Testing of new adhesive and CFRP laminate for steel-CFRP joints under sustained loading and temperature cycles

Composites Part B 99 (2016) 235e247

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Composites Part B journal homepage: www.elsevier.com/locate/compositesb

Testing of new adhesive and CFRP laminate for steel-CFRP joints under sustained loading and temperature cycles Ankit Agarwal a, b, *, Stephen J. Foster c, Ehab Hamed a a

Centre for Infrastructure Engineering and Safety, School of Civil and Environmental Engineering, University of New South Wales, Sydney, 2052, Australia Cooperative Research Centre for Advanced Composite Structures, 1/320 Lorimer Street, Port Melbourne, Victoria, Australia c School of Civil and Environmental Engineering, University of New South Wales, Sydney, 2052, Australia b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 February 2016 Received in revised form 27 April 2016 Accepted 3 June 2016 Available online 6 June 2016

The influence of combined mechanical and environmental loads on the behaviour of CFRP strengthened steel structures is crucial for their safe use and effective design, and requires further fundamental investigations. Past studies have shown that ‘catastrophic’ failure of steel-CFRP joints can occur under just 15% of the ultimate load during long term exposure to wet thermo-mechanical loading. In this paper, the results of an experimental investigation into the effects of thermo-mechanical loading in both wet and dry conditions on steel-CFRP single-lap joints are presented. Two types of CFRP and a new adhesive that has a glass transition temperature (Tg) of 82
C are used in the study. The results revealed no failure in the joint during exposure to thermal cycling between 10
C and 50
C, with and without an applied sustained load, in both wet and dry conditions and with little degradation in the residual bond strength. It was recommended that the Tg of the adhesive should be at least 30
C greater than the highest service temperature in order to avoid failure under thermo-mechanical loading. It was also recommended to seal the adhesive joint with water-proof sealant in the CFRP repair system to prevent the ingress of moisture. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Debonding Thermo-mechanical Steel-CFRP joints Glass transition temperature Moisture

1. Introduction Carbon fiber reinforced plastics (CFRPs) composites are increasingly being used as a repair material for steel structures due to their high strength to weight ratio, installation flexibility, and long term durability [1,2]. Traditional methods of repairing steel structures such as steel sleeve repair are usually bulky, difficult to apply, and prone to corrosion [3]. Even though considerable research has been conducted to clarify the suitability of using FRP composites as a repair material at different environmental conditions [4e10], the effects of fluctuating thermal conditions under sustained load are not well investigated [11]. Recent state of the art reviews by Fawzia and Kabir (2012) [12] and Heshmati et al. (2015) [13] confirm that the available literature on environmental durability, especially thermo-mechanical loading, of CFRP strengthened steel members is minimal compared to FRP repaired concrete structures. As CFRP repaired members can be subjected to different

* Corresponding author. Centre for Infrastructure Engineering and Safety, School of Civil and Environmental Engineering, University of New South Wales, Sydney, 2052, Australia. E-mail address: [email protected] (A. Agarwal). http://dx.doi.org/10.1016/j.compositesb.2016.06.039 1359-8368/© 2016 Elsevier Ltd. All rights reserved.

fluctuating thermal conditions and loading scenarios during their service life including sustained loads, it is critical to clarify and investigate the influence of these environmental conditions on the behaviour and bonding capacity of CFRP bonded joints. One of the main failure modes that characterize CFRP strengthened steel members is debonding failure, which is mainly governed by the weak adhesive layer that can be subjected to both shear and peeling stresses. Therefore, the durability of the repaired system does not only depend on the mechanical properties of the components (CFRP, adhesive, and steel) but mainly on the interfacial bonding between the components [13,14]. Because, it is not always possible to perform durability tests on full scale strengthened steel structures, small scale testing of adhesively bonded lap joints are typically conducted, which is the focus of this study. A number of studies have been undertaken to investigate the impact of moisture content, elevated temperature and other environmental conditions on the behaviour of steel-FRP joints [4e9]. In all of these studies, degradation in the residual strength and stiffness of steel-FRP joints was observed upon exposure to different environmental conditions. Only the degree of degradation varied, from case to case, depending on the type of materials used, the environmental exposure and the loading scenario. Recently, Korta

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et al. (2015) [15] observed significant bond strength degradation in CFRP-aluminium joint upon exposure to combined moderate humidity and temperature with no external force. Jiang et al. (2015) [16] investigated the impact of hygrothermal ageing (40
C under water for four months) on steel-CFRP adhesive joints and found significant reduction in the ultimate strength and stiffness of the adhesive joint. The hygrothermal ageing also changed the failure mode from partial interfacial failure to complete FRP delamination. Heshmati et al. (2016) [17] concluded based on experimental investigation that moisture have a beneficial effect on the strength of the steel-FRP joint for shorter duration but may cause localized interfacial damage during longer period. Oudad et al. (2012) [18] also found that humidity absorption reduces the rigidity of the joint. The effects of cyclic thermal environments such as freezethaw cycles and thermal cycles on steel-CFRP joints have also been studied [10,19e21]. Nguyen et al. (2012) [20] and Agarwal et al. (2014) [10] found strength reduction in steel-CFRP joints upon exposure to freeze-thaw cycles and thermal cycles, respectively, while Kim et al. (2012) [19] and Yong-xin et al. (2005) [21] found an increase in the load carrying capacity of steel-CFRP joints upon exposure to freeze-thaw cycles and hot/wet cycle, respectively. In all these studies, testings to failure of the specimens were performed after exposure to the respective environmental scenarios. The influence of simultaneous exposure to environmental conditions and mechanical loading, particularly sustained loading, that is a close representation of the on-site conditions was not investigated. There have been only a few studies, such as those of Dawood and Rizkalla (2010) [5], Nguyen et al. (2013) [22], Nguyen et al. (2012) [23] and Agarwal et al. (2015) [11] that investigated the impact of combined sustained loading and environmental conditions. With the exception of Dawood and Rizkalla (2010) [5], who did not find any significant impact of sustained load on steel-CFRP joints that were exposed to wet-dry cycles, the other studies showed results that raise concerns regarding the use of FRP repair system under sustained loads. Nguyen et al. (2012) [23] measured a reduction of 30% in the bond strength of steel-CFRP joint upon 400 min of exposure to sustained load that equals to only 20% of the instantaneous failure load, combined with thermal cycling between room temperature and 50
C. Nguyen et al. (2013) [22] found that all steel-CFRP joints failed under a sustained load that equals to only 15% of the instantaneous failure load within two hours of exposure to cyclic temperature between 20
C and 50
C at 90% relative humidity (RH). Based on the review of environmental durability of CFRP repair systems, Bai et al. (2014) [24] inferred that the combination of load, temperature, and moisture is the most critical scenario for CFRP strengthened steel members that may lead to catastrophic failure of strengthened members. Similar observations, with more detailed results, were made in a recently conducted experimental investigation by Agarwal et al. (2015) [11] regarding the impact of thermo-mechanical loading on steelCFRP adhesively bonded joints. They found that no significant degradation in the bond strength was observed when the specimens were exposed to sustained load or thermal cycling separately. However, when both the mechanical and thermal loads were applied simultaneously in immersed conditions, all specimens that were under a sustained load that equals to only 30% of the instantaneous failure load failed within three days of the thermomechanical load application, while specimens under 50% of the instantaneous failure load failed within one day. The adhesives used by Agarwal et al. (2015) [11] had a glass transition temperature Tg that was only 20
C higher than the service temperature range. Therefore, it is important to understand whether all adhesives fail under combined sustained load and thermal cycles irrespective of their Tg. It is also crucial to conduct the experiments in

both wet as well as dry conditions in order to understand the role of water since the thermo-mechanical loads were applied in immersed conditions in Agarwal et al. (2015). In this study, an experimental program was designed that aims to investigate the effects of four different environmental and loading scenarios on the long term strength and durability of steelCFRP adhesively bonded joints with new adhesive and CFRP. The idea of testing new materials is a part of an ongoing research work that aims to develop new systems that overcome the premature failures under combined sustained loads and temperature cycles. As per ACI (2008) [25], the glass transition temperature (Tg) of the adhesive must be at least 15
C above the service temperature in dry environments. It was also mentioned that further research is needed to identify the critical service temperature for FRP systems in other environments [25]. In Agarwal et al. (2015) [11], failure in the adhesive joint was observed even when the Tg was 20
C above the testing temperature. In this testing, the testing temperature range was chosen such that the upper temperature range is at least 30
C below the Tg of the adhesive in order to clarify the acceptable range of temperature for structural adhesives. 2. Experimental program A total of 45 steel-CFRP single lap shear specimens were tested under different environmental and loading conditions. A new kind of adhesive that has a higher Tg compared to most commercially existing ones is used for the preparation of the specimens. This adhesive, which is referred to here as Adhesive C, is newly manufactured non-commercial adhesive. Two types of CFRP were used in this testing; one is a commercial CFRP (or CFRP1) used in Agarwal et al. (2015) [11], and other is a newly manufactured noncommercial CFRP (or CFRP2). A total of 26 of the 45 specimens were prepared using CFRP1, while the remaining 19 specimens were prepared using CFRP2. Control tests were first conducted, where the specimens were tested to failure immediately after the curing stage at a displacement control rate of 1.27 mm/min, as per ASTM D1002 [26], in a 10 kN capacity testing machine (see Fig. 1). The environmental conditions investigated in this study include a) sustained load only under ambient temperature, b) wet thermal cycling only, c) both sustained-load and wet thermal cycling, and d) both sustained load and thermal cycling in dry conditions. Testing of joints under dry conditions is new and has not been conducted in Agarwal et al. (2015) [11]. Two levels of sustained loads (i.e. 30% and 50% of the instantaneous failure load), and one range of thermal cycling (10
C to 50
C) were investigated. In general, three specimens were tested for each series of testing. The geometrical properties of the lap-shear specimens are shown in Fig. 2. The bonded area of the steel-CFRP joint is 25 mm  25 mm and the thickness of the adhesive layer is about 0.5 mm. The specimens were numbered as [AA]-[BB]-[CC]-[DD], where ‘AA’ represents the type of specimens (‘LS3’ for lap-shear joint with CFRP1; ‘LS4’ for lap-shear joint with CFRP2); ‘BB’ represents the thermal condition (‘0’ for no thermal cycling, ‘T1’ for thermal cycling between 10
C and 50
C in immersed conditions, and ‘T2’ for thermal cycling between 10
C and 50
C in dry conditions); each thermal cycle consisted of two and a half hours of a temperature of 10
C, followed by two and a half hours of a temperature of 50
C. ‘CC’ represents the applied sustained load (‘0’ for no sustained load, ‘30’ and ‘50’ for a sustained load that equals to 30% and 50% of the instantaneous failure load, respectively); and ‘DD’ represents the specimen’s number. For example, LS4-T1-50-1 is the first lap shear specimen of the series with steel and CFRP2 adherends, glued together with Adhesive C, exposed to thermal cycling between 10
C and 50
C under 50% of the instantaneous failure load in immersed conditions. LS3-T2-30-2 is the second

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test includes three specimens that were loaded to 30% of the instantaneous failure load, and three specimens to 50% of the instantaneous failure load. The load was then kept constant for 21 days along with the exposure to thermal cyclic environment (10
C to 50
C) in immersed conditions. Test series 5 includes six lapshear specimens with CFRP1 that were also loaded to similar sustained loads (three specimens under 30% and three under 50% of the instantaneous failure loads) but exposed to thermal cycling between 10
C and 50
C in dry conditions. The specimens that had not failed during the long term exposure were then tested for the residual strength after seven days of recovery period. 2.1. Preparation of steel-CFRP joints The preparation of steel-CFRP joints is the most critical step of lap-shear testing due to the sensitivity of single-lap joint to different thickness profiles of the adhesive layer [27]. 10 steel-CFRP specimens (25 mm wide) were prepared at a time from each 300 mm wide steel and CFRP plates. In this testing, the specimens were carefully prepared using the six following steps: i. Cutting and preparation of CFRP Strips: The CFRP laminates were first cut to size 300 mm  200 mm using a 2.9 mm rotary saw blade with air cooling. ii. Preparation of steel plates

Fig. 1. 10 kN INSTRON testing machine.

Mild steel plates of size 200 mm  300 mm  3 mm were ordered from the manufacturer. The plates were cleaned with acetone and lint free cloth. A 40 mm  300 mm strip along the edge of one of the surfaces was grit blasted according to ISO 8501e1:2007 [28] in order to achieve steel surface essential for adhesive bonding. The grit blasting was carried out using a garnet grit (grit size of 30e60) in a compressed air regulator set to 550 kPa, a nozzle angle of approximately 45
, and a nozzle diameter of 7 mm positioned approximately 100 mm from the surface under preparation using a Hafco Metalmaster SB-420 sand blasting cabinet. After grit blasting, the steel surface was wiped with dry lint free cloth. iii. Preparation of tooling plate for layup

Fig. 2. Dimension of steel-CFRP single lap shear specimen.

specimen with steel and CFRP1 adherends, glued with Adhesive C, exposed to dry thermal cycling between 10
C and 50
C under 30% of the instantaneous failure load. The experimental plan for steel-CFRP single lap shear specimens with CFRP1 and CFRP2 are shown in Table 1 and Table 2, respectively. As shown in the tables, test series 1 and 6 were control testing of the joints with CFRP1 and CFRP2, respectively. Test series 2 and 7 include three specimens with CFRP1 and CFRP2, respectively, each under sustained loads equal to 30% and 50% of the instantaneous failure load. The loads were kept constant for 21 days under ambient temperature. After 21 days, the specimens were unloaded, allowed to recover for 7 days, and were then tested for their residual strength, similar to the control specimens. Test series 3 and 8 include three specimens each with CFRP1 and CFRP2, respectively, that were exposed to wet thermal cycling between 10
C and 50
C only without any sustained load for 21 days. After 7 days of recovery, the specimens were tested for their residual strength. Test series 4 and 9 include testing of specimens with CFRP1 and CFRP2, respectively, under wet thermo-mechanical loading. Each

A 300  400 mm aluminium plate was cleaned with acetone and was checked to ensure that the surface had a smooth profile. An Airtech WL5200 release film was placed on the aluminium surface, and secured in position using Henkel Clingtape 48 mm waterproof gaffer tape. The surface of the release film was wiped by a clean dry cloth. The aluminium plate was placed on a flat surface and the 200  300  3 mm steel adherend plate was placed on the tool. The steel tool plate was cleaned with acetone and lint-free cloth, and a layer of Airtech WL5200 was placed over the top. The 300 mm edges of both the plates were placed against one another. A 175  300  10 mm aluminium spacer was placed over the steel adherend such that an area of 25 mm  300 mm of steel adherend remained exposed. iv. Application of Adhesive C and placement of CFRP adherend on steel adherend Two layers of Adhesive C, which was mixed as per the specifications, were applied both on the steel and CFRP adherends. The CFRP adherend was then immediately placed onto the steel adherend thus making the bonded area of 300 mm  25 mm. A 200  300  25 mm steel mass was placed over the layup to apply

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Table 1 Experimental plan and bond strengths of steel-Adhesive C-CFRP1 joints under different thermo-mechanical conditions.

Table 2 Experimental plan and bond strengths of steel-Adhesive C-CFRP2 joints under different thermo-mechanical conditions.

Test series

Conditions

Specimens

Bond strength (MPa)

Test series

Conditions

Specimens

Bond strength (MPa)

1

Control

Control

7

30%a Sustained load

30%a Sustained load

11.66 8.98 11.36 9.26 8.60 9.97 7.79 9.06 8.30 8.38 8.87 10.91 9.00 9.60 8.18 9.72 11.33 9.74 e 9.29 8.62 9.00 9.91 9.33 10.00 9.75 10.18 11.05 9.72 10.32 Failed 9.07 9.5 9.30

6

2

LS3-0-0-1 LS3-0-0-2 LS3-0-0-3 LS3-0-0-4 LS3-0-0-5 Average LS3-0-30-1 LS3-0-30-2 LS3-0-30-3 Average LS3-0-50-1 LS3-0-50-2 LS3-0-50-3 Average LS3-T1-0-1 LS3-T1-0-2 LS3-T1-0-3 Average LS3-T1-30-1 LS3-T1-30-2 LS3-T1-30-3 Average LS3-T1-50-1 LS3-T1-50-2 LS3-T1-50-3 Average LS3-T2-30-1 LS3-T2-30-2 LS3-T2-30-3 Average LS3-T2-50-1 LS3-T2-50-2 LS3-T2-50-3 Average

LS4-0-0-1 LS4-0-0-2 LS4-0-0-3 LS4-0-0-4 Average LS4-0-30-1 LS4-0-30-2 LS4-0-30-3 Average LS4-0-50-1 LS4-0-50-2 LS4-0-50-3 Average LS4-T1-0-1 LS4-T1-0-2 LS4-T1-0-3 Average LS4-T1-30-1 LS4-T1-30-2 LS4-T1-30-3 Average LS4-T1-50-1 LS4-T1-50-2 LS4-T1-50-3 Average

14.29 13.36 13.88 13.78 13.83 13.36 14.59 13.23 13.73 13.82 14.35 12.25 13.48 11.07 11.03 12.77 11.62 e 13.92 14.78 14.3 13.13 13.80 Failed 13.46

50%a Sustained load

3

Only Thermal cycle

4

Wet thermal cycle þ 30%a sustained load

Wet thermal cycle þ 50%a sustained load

5

Dry thermal cycle þ 30%a sustained load

Dry thermal cycle þ 50%a sustained load

a The percentage is with respect to the control strength obtained under instantaneous loading only at ambient conditions.

sufficient weight for bonding of the two adherends. The final set-up is shown in Fig. 3.

50%a Sustained load

8

Only Thermal cycle

9

Wet thermal cycle þ 30%a sustained load

Wet thermal cycle þ 50%a sustained load

a The percentage is with respect to the control strength obtained under instantaneous loading only at ambient conditions.

of pure adhesive coupon specimens as per ASTM D638-10 [29]. Adhesive C is an epoxy based primer, and is a mixture of two parts. The average tensile strength of the adhesive was 18 MPa, and the average elastic modulus of the adhesive specimens was 3320 MPa. The glass transition temperature (or Tg) of the adhesive is 82
C as per manufacturer’s data sheet, which is higher than the glass transition temperatures of Adhesives A and B (50
C and 62
C) used in Agarwal et al. (2015) [11]. 2.3. Thermal cycling apparatus and instrumentation

v. Curing Operation The specimens were cured at 55
C for 48 h in an environmental chamber. The temperature of the chamber was recorded and monitored using DT50 DataTaker. After curing, the specimens were removed from the chamber and allowed to cool. vi. Cutting Operations The single-lap shear specimens were cut-to-size 25 mm  375 mm using water jet cutting where 10 specimens were obtained from each 300 mm  375 mm set-up. The final specimen’s geometry is shown in Fig. 2. 2.2. Material properties The material properties of steel, CFRP1, and CFRP2 were obtained from their respective manufacturer’s specifications. The elastic modulus and the tensile strength of CFRP1 laminate were 165 GPa and 2800 MPa, respectively. The elastic modulus and tensile strength of CFRP2 were 50 GPa and 650 MPa, respectively. The elastic modulus of the steel was 207 GPa, and the yield and the ultimate strengths were 350 MPa and 430 MPa, respectively, as per the manufacturer’s data sheet. The tensile properties of the adhesive were obtained by testing

The thermal cycling equipment used in this study is shown in Fig. 4. The apparatus consists of four units e hot cycle unit, cold cycle unit, test tank and the controller. The equipment can apply sustained loading with wet thermal cycling on six specimens simultaneously. In this experiment, the cycle time for each of the cold and hot water cycle is 150 min (2 h and 30 min); which makes the time period of the thermal cycle to be five hours. A typical thermal cycle profile as measured in this test is shown in Fig. 5. It can be seen that even though the targeted temperature during the hot cycle was 50
C, a temperature of up to 48
C was achieved from the apparatus. This was mainly due to loss of energy to the surroundings, through pipe, ducts, and other connections in the system. Fig. 6 shows the experimental set-up of the steel-CFRP single lap joint for thermo-mechanical testing and a photo of a loaded specimen. For test series 5, the specimens that were tested under dry conditions were coated with silicon sealant to make it waterproof before the testing. The loads on the specimens were measured using 20 kN range load cells. For the long-term tests, the displacements were measured using two 2.5 mm range LVDTs which have an accuracy of ±2.5 mm. For the residual strength and short-term tests, 10 mm range LVDTs were used. The gauge length was the full 225 mm length of the specimen, as shown in Fig. 6. The temperatures of the test tank and of the specimens were measured using two

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after a 7 day of recovery period. None of the specimens either with CFRP1 or with CFRP2 failed during the long term exposure.

Fig. 3. Final set-up for the preparation of single lap-shear specimen.

thermocouples e one thermocouple measured the temperature of the test tank, and the other measured the temperature at the surface of the specimen. 3. Results The results of the steel-CFRP single lap shear specimens under different thermo-mechanical conditions are described in this section. Detailed results of the individual specimens are given in Tables 1 and 2. 3.1. Control specimens (instantaneous loading e no thermal cycling) e test series 1 and 6 3.1.1. CFRP1 (test series 1) The stress versus displacement curves of the steel-CFRP1 lap joint are shown in Fig. 7(a). The average failure stress, which is calculated as failure load divided by the bonded area, was 10 MPa and the average displacement at failure load was 0.49 mm. It can be seen that the response of the adhesive joint is mostly linear for all specimens. Three specimens (LS3-0-0-2, LS3-0-0-4 and LS3-0-0-5) failed by delamination of the fibers within the CFRP but other two specimens (LS3-0-0-1 and LS3-0-0-3) failed by a mixed mode failures (CFRP delamination and adhesive failure at steel-adhesive interface) under higher loads. It is important to note that these are similar specimens but with different behaviour in terms of failure, which can be attributed to the sensitivity of single lap joints to different thickness profiles of the adhesive layer as well as to other parameters in the system [27]. The typical failure modes observed in this set of testing are shown in Fig. 8(b) and (c). Fig. 8 also shows other failure modes observed in this testing program as discussed in the subsequent. 3.1.2. CFRP2 (test series 6) The stress versus displacement curves of the steel-CFRP2 lap joint are shown in Fig. 7(b). The average failure stress was 13.8 MPa and the average displacement at failure load was 1.19 mm. Again here, the response is mostly linear similar to the previous case. However, the predominant mode of failure in this case was adhesive failure at steel-adhesive interface, as opposed to dominant delamination failure in the previous case with CFRP1. This indicates that the bond between steel and adhesive was the critical link in this case. It can be noted from Fig. 7 that the displacements are larger in case of CFRP2 as compared to joints with CFRP1. This is attributed to the lower elastic modulus of CFRP2 compared to CFRP1 (50 GPa versus 165 GPa), which allowed larger displacements to develop. 3.2. Sustained load only e test series 2 and 7 Six steel-CFRP joints were kept under 30% and 50% of the instantaneous failure load (3 each) for both CFRP1 and CFRP2 at ambient conditions for 21 days, and tested for residual strength

3.2.1. CFRP1 (test series 2) The stress versus displacement curves that are obtained during the residual strength testing are shown in Fig. 9. The average curve of the control specimens is also shown for comparison. The average residual strength for the specimens under 30% sustained load is 8.4 MPa, which is about 16% lower than that of the control specimens (10.0 MPa). The average displacement at failure load is 0.403 mm, which is about 17% lower than that of the control specimens (0.488 mm). The average residual strength of the specimens under 50% sustained load is 9.6 MPa, which is close to the strength of the control specimens (10.0 MPa). The average displacement at failure load is about 0.397 mm. Although the reduction in the strength is relatively small and within the tolerances of the experimental program, it is expected that the specimens under 50% sustained load would exhibit a larger reduction in the residual strength. Yet, the average response shows an opposite trend, which is mainly attributed to the sensitivity of the bonded joint to many parameters including the preparation procedure and the bond line thickness [27]. The dominant mode of failure for all specimens in this series is a CFRP delamination failure with minor debonding near the edges. This failure is similar to the failure mode observed in the control specimens. Thus, exposure of lap joints made with CFRP1 to sustained loads that are less than 50% of the instantaneous failure load does not influence their failure mode and bonding strength. This observation was also made by Agarwal et al. (2015) [11]. 3.2.2. CFRP2 (test series 7) The stress versus displacement curves obtained during the residual strength testing of steel-CFRP2 specimens that were exposed to 30% and 50% of the instantaneous failure load for 21 days at ambient conditions are shown in Fig. 10. The average failure stress of the specimens exposed to 30% sustained load was 13.7 MPa, which is almost equal to the strength of the control specimens (13.8 MPa). As can be seen from the figure, the average failure stress of the specimens exposed to 50% sustained load was 13.5 MPa, which is about 2.5% lower than that of the control specimens. The dominant failure mode in this case is similar to that of the control specimens (adhesive failure at the interface between steel and adhesive), indicating that exposure to sustained loads that are less than 50% of the instantaneous failure load does not have a significant influence on either the bond strength or the failure mode. This conclusion is supported by similar observations made in Agarwal et al. (2015) [11]. 3.3. Wet thermal cycling only e test series 3 and 8 Three specimens of steel-CFRP1 joints and three specimens of steel-CFRP2 joints were exposed to thermal cycling between 10
C and 50
C in immersed conditions for 21 days (108 thermal cycles). The specimens were then allowed to recover for 7 days and tested for their residual strength. 3.3.1. CFRP1 (test series 3) The stress versus displacement curves are shown in Fig. 11(a). An average curve for the control specimens is also shown for comparison. The average failure stress after exposure to thermal cycles is 9.7 MPa and is close to the bond strength of the control specimens (10 MPa). The average displacement at failure load is 0.461 mm, which is about 5% lower than that of the control specimens (0.488 mm). Thus, there is no significant reduction in the joint strength upon exposure to thermal cycles only.

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Fig. 4. Thermal cycle-sustained load apparatus.

Fig. 5. Typical thermal cycle profile (10
Ce50
C) obtained from the thermal cycling apparatus.

The failure mode in this case is adhesive failure (complete debonding at steel-adhesive interface), which is different than the failure observed in control specimens (delamination failure). Thus, exposure to wet thermal cycles alone weakens the interface between steel and adhesive. This weakening of the interface is just sufficient to let debonding to dominate over delamination failure observed in control specimens; yet, the change in the bond strength was negligible.

3.3.2. CFRP2 (test series 8) The stress versus displacement curves of the steel-CFRP2 joints are shown in Fig. 11(b). The average failure stress after exposure to thermal cycles was 11.6 MPa, which is about 16% lower than that of the control specimens (13.8 MPa). The failure mode in this case was also adhesive failure (complete debonding at steel-adhesive interface), similar to the failure observed in control specimens. In both types of specimens (with CFRP1 and CFRP2), the observed failure mode after exposure to thermal cycling was debonding failure at the interface between steel and adhesive (irrespective of the failure in their respective control specimens). Agarwal et al. (2015) [11] also observed similar failure mode with both types of adhesives (Adhesive A and Adhesive B) after exposure

to wet thermal cycling only. This indicates that application of only wet thermal cycles weakens the interface between steel and adhesive, and makes it more susceptible to failure. However, it is questioned if it is the temperature cycle alone or water absorption that causes this deterioration. This is investigated in section 3.5, where the specimens are tested in dry conditions but under the same temperature cycles. 3.4. Both sustained load and wet thermal cycling e test series 4 and 9 In this section, the results of specimens from each CFRP group that were exposed to thermal cycling between 10
C and 50
C along with sustained loading of 30% and 50% of the instantaneous failure load are presented and discussed. The specimens that did not fail during 21 days of exposure were allowed to recover for seven days and were then tested for their residual strength. 3.4.1. CFRP1 (test series 4) Three specimens each under 30% and 50% of the instantaneous failure load were exposed to wet thermal cycling for 21 days. The longitudinal displacement versus time curves for specimens LS3-

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Movable Grip

Filler Grips

CFRP (1.4 mm)

225 mm LVDT clamp

LVDT clamp Adhesive

LVDT

LVDT

Steel (3 mm) Filler Grip

Fixed Grip

Fig. 6. Thermal cycling experimental set-up for steel-CFRP single lap shear specimen.

Fig. 7. Stress versus displacement curves for control steel-CFRP joints e (a) CFRP1; and (b) CFRP2.

T1-30-1 and LS3-T1-50-1 are shown in Fig. 12. The displacement data and the temperature data between day 12 and day 18 of testing could not be recorded due to a logger fault. Curves for other specimens were similar to these curves and hence, are not shown. The average initial displacements when the load was applied were 98 mm (0.098 mm) for 30% load and 125 mm (0.125 mm) for 50% load. None of the six specimens had failed after 21 days of exposure to thermo-mechanical loading; while, Agarwal et al. (2015) [11], who tested different kind of adhesives but under the same conditions, observed failure within 15 thermal cycles at 30% of the instantaneous failure load and within one thermal cycle at 50% of the instantaneous failure load. Nguyen et al. (2013) [22] also observed similar failure under only 15% of the instantaneous failure load at the same thermal conditions. While the modulus of elasticity and tensile strength of Adhesive C are lower than those of Adhesives A and B that were tested in Agarwal et al. (2015) [11], it is believed that the higher performance of the joints made with Adhesive C is attributed to the higher Tg of

the adhesive (82
C for Adhesive C compared with 50
C and 62
C for Adhesives A and B, respectively). At service temperatures closer to the Tg, the epoxy adhesive is in a “rubbery” state and possess lower elastic modulus and strength as compared to service temperatures that are significantly lower than the Tg of the adhesive [30]. This hypothesis is supported by the previous results, which showed no significant difference in the performance of the joints made with Adhesive C compared to Adhesives A and B under sustained loading only or wet cyclic temperature only. Thus, it is not about a different viscoelastic or moisture absorption properties of Adhesive C compared to A and B. Rather, it is about degradation of the adhesive performance under temperatures that are close to its Tg when the temperature is combined with sustained mechanical loading. The combination of sustained loading and thermal cycles is more critical than when each of them is applied separately mainly because the mechanical load can restrain/change the free thermal expansion or contraction of the joint and leads to the development of additional interfacial stresses that may significantly reduce the

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Fig. 8. Typical failure modes observed in this testing program: (a) adhesive failure or complete debonding from the steel substrate; (b) complete CFRP delamination; (c) Mixedmode failure; and (d) delamination failure with slight debonding near the edges.

Fig. 9. Residual strength testing of steel-CFRP1 joints that were under (a) 30% sustained loading, and (b) 50% sustained loading.

bonding strength when the applied temperature is close to Tg (as in Ref. [11]). 3.4.1.1. Residual strength testing. Since none of the specimens in test series 4 failed upon exposure to 21 days of thermo-mechanical loading, the specimens were unloaded and allowed to recover for seven days. After the recovery period, the specimens were tested for their residual strength. During unloading, specimen LS3-T1-301 got damaged due to twisting inside the test tank and could not be tested for the residual strength. The stress versus displacement curves for all other specimens are shown in Fig. 13. The average failure stresses were 9.0 MPa and 9.8 MPa in case of specimens that

were loaded to 30% and 50% of the instantaneous failure load, respectively. These stresses are 10% and 2% lower than the strength of the control specimens, respectively. The failure observed in all six specimens was complete debonding at steel-adhesive interface (adhesive failure) as opposed to dominant delamination failure in the control specimens. This change in failure mode is attributed to the application of wet thermal cycle that weakens the interface between steel and adhesive, as discussed in the case of thermal cycling only (section 3.3). 3.4.2. CFRP2 (test series 9) All three specimens in this series of testing under 30% of the

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Fig. 10. Residual strength testing of steel-CFRP2 joints that were under (a) 30% sustained loading, and (b) 50% sustained loading.

Fig. 11. Residual strength testing of steel-CFRP joints exposed to thermal cycling only e (a) CFRP1, and (b) CFRP2.

load failed within 15 thermal cycles. Thus, lap joints made with CFRP2 and Adhesive C performed better despite relatively low modulus of elasticity and tensile strength as compared to the joints tested in Agarwal et al. (2015) [11]. This high performance of the joint with Adhesive C is mainly attributed to the higher Tg of Adhesive C. The hypothesis made earlier is again supported here that it is not about different viscoelastic properties and moisture absorption properties; rather, it is about the degradation of the adhesive performance under temperatures that are close to Tg. The average initial displacement of the specimens under 30% and 50% sustained load were 0.134 mm and 0.236 mm, respectively, and the average recovered displacement in the specimens after unloading were 0.156 mm and 0.258 mm, respectively, which is almost equal to the initial displacement. As seen from Fig. 14, the residual displacement in the joints is almost equal to the creep of the adhesive layer between steel and CFRP. Similar observations were also made in Agarwal et al. (2015) [11].

Fig. 12. Displacement versus time curves for specimens LS3-T1-30-1 and LS3-T1-50-1 under thermo-mechanical loading.

instantaneous failure load survived all 108 thermal cycles (21 days). Two out of three specimens that were subjected to 50% of the instantaneous failure load survived all 108 thermal cycles, while one specimen failed after twelve thermal cycles. It should be noted that in Agarwal et al. (2015) [11], all specimens under 50% load failed within one thermal cycle while the specimens under 30%

3.4.2.1. Residual strength testing. The five of the six specimens that did not fail during the thermo-mechanical loading phase were tested for their residual strength and the results are shown in Fig. 15. Data for LS4-T1-30-1 could not be recorded due to a logger error and is not reported here. The average failure stress of the specimens under 30% sustained load was 14.3 MPa (Fig. 15(a)), which is about 4% higher than the average strength of the control specimens (13.8 MPa). The average failure stress in case of specimens under 50% failure load is obtained from Fig. 15(b), and is equal to 13.5 MPa, which is about 2% lower than that of the control specimens. The dominant failure mode in all specimens was adhesive failure at the interface between the steel and the adhesive, similar to other cases where wet thermal cycling was applied and to that of the control specimens.

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4000

Load (N)

These results with both types of CFRP indicate that exposure to thermo-mechanical loading does not have any significant influence on the bond strength of steel-CFRP joints as degradation of less than 10% in the bond strength was observed. This is similar to the observation made in the case of thermal cycling only, where a reduction of only about 16% in the bond strength was observed (section 3.3.2). As observed in the case of wet thermal cycle only, the mode of failure in this case of thermo-mechanical loading was adhesive failure. The failure modes in the control specimens and that of the sustained load only specimens were mixed-mode failure in steelCFRP1 joints and debonding failure in steel-CFRP2 joints. This suggests that it is the exposure to the wet thermal cycle that weakens the interface between steel and adhesive irrespective of the presence of sustained load. This weakening of interfaces is either due to intrusion of moisture or due to thermal expansion and contraction during thermal cycle. This is discussed in the next section.

30% Load

0

8 6

300 450 Longitudinal Displacement (μm)

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4. Summary and conclusions In this paper, steel-CFRP adhesively bonded joints with two types of CFRP (commercial CFRP1 and newly manufactured CFRP2)

14

LS3-T1-50-1

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LS3-T1-50-2 LS3-T1-50-3

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Control

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4

4

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average failure stress was 10.6 MPa in case of 30% loaded specimens, which is about 6% higher than that of the control specimens and the specimens under wet thermo-mechanical loading (10.0 MPa). The average failure stress in case of 50% loaded specimens was 9.0 MPa, which is about 10% lower than that of the control specimens. Thus, there is no significant degradation in the bond strength of steel-CFRP joints upon exposure to dry thermomechanical loading. Similar observations were made for specimens exposed to wet thermo-mechanical loading in test series 4. The failure in all the dry specimens (including the one that failed with time) was due to CFRP delamination similar to that of the control specimens and specimens under sustained load only, but different than the debonding failure observed in case of specimens that were exposed to wet thermal cycles (with or without sustained load). The failure modes observed in different cases suggest that it is not the thermal cycle or sustained load but it is the moisture that alters the failure mode from delamination to debonding failure. Similar changes in failure mode upon water ingress were also observed in other studies [16,31e33]. This may be attributed to ingress of water at the interface that makes it the critical link in the bonded joint. It is important to note that no rusting was observed in any specimen, whether protected by silicon or otherwise, which indicates that any change in the joint properties was not due to corrosion on the steel substrate but was due to applied thermomechanical conditions and material properties.

Stress (MPa)

Stress (MPa)

Control

Unloading

Fig. 14. Load versus displacement curves obtained from loading and unloading of steel-CFRP2 specimens during thermo-mechanical testing.

LS3-T1-30-2

10

Residual Displacement

0

3.5.1. Residual strength testing All five specimens (three under 30% and two under 50% load) that survived the dry thermo-mechanical loading were unloaded, allowed to recover for 7 days and tested for the residual strength. The stress versus displacement curves are shown in Fig. 17. The

LS3-T1-30-3

50% Load

1000

In this series of tests, the bonded region of steel-CFRP1 joints were sealed with water-proof silicon sealant to prevent the entry of water within the adhesive joint. Six water-proofed specimens were exposed to thermo-mechanical loading (three specimens each under sustained loads that equal to 30% and 50% of the instantaneous failure load) and tested for the residual strength. None of the three specimens under 30% of the instantaneous failure load failed during long term exposure for 21 days. Two out of the three 50% loaded specimens survived all 108 thermal cycles while one specimen (LS3-T2-50-1) failed at 45th thermal cycle (9 days) via CFRP delamination. It is again worthwhile to note here that all the specimens under 30% and 50% of the instantaneous failure load in Agarwal et al. (2015) [11] failed within 15 thermal cycles and one thermal cycle, respectively. Displacement versus time curves for the specimen LS3-T2-50-1 and for one of the two other 50% loaded specimens that survived the full 21 days of thermal cycles are shown in Fig. 16. The average initial displacement of 30% and 50% loaded specimens were 0.046 mm and 0.124 mm, respectively, and the average recovered displacements after unloading were 0.034 mm and 0.189 mm, respectively.

12

3000

2000

3.5. Dry thermo-mechanical loading (test series 5)

14

Thermal Cycle applied

0 0

0.3

0.6 0.9 1.2 1.5 Longitudinal Displacement (mm)

(a)

0

0.3

0.6 0.9 1.2 1.5 Longitudinal Displacement (mm)

(b)

Fig. 13. Stress versus displacement curves obtained during the residual strength testing of steel-CFRP1 joints exposed to thermal cycling with (a) 30% and (b) 50% sustained loading.

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LS4-T1-50-2

LS4-T1-30-3

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Stress (MPa)

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6

6

3

3

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Control

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0.8 1.2 1.6 Longitudinal Displacement (mm)

2

0

0.4

(a)

0.8 1.2 1.6 Longitudinal Displacement (mm)

2

(b)

Fig. 15. Stress versus displacement curves of the residual strength testing of steel-CFRP2 joints exposed to thermal cycling with (a) 30% and (b) 50% sustained loading.

of sustained loads (30% and 50% of the instantaneous failure load) were applied. The thermal cycle ranged between 10
C and 50
C. The results of joints with both CFRPs are summarized in Fig. 18. The bond strength of the control specimens with CFRP1 and CFRP2 were 10.0 MPa and 13.8 MPa, respectively; and the failure modes were delamination failure and debonding failure, respectively. On application of sustained load only, no failure was observed in the joints with both types of CFRP during the long term testing for 21 days. A reduction of less than 16% in the residual bond strength of the joints made with CFRP1 was observed while little or no degradation of the bond was observed for the joints made with CFRP2. Also, there was no change in the failure mode compared to the control specimens. This shows that sustained load of up to 50% of the instantaneous failure load has little or no impact on the bonding capacity of steel-CFRP joint investigated in this study. When the specimens were exposed to wet thermal cycling between 10
C and 50
C for 21 days and tested for their residual strength, a reduction of less than 16% was observed in both types of joints, which is similar to the 15% degradation observed in Agarwal et al. (2015) [11]. The failure mode in all the specimens upon application of wet thermal cycling was debonding at steel-adhesive interface. In the case of combined wet thermo-mechanical loading, significant differences were observed between the results obtained in the current study and the ones presented in Agarwal et al. (2015) [11] due to differences between the adhesives used. While, all the specimens in Ref. [11] failed with time under wet thermal cycling with maximum temperature that was at least 22
C below the Tg of the adhesive, combined with sustained load of up to 50% of the instantaneous strength, no failure with time or degradation of the residual bond strength was observed in the current study under

Fig. 16. Displacement versus time curves for steel-CFRP1 joints under dry thermal cycling and 50% sustained loading.

were prepared using newly manufactured non-commercial Adhesive C, and were tested under different thermo-mechanical conditions. These conditions included: (a) control specimens under instantaneous load and ambient conditions; (b) sustained load only under ambient conditions; (c) wet thermal cycling only; (d) both sustained load and thermal cycle in immersed conditions; and (e) both sustained load and thermal cycle in dry conditions. Two levels

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LS3-T2-30-1 LS3-T2-30-2

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LS3-T2-30-3 Control

8 6

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Stress (MPa)

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Control

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LS3-T2-50-2

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0.2

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0.6 0.8 1 1.2 1.4 Longitudinal Displacement (mm)

(a)

0

0.2

0.4 0.6 0.8 1 1.2 Longitudinal Displacement (mm)

(b)

Fig. 17. Stress versus displacement curves of the residual strength testing of steel-CFRP1 single lap joints exposed to thermal cycling and 30% sustained loading in dry conditions.

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Control SL (50%)

TC only TML 30%

SL(30%)

TML 50%

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SL(30%)

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8

4

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(b)

Control = Control specimens with no thermal cycle and no sustained load SL (30%) = 30% Sustained Load only SL (50%) = 50% Sustained Load only TC only = Thermal cycle only between 10°C and 50°C TML 30% = Thermo-mechanical loading with 30% sustained load TML 50% = Thermo-mechanical loading with 50% sustained load TML 30% (NM) = Thermo-mechanical loading with 30% sustained load and No Moisture TML 50% (NM) = Thermo-mechanical loading with 50% sustained load and No Moisture

Fig. 18. Summary of the residual strength of steel-CFRP joints under different thermo-mechanical conditions: (a) CFRP1 and (b) CFRP2.

similar conditions. This is attributed to the higher Tg of the adhesive that was 32
C higher than the applied maximum temperature, which is similar to the one applied in Ref. [11]. Despite the need for more tests, based on these test results and the ones presented in Refs. [11], it may be concluded that failure is likely to occur with time for lap-joints that are under a combination of sustained loading and wet thermal cycling with temperatures that are up to 20
C below Tg of the adhesive. This is supported by the recommendations of Hollaway (2010) [34], who suggested that the service temperature should be at least 30
C below the Tg of the adhesive. The failure mode observed in all specimens under wet thermal cycling (with or without sustained load) was debonding failure at the interface between the steel and the adhesive. This highlights that application of wet thermal cycling degrades the steel-adhesive interface and makes it the critical link for failure. However, it was also crucial to know if this degradation was due to thermal cycle alone or due to moisture ingress. Hence, lap joints were investigated under thermo-mechanical loading in dry conditions. Here, specimens were coated with waterproof silico n sealant to prevent the entry of water in the adhesive joint during thermo-mechanical loading. The obtained results were similar to the results of wet thermo-mechanical testing, i.e. no failure during long term exposure and no significant degradation in the residual joint strength. However, the dominant failure mode in the dry thermo-mechanical conditions was delamination failure, similar to the failures obtained in other dry conditions (control and sustained load only). This failure was different to debonding failure obtained in the case of wet thermal cycle testing (with and without sustained load). This indicates that it is the moisture that degrades steel-adhesive interface and not thermal cycling. Thus, it is recommended to seal adhesive joints of CFRP repaired steel members with waterproof sealant to prevent moisture ingress in the bonded region.

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