Experimental study on tensile behaviour of steel plates with centre hole strengthened by CFRP plates under marine environment

Author’s Accepted Manuscript Experimental study on tensile behaviour of steel plates with centre hole strengthened by CFRP plates under marine environment Yi Wang, Yifeng Zheng, Junhui Li, Lide Zhang, Jun Deng www.elsevier.com/locate/ijadhadh

PII: DOI: Reference:

S0143-7496(18)30020-4 https://doi.org/10.1016/j.ijadhadh.2018.01.017 JAAD2123

To appear in: International Journal of Adhesion and Adhesives Received date: 10 October 2017 Accepted date: 21 January 2018 Cite this article as: Yi Wang, Yifeng Zheng, Junhui Li, Lide Zhang and Jun Deng, Experimental study on tensile behaviour of steel plates with centre hole strengthened by CFRP plates under marine environment, International Journal of Adhesion and Adhesives, https://doi.org/10.1016/j.ijadhadh.2018.01.017 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Experimental study on tensile behaviour of steel plates with centre hole strengthened by CFRP plates under marine environment Yi Wang, Yifeng Zheng, Junhui Li, Lide Zhang, Jun Deng* School of Civil and Transportation Engineering, Guangdong University of Technology, Guangzhou 510006, China * Corresponding author. [email protected]

Abstract Carbon fibre reinforced polymer (CFRP) is effective in strengthening steel structures with defects. However, when the composite structures are exposed to marine environment, their mechanical properties can be degraded to a degree that has not been thoroughly elucidated to date. In this paper, tensile behaviour of steel plates with a centre hole strengthened by CFRP plates under marine environment were experimentally studied. The marine environment was simulated by wetting/drying cycles (WDCs) with 3.5%wt sodium chloride solution. Different patch configurations and adhesive types were considered. After the environmental exposure, tensile tests were conducted. From the test results, load-displacement curves, load-strain curves, yield load, ultimate load and failure displacement change with 0, 90 and 180 WDCs were obtained. Based on particle image velocimetry (PIV), the failure modes of specimens were also analysed. After CFRP strengthening, the cracking propagation of steel plates was impeded, and the bearing capacity and ductility were significantly improved. The mechanical degradation degree was more than 20% after 180 WDCs exposure, while the strengthening effect reduction was even worse, which shows poor bond durability. Deterioration of adhesive not only has a negative effect on the mechanical performance of composite specimens but because of the stiffness reduction of the adhesive layer, a beneficial effect was also observed in the first 90 WDCs. This phenomenon plays an essential role in bond durability of the strengthening system, which warrants further investigation. Keywords: steel plate; CFRP; wetting drying cycles; marine environment; tensile behavior 1. Introduction Carbon fibre reinforced polymer (CFRP) is a composite material, consisting of high strength continuous carbon fibers arranged in certain rules and impregnated or pultruded with resin. It is widely accepted as an excellent strengthening material for steel structures due to its high strength to weight ratio and corrosion resistance advantages [1-2]. When suffering from overloading or harsh environments, steel structures may be damaged and fail to fulfil the requirements of structural safety [3]. After CFRP laminate strengthening, the strength and stiffness can be considerably higher than the unstrengthened steel structural members in a short time because of the higher stiffness of CFRP. However, for long-term harsh environmental exposure, the mechanical properties may show further degradation, which has not been fully examined to date [4]. Although some mechanical devices were proposed for debonding failure prevention [5-6], the environmental deterioration is difficult to impede. Accurate service life prediction and durability evaluation methods are still in need [7]. Thus, it is necessary to clarify the damage mechanism and predict the mechanical degradation of damaged steel structures strengthened with CFRP under harsh environment. Due to the importance of the durability problem of steel structures after strengthening, this area has received significant attention from researchers [4]. According to the existing studies, harsh environment (e.g., elevated temperatures [8], high humidity, hygrothermal environment [9], freeze-thaw cycles [10], salt attack and marine environment [3]) can deteriorate the mechanical properties of the composite structures, and the adhesive plays a key role in this deterioration. Generally, CFRP has good durability and is non-corrosive in theory. The deterioration primarily occurs in the adhesive layer and the interface of adhesive and adherend. 1

When the steel members strengthened by CFRP were exposed to marine environment, the moisture could penetrate into the adhesive layer, which can decrease the strength and stiffness of the adhesive [11]. Because of the moisture penetration, the epoxy resin adhesive could be plasticized [12] and swell due to various chemical and physical reasons. When the epoxy in CFRP contains C-N bond, the sodium chloride salt can also break the C-N bond and result in the loose of CFRP [13], which may be the reason for CFRP delamination failure. With the existence of the salt solution, the further corrosion of steel could be another reason. Even though the possibility of steel corrosion becomes lower after covering by CFRP, the galvanic effect may be effective [14] and the porous nature of adhesive can promote the transport of ions and oxides. It seems that the continuous immersion in salt solution can cause severe degradation. With regard to the galvanic corrosion, the wetting/drying cycles (WDCs) may have a more serious detrimental effect, while the physical damage may be partially recovered during drying periods. Therefore, to simulate the actual marine environment, the WDCs condition should be included in research of the composite structures’ durability. In addition, the CFRP strengthening is mostly applied for damaged steel structures; however, the existing studies focused either on the bond behaviour of double strap joints or strengthening materials. For the steel structures with defects under harsh environments, few studies were conducted to investigate the mechanical behaviour deterioration. Since the damage mechanisms of steel structures with defects may be different from the sound steel structures, it is necessary to perform this work to achieve accurate life cycle evaluation. Steel members suffering from fatigue loading may have lower strength and stiffness. Even worse, cracks and holes may be induced. Fatigue performance of steel structures strengthened with CFRP has been widely studied [15-16]. Notches or holes have been applied to simulate the steel damage [17]. In this study, to evaluate the defects of steel structures, a hole in the centre of steel plates with notches on the width direction was manufactured for testing. The main purpose is to understand how the mechanical properties of steel plates with a centre hole strengthened by CFRP plates degrade under marine environment. Different adhesive types (with different elastic modulus) and patch configurations (centre hole fully covered by CFRP plates and two CFRP plates straddling the centre hole) were studied to elucidate the interface mechanical deterioration mechanism and service life prediction. 2. Test programme After marine environmental exposure, a tensile test of steel specimens with a centre hole strengthened with CFRP was conducted in this study. Mechanical degradation of the specimens can be analysed from the test results. The composite specimen contains three parts: steel plate, CFRP plates and adhesives. The mechanical behaviour of the specimens is highly dependent on the mechanical properties of each part. The specimen preparation procedures and environmental exposure are also essential for the testing results. 2.1 Materials Type Q235 steel was applied in this study for the steel plates with a centre hole. The designed thickness of the steel plates is 4 mm and the width is 45 mm. From the specification, the elastic modulus of the steel plate is 210 GPa and the yield strength is 235 MPa. However, the actual value could be different from the designed one. According to the standard (GB/T2975, 1998), three standard specimens were tested. The average elastic modulus is 207 GPa, yield strength is 297 MPa and tensile strength is 427 MPa. The CFRP plates in this study were produced by Dezhenghang Architectural Technique Limited Liability Company in Guangzhou (DZH) based on standard (GB50367-2006) [18], for which one directional carbon fibres were impregnated with epoxy resin to cure as the CFRP plates. The fibre volume fraction is about 65%. The width and thickness of CFRP plate are 15mm and 1.4mm. The laminate is an orthotropic material. 2

According to the manufactures’ instruction, in the tensile direction, while the elastic modulus and tensile strength are 165 GPa and 2400 MPa, the interlaminar shear strength and elongation are 500 MPa and 1.7, respectively. Since the CFRP plate in this study was mainly under tension, it has good durability because its tensile strength and elastic modulus will negligibly be affected by seawater immersion [19]. Adhesives play an important role in the interfacial stress transfer, which is the key component of the tensile capacity of the composite specimens. Because epoxy resin has good durability and excellent physical and mechanical properties, it is widely applied as an adhesive in construction. In this study, two types of adhesives were applied. One is an internationally used adhesive produced by Sikadur (China) Limited Liability Company, the adhesives for Sikadur® 30 structural strengthening, which is relatively harder (higher elastic modulus). It contains two proportions and the mix ratio is 3:1 for CFRP plates. The other is a domestic CFRP adhesive produced by DZH, which is relatively softer (lower elastic modulus). It also contains two parts and the mix ratio is 3:1. The physical and mechanical properties of the two adhesives were provided by the manufacturer, as seen in Table 1. In addition, the bond strength (adhesive strength on steel) was also provided. They were 30 MPa and 18 MPa for Sikadur® 30 and DZH adhesives, respectively. Table 1 Physical and mechanical properties of structure strengthening adhesive Adhesives Shear strength (MPa) Tensile strength (MPa) Elastic modulus (GPa) ® Sikadur -30 18(+23℃) 24-27(+15℃) 11(+23℃) DZH 18 46 3.6(+23℃) 2.2 Specimens preparation Three types of specimens were prepared. As shown in Figure 1, type I-steel plates with a centre hole (Figure 1 (a), type II-steel plates with centre hole strengthened by CFRP plates (CFRP fully covering the initial cracks) (Figure 1(b)), type III-steel plates with centre hole strengthened by CFRP plates (CFRP straddling the initial cracks) (Figure 1(c)). The type I specimen was designed as the control specimen, while the other two types were applied to understand the effect of marine environment on mechanical deterioration after strengthening. To simulate the actual damaged steel structures, a hole with diameter 5 mm was manufactured in the centre of the steel plates. In the two sides of the pore, a notch was produced with length 5 mm and width 0.1 mm (see Figure 1(a)). To compare the mechanical performance of the three types of specimens, the strengthening effect of CFRP in different styles can be understood. To study the marine environment effect on the mechanical performance of the composite system, two different adhesives and wetting-drying cycles were used. The summary of parameters and number of specimens are shown in Table 2. The total number of specimens is 33.

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Figure 1. Specimen size (a) control specimen (non-strengthened) (Type I) (b) CFRP strengthened specimen (CFRP fully covering the initial cracks) (Type II) (c) CFRP strengthened specimen (CFRP straddling the initial cracks) (Type III)

Specimen code U A-S A-S-3 A-S-6 A-D A-D-3 A-D-6 B-S B-S-3 B-S-6 B-D B-D-3 B-D-6

Table 2 Summary of parameters and number of specimens Specimen type Adhesive type WDCs Type I 0 Type II Sikadur® 30 0 ® Type II Sikadur 30 90 ® Type II Sikadur 30 180 Type II DZH 0 Type II DZH 90 Type II DZH 180 ® Type III Sikadur 30 0 ® Type III Sikadur 30 90 ® Type III Sikadur 30 180 Type III DZH 0 Type III DZH 90 Type III DZH 180

Specimen numbers 3 3 3 3 2 2 2 3 3 3 2 2 2

Note: For specimen code, U means specimens without strengthening (Type I), A means Type II specimens, B means Type III specimens. S means the adhesive type is Sikadur® 30 while D represents DZH adhesive.

The composite specimen preparation procedure is shown in Figure 2. The steel plates with centre hole and notches were manufactured as Figure 1(a). Next, the plates were sandblasted by silicon carbide F60 to enhance the surface roughness and bond activity. Afterwards, the surface of the steel plates was cleaned by acetone. Meanwhile, the CFRP plates were sandblasted by white fused alumina F240 and cleaned by acetone. Each epoxy resin adhesive was weighted as the proportion required and mixed properly. To assure the adhesive thickness equals 1 mm, a steel ball with diameter 1 mm was mixed in the adhesives (the steel ball is approximately 1% of the weight of the adhesive). During stirring of adhesives, to prevent air bubbles from 4

forming in the adhesives, the speed was kept constant and low. After the adhesive’s colour changed to even grey, the CFRP plates were bonded to the steel plates. During the bonding, the composite specimens were pressed evenly from the surrounding and extruding the extra adhesives from the two sides of steel plates to avoid the voids and flowing of adhesives. To improve the strengthening effect, the adhesives at the CFRP plate ends were retained. Finally, the extra adhesives at the steel plates’ surrounding were removed in time. As shown in Figure 2, two types of clamps were used for specimens curing under pressure, wooden sticks were also applied to provide an even pressure distribution. In this case, the bond between the CFRP and steel could be good and the adhesives thickness can be uniform. Since only the bonded area will be examined, the remaining area was coated with the anticorrosive materials. Otherwise, the steel plates could be heavily corroded in the area without CFRP bonding, and the tensile failure would occur at the corroded area such that the effect of CFRP strengthening cannot be known. A typical prepared type II specimen can be seen in Figure 2. !

Curing under pressure G-shaped clamp

Steel plates sandblasting (Silicon carbide F60) and cleaning with acetone

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CFRP plates sandblasting (White fused alumina F240) and cleaning with acetone Type II specimen before environmental exposure

Figure 2. Specimen preparation procedure 2.3 Environmental exposure The marine environment was simplified by WDCs with sodium chloride solution [20]. The period of one WDC is 24 hours, which was under the drying condition for 14 hours and the wetting condition for 10 hours. For the wetting case, the specimen was immersed in 3.5% by weight sodium chloride solution. The temperature was approximately 40℃ with accuracy ±1℃. In this study, to understand the effect of exposure periods, specimens exposed to 0, 90 and 180 WDCs were prepared. Typical specimens subjected to the marine environment can be seen in Figure 3. The specimens were corroded after the environmental exposure, especially for the type III specimens with increased steel area without anti-corrosive material protection. From the observation, it seems the specimens with Sikadur®-30 adhesive have more severe corrosion. The reason is likely due to the more porous nature of Sikadur®-30 adhesive. Because it is harder and more difficult to flow after CFRP plates bonded to steel plates, as a result, more connected pores remained, which provided paths in 5

which the salt solution could transport.

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Figure 3. Specimens suffered from 180 WDCs (a) typical type II specimen (DZH adhesive) (b) typical type II specimen (Sikadur®-30 adhesive) (c) typical III specimen (DZH adhesive) (d) typical III specimen (Sikadur®-30 adhesive)

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2.4 Tensile test After the environmental exposure, the strain gauges were attached to the CFRP plate surface to obtain the interfacial stress distribution during loading. For different types of specimens, the strain gauge arrangements were different, but all were attached along with the longitudinal central line of CFRP plates for two sides, as shown in Figure 4. A data logger (TDS-530) was applied to collect the strain development information. The tensile test was conducted in the Structural Engineering lab of Guangdong University of Technology. The testing machine is shown in Figure 5. During the test, the loading process is displacement control and the loading rate is 0.5 mm/min until the specimen failure. The loading was programme controlled by a PC. Before the test, 10% of ultimate load pre-loading was applied to achieve specimen centring. From the test results, the yield load, ultimate load, load-displacement and load-strain relationships can be obtained.

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Figure 5. Tensile test set up 2.5 Particle image velocimetry The relative displacement of CFRP plate ends obtained by the loading machine may be not reliable. This finding is observed because the specimen and clamps, clamps and the loading machine could have slips, which can affect the accuracy of the results. Thus, in this study, particle image velocimetry (PIV) was also applied to measure the specimen displacement. As shown in Figure 5, a camera (Canon® EOS 5D Mark II) (5616×3744 pixel) was set up in front of the loading machine and placed perpendicular to the sample. The distance between the camera and the specimens is approximately 1 m. By collecting the image once per second automatically, the deformation and failure of specimen can be captured and analysed by Matlab image analysis tool. The application of PIV technique followed the study by White et al. (2003)[21]. Since the size of the sub-area is important for the accuracy of the results analysis, calibration with strain development in steel plates was conducted, as shown in Figure 6. By comparing the results from PIV and strain gauges, it was found that the sub-area size with 10 mm×10 mm is accurate enough for image analysis (see Table 3). Table 3 Elastic modulus of steel plate under two kinds of monitoring method Monitoring methods Elastic modulus (GPa) Deviation (%) Strain measurement 206 -PIV-20mm 109 -47.1 PIV-10mm 209 1.4 Note：Deviation=（EPIV-Estrain）/Estrain×100%, EPIV is the elastic modulus calculated from PIV and Estrain is the elastic modulus calculated from the strain measurement.

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Matlab image analysis tool. The application of PIV technique followed the study by White et al. (2003). Since the size of sub-area is important for the accuracy of the results analysis, calibration with strain development in steel plates was conducted, as shown in Fig. 6. By comparison results found that the sub-area size with 10mm×10mm is accurate enough for image analysis.

PIV analysis area

Figure 6. PIV analysis area 3. Test results and discussions

3. Test results and discussions 3.1 Load-deflection relationship The tensile test was conducted to understand the mechanical degradation of steel plates with a centre hole 3.2 Load-strain relationship strengthened by CFRP plates under marine environment. From the tensile test, the yield load and ultimate load can be observed. Based on the PIV technique, the displacement at break, load-displacement curve and the 3.3 Failure modes failure modes could be analysed. The load-strain curve can be obtained from the strain gauge measurements. 4. Mechanical deterioration mechanism

3.1 Load-displacement relationship By applying the PIV technique, the displacement development (length change of the CFRP plate) with 5. Conclusions load can be obtained. From the test results, the load-displacement relationship was plotted in Figures 7 and 8. In this paper, These findings indicate that the results of specimens with Sikadur® 30 adhesive were more scattered than the Acknowledgements DZH adhesive cases. For different environmental exposure cases, there were no big differences. In addition to This work is supported by the National Natural Science Foundation of China through Grant 51278131, the U cases in Figure 7, load-displacement curves can be divided into six stages. First, in the elastic stage, the Program for New Century Excellent Talents in University through Grant NCET-13-0739, and Fok Ying Tong displacement increases linearly with the load. In this stage, the stiffness of specimens can be analyzed because Education Foundation through Grant 131073. the slope of the curve was consistent, but no clear trend was observed from the curves. The second stage is the yield stage,References where the period is rather short and the slope of the curves changes rapidly. The turning point can Zhao, X.-L., andAfter Zhang, L. it(2007). review on FRP where strengthened steelincreases structures."slowly with be defined as yield load. that, enters"State-of-the-art the initial hardening stage, the load Engineering Structures, 29(8), 1808-1823. the displacement. The slope decreases gradually to approximately zero. For a period, the load remained constant while the displacement increases continuously. Next, the load drops suddenly and the displacement stopped increasing. From the observation of strain development, the sudden load decrease is due to the debonding. Therefore, the fourth stage is the debonding initiation and the debonding load can be defined as the second turning point. Next, the load increases again as the hardening of steel, which is the second hardening stage. Finally, the load decreases with the displacement until failure, which can be described as the softening stage. For both strengthening types, the obvious strengthening effect can be observed since the ultimate load was largely increased after strengthening. According to the characteristics of the load-displacement curves in Figure 8, how the marine environment affects the mechanical behaviour of the composite specimens can be understood. For the steel plates with a centre hole strengthened by CFRP, the strength and stiffness were enhanced compared with the unstrengthened cases (see Figure 7). The strengthening effect can be viewed from the slope of the load-displacement curve in the elastic stage. Suffering from the harsh environment, the adhesives could swell and become softer while the steel could be corroded [13]. On one hand, the loosening of the CFRP plates due to adhesive deterioration and corrosion of steel plates can decrease the strength and stiffness of the specimen. If the mechanical properties of the adhesives decreased dramatically, the shear strength would be easily reached which can cause debonding failure. On the other hand, the adhesives could be softer after absorbing salt solution and the bond strength would be higher with softer adhesives due to the increasing of interfacial fracture energy and interfacial load transfer capacity [22]. The mechanical deterioration can be known from the yield load and ultimate load. After debonding of CFRP plates, the strengthening effect would be weak. 8

Thus, the second hardening stage and softening stage should be similar to the unstrengthened case. If the steel corrosion was a serious problem, the performance of the last two stages would be different, especially for the ultimate load in the fifth stage. In this study, the mechanical degradation was not significant after 180 WDCs exposure. The stiffness and strength reduction were not obvious from a qualitative point of view, especially for the case with DZH adhesive. However, because of heavier steel corrosion, type III specimens were confirmed that had a higher degree of mechanical degradation than type II specimens. 70

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Figure 8. Load-displacement relationship of specimens after marine environmental exposure (a) Type II specimens with Sikadur® 30 adhesive (b) Type II specimens with DHZ adhesive (c) Type III specimens with Sikadur® 30 adhesive (d) Type III specimens with DHZ adhesive

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3.2 Bearing capacity and failure displacement From the tensile test, the yield load and ultimate load can be collected. These loads are the bearing capacity of the specimens for elastic and elasto-plastic stages. Based on the PIV technique, the displacement can also be analysed. The failure displacement was compared for each case since it is important to understand the ductility of specimens. The mechanical behaviour of specimens without environmental exposure is presented in Figure 9. As shown, after strengthening by CFRP plates, both the yield load and ultimate load were improved significantly. For the yield load of type II specimens, there was not a large difference, except the larger scatter of Sikadur® 30 adhesive bonded specimens. However, in terms of the cases of type III specimens with Sikadur® 30 adhesive, both the yield load and ultimate load were lower than the DZH adhesive case, especially for the yield load. The reason may be attributed to the better stress transfer of lower elastic modulus adhesives. Surprisingly, for DZH adhesive cases, mechanical performance of type II specimens was not superior to type III specimens, which is different from the fatigue test results in the study of Yu et al. (2014)[16]. In addition, the failure displacement was increased after CFRP plates strengthening. Although the displacement of type II specimens with Sikadur® 30 adhesive was not stable, the others show similar average values, which means that the ductility was increased by CFRP strengthening. With regard to the mechanical properties of specimens subjected to marine environmental exposure, normalized values were considered to understand the significance of the environment effect. As shown in Figure 10, yield strength, tensile strength and failure displacement were normalized with the value of the unexposed case. Mechanical degradation can be observed in the case of Sikadur® 30 adhesives bonded specimens, the reduction could be more than 20% after 180 WDCs exposure. It is worth noting that the yield strength and tensile strength of the case of Sikadur® 30 adhesive bonded type III specimens increased during the first 90 WDCs and later decreased significantly during the next 90 WDCs. After marine environmental exposure, the stiffness of Sikadur® 30 adhesives was reduced and the stress transfer efficiency was improved. As a result, the strength of the composite system was increased. Afterwards, because of the further deterioration of adhesives, the adhesive layer easily failed and the debonding premature failure occurred. Additionally, the reduction of failure displacement was obvious for all the cases. Specifically, while the displacement decreased consistently with the WDCs for the cases with DZH adhesives, it dropped significantly after 90 WDCs exposure and thereafter had no clear change for the cases with Sikadur® 30 adhesive. 70

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Figure 10. Degradation of mechanical properties of specimens exposed to marine environment (a) yield strength (b) tensile strength (c) failure displacement 3.3 Load-strain relationship Special attention was paid to the strain development at the centre of specimens because it is the position where stress concentrated due to the existence of the centre hole. For different cases, the strain developing in the centre of specimen with load is presented in Figure 11. Since strain gauges were attached in the two sides, average values were considered. As shown, the strain increased linearly at the first stage and then it became nonlinear. However, no obvious turning point can be observed. When the CFRP plates debonded, the strain would drop rapidly and this part was removed as the routine was inconsistent. While type II specimens show large variations for each case, the type II specimen cases agreed rather well with each other, especially for the cases with DZH adhesives. After environmental exposure, the load-strain relationship showed no obvious difference, which means the loosening of CFRP was not significant and the adhesives were strong enough for stress transfer in this study. Besides, the results could be useful for modeling of strengthened steel structures subjected to marine environment. Since the effect of environmental exposure on the load-strain relationship of specimens was marginal, the cohesive law of the CFRP-to-steel bonded joints obtained from normal cases could be applied for the marine environmental case.

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Figure 11. Load-strain relationship of specimens (strain of CFRP plate centre) before and after marine environmental exposure (a) Type II specimens with Sikadur® 30 adhesive (b) Type II specimens with DHZ adhesive (c) Type III specimens with Sikadur® 30 adhesive (d) Type III specimens with DHZ adhesive 3.4 Failure modes Although the stress-strain relationship has no notable difference with the marine environment exposure, the failure modes could be affected. As discussed in Section 3.1, the salt solution may deteriorate the mechanical properties of adhesive and CFRP. How the composite systems failure occurred is of significant concern. The failure would initiate at the interface of CFRP/steel composites if there was no debonding prevention device application. There are five weak positions, steel cracks, steel/adhesive interface, adhesive, adhesive/CFRP interface and CFRP plates [2]. In this study, the total debonding of the CFRP was considered to be specimen failure. The debonding failure mainly occurred in the steel/adhesive interface, as shown in Figure 12. The CFRP delamination was also observed. From the failure mode, it is clear that the steel plate crack propagation was impeded by CFRP strengthening. Because of the contribution of CFRP, the failure did not occur at the centre hole of the steel plate. Another, there was no obvious change for the failure modes of specimens subjected to marine environmental exposure. A typical debonding failure process of environmental exposed specimen is shown in Figure 13, where the crack initiation, propagation and resulting debonding failure with load increase can be observed. Although an accelerating wetting/drying condition was applied in the test, the mechanical degradation was not detrimental. Longer periods of environmental exposure should be investigated. To prevent the debonding failure to improve the bearing capacity, measures will be taken in future research.

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Figure 12. Failure schematic diagram of specimens without environmental exposure (a) A-S-1 (b) A-D-1 (c) B-S-2 (d) B-D-1

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Final failure

52.35 kN

Figure 13. Typical debonding failure process of specimens after environmental exposure (A-S-3) 4. Conclusions Tensile behaviour of steel plates with a centre hole strengthened by CFRP plates subjected to marine environment was examined in this study. By conducting tensile tests after environmental exposure, the load-displacement curve, load-strain curve, yield load, ultimate load, failure displacement and failure mode were obtained. From the discussions, the following conclusions can be reached. (1) Both CFRP strengthening forms were effective in enhancing the bearing capacity of the notched steel plates. After CFRP strengthening, the crack propagation of the steel plate was impeded. Yield load and ultimate load of steel plates were largely improved, as well as ductility. For the yield load, the increase was over than 40% for each strengthening form. (2) After environmental exposure, the mechanical properties of the composites could be degraded. The degradation degree was significant for 180 WDCs exposure (as high as 20%), which indicates poor durability performance. It also indicates that the strengthening technique must be improved when it is applied in marine environment. (3) Although the deterioration of adhesives has a negative effect on the mechanical performance of the composite specimens, the stiffness reduction may enhance the bond strength. The benefits and negative effects of adhesive deterioration play a key role in the mechanical performance of the composite structures under 13

Adhes cracki

marine environment. (4) There was no clear failure mode change due to marine environmental exposure, which were basically mixed between adhesive debonding and CFRP delamination. To improve the bearing capacity of the composite structure, debonding failure should be prevented in future study. Acknowledgements This work is supported by the National Natural Science Foundation of China through Grants (Project No. 51778151, 51708133, 51278131), the Department of Education of Guangdong Province, China (Project No. 2016KZDXM051), and by China Postdoctoral Science Foundation (Project No. 2017M622633). References 1. Hollaway, L.C.; Cadei, J. Progress in the technique of upgrading metallic structures with advanced polymer composites. Progress in Structural Engineering and Materials 2002, 4, 131-148. 2. Zhao, X.-L.; Zhang, L. State-of-the-art review on frp strengthened steel structures. Engineering Structures 2007, 29, 1808-1823. 3. Nguyen, T.-C.; Bai, Y.; Zhao, X.-L.; Al-Mahaidi, R. Durability of steel/cfrp double strap joints exposed to sea water, cyclic temperature and humidity. Composite Structures 2012, 94, 1834-1845. 4. Gholami, M.; Sam, A.R.M.; Yatim, J.M.; Tahir, M.M. A review on steel/cfrp strengthening systems focusing environmental performance. Construction and Building Materials 2013, 47, 301-310. 5. Fu, B.; Teng, J.G.; Chen, J.F.; Chen, G.M.; Guo, Y.C. Concrete cover separation in frp-plated rc beams: Mitigation using frp u-jackets. Journal of Composites for Construction 2017, 21. 6. Wang, Y.; Zhou, C. Bond characteristics of cfrp/steel interface end-anchored with g-shaped clamps. Polymer & Polymer Composites 2017, 25, 661-667. 7. Heshmati, M.; Haghani, R.; Al-Emrani, M. Environmental durability of adhesively bonded frp/steel joints in civil engineering applications: State of the art. Composites Part B: Engineering 2015, 81, 259-275. 8. Feng, P.; Hu, L.; Zhao, X.L.; Cheng, L.; Xu, S. Study on thermal effects on fatigue behavior of cracked steel plates strengthened by cfrp sheets. Thin-Walled Structures 2014, 82, 311–320. 9. Heshmati, M.; Haghani, R.; Al-Emrani, M. Effects of moisture on the long-term performance of adhesively bonded frp/steel joints used in bridges. Composites Part B Engineering 2016, 92, 447-462. 10. Agarwal, A.; Foster, S.J.; Hamed, E.; Ng, T.S. Influence of freeze–thaw cycling on the bond strength of steel–frp lap joints. Composites Part B: Engineering 2014, 60, 178-185. 11. Shrestha, J.; Zhang, D.; Ueda, T. Durability performances of carbon fiber–reinforced polymer and concrete-bonded systems under moisture conditions. Journal of Composites for Construction 2016, 04016023. 12. Apicella, A.; Nicolais, L. Effect of water on the properties of epoxy matrix and composite. Springer Berlin Heidelberg: 1985; p 69-77. 13. Sun, H.; Wei, L.; Zhu, M.; Han, N.; Zhu, J.-H.; Xing, F. Corrosion behavior of carbon fiber reinforced polymer anode in simulated impressed current cathodic protection system with 3% nacl solution. Construction and Building Materials 2016, 112, 538-546. 14. George, K.; Theodoros, M.; Stefanos, K.; Fyllas, N.M.; Jones, G.V. Galvanic corrosion of carbon and steel in aggressive environments. Journal of Composites for Construction 2001, 5, 200-210. 15. Deng, J.; Lee, M.M. Fatigue performance of metallic beam strengthened with a bonded cfrp plate. Composite Structures 2007, 78, 222-231. 14

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