid installation time and lower life-cycle costs [1-3]. For these reasons, this material is

19

becoming a valuable alternative to conventional ones, also in civil engineering. For example, it can be used

20

in many fields of civil engineerings, such as bridges and buildings [4], electricity transmission towers [1],

21

and windows frames [5, 6]. On the other hand, composite materials present some disadvantages: orthotropy

22

[7], brittleness in bolted connections [8], and low elastic modulus if compared to steel (up to ten times lower)

23

[3, 9-11]. In particular, this latter condition impedes the use of GFRP pultruded profiles in structural

24

applications, especially when the concentrated bearing load is applied [12-14]. For this reason, it is important

25

to improve the stiffness of the GFRP pultruded profiles, and some authors demonstrated that such

26

improvement can be obtained by coupling GFRP profiles to materials with higher mechanical performances,

27

i.e. hybridization systems [2, 3, 15].

2

1

Kim and Lee [10] developed a steel-reinforced hybrid GFRP deck panel for temporary bridges. Two types of

2

cross-sectional profiles were proposed: one where the flanges at the top and the bottom were reinforced with

3

steel wires and one where the web is reinforced with a steel plate. The results confirmed that the flexural

4

stiffness of the GFRP deck panel was effectively increased by the proposed methods of hybridization.

5

Wu et al. [3] explored two strengthening methods to improve the bearing capacities of GFRP pultruded SHS

6

sections, by bonding alternatively CFRP plates and steel sections on the external surfaces of the GFRP

7

profiles. Both the strengthening methods demonstrated considerable enhancement of the bearing capacities,

8

respectively of 70 % and of 200 %.

9

However, to the authors knowledge, validated data on this topic are still scarce and there has been little

10

research on the comparison of different types of adhesives used for the joining of GFRP and steel profiles.

11

Most of the existing studies examine the behavior of a single adhesive used to connect GFRP decks to steel

12

girders and do not look at the improving of the GFRP profiles stiffness [16-19]. Furthermore, the influence

13

of environmental aging has not been studied, even if it is well known that bonding connections are strongly

14

influenced by environmental conditions, which can reduce mechanical properties of the adhesives [20-22].

15

In this paper, the connections of GFRP pultruded profiles and steel laminates using three different epoxy

16

adhesives were studied. In fact, our research group has designed a “Structural Member for curtain walls”

17

(patent application n. 102015000087569), a linear construction element which allows the realization of

18

curtain walls with high glazing areas. The innovative principle consists of the use of GFRP profiles, with

19

small sectional areas, adhesively bonded to steel laminates, with the objective of containing deformations

20

and facilitating the fixing of the curtain panels.

21

In the experimental campaign, the following topics were investigated:

22

- the compatibility of the bonding system, through shear tests conducted on small-scale specimens (GFRP-

23

steel single lap joints), looking at the effects of two environmental aging;

24

- the response to localized stresses on the bonding system, through puncture resistance tests on full-scale

25

specimens (GFRP-steel squared tubular short profiles), which simulate the local stress transmitted by curtain

26

panels bolted to the structural members; the effect of two environmental aging was also evaluated;

3

1

- the response to flexural loads on the bonding system, through three-point bending tests on full-scale

2

specimens (GFRP-steel squared tubular long profiles), which simulate the flexural loads that the whole

3

structural members in curtain walls undergo (i.e. the wind load).

4

The results presented in this paper confirmed the reliability of the Structural Member for curtain walls basic

5

principle.

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2. Materials and methods

8

The aim of the study is to evaluate both the GFRP-steel compatibility and the enhancement of the

9

mechanical properties of GFRP profiles caused by the steel reinforcement. The experimental tests include: (i)

10

tensile tests for material characterization, (ii) shear tests on adhesively bonded single lap joints of GFPR and

11

steel, (iii) puncture tests on short squared tubular specimens of GFRP reinforced with steel and (iv) three-

12

point bending tests on long squared tubular specimens of GFRP reinforced with steel. The GFRP specimens

13

used in the two latter tests (iii, iv) were reinforced attaching with suitable adhesives steel plates in three

14

configurations: with the steel plates positioned in the “upper”, “lower” and “upper-inner” sides of the GFRP

15

profiles respect to the loading direction (Fig. 1). Furthermore, the effect of two environmental aging, namely

16

continuous condensation and UV exposures, was analyzed in steps (ii) and (iii) of the experimental program.

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2.1. Material characterization tests

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As the first step, the mechanical properties of each material used in the hybrid system were identified

20

separately, i.e. the elastic modulus, the tensile strength, and the maximum elongation.

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Adherents

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The first material used in this work was the pultruded E-glass fiber reinforced polyester composite,

24

manufactured by Fibrolux (Germany), using fiber-glass and an isophtalic polyester resyn. The profiles

25

are composed of a succession of layers: fiber roving, mat and surface veil, embedded in the polyester

26

matrix. The second material used was the S235JR steel grade supplied by Termoforgia (Italy). The properties

27

of the two materials, according to the manufacturers, are reported in Table 1.

4

1

The mechanical properties of the used GFRP profiles were also tested in the laboratory according to [23, 24].

2

The dimensions of laminates were 25x100x5 mm3. The free length of the specimens was 70 mm and the

3

gauge length of the extensometer was set to 50 mm. The tests were done applying a displacement rate of 5

4

mm/min with the aid of a Zwick/Roell Z050 testing machine, using an extensometer to measure the strain.

5

Three repetitions were employed and the results are summarized in Table 2.

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Adhesives

8

Three epoxy adhesives were considered in the experimental program, designated EPX1, EPX2, and EPX3.

9

The relative technical and mechanical characteristics, reported from manufacturers, are presented in Table 3.

10

The mechanical properties were also tested in the laboratory and five specimens of each type of adhesive

11

were subjected to tensile tests, according to [23, 25]. The dimensions of the dumbbell specimens are shown

12

in Fig. 2. All specimens were cured at room temperature for about one month. The results are reported in

13

Table 4 and confirmed the highest performance of the first epoxy adhesive and the worst load bearing

14

capacity of EPX3, which showed the highest deformability among the tested adhesives.

15

Furthermore, the glass transition temperature (Tg) of the adhesives was investigated and three samples of

16

each type of adhesive were tested with a differential scanning calorimeter (DSC), according to the procedure

17

of the standard [26]. The Tg was determined as the midpoint temperature between the extrapolated onset

18

temperature (Teig) and the extrapolated end temperature (Tefg). The results (Table 4) showed that among

19

structural adhesives, EPX2 had the highest Tg (on average 67 °C) and similar temperatures were obtained for

20

EPX1.

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2.2. Artificial ageing

23

In this experimental work, two different types of artificial aging were employed: the first simulated the

24

external environment exposure (Tcc) and the second the effect of the UV irradiation (Tuv).

25

The simultaneous exposure to heat and high humidity levels is one of the most harmful conditions of

26

adhesives and plastics [21, 22]. This external environment exposure (Tcc) was simulated using a climatic

27

chamber “Angelantoni” CST-130 S model (Fig. 3a). The specimens were aged at the constant temperature of

5

1

40 °C and at the relative humidity of 100 %, according to ISO 6270-2 [27], for six months without

2

interruption.

3

The exposition to UV radiations can dissociate the molecule bonds in most polymers, leading to the

4

degradation of polymeric materials [28]. This aging type (Tuv) was simulated using eight fluorescent UV

5

lamps (Philips Actinic BL TL-D). The specimens were aged according to [29], avoiding the exposition to

6

high temperatures (40 °C). In fact, as highlighted in a previous work [20], the effects of the exposition to

7

high temperatures are higher than those due to UV rays. Therefore, the samples were placed inside a wooden

8

structure and they were subjected to high UV radiations in laboratory conditions (temperature of 21 °C,

9

relative humidity of 33 %). The temperature on specimens surface was recorded through a radiometer and it

10

was registered of about 26 °C. In order to guarantee a uniform distribution of irradiation on the bonding

11

areas, the wooden structure was equipped with three lamps at the top, three at the bottom and two at the sides

12

(Fig. 3b). The lamps emitted on a wavelength in the range of 340-400 nm [30], with a peak at 370 nm,

13

producing a UV irradiance between 41 and 45 W/m² on the specimens surfaces. Cycle II of 24 hours [29]

14

was repeated 42 times (overall 1000 hours) without interruption.

15

Furthermore, the coupling of the two previous artificial exposures was investigated, in order to analyze the

16

effects of the combined aging conditions. The study was conducted on single lap joints bonded with the two

17

adhesives (EPX1 and EPX2), that showed the best performance. Two sequences of aging were adopted, in

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the first (Tcc+Tuv), the samples were firstly subjected to high humidity and temperatures levels for six

19

months, and afterward to UV radiations for 1000 hours; in the second sequence (Tuv+Tcc) the opposite order

20

was used.

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2.3. Test programme

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The experimental tests, as shown in Table 5, comprised: (ii) shear tests on adhesively bonded GFPR-steel

24

single lap joints, (iii) puncture tests on GFPR-steel squared tubular short specimens and (iv) three-point

25

bending tests on GFPR-steel squared tubular long specimens. In the shear tests, a series of twelve specimens

26

per adhesive type, subdivided according to the aging condition, were tested: four without aging (T0), four

27

after exposure to a hot-wet environment (Tcc), and four after exposure to UV radiations (Tuv). Furthermore,

28

since the joints bonded with the EPX1 and EPX2 adhesives showed the best performances, further specimens 6

1

were subjected to combined aging, as described in the previous section, and three repetitions for each

2

configuration were used.

3

In the puncture tests, a series of twenty-one specimens per adhesive type, subdivided according to the aging

4

condition, were tested: nine without aging (T0), nine after exposure to a hot-wet environment (Tcc), and three

5

after exposure to UV radiations (Tuv). The influence of the UV exposure was analyzed only on the specimens

6

in the “upper-inner” configuration, which presented the highest amount of adhesive. Three GFRP squared

7

tubular specimens without the steel reinforcement were also tested as a reference.

8

In the three-point bending tests, a series of nine specimens subdivided according to the geometry

9

configuration were employed: three with the steel plates positioned on the “upper” side, three on the “lower”

10

side and three on the “upper-inner” side of the GFRP profiles (Fig.1). As in the previous case, three GFRP

11

squared tubular specimens without the steel reinforcement were tested as a reference.

12 13

3. Experiments

14

3.1. Shear tests

15

The shear tests allowed to evaluate the compatibility between the GFRP and steel and to compare the

16

mechanical behavior of the single lap joints bonded with three epoxy adhesives, namely their load carrying

17

capacity, displacement, and stiffness.

18

The geometry of the specimens was manufactured according to [31]. The width of laminates was 25.4 mm

19

and the length 100 mm; the overlap length was 25.4 mm. The thickness of the GFRP and steel adherents

20

were respectively 5 mm and 3 mm.

21

Different bonding thickness among the three epoxy adhesives was employed as recommended by the

22

manufacturer, i.e. 0.3 mm, 2 mm and 1mm for EPX1, EPX2 and EPX3, respectively.

23

All tests were carried out on a Zwick/Roell Z050 testing machine of 50 kN capacity under displacements

24

control. Fig. 4a shows the setup where specimens were subjected to the shear test. The load was applied at

25

the slow rate of 1.25 mm/min. The elongation was measured by extensometer and the gauge length was set

26

to 55 mm. All specimens were loaded up to the joint failure.

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3.2. Puncture tests 7

1

The puncture tests aim to simulate the local stress transmitted by the curtain panels bolted to the GFRP

2

structural members. The steel plates are used with aim of reinforcing the GFRP bolted area, weakened by the

3

drilling that cuts the composite’s fibers.

4

The specimens were prepared according to the guidelines of a previous study, which investigated different

5

joining configuration of steel profiles and CFRP laminates [32]. In the present study, the GFRP tubular

6

profiles were adhesively joined to steel plates in three different configurations, see Section 2 and Fig.1. The

7

section area of the GFRP profiles is 50×80 mm2, the thickness is 5 mm, and the length of the profiles is 400

8

mm. The geometry of the steel plates is 35×400 mm2, 2 mm thick. For the three adhesives, the same bonding

9

thickness used in the single lap joints was employed.

10

All the tests were performed on the Zwick/Roell Z050 testing machine under displacements control. Fig.4b

11

shows the three-point bending set-up. The load was applied at a displacement rate of 3 mm/min. The

12

displacement in the lower side of the profiles was obtained by stereo-DIC using images recorded every 2s by

13

two synchronized cameras, until the failure of the specimen. Figure 5 shows a colour map of the

14

displacements measured in mm.

15 16

3.3. Three-point bending tests

17

The three-point bending tests were performed to reproduce the stress of the whole structural member when it

18

is subjected to the wind load. This is the typical load condition used to design the vertical structures

19

(mullions) constituting the curtain walls. The steel plates are used to enhance the flexural stiffness of GFRP

20

profiles. The same manufacture method of the short specimens used in the puncture test was followed. The

21

section area of the GFRP profiles is 50×80 mm2, the thickness is 5 mm, and the length of the profiles is 1000

22

mm. The geometry of the steel plates is 35×1000 mm2, with a thickness of 2 mm. The adhesive bonding

23

thickness for all specimens is 0.3 mm.

24

Fig. 4c shows the three-point bending set-up, the displacement was applied with a rate of 3 mm/min. As for

25

the puncture test, the displacement was measured using DIC.

26 27 28

4. Results and discussion 8

1

In this section, the mechanical responses and the failure modes of the GFRP-steel specimens (single lap

2

joints, squared tubular short and long profiles) are presented and analyzed. The results are subdivided

3

according to test type.

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4.1) Shear tests

6

4.1.1) Mechanical performance

7

Table 6 shows the average results of un-aged (as produced) and aged GFRP-steel single lap joints, subjected

8

to shear tests: the load carrying capacities (N), the maximum displacement recorded at maximum load break

9

(mm) and the stiffness (N/mm) are presented. The corresponding load-displacement curves are showed in

10

Figure 6.

11

With regard to un-aged samples (T0), the EPX1 joint has the highest load carrying capacity, twice than

12

EPX3. The stiffness of EPX1 and EPX2 joints are similar, while EPX3 showed the highest value (around

13

32500 N/mm), presenting a linear trend until the brittle failure (Fig. 6).

14

With regard to aged samples, the continuous condensation treatment (Tcc) produced more negative effects on

15

the mechanical responses than the UV exposure (Tuv). All the samples registered displacements increments,

16

especially those bonded by the EPX3 adhesive. Consequently, the stiffness had a large reduction (on average

17

of 90%) in all the tested adhesive, see Fig. 6. These results are in accordance with other authors founding,

18

which demonstrated that adhesively bonded joints exposed to different temperatures and humidity levels

19

registered high stiffness degradation, even without reaching the adhesive’s glass transition temperature [21].

20

After the UV exposure, the stiffness reduction was on average of 30%. This is caused by the wavelength of

21

UV radiation that, in the range of 90–400 nm, usually cause polymers bond dissociation and the consequent

22

decrease in adhesives elastic modulus [28, 33]. Regarding the load carrying capacity, all the samples showed

23

an increase of around 15-20%. It is fair to say that this outcome could be simply due to the variance of the

24

experimental results, nonetheless, as showed in a previous study [20], during the UV aging the specimens

25

were inevitably heated by radiations, reaching surface temperature values of about 26 ˚C. This even small

26

temperature increase could have determined a slight increment of the load carrying capacity, thanks to the

27

cross-linking of the polymer network [28].

9

1

With regard to the combination of the environmental conditions, both sequences (Tcc+Tuv and

2

Tuv+Tcc) produced an increase of the mechanical properties of the joints respect to those observed in the

3

previous aging conditions (Tcc and Tuv). In terms of stiffness, the best outcome was obtained after the Tcc+Tuv

4

exposure for EPX1 samples, with an increment of about 20 %. In the other environmental condition

5

(Tuv+Tcc), the load carrying capacity of EPX2 joints increased up to 36 %. As discussed, the UV exposure

6

could have led to increasing the load carrying capacity of the joints, even before and after the exposition to

7

the hot-wet condition, as previously mentioned. Even considering the data dispersion, the beneficial effect of

8

UV in the bonding is evident, because the Tcc produces a significant stiffness reduction that is not observed

9

in the combined sequences. Further studies are warranted in order to deeply analyze this aspect, which could

10

be the aim of a future work.

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4.1.2) Failure modes

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Figure 7 shows the four types of failure modes occurred during the tests and classified according to [30]. The

14

first is an “Adhesive Failure” (AF - Fig. 7a) and occurred at the interface between the adherend and the

15

adhesive, this is usually not acceptable in adhesive technology [34]. The second is a “Cohesive Failure” (CF

16

- Fig. 7b), occurring within the adhesive layer: this reveals a good compatibility between adhesive and

17

adherents. In the third picture (Fig. 7c), a “Light-Fiber-Tear Failure” (LFTF) is showed that occurred within

18

the GFRP adherent, with few glass fibers transferred from the adherent to the adhesive. In Fig. 7d there is an

19

example of “Mixed Failure” (MF), which combines two of the failure modes described above (CF and

20

LFTF).

21

The distribution of fracture modes in MF was evaluated by a graphical estimation. Table 7 shows the results

22

of un-aged and aged specimens.

23

In the un-aged condition (T0 ), the EPX1 showed the best results since the LFTF failure modes

24

mostly occurred. In this case, in fact, the optimal adhesion between the adherents led to the delamination of

25

the GFRP profiles. The poorest behavior was observed using EPX3, where all adhesive failures occurred.

26

After the single aging treatments (Tcc and Tuv), all adhesives showed very similar results to the un-

27

aged condition. EPX1 showed a slight worsening of the performance, especially after the UV exposure, with

10

1

almost mixed (adhesive – light fiber tear) failures. EPX2 presented a decrease of the performances with all

2

AF modes and EPX3 confirmed the un-aged results.

3

After the combination of the aging treatments (Tcc+T uv and Tuv+Tcc), all the specimens showed a

4

slight enhancement of the performance respect to the previous aging conditions (Tcc and Tuv). EPX1

5

presented almost all mixed failure modes, with a high percentage of the LFTF type, EPX2 epoxy showed the

6

little percentage of cohesive failure even if the main fracture type remained the adhesive one.

7 8

4.2 Puncture tests

9

4.2.1 Mechanical performance

10

Figure 8 represents the typical load-displacements curve obtained during a puncture test for a reinforced and

11

un-reinforced specimen. A two-stage response is observed, an almost linear elastic response up to a certain

12

level of force and, subsequently, a non-linear behavior with a lower stiffness.

13

In the case of reinforced specimens, after the linear stage, the load increases with many fluctuations, finally

14

exhibiting a plateau. This response is associated with the corresponding failure process of steel strengthened

15

sections. The linear stage ends when the initial failure at web–flange junction occurred, i.e. when the steel

16

plate penetrated the GFRP profile. The subsequent progressive failure in the webs is evidenced by the

17

plateau of the load–displacement response.

18

Since the section loses its structural integrity after the initial web–flange junction failure, the load at the end

19

of the linear stage is defined as the bearing capacity of the specimen. The same criterion can be used also for

20

un-reinforced specimens.

21

Table 8 shows the average results of un-aged (as produced) and aged GFRP-steel squared tubular short

22

specimens, subjected to puncture tests: the load carrying capacities (N), the displacements (mm) and the

23

stiffness (kN/mm) are listed. For each adhesive type, all the specimen’s configurations (upper, lower and

24

upper-inner) are compared to un-reinforce specimens. The corresponding load-displacement curves for the

25

three epoxy adhesives, before and after the two aging treatments, are showed in Figure 9, 10 and 11. The

26

non-linear stage was not showed in the graphs.

27

With regard to un-aged samples (T0), the stiffness of the reinforced specimens was always higher

28

than the un-reinforced ones, for every configuration and tested adhesives. The best result was obtained using 11

1

the GFRP profiles with the steel plate positioned in the upper-inner configuration (U-I), and the highest value

2

was registered by EPX1 adhesive. This result is consistent with other author’s findings, which demonstrated

3

the higher performance of hybrid specimens with respect to those without reinforcements [9, 10].

4

On the other hand, the load carrying capacity of the hybrid specimens resulted lower than the un-reinforced

5

GFRP profiles: this is because, during the load application, the steel plates penetrated inside the samples,

6

indenting the composite’s surfaces and weakening the performance of the bonding systems. The only

7

exception was the specimen in the upper configuration (U) bonded with EPX1 adhesive.

8

The two artificial aging (Tcc and Tuv) had similar effects on the mechanical response of the hybrid

9

specimens, which, in general, was deteriorated. The displacement was increased in almost all the GFRP-steel

10

specimens, with a maximum increase of about 60% for the specimens bonded with EPX1 adhesive.

11

Consequently, a large stiffness decrement was observed for all configurations. Nonetheless, as showed in

12

Figures 10 and 11, aged reinforced specimens still maintained a higher stiffness than un-reinforced ones,

13

especially after UV radiations. These results confirm the advantage of using such hybridization method, even

14

when the bonding system is exposed to adverse environmental conditions.

15 16

4.2.2) Failure modes

17

Fig. 12a presents the typical cracks occurred in GFRP profiles, both in un-reinforced and hybrid specimens

18

after puncture tests: in the compression zone, the loading member caused structural deformations along the

19

z-axis and web-flange separations occurred, with inter-laminar shear cracks at the corners of the pultruded

20

GFRP sections [3]. As showed in Fig. 12b, the GFRP-steel specimens in the upper (U) and upper-inner (U-I)

21

configurations presented a larger structural deformations since the steel laminate penetrated inside the GFRP

22

profile during the load application.

23

Looking at the bonding between GFRP and steel, the failure modes occurred always in the “adhesive” type

24

(according to the classification of ASTM D5573-99 [30]). The hybrid specimens, for the tested adhesive in

25

three different configurations, showed different behaviors. In Table 9, the Partial Detachment (PD), the Total

26

Detachment (TD) and No Detachment (ND) of the steel laminates are reported, before and after the aging

27

treatments.

12

1

In the un-aged condition (T0), EPX1 and EPX2 have a very similar behavior: in the upper (U) and

2

lower (L) configurations, no detachment occurred between GFRP and steel, while in the upper-inner (U-I)

3

configuration EPX1 showed a better performance of about 25%. The worst results were obtained using

4

EPX3 with the separation of the steel laminate in all the cases.

5

After the continuous condensation (Tcc) treatment, very similar results than the un-aged condition

6

were observed using EPX1 and EPX2, with a slight worsening of the failure modes in the upper-inner (U-I)

7

configuration. An unexpected behavior was observed for EPX3 that improved its performance in all the

8

specimens configurations.

9

After the UV (Tuv) exposure, better results than the un-aged condition were observed with EPX1 and EPX2,

10

while EPX3 showed the same behavior.

11 12

4.3) Three-point bending tests

13

4.3.1) Mechanical performance

14

Table 10 shows the average results of GFRP-steel squared tubular long specimens subjected to three-point

15

bending test: the load carrying capacities (N), the displacements (mm) and the stiffness (kN/mm) are

16

presented. As for the puncture test, only the data in the elastic range were taken into consideration. All

17

specimen’s configurations (upper, lower and upper-inner) were tested while a single adhesive, EPX1, was

18

used, i.e. the best adhesive according to the previous experiments.

19

The mechanical performance of the GFRP squared tubular profiles was increased by the steel reinforcements

20

in all configurations, especially in the lower (L) one. During this loading condition, in fact, the lower web is

21

the most stressed side of the GFRP profile, and the combination with the steel reinforcement resulted in the

22

most advantageous.

23

The stiffness of the lower hybrid samples resulted higher than the un-reinforced profiles, of about 80 %,

24

followed by the upper-inner configuration (+48 %).

25

The load carrying capacity of the lower hybrid specimens was again the highest among the other

26

configurations (+ 10 %) while, for the samples with the steel laminates positioned in the upper-inner (U-I)

27

side, the result was very similar to the un-reinforced ones.

28 13

1 2 3

4.3.2) Analytical model

4

A simple analytical model was used to validate the results obtained through the previous experimental tests

5

and to evaluate the effectiveness of the steel reinforcement. The elastic modulus EGFRP of the GFRP profiles

6

can be computed using the formula of the elastic deflection of a simply supported beam:

7


 = 

8

where l is the span length, F is the load and S is the displacement of the GFRP profiles. In this case, F and S

9

are the maximum values obtained at the end of the elastic ranges. IGFRP is the moment of inertia of the GFRP



(1)

 

10

profiles computed through the following equation:

11

 =

12

where Be and He are the dimensions of the external area of the tube and Bi and Hi are the corresponding

13

dimension of the internal area.

14

The stiffness kGFRP of a GFRP specimen in three-point bending, without the steel reinforcement, is given by:

15

 = 48 

16

(3)

17

In order to compute the stiffness kGFRP-STEEL of the hybrid system, two hypotheses can be assumed: perfect

18

bonding, in this case, the GFRP and the steel work together (k1GFRP-STEEL); no bonding, in this case, the two

19

materials do not collaborate (k2GFRP-STEEL).

20

A further distinction is necessary between upper and lower configurations and upper-inner configuration.

  

−

!

 

(2)



 

For upper and lower (UL) configurations, the total stiffness is given by:

21

./ 0122/

"#$ %&'

$ = ( )* 

23

(4)

24

where ESTEEL is the elastic modulus of the steel. IULGFRP and IULSTEEL [mm4 ] are the moments of inertia of the

25

GFRP and steel sections, computed through the following equations:

26

#$  = 3

27

(5)

!

+ 4

!

! 5

+ ( )* ,--* = 48 

./ 

22



!



+ 48 ,--*







14



0122/



0122/

#$ &'

$ = 3,--* + 4,--* [(

2

(6)

3

where AGFRP and ASTEEL are respectively the areas of the GFRP and steel sections, SSTEEL is the thickness of

4

the steel profile; XUL is the distance of the neutral axis from the barycentre obtained from the following

5

equation (see Fig. 13a and 13b):

6

=#$ = (

7

(7)

8

where SGFRP is the thickness of the GFRP profile.

9

If the two sections do not collaborate, the total stiffness is just the sum of the stiffness of the two separate

- 0122/  0122/



– 5) + (



)] =

(  0122/ )

1

>? B A -0122/   @ 0122/ @ @

-    A-   A-0122/  0122/

10

sections:

11

C#$ %&'

$ = (

!

+ (,--* = 48 

!

 



+ 4,--* [;  – 5< + (



)]

)

+ 48 ,--*

0122/ 



(8)

For the upper-inner (UI) configuration, the computation of the moment of inertia is different,

12 13

therefore the following equations have to be used:

14

"# %&'

$ = ( ) 

15

(9)

16

where IUIGFRP and IUISTEEL are computed using the following equations:

17

#  = 3

18

(10)

19

# &'

$ = 3#" ,--* + 3#C ,--* =

20

3,--* + 4,--* [(

21

(11)

22

where XUI [mm] is the distance from the neutral axis from the barycentre in upper-inner case, calculated

23

through the following formula (see Fig. 13c):

24

=# = (

25

(12)

26

If the two sections do not collaborate, the total stiffness is:

!

+ 4

 

!

! 5

+ ( ) ,--* = 48 



!

.D  

+ 48 ,--*

.D 0122/ 





– 5) + (

0122/ 

)] + 3,--* + 4,--* (

- 0122/  0122/  % -0122/   0122/

-    A-   A-0122/  0122/

 

– 5 − E

!

−

0122/  ) 

)

15

1

C# %&'

$ = (

2

(13)

!

+ 2(,--* = 48 

!

 

+ 2 (48 ,--*

0122/ 

)

3

The force-displacement curves obtained according to the analytical model were compared with those

4

observed in the experimental tests (Figure 14). The reinforced specimens have a behavior that is in between a

5

perfect collaboration and no collaboration, in particular, the lower configuration has the highest stiffness that,

6

for the first part of the test, is almost equal to the one predicted with perfect collaboration. Looking at the un-

7

reinforced GFRP profiles, the curves fit the analytical model with no collaboration, this is also consistent

8

with what expected.

9

The results confirm the effectiveness of the hybridization method [9, 10]. The long specimens were used to

10

look at the bending performance of the reinforced profiles. In this case, the failure modes observed were very

11

similar to the one obtained in the puncture tests and will not be further discussed here.

12 13

5. Conclusions

14

In the present study, an experimental campaign to study the bonding connection of GFRP pultruded profiles

15

and steel laminates through three different epoxy adhesives is proposed. The stiffness increase and

16

deformations reduction obtained by coupling steel laminates to GFRP pultruded profiles was analyzed with

17

the objective to verify the applicability of this hybrid system in structural members for curtain walls. The

18

compatibility of the two materials and the mechanical responses to local and global stresses were verified.

19

Different mechanical tests (i.e. shear, puncture and flexural) were performed and the effects of two

20

environmental conditions, continuous condensation and UV radiations, were investigated.

21

The main outcomes are:

22

- Shear tests demonstrated the compatibility of the GFRP-steel bonding system and the best mechanical

23

performance of the first epoxy adhesive (EPX1) was observed, both in un-aged and aged conditions.

24

Between the two aging treatments, the continuous condensation demonstrated higher negative effects on the

25

joints, registering stiffness decreasing values up to 93 %. After the UV radiations, the load carrying capacity

26

of the joints was instead increased thanks to a further polymerization of adhesives.

27

The specimens were also subjected to the combinations of the continuous condensation and the UV

28

radiations, in two different sequences, (Tcc+Tuv) and (Tuv+Tcc). Both the aging treatments showed better 16

1

effects on the mechanical responses of the joints than those registered in the aging conditions separately

2

analyzed, and the best results were obtained after the Tcc+Tuv exposure, in terms of stiffness increases.

3

- Puncture tests permitted to simulate the local stress transmitted by the curtain panels bolted to the GFRP

4

structural members. It was demonstrated that the stiffness of the GFRP reinforced profiles, with the steel

5

laminates positioned in the three different configurations (upper, lower, upper-inner sides of the GFRP

6

profiles), was higher than the un-reinforced ones. With regard to the load carrying capacity, the un-

7

reinforced GFRP profile showed better performance than the hybrid ones. This is because of the steel plates

8

that, during the loading condition, penetrated the GFRP surfaces. This fact suggests the authors to evaluate a

9

different geometry of the steel plate, in order to avoid this undesirable result.

10

EPX1 showed the best mechanical characteristics in un-aged condition. The two aging treatments

11

demonstrated negative effects on the specimens, especially the continuous condensation one. However,

12

hybrid squared tubular profiles subjected to aging treatments maintained higher stiffness in the elastic range

13

than un-reinforced GFRP profiles, in almost all the adhesives.

14

- Flexural tests were performed to simulate the stress of the whole structural member that occurred in curtain

15

walls (when it is subjected to the wind load). The hybrid specimens showed a significant increase in terms of

16

both load carrying capacity and stiffness, especially when the steel laminate was positioned in the lower

17

configuration. In fact, the lower side of the GFRP profile was subjected to tensile stresses and the steel

18

reinforcement allowed a higher containment of the deformations than in the other positions. The quality of

19

the reinforcement was also verified using an analytical model that distinguishes between perfect bonding and

20

no bonding of the steel plate.

21

The above mentioned results confirmed the reliability of the “Structural Member for curtain walls” basic

22

principle, i.e. the use of GFRP profiles as vertical structures (mullions), adhesively bonded to steel laminates,

23

with the objective of containing deformations and facilitating the fixing of the curtain panels.

24 25

References

26

[1] Godat A, Legeron F, Gagne V, Marmion B. Use of FRP pultruded members for electricity transmission

27

towers. Composite Structures 105 (2013) 408–421.

17

1

[2] Hollaway L.C. A review of the present and future utilisation of FRP composites in the civil infrastructure

2

with reference to their important in-service properties. Construction and Building Materials 24 (2010) 2419–

3

2445.

4

[3] Wu C, Bai Y, Zhao X-L. Improved bearing capacities of pultruded glass fibre reinforced polymer square

5

hollow sections strengthened by thin-walled steel or CFRP. Thin-Walled Structures 89 (2015) 67–75.

6

[4] Keller T. Recent all-composites and hybrid fibre-reinforced polymer bridges and buildings. Progress in

7

Structural Engineering and Materials 3 (2001) 132-140.

8

[5] Appelfeld D, Hansen C.S, Svendsen S. Development of a slim window frame made of glass fibre

9

reinforced polyester. Energy and Buildings 42 (2010) 1918-1925.

10

[6] Dispenza C, Pisano A.A, Fuschi P. Numerical simulations of the mechanical characteristics of glass fibre

11

reinforced C-profiles. Composites Science and Technology 66 (2006) 2980-2989.

12

[7] Turvey G.J. Testing of pultruded glass fibre-reinforced polymer (GFRP) composite materials and

13

structures. Woodhead Publishing Series Civil Structure Engineering (2013) 440–508.

14

[8] de Castro J, Keller T. Ductile double-lap joints from brittle GFRP laminates and ductile adhesives, part I:

15

experimental investigation. Composites part B 39 (2008) 271–81.

16

[9] Haoa Q, Wangb Y, Oua J. Design recommendations for bond between GFRP/steel wire composite rebars

17

and concrete. Engineering Structures 30 (2008) 3239-3246.

18

[10] Kim H-Y, Lee S-Y. A steel-reinforced hybrid GFRP deck panel for temporary bridges. Construction

19

and Building Materials 34 (2012) 192–200.

20

[11] Qureshi J, Mottramb J. T. Behaviour of pultruded beam-to-column joints using steel web cleats. Thin-

21

Walled Structures 73 (2013) 48–56.

22

[12] Wu C, Bai Y. Web crippling behavior of pultruded glass fibre reinforced polymer sections. Composite

23

Structures 108 (2014) 789–800.

24

[13] Borowicz D.T, Bank L.C. Behavior of pultruded fiber-reinforced polymer beams subjected to

25

concentrated loads in the plane of the web. Journal of Composites for Constructions 15(2) (2010) 229–38.

26

[14] Turvey G.J, Zhang Y. Characterisation of the rotational stiffness and strength of web–flange junctions

27

of pultruded GRP WF-sections via web bending tests. Composites Part A 37(2) (2006)152–64.

18

1

[15] Hai N. D, Mutsuyoshi H, Asamoto S, Matsui T. Structural behavior of hybrid FRP composite I-beam.

2

Construction and Building Materials 24 (2010) 956–969.

3

[16] Keller T, Gurtler H. Design of hybrid bridge girders with adhesively bonded and compositely acting

4

FRP deck. Composite Structures 74 (2006) 202–212.

5

[17] Keller T, Zhou A. Fatigue behavior of adhesively bonded joints composed of pultruded GFRP

6

adherends for civil infrastructure applications. Composites: Part A 37 (2006) 1119–1130.

7

[18] Keller T, Schollmayer M. Through-thickness performance of adhesive joints between FRP bridge decks

8

and steel girders. Composite Structures 87 (2009) 232–241.

9

[19] Schollmayer M, Keller T. Modeling of through-thickness stress state in adhesive joints connecting

10

pultruded FRP bridge decks and steel girders. Composite Structures 90 (2009) 67–75.

11

[20] Stazi F, Giampaoli M, Rossi M, Munafo P. Environmental ageing on GFRP pultruded joints:

12

Comparison between different adhesives. Composite Structures 133 (2015) 404–414.

13

[21] Zhang Y, Vassilopoulos A.P, Keller T. Environmental effects on fatigue behavior of adhesively-bonded

14

pultruded structural joints. Composite Science and Technology 69 (2009) 1022–1028.

15

[22] Zhang Y, Vassilopoulos A.P, Keller T. Effect of low and high temperatures on tensile behavior of

16

adhesively-bonded GFRP joints. Composite Structures 92 (2010) 1631–1639.

17

[23] EN ISO 527-1:2012. Plastics -Determination of tensile properties – Part 1: General principles.

18

[24] ISO 527-4:2012. Plastics -Determination of tensile properties – Part 4: Test conditions for isotropic and

19

orthotropic fibre-reinforced plastic composites.

20

[25] EN ISO 527-2:2012. Plastics -Determination of tensile properties – Part 2: Test conditions for moulding

21

and extrusion plastics.

22

[26] ASTM D 3418-03. Standard test method for Transition Temperatures of Polymers By Differential

23

Scanning Calorimetry.

24

[27] ISO 6270-2:2005. Paints and varnishes — Determination of resistance to humidity — Part 2: Procedure

25

for exposing test specimens in condensation-water atmospheres

26

[28] Nguyen T. Bai Y. Zhao X. Al-Mahaidi R. Effects of ultraviolet radiation and associated elevated

27

temperature on mechanical performance of steel/CFRP double strap joints. Composite Structures 94 (2012)

28

3563–3573. 19

1

[29] ASTM D904-99. Standard Practice for Exposure of Adhesive Specimens to Artificial Light.

2

[30] ASTM D5573-99. Standard practice for classifying failure modes in fiber-reinforced-plastic (FRP)

3

joints.

4

[31] ASTM D5868 – 01. Standard Test Method for Lap Shear Adhesion for Fiber Reinforced Plastic (FRP)

5

Bonding.

6

[32] Elchalakani M. CFRP strengthening and rehabilitation of degraded steel welded RHS beams under

7

combined bending and bearing. Thin-Walled Structures 77 (2014) 86–108.

8

[33] Grassie N. Scott G. Polymer degradation and stabilization. Cambridge: Cambridge University Press.

9

(1988).

10

[34] De Castro J. Experiments on double-lap joints with Epoxy. polyurethane and ADP adhesives.

11

Composite Construction Laboratory. Appendix B – Technical Report n. CCLab2000.1 b/2.

12 13

Figure captions

14

Figure 1. GFRP-steel squared tubular specimens in three different configurations, with the steel plates

15

positioned in the upper, lower and “upper-inner” sides of the GFRP profiles.

16

Figure 2. Dumbbell specimens dimensions.

17

Figure 3. Specimens subjected to the aging conditions: the hot-wet environment in the climatic chamber (a)

18

and the UV radiation in the wooden structure equipped with lamps (b).

19

Figure 4. Experimental setup: tensile test on GFRP-steel single lap joints (a), puncture resistance test on

20

GFRP-steel squared tubular short specimens (b), bending test on GFRP-steel squared tubular long specimens

21

(c).

22

Figure 5. A colour map of the displacements (measured in mm) of the GFRP-steel tubular specimens during

23

three-point bending tests.

24

Figure 6. Representative load-displacement trends of GFRP-steel single lap joints before (T0) and after the

25

aging treatments (Tcc, Tuv, Tcc+Tuv, Tuv+Tcc): comparison of three different epoxy adhesives.

26

Figure 7. Failure modes of GFRP-steel single lap joints: adhesive AF (a), cohesive CF (b), light-fiber-tear

27

LFTF (c) mixed MF (d) failures.

20

1

Figure 8. The load-displacements trend of GFRP squared tubular short specimens subjected to puncture test,

2

with and without the steel reinforcement.

3

Figure 9. Representative load-displacement curves of GFRP-steel squared tubular short specimens, bonded

4

with three epoxy adhesives, in the elastic range: comparison of the three different configurations with the un-

5

reinforced profile, in un-aged condition (T0).

6

Figure 10. Representative load-displacement curves of GFRP-steel squared tubular short specimens bonded

7

with three epoxy adhesives, in the elastic range: comparison of the three different configurations with the un-

8

reinforced profile after the continuous condensation treatment (Tcc).

9

Figure 11. Representative load-displacement curves of GFRP-steel squared tubular short specimens in the

10

upper-inner configuration in the elastic range: comparison of three epoxy adhesives with the un-reinforced

11

profile after the UV exposure (Tuv).

12

Figure 12. Failure modes of GFRP-steel squared tubular short specimens: typical cracks (a) and comparison

13

of un-reinforcement samples with reinforced ones in the three different configurations (b).

14

Figure 13. GFRP-steel squared tubular specimens in three different configurations: different positions of the

15

neutral axis (n) with respect to the barycentre (b).

16

Figure 14. Comparison of experimental and analytical stiffness results of GFRP squared long specimens.

17 18 19 20

21

1 2 3 4

Fig. 1

5 6

22

1 2 3 4

Fig. 2

5 6

23

1 2 3 4 5

Fig. 3

6 7

24

1 2 3 4 5

Fig. 4

6 7

25

1 2 3 4

Fig. 5

5 6

26

1 2 3 4 5

Fig. 6

6 7

27

1 2 3 4 5

Fig. 7

6 7

28

1 2 3 4

Fig. 8

5 6

29

Fig. 9

Fig. 10

Fig. 11

Fig. 12

Fig. 13

Fig. 14

Nomenclature Adhesive failure AF AGFRP ASTEEL At Be Bi CF DIC DSC EPX1 EPX2 EPX3 Et EGFRP ESTEEL

Section area of the GFRP profile Section area of the steel plate Application temperature External dimension of the section area’s base Internal dimension of the section area’s base Cohesive failure Digital Image Correlation Differential scanning calorimeter First epoxy adhesive Second epoxy adhesive Third epoxy adhesive Young modulus in tension Young modulus of GFRP profiles in tension

F

Load carrying capacity

U-I

He

External dimension of the section area’s height Internal dimension of the section area’s height Moment of inertia of the GFRP profiles section area Moment of inertia of the steel plate section area Stiffness of the GFRP sample Stiffness of the steel plate Span length

UV

Steel plate positioned on the lower side of the GFRP profile Light-Fiber-Tear Failure Mixed failure No detachment Partial detachment Displacement Service temperature Total detachment Un-aged conditions Artificial aging in climatic chamber Extrapolated onset temperature Extrapolated end temperature Glass transition temperature Artificial aging under UV rays Steel plate positioned on the upper side of the GFRP profile Steel plates positioned in the upper-inner side of GFRP profile Ultraviolet radiations

Wt

Working time at 22 °C

X εt

The distance of the neutral axis from the barycentre Tensile strain

σt σys τ

Tensile strength Tensile yield strength Average shear strength

Hi

IGFRP ISTEEL kGFRP kSTEEL l K1GFRP-

Young modulus of steel plates in tension

L LFTF MF ND PD S St TD T0 Tcc Teig Tefg Tg Tuv U

Stiffness of the GFRP-steel specimen: materials work together

STEEL

k2GFRP-

Stiffness of the GFRP-steel specimen: materials do not collaborate

STEEL

1 2 3 4 5

9 10

Table 1. GFRP profiles and steel mechanical properties according to manufacturer’s data sheets. GFRP PROFILES a STEEL PROFILES b Et (GPa) σt (Mpa) εt (%) Et (GPa) σys (Mpa) σt (Mpa) 25.0 250-450 1.0-1.8 198.0 326.7 385.5 a According to ASTM D638 / UNI 5819 b According to EN 10025-2: 2004

6 εt (%) 7 29.1 8

11 12 13

Table 2. Mechanical properties in tension of GFRP material.

14

GFRP PROFILES a Et (GPa) σt (Mpa) εt (%) 29.8 ±2.6 168.8 ±31.1 0.7 ±0.2 a According to EN ISO 527-1,4: 2012

15 16

Table 3. Technical and mechanical characteristics of the epoxy adhesives reported by manufacturers.

38

1 Et (MPa) σt (Mpa) εt (%) Tg (°C) 2 Series 3 EPX1 2966.39 ± 44.12 27.34 ± 0.77 2.39 ± 0.65 61.07 ± 3.34 4 EPX2 1774.03 ± 30.28 17.11 ± 0.70 3.81 ± 0.23 66.87 ± 0.45 5 EPX3 648.60 ± 29.56 11.13 ± 1.02 7.26 ± 0.56 46.90 ± 0.63 6 7 8 Adhesive

EPX1

EPX2

GHIJKLMN base

two-part epoxy adhesive controlled flow >300 15 to 25 -40+120 /

two-part epoxy adhesive

GOPQKQRIPLS TR (min) UR (°C) &R (°C) 'V (°C) &WXYMLI RXIMRJIPR τ* (MPa) σt (Mpa)
R (MPa) εt (%) #se

sand 33.5 / / 3 structural

pasty 120 15 to 25 -40+80 71 sand and degrease 15 17 1700 5 semistructural

9 10 two-part 11 epoxy adhesive12 13 fluid 14 15 5-20 20 16 -40+100 17 / 18 EPX3

sand and degrease19 / 20 17 / 21 / semistructural22

* On aluminum/steel adherents

23 24 25 26 27 28 29 30 31

Table 4. Mechanical properties in tension and glass transition temperatures of the adhesives.

Table 5. Test programme Samples Single lap joints

Adhesive EPX1 EPX2 EPX3

GFRP-steel squared tubular short specimens EPX1

EPX2

EPX3 without steel GFRP-steel squared tubular long specimens EPX1

Configuration / / / Ua Lb U-Ic U L U-I U L U-I / U L U-I

T0 4 4 4 3 3 3 3 3 3 3 3 3 3 3 3 3

Tcc 4 4 4 3 3 3 3 3 3 3 3 3 -

Tuv 4 4 4 3 3 3 -

-

-

Tcc+Tuv 3 3 -

Tuv+Tcc 3 3 -

-

-

-

-

-

-

-

-

-

-

39

1 2 3 4

without steel / the steel plate positioned on the upper side of the GFRP profile b the steel plate positioned on the lower side of the GFRP profile c the steel plates positioned in the upper-inner side of the GFRP profile

3

-

-

-

-

a

5 6 7 8

Table 6. Mechanical properties of GFRP-steel single lap joints bonded with three different adhesives: results before

9

(T0) and after (Tcc, Tuv, Tcc+Tuv, Tuv+Tcc) the aging treatments.

STIFFNESS

DISPL.

LOAD

Adhesive Ageing condition EPX1 EPX2 EPX3 T0 (N) 5052.70 ± 1505.86 2818.54 ± 774.91 2480.49 ± 253.30 Tcc (%) - 15.20 - 25.70 - 51.49 Tuv (%) + 15.54 + 21.37 + 24.63 Tcc + Tuv (%) + 29.12 - 15.95 Tuv + Tcc (%) + 21.65 + 36.60 T0 (mm) 0.20 ± 0.06 0.12 ± 0.03 0.10 ± 0.01 Tcc (%) + 92.96 + 68.94 + 561.97 Tuv (%) + 6.58 + 88.82 + 78.71 Tcc + Tuv (%) + 18.70 - 15.39 Tuv + Tcc (%) + 12.79 + 20.62 T0 (N/mm) 30694.25 ± 4291.36 30909.75 ± 2557.24 32549.75 ± 2736.74 Tcc (%) - 93.53 - 84.96 - 92.45 Tuv (%) - 20.03 - 32.13 - 37.63 Tcc + Tuv (%) + 19.80 - 14.41 Tuv + Tcc (%) + 11.26 + 6.71 -

10 11 12

Table 7. Failure modes of GFRP-steel single lap joints: results before (T0) and after the aging (Tcc, Tuv, Tcc+Tuv, Tuv+Tcc) treatments: adhesive (AF), cohesive (CF), mixed (MF), light-fiber-tear (LFTF) failures. T0

Tcc 2 LFTF 1 MF (90% LFTF, 10% AF) 1 MF (85% LFTF, 15% AF)

Tuv 1 LFTF 2 MF (40% LFTF, 60% AF) 1 MF (20% LFTF, 80% AF)

4 AF

4 AF

EPX2

1 LFTF 3 AF

EPX3

4 AF

4 AF

4 AF

EPX1

3 LFTF 1 MF (20% LFTF, 80% AF)

Tcc + Tuv 1 LFTF 1 MF (50% LFTF, 50% AF) 1 MF (95% LFTF, 5% CF) 1 AF 1 MF (90% AF, 10% CF) 1 M (95% AF, 5% CF) /

Tuv + Tcc 1 LFTF 2 MF (80% LFTF, 20% AF)

2 AF 1 MF (95% AF; 5% CF)

/

13 14 15 16 17 18 40

1 2

Table 8. Mechanical properties of GFRP squared short specimens, with and without the steel reinforcement: results before (T0) and after (Tcc, Tuv) the aging treatments. Adhesi Configurat ve ion

Load T0 (N)

U EPX1

L U-I U

EPX2

L U-I U

EPX3

L U-I

Witho ut steel

15859. 21 12338. 46 11238. 64 11900. 37 13874. 00 12351. 41 7959.9 6 13974. 55 7643.2 2

Displacement

Tcc Tuv T0 (%) (%) (mm) ±2434. 0.98 83 17.38 ±1603. -3.26 0.79 75 ±2363. +18.8 +5.39 0.61 16 1 ±782.5 0.78 3 25.95 ±1173. 1.01 74 16.71 ±810.2 0.62 1 31.74 22.13 ±1794. 0.48 53 29.02 ±417.8 1.76 6 24.56 ±313.2 +1.78 0.50 1 20.42 dev.st.

14556. ±347.1 56 0

-

-

1.06

dev. st. ±0.2 1 ±0.1 4 ±0.0 9 ±0.0 7 ±0.0 4 ±0.0 4 ±0.0 6 ±0.1 4 ±0.2 1 ±0.0 6

Stiffness

Tcc Tuv T0 Tcc Tuv dev.st. (%) (%) (N/mm) (%) (%) +28.3 18599.8 ±1906. 2 9 98 30.41 +74.4 18249.4 ±2611. 8 6 05 36.05 +60.3 +57.7 22552.0 ±4389. 6 2 5 90 43.63 40.04 +11.3 16282.8 ±1630. 4 6 84 40.33 +37.2 15041.1 ±816.0 -9.35 8 2 1 21554.1 ±991.6 -7.27 13.54 2 9 30.17 19.73 19861.3 ±2744. -3.81 -9.31 4 33 +17.1 15512.1 ±264.0 5 0 5 49.71 20345.1 ±6162. -8.38 -1.97 3 22 28.23 17.56 -

-

12647.5 ±92.63 0

3 4 5 6 7

Table 9. Failure modes of GFRP squared short specimens with the steel reinforcement: results before (T0) and after (Tcc. Tuv) the aging treatments: Partial Detachment (PD), Total Detachment (TD), No Detachment (ND) of the steel laminates.

U

T0 L

EPX1

ND

ND

EPX2

ND

ND

U-I* 75% PD 25 %ND TD

U

Tcc L

U-I*

ND

ND

TD

ND

ND

50% PD 50% ND

Tuv U-I* 50% PD 50%ND 50% PD 50%ND

35% PD TD TD ND ND ND TD 65%ND *The detachment occurred only for the steel laminate positioned in the inner side of the squared tubular profiles. EPX3

8 9 10 11 12 13 14

41

1 2

Table 10. Mechanical properties of GFRP squared long specimens, with and without the steel reinforcement. Adhesive

Load (N)

Configuration medium

EPX1

Displacement (mm)

Stiffness (N/mm)

∆*(%) medium dev.st. ∆*(%) medium

dev.st.

∆*(%)

U

15493.99 ±1147.18 +7.14

6.60

±0.59 -16.82 3466.85 ±42.64 +30.52

L

15909.07 ±846.70 +10.01

6.92

±0.67 -12.75 4834.80 ±685.26 +82.02

14429.68 ±531.00

5.64

±0.82 -28.92 3925.90 ±14.00 +47.80

U-I

3

dev.st.

-0.22

14462.12 ±766.92 7.94 ±1.29 Without steel * the percentage variation is respect to the un-reinforced specimens (without steel).

2656.15 ±21.28

-

4 5 6

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