- Email: [email protected]
Influence of corrosion on the bond behavior in CFRP-steel single lap joints
Construction and Building Materials 236 (2020) 117607
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Influence of corrosion on the bond behavior in CFRP-steel single lap joints Shanhua Xu a,b,c, Han Li a,⇑, Youde Wang a,b,c,⇑, Yujiao Wang a, Yan Wang a a
School of Civil Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China Key Lab of Engineering Structural Safety and Durability (XAUAT), Xi’an 710055, China c State Key Laboratory of Green Building in Western China, Xi’an 710055, China b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The irregular corroded steel surface
can change the failure mode near the loaded end. Only the moderately rough surface can maximize the CFRP-steel bonding behavior. Corrosion can improve the load transfer efficiency between CFRP and steel.
a r t i c l e
i n f o
Article history: Received 19 July 2019 Received in revised form 9 November 2019 Accepted 12 November 2019
Keywords: Bond behavior Corrosion characteristics Carbon fiber reinforced polymer Corroded steel
a b s t r a c t The purpose of this paper is to investigate the corrosion influence on the bond behavior of carbon fiber reinforced polymer (CFRP) strengthened steel joints. The corrosion was characterized by the mass loss ratio n, the average corrosion pit aspect ratio l and the developed interfacial area ratio Sdr. Through the shear test of CFRP-steel single lap joints, the bond behavior was obtained and its relationships with corrosion parameters were established. Results show that the corroded specimens have a higher ultimate load and a general smaller effective bond length than uncorroded specimens. As the corrosion level increases, the failure mode near the loaded end changes from the steel-adhesive adhesion failure to the cohesive failure; the ultimate load and the interfacial fracture energy also increase. In addition, when the l and the Sdr take the intermediate value, the ultimate load and the interfacial fracture energy reach the maximum, which indicates that only the proper degree of surface irregularity can maximize the bond behavior. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Carbon fiber reinforced polymer (CFRP) materials have shown a growth in popularity in the steel strengthening field. Due to its ⇑ Corresponding authors at: School of Civil Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China (H. Li and Y. Wang). E-mail addresses: [email protected] (H. Li), [email protected] (Y. Wang). https://doi.org/10.1016/j.conbuildmat.2019.117607 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
high strength to weight ratio and the excellent resistance to fatigue and corrosion [1], CFRP materials can be used in the bending and fatigue reinforcement [2–4], buckling reinforcement [5] and durability reinforcement. Existing studies indicate that through CFRP bonding method, the load bearing capacity and the durability of steel members have been improved with a minimal dead weight increment, the stress concentration effects are also weakened. However, it has been found that the debonding of CFRP from steel
2
S. Xu et al. / Construction and Building Materials 236 (2020) 117607
substrate is one of the main failure modes of these strengthening systems, which demonstrates the importance of studying the bond behavior. There exist many literatures focusing on the factors affecting the bonding properties between CFRP and steel. Both Xia [6] and Yu [7] studied the influence of adhesive thickness on the bond behavior through a similar single shear test. They found that the specimens with thin adhesive layer are prone to the cohesive failure while those with thick adhesive layer are tend to the delamination of CFRP plates. Yu also found an increment of the bonding strength with the CFRP plate rigidity. Through a butt-joint tensile test and a single-lap shear test, Teng [8] studied the effects of the surface treatment and claimed that the adhesion failure between steel and adhesive could be avoid through grit-blasting. Li [9] conducted a tensile test of CFRP-steel specimens and found a close relationship between the bond behavior and the mechanical properties of adhesive and CFRP plate. It is noticed that the steel substrates adopted in most of the current researches are uncorroded. However, in actual conditions, CFRP materials are mainly applied on the existing structures which are inevitably eroded by environment. In literature [10], the volume loss caused by corrosion was divided into uniform and non-uniform volume loss, the former causes the cross-section weakening while the later results in the irregular surface topography. Due to the negligible relationship between cross-section and the bonding interface, only the corrosion topography should be considered to affect the bonding properties. In the study of Chotickai [11], corrosion was preinstalled on the steel plates and the ultimate load of CFRP-steel bonded joints was increased. Li [12] conducted a more systematic and comprehensive research and found that the failure mode, the ultimate load and the effective bond length are all influenced by corrosion duration when the adhesive layer takes a critical thickness. This paper focused on the corrosion effects on the bond behavior of CFRP strengthened steel joints. The corroded steel plates were cut from the web of U channels served in a steel plant and suffers aggressive internal environment. Based on the corrosion topography obtained by a photographic 3D scanner, the corrosion parameters such as the mass loss ratio n, the average corrosion pit aspect ratio l and the developed interfacial area ratio Sdr were obtained. In addition, the CFRP-steel single lap joint was chosen to investigate the bond behavior such as the failure mode, the ultimate load, the effective bond length and the interfacial fracture energy. Finally, the relationships between the corrosion parameters and the bonding properties were established.
2. Experimental study In order to investigate the influence of corrosion on the bond behavior between CFRP plate and steel, a surface scanning test and a single shear test was conducted respectively. The former intends to acquire the corrosion topography while the later aims to obtain the bond behavior of CFRP-steel single lap joints. 2.1. Materials In the current study, three materials were chosen to manufacture the specimens: corroded steel, adhesive and CFRP plates. The steel adopted in this study was taken from a steel mill located in northeastern China, the web of U-shaped steel on both sides of the aisle beam in the steel slag warehouse (as shown in Fig. 1). It has been subjected to high temperature, high humidity – and high concentration of SO24 and NO3 during its over ten-year service period. The mechanical properties of steel were acquired by tensile test and are listed in Table 1. A total of 6 steel plates with dimensions of 280 mm 80 mm 5 mm (uncorroded thickness) were then taken according to the appearance (shown in Fig. 2) under different corrosion level. These steel plates were named after ‘‘CL X”, where X is a number indicating the corrosion level. The adhesive utilized in this study was an elastic two-part epoxy adhesive. In addition, the CFRP plate was a unidirectional plate produced with pultrusion technology, the width of which was 30 mm and the thickness was 1.4 mm. The mechanical properties of adhesive and CFRP plates were provided by manufacturer and are listed in Table 1. 2.2. Specimens The CFRP-steel single lap joints were chosen as the specimen in this study. Compared with double lap joints, it has only one path for debonding which allows easy monitoring and inspection [6].
Table 1 Mechanical properties of materials.
Elastic modulus (GPa) Shear strength (MPa) Tensile strength (MPa) Yield stress (MPa) Ultimate elongation (%)
Fig. 1. The source of the steel and the aggressive environment.
Steel
Adhesive
CFRP plate
200 – 359 250 22.08
3.2 14 40 – –
168 60.7 2454 – 1.66
S. Xu et al. / Construction and Building Materials 236 (2020) 117607
3
Fig. 2. Appearances of steel plates under different corrosion level (CL).
Fig. 3. (a) CFRP-steel single lap joint, (b) device for topography measurement, (c) scanning parameters and (d) loading instrument and sensors.
Prior to the fabrication, the corrosion production was removed by mechanical polishing and the steel plates were cleaned by pure ethanol to remove contaminations. Fig. 3(a) depicts the schematics of the specimen, a 700 mm 30 mm piece of CFRP plate was adhesively bonded to a 240 mm 30 mm rectangle area on the steel plate. The bond length was chosen as 240 mm which is larger than the effective bond length obtained from other literatures [6–7]. After the fabrication, the specimens were cured at room temperature for 2 weeks according to the product manual. In order to reduce the experimental discreteness, each steel plate was used for 3 times. In other words, after the specimen was loaded to destruction, the steel plate was taken out and used to make another two specimens after the surface was cleaned. Finally, a total of 18 CFRP-steel single lap joints were manufactured. The specimens were named with ‘‘CL” and two numbers, the former refers to the corrosion level while the later represents the batch. For example, CL2-3 indicates the third specimen made of the steel plate CL2.
2.3. Corrosion topography measurement Prior to the fabrication of specimens, the surface topography of steel plate was measured by a non-contact 3D scanning system (as shown in Fig. 3(b)). The mechanism and scanning parameters are shown in Fig. 3(c), the grating projection device projects a plurality of structured lights onto the steel plate, through the analysis of the phase difference of these images captured by multiple cameras, the three-dimensional point cloud data can be acquired. The measurement region was identical to the bonding area (240 mm 30 mm), both the horizontal and vertical resolution was 0.15 mm. 2.4. Instrument and loading procedure The single shear test was implemented by a loading device designed for this study. As Fig. 3(d) shows, the specimen was bolted to the base to prevent the out-of-plane bending. The CFRP plate was clamped by two plates and connected to the slider by
4
S. Xu et al. / Construction and Building Materials 236 (2020) 117607
a tension sensor. Through the pression of a hydraulic jack, the slider moved along the track and applied tension to the CFRP plate. The loading increments was 0.5 kN when the load–displacement curve was in the linear phase and reduced to 0.25 kN while it entered the nonlinear phase. To acquire the strain distribution along the interface, a total of 13 strain gauges at intervals of 20 mm were attached to the CFRP surface, in which two strain gauges were arranged in the first row near the loaded end. Both the strain and the load data were collected by a data logger. 2.5. Digital image Correlation (DIC) method The relative displacement between CFRP and steel plate at loaded end was measured using Digital Image Correlation (DIC) technology. As shown in Fig. 4(a), the measured process includes three phases: speckle production, image calibration and image processing. Before the loading procedure, the speckle on the specimen was produced by painting, then the HD camera was adjusted to make sure that the loaded end and the calibration plate was in the view. The calibration was performed by establishing the relationship between the pixel and the actual size. During the loading procedure, the deformation images were captured by HD camera. After the single shear test, those images were analyzed by software and the displacement field was obtained. In order to verify the reliability, the slip of the specimen CL4-3 at x = 10 mm obtained by DIC method was compared with that calculated by Equation (11), as shown in Fig. 4(b). It can be seen that the two results are basically consistent, which indicates that the DIC method is reliable. 3. Corrosion characteristics of steel plates For an CFRP-steel bonded interface, adhesion failure is more likely to occur at the steel-adhesive interface than at the CFRPadhesive interface, which makes the steel surface an important factor affecting the bond behavior. This view has been confirmed by several studies which found that the shear strength of the steeladhesive interface increases with the roughness parameter Ra [13–14]. Based on the above points, this paper focuses on the corrosion effects on the steel surface. The surface topography of steel plates is shown in Fig. 5. In order to quantify the corrosion degree, the steel plates were weighed and the mass loss ratio n was calculated by:
n¼
W W0 100% W0
ð1Þ
where W and W0 refer to the weight of the corroded and uncorroded steel plate, respectively. The results are listed in Table 2.
3.1. The statistics of corrosion pits Unlike sandblasted or mechanically worn surfaces, corroded surfaces are more complex due to the presence of pits with varying sizes. In order to evaluate the irregularity of these corroded surfaces, the pit identification and the aspect ratio (depth to width ratio) statistics were performed through programming. It is worth noting that due to the neglectable effect of the profiles perpendicular to the loading direction on the bonding strength, only the profiles along the loading direction were concerned. As shown in Fig. 6(a), the surface topography was divided into profiles along the loading direction, in which the minimum points were regarded as the bottom of pits while the two adjacent maximum points were considered as the top points. On the other words, the corrosion pit was approximated by a triangle containing three extreme points, the distance between the two maximum points was defined as the width and the height difference between the larger maximum and the minimum point was the depth. The statistical results of the pit aspect ratio on corroded surfaces are expressed in the form of distribution histogram, and CL2 is taken as an example in Fig. 6(b). By fitting the distribution histograms, the authors found that they all agree well with the following function: ðxlÞ2 A y ¼ y0 þ pffiffiffiffiffiffiffi e 2r2 2pr
ð2Þ
where l is the average value of the aspect ratio, r refers to the standard deviation, the y0 represents the baseline and A represents the amplitude. The mean value l and the standard deviation r of the pit aspect ratio are shown in Fig. 6(c) as a function of mass loss ratio n and are listed in Table 2. As it can be seen, both l and r show an increasing–decreasing tendency which can be explained by the evolution of corrosion pits. In the early corrosion stage, the pits are mainly caused by corrosion and initial defects, they are shallow and close in size (Profile 1 and 2 in Fig. 6(c)), resulting in a small l and r. As corrosion progresses, the pits mainly expand toward the depth direction and become deep and narrow (Profile 3 and 4 in Fig. 6(c)). Meanwhile, the size difference become larger due to the unequal expansion degree. As a result, both l and r increase in this period. In the late stage of corrosion, the pits become shallow and wide due to the coalescence of the neighboring pits (Profile 5 and 6 in Fig. 6(c)), which lead to a decrease in l. It is worth noting that in [15–16], the aspect ratio decreased monotonically with corrosion degree, which may be due to the fact that the uncorroded specimens were not considered.
Fig. 4. (a) The process of DIC method and (b) the comparison of the slip of CL4-3 at x = 10 mm between DIC readings and strain calculations.
5
S. Xu et al. / Construction and Building Materials 236 (2020) 117607
Fig. 5. The surface topography of corroded steel plates.
Table 2 Corrosion characteristics and the failure mode near the loaded end. Specimen
CL0-1 CL0-2 CL0-3 CL1-1 CL1-2 CL1-3 CL2-1 CL2-2 CL2-3 CL3-1 CL3-2 CL3-3 CL4-1 CL4-2 CL4-3 CL5-1 CL5-2 CL5-3
Corrosion Level
n/%
Aspect ratio
l
r
Sdr/%
Failure mode
(b) (a) (b) (a) (a) (a) (c) (a) (b) (a) (a) (c) (a) (b) (b) (b) (b) (b)
0
0
0.0250
0.0119
0.2776
1
12.48
0.0228
0.0116
0.2000
2
19.10
0.0412
0.0246
0.7233
3
25.72
0.0380
0.0243
0.7202
4
27.10
0.0341
0.0190
0.4427
5
28.74
0.0336
0.0175
0.4605
6
S. Xu et al. / Construction and Building Materials 236 (2020) 117607
Fig. 6. (a) Corrosion pits identification, (b) the statistics of corrosion pits on CL2 and (c) the aspect ratio curve of corrosion pits.
ðS AÞ A
3.2. Roughness parameters
Sdr ¼
There are over thirty surface roughness parameters suggested in ISO 25178-2 [17] and each of them could only describe the characteristics of a certain aspect. Therefore, only the parameter that may be related to the bond behavior was selected in this paper. From the perspective of bonding theory, the adhesion between steel and adhesive is related to the physical and chemical bonding, the mechanical interlocking effect and the bonding area [18–19]. Since the physical and chemical bonding is primarily related to the materials make up the interface which are the same in this study (steel and adhesive), this part is not considered. The mechanical interlocking effect is mainly affected by the shape of the interface, which has been evaluated by the pit statistics in section 3.1. In order to reveal the influence of the bonding area, the developed interfacial area ratio Sdr was adopted. As Fig. 7(a) shows, the physical meaning of Sdr refers to the ratio of the increment of the surface area within the definition area over the definition area, which can be calculated by Eq. (3).
Both the acreage of the corroded surface S and the definition area A were calculated by a surface treatment software and then Sdr was obtained (shown in Fig. 7(b) and listed in Table 2). A general law of increase–decrease was observed and was also related to the corrosion evolution.
Sdr ¼
1 A
" RR A
ffi sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ! # 2 2 @zðx;yÞ @zðx;yÞ 1 þ @x 1 dxdy þ @y
ð3Þ
In this study, since the geometric model of the corroded surface was composed of discrete points, it was difficult to find the expression of the surface z (x, y) and calculate the partial derivative. Moreover, as the surface area could be easily obtained by software, a simplified calculation Eq. (4) was proposed by authors based on the the definition of Sdr.
ð4Þ
4. Results and discussion 4.1. Corrosion effects on failure mode It has been demonstrated that the steel surface topography would affect the failure mode of CFRP-steel single or double lap joints. For the CFRP bonded steel systems, there are 6 typical failures [1]: (a) adhesion failure between steel and adhesive; (b) cohesive failure of adhesive layer; (c) adhesion failure between CFRP and adhesive; (d) CFRP delamination; (e) CFRP rapture; (f) steel yielding. Fig. 8 shows the typical failure modes of the specimens in this study. As it can be seen, the failure mode near the loaded end is quite different from that near the free end. At the loaded end, a total of 3 types of failure modes (a) (b) and (c) are observed. However, there is only one failure mode combined with (c) and (d) near the free end. This difference is mainly related to the failure process of the specimens. As will be discussed in 4.2, when the load reaches the ultimate load, the failure expands from the loaded end to the free end until the specimen sudden break. During the damage expansion stage, the failure near the loaded end arise and the bonding interface is mainly affected by the shear stress; with
S. Xu et al. / Construction and Building Materials 236 (2020) 117607
7
Fig. 7. (a) The calculation of Sdr and (b) the Sdr curve with n.
Fig. 8. Typical failure modes of specimens.
the expansion of damage area, the interfacial peeling effect increases significantly and finally leads to the sudden break. Since the corroded surface has little effect on the peeling failure, it can be inferred that only the failure near the loaded end could reflect the bond behavior under the influence of corrosion. Therefore, only the failure mode near the loaded end is studied in this paper.
It can be seen from Table 2 that as the corrosion level increases, the failure mode near the loaded end changes from adhesion failure to cohesive failure. For the specimens with lower corrosion levels (0 and 1), the l, r and the Sdr are small, which means that the effects of pits can be neglected and the contact area is small, resulting in an weak interface between steel and adhesive and lead
8
S. Xu et al. / Construction and Building Materials 236 (2020) 117607
to adhesion failure there. As the corrosion level increases (2 and 3), the l, r and the Sdr are the largest but the main failure mode is still the adhesion failure between steel and adhesive. The reason for this phenomenon may be that the excessive aspect ratio and the Sdr would cause an extremely uneven adhesive layer which enlarge the peeling effect of the interface between steel and adhesive. For the specimens with higher corrosion level (4 and 5), the l is still large and the r is reduced, which means that the pits can provide sufficient mechanical interlocking without excessively enlarging the interface peeling effect. As a result, the strength of the steeladhesive interface is greater than that of the adhesive layer, which lead to the cohesive failure. In summary, extreme value of the l, r and the Sdr, whether it was too big or small, would make the steeladhesive interface weaker than adhesive layer and lead to adhesion failure. 4.2. Corrosion effects on load–displacement behavior The relationships between load and the displacement at the loaded end are shown in Fig. 9. It is similar to [6] that the load–displacement curves can be divided into three parts. Initially, the curves are in the elastic stage, the load increases linearly with displacement. As the load is close to the ultimate load, the curves enter the softening stage and become nonlinear due to the softening of adhesive layer. Finally, when the load reaches the ultimate load, the failure occurs at the loaded end and expands toward the free end until the sudden break with a limitedly varying load and a continuously increasing displacement, resulting in a plateau in the curves. It is noted that no plateau is observed in the curves of specimens with corrosion levels 0, 1, 2 and 3. As mentioned before, uniform or extremely uneven surfaces may lead to the adhesion failure on the steel-adhesive interface, which is brittle and does not fully exert the ductility of the specimens. The ultimate loads of specimens are listed in Table 3. In order to eliminate the difference in results, the average ultimate load is adopted. Considering the ignorable influence of corrosion on the
specimens with adhesion failure between CFRP and adhesive (CL2-1 and CL3-3), their ultimate loads are not counted in the average value. Fig. 10 depicts the effect of corrosion on the average ultimate load and the following points can be summarized: (1) The average ultimate load of corroded specimens is generally greater than that of the uncorroded specimens, which is similar to [12] which indicates the positive impact of corrosion on the ultimate load. As can be seen from Fig. 10(a), with the increase of the n, the average ultimate load decreases slightly and then increases. In addition, it can be drawn that only when the corrosion is serious (n > 15%), the steel surface will have an obvious impact on the average ultimate load. Compared with the uncorroded specimens, the average ultimate load of specimens with a 12.48% mass loss ratio only decreases by 0.39%, while that with a 28.74% mass loss ratio increases by 10.45%. By fitting the curve, the Eq. (5) can be obtained with a good coincidence (R2 = 0.94):
F u ¼ 0:0191 en=5:213 þ 40:672
ð5Þ
(2) Fig. 10 (b)(c) depict the relationships between the average ultimate load and the corrosion parameters l and Sdr. It is worth noting that the average ultimate load shows an increasing–decreasing tendency whether the argument is l or Sdr, indicating that only the appropriate surface irregularity can provide the maximum reinforcement to the ultimate load. In the results of Teng and Fernando [8], with the increase of the fracture energy and fractal dimension of the steel surface, the ultimate load increased firstly and then stabilized, no decline was observed because the grinding and sand blasting treatments adopted in their studies were not enough to enlarge the peeling effect like corrosion effect. The fitting results are shown in Eqs. (6) and (7) (R2 = 0.99 and 0.84, respectively):
F u ¼ 4:338 e
ðl0:0337Þ2 2:723105
þ 40:687
F u ¼ 54:476S2dr þ 54:527Sdr þ 31:200
Fig. 9. Load-displacement curves of specimens.
ð6Þ ð7Þ
9
S. Xu et al. / Construction and Building Materials 236 (2020) 117607 Table 3 Bond behavior of specimens. specimen
CL0-1 CL0-2 CL0-3 CL1-1 CL1-2 CL1-3 CL2-1 CL2-2 CL2-3 CL3-1 CL3-2 CL3-3 CL4-1 CL4-2 CL4-3 CL5-1 CL5-2 CL5-3
Ultimate load (kN)
Effective bond length (mm)
Interfacial fracture energy (N/mm)
Fu
Average Fu
Le
Average Le
G
Average G
37.13 39.01 46.41 35.41 39.54 47.11 35.17* 39.24 43.71 44.4 41.11 40.25* 43.3 43.5 48.02 45.95 42.76 46.65
40.85
110 110 110 90 130 100 90 90 130 130 90 90 130 50 90 90 90 70
110.00
2.89 1.94 2.49 2.04 2.64 1.89 0.63* 0.52 2.77 3.05 1.78 1.42* 6.3 8.79 10.62 9.32 1.20# 11.21
2.44
40.69
41.48
42.76
44.94
45.12
106.67
103.33
103.33
90.00
83.33
2.19
1.65
2.42
8.57
10.26
* indicates the specimens with failure mode (c) near the loaded end and # indicates the specimens having problems with the strain gauges. These specimens were not counted in this study.
Fig. 10. Relationships between the average ultimate load ¯Fu and the corrosion parameters: (a) the mass loss ratio n, (b) the average aspect ratio l and (c) the developed interfacial area ratio Sdr.
4.3. Corrosion effects on shear stress distribution The average interfacial shear stress between two adjacent strain gauges can be calculated by dividing the force difference by the bonding area by Eq. (8) [20]:
siþ1=2 ¼ ðei eiþ1 ÞEp tp =Dl
ð8Þ
Where ei and ei+1 are the strain values at the ith and i + 1th strain gauges from the loaded end (i = 1 at the loaded end), Ep and tp are the elastic modulus and thickness of the CFRP plate respectively, Dl is the spacing of the two strain gauges, si+ 1/2 is the average interfacial shear stress between the two strain gauges. Fig. 11 depicts the interfacial shear stress distributions during the loading procedure. Since the results of different batches of specimens are similar, only the third batch is shown. It can be seen that the interfacial shear stress distribution of corroded specimens is different from that of uncorroded specimens. Compared with the gentle transition of uncorroded specimens, the shear stress of the corroded specimens exhibits a concussive reduction toward the free end. The reason for this phenomenon may be that the irregular surface changes the interfacial stress from shear stress to a composite stress, which interferes the stress distribution along the bonding area. It is notable that the shear stress is only distributed over a certain length beyond which the shear stress can be ignorable, this
length is generally called the effective bond length Le [21] and is listed in Table 3. The Le refers to the load transfer efficiency between CFRP plate and steel, which is mainly affected by the elastic modulus and the thickness of CFRP plate and steel, the width of CFRP plate, the shear modulus and the extreme shear stress of adhesive [22–23]. Since the materials and the size of specimens are the same in this study, the variation of Le can be attributed to the uneven thickness of steel and the stress state transformation of the adhesive caused by interlocking effect. Fig. 12(a) and (b) depict the relationships between the average effective bond length and corrosion parameters. Since there is no obvious law in the average Le-n curve, only the average Le-l and the average Le-Sdr curves are shown. As can be seen from Fig. 12 (a), the average effective bond length decreases with the l and the decreasing rate increases, which indicates that the sharper corrosion pits can improve the load transfer efficiency between CFRP plate and steel. In Fig. 12(b), a similar trend between the average effective bond length and the Sdr can be observed and a negative impact of bonding area on the effective bond length can be drawn. Through the fitting method, the quantitative relationships between the average effective bond length and the corrosion parameters Eqs. (9) and (10) can be obtained (the R2 of both equations are 0.96):
Le ¼ 0:0558 el=0:0066 þ 111:028
ð9Þ
10
S. Xu et al. / Construction and Building Materials 236 (2020) 117607
Fig. 11. Interfacial shear stress distribution of (a) CL0-3 (b) CL1-3 (c) CL2-3 (d) CL3-3 (e) CL4-3 (f) CL5-3.
Fig. 12. Relationships between the average effective bond length ¯Le and corrosion parameters: (a) the average aspect ratio l and (b) the developed interfacial area ratio Sdr.
Le ¼ 1:837 eSdr =0:271 þ 112:940
ð10Þ
4.4. Corrosion effects on bond-slip behavior The slip (relative displacement of CFRP plate and streel) in the middle of two strain gauges was obtained based on Eq. (11):
Siþ1=2 ¼ S0
i1 X 1 1 ðem þ emþ1 Þ Dl ðei þ eiþ1 Þ Dl 2 4 m¼1
ð11Þ
where Si+1/2 is the slip in the middle of the ith and i + 1th strain gauges, S0 is the slip at the loaded end, ei and Dl has the same meaning as in Equation (8). The relationships between interfacial shear stress and slip, i.e., the bond-slip curves are shown in Fig. 13. It can be seen that only the specimens with corrosion level 4 and 5 have a complete ascending-descending bond-slip curve like other studies [6–7]. As for specimens with corrosion level 0,1,2 and 3, however, no drop is observed in the bond-slip curves. This difference is mainly caused by the different adhesive properties. Compared with other
studies, the adhesive used in this paper has a higher ultimate strength and a lower elastic modulus (see Table 1), resulting in better load bearing and deformation ability. Therefore, unless the corroded surface can provide sufficient reinforcement to the interface, the specimens are more prone to the steel-adhesive adhesion failure than cohesive failure, which does not fully exert the bond-slip properties of the adhesive layer. The key parameter of the bond-slip curve, the interfacial fracture energy G which indicates the area enclosed by the bond-slip curve and the slip axis, is listed in Table 3. In order to eliminate the difference, the average value ¯G is adopted to investigate the relationship with corrosion parameters, as shown in Fig. 14. As can be seen in Fig. 14(a), the average G decreases first and then increases with n, and then increases sharply when n reaches 27.1%. As explained before, in the beginning of corrosion, the peeling effect caused the decrease in the average G; while in the later stage, the corroded surfaces could provide sufficient reinforcement to the interface, resulting in an increase in the average G; when the interface strength exceeded the cohesive strength, the cohesive failure occurred and the average G increased sharply.
S. Xu et al. / Construction and Building Materials 236 (2020) 117607
11
Fig. 13. Bond-slip curves of specimens.
Fig. 14. Relationships between the average interfacial fracture energy ¯G and corrosion parameters: (a) the mass loss ratio n, (b) the average aspect ratio l and (c) the developed interfacial area ratio Sdr.
In Fig. 14 (b) and (c), the average G exhibits a sharp law of increase–decrease as the independent variable increases. It is worth noting that the average G reached the maximum value when the l and the Sdr take the intermediate value, which implies that only the proper surface irregularity can maximize the bond-slip behavior. 5. Conclusions In the present study, the influence of corrosion on the bond behavior of CFRP-steel single lap joints was investigated. The corrosion parameters which consist of the mass loss ratio n, the average pit aspect ratio l and the developed interfacial area ratio Sdr were proposed to evaluate the bond behavior such as the failure mode, the ultimate load, the effective bond length and the interfacial fracture energy. Based on the experimental results, the following conclusions can be made: (1) Only the failure near the loaded end is affected by corrosion. With the increase of the n, the failure mode near the loaded end transforms from the adhesion failure between steel and
adhesive to the cohesive failure on adhesive layer. When the l and the Sdr take the intermediate value, the failure mode near the loaded end is the cohesive failure; when the l and the Sdr take the extreme value, whether it was too big or small, the failure mode is the adhesion failure between steel and adhesive. (2) The ultimate load of corroded specimens is larger than that of the uncorroded specimens. The average ultimate load exhibits an exponential increase with the n. An increasing– decreasing tendency of the average ultimate load is observed and reached the maximum value when the the l and the Sdr take the intermediate value, which indicates that only the proper degree of surface irregularity can maximize the bonding strength. (3) The interfacial shear stress distribution of the uncorroded specimens is relatively gentle compared with that of the corroded specimens. The average effective bond length decreases with the l and the Sdr, indicating that both the sharper corrosion pits and the larger bonding area can improve the load transfer efficiency between CFRP plate and steel.
12
S. Xu et al. / Construction and Building Materials 236 (2020) 117607
(4) The average interfacial fracture energy decreases first and then increases with n. As for the l and the Sdr, the average G increases first and then decreases. Similar to the average ultimate load, the average G reaches the maximum value as the l and the Sdr take the intermediate value, which implies that the moderately rough surface can maximize the bond-slip behavior. CRediT authorship contribution statement Shanhua Xu: Conceptualization, Methodology, Funding acquisition, Project administration. Han Li: Methodology, Software, Validation, Formal analysis, Investigation, Resources. : . Youde Wang: Software, Writing - review & editing, Supervision. Yujiao Wang: Writing - original draft. Yan Wang: Data curation. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors are grateful to the National Natural Science Foundation of China (Grant Nos. 51678477, 51908455) and the Scientific Research Project of Shaanxi Provincial Department of Education Key Laboratory (Grant No. 17JS061) for their financial supports. The authors would also like to express gratitude to the viewers for their comments. References [1] X.L. Zhao, L. Zhang, State-of-the-art review on FRP strengthened steel structures, Eng. Struct. 29 (2007) 1808–1823. [2] J. Deng, M.M.K. Lee, Behaviour under static loading of metallic beams reinforced with a bonded CFRP plate, Compos. Struct. 78 (2007) 232–242.
[3] J. Deng, M.M.K. Lee, Fatigue performance of metallic beam strengthened with a bonded CFRP plate, Compos. Struct. 78 (2007) 222–231. [4] P. Colombi, C. Poggi, An experimental, analytical and numerical study of the static behavior of steel beams reinforced by pultruded CFRP strips, Compos. Part B Eng. 37 (2006) 64–73. [5] J. Haedir, X.L. Zhao, Design of short CFRP-reinforced steel tubular columns, J. Constr. Steel Res. 67 (2011) 497–509. [6] S.H. Xia, J.G. Teng, Behavior of FRP-to-steel bond joints, Proceedings of the international symposium on bond behaviour of FRP in structures (BBFS 2005), Hong Kong, China, 2005. [7] T. Yu, D. Fernando, J.G. Teng, X.L. Zhao, Experimental study on CFRP-to-steel bonded interfaces, Compos. Part B Eng. 43 (2012) 2279–2289. [8] J.G. Teng, D. Fernando, T. Yu, X.L Zhao, Treatment of steel surfaces for effective adhesive bonding, in: The 5th International Conference on FRP Composites in Civil Engineering (CICE 2010), Beijing, China, 2010. [9] C. Li, L. Ke, J. He, Z. Chen, Y. Jiao, Effects of mechanical properties of adhesive and CFRP on the bond behavior in CFRP-strengthened steel structures, Compos Struct 211 (2019) 163–174. [10] Y. Wang, S. Xu, H. Wang, A. Li, Predicting the residual strength and deformability of corroded steel plate based on the corrosion morphology, Constr. Build. Mater. 152 (2017) 777–793. [11] P. Chotickai, Effect of pre-installed corrosion on steel plate-CFRP bond characteristics, Int. J. Adhes. Adhes. 84 (2018) 431–437. [12] A. Li, S. Xu, H. Wang, H. Zhang, Y. Wang, Bond behaviour between CFRP plates and corroded steel plates, Compos. Struct. 220 (2019) 221–235. [13] M. Shahid, S.A. Hashim, Effect of surface roughness on the strength of cleavage joints, Int. J. Adhes. Adhes. 22 (2002) 235–244. [14] W. Zielecki, P. Pawlus, R. Perłowski, A. Dzierwa, Surface topography effect on strength of lap adhesive joints after mechanical pre-treatment, Arch. Civ. Mech. Eng. 13 (2013) 175–185. [15] S. Xu, Y. Wang, Estimating the effects of corrosion pits on the fatigue life of steel plate based on the 3D profile, Int. J. Fatigue 72 (2015) 27–41. [16] K. van der Walde, B.M. Hillberry, Characterization of pitting damage and prediction of remaining fatigue life, Int. J. Fatigue 30 (2008) 106–118. [17] ISO, 25178–2, Geometrical product specifications (GPS)-surface texture: areal-part 2: terms, definitions and surface texture parameters, International Organization of Standardization, Geneva, 2012. [18] A.J. Kinloch, Adhesion and adhesives: science and technology, Chapman and Hall, London, 1987. [19] W.C. Wake, Adhesion and the formulation of adhesives, Applied Science Publishers, London and New York, 1976. [20] H.B. Pham, R. Al-Mahaidi, Modelling of CFRP-concrete shear-lap tests, Constr. Build. Mater. 21 (2007) 727–735. [21] X.L. Zhao, FRP-Strengthened Metallic Structures, CRC Press, New York, 2014. [22] L.J. Hart-Smith, Adhesive-bonded double-lap joints, Technical Report NASA CR-112235, Long Beach, Douglas Aircraft Company, CA, 1973. [23] H. Yuan, J.G. Teng, R. Seracino, Z.S. Wu, J. Yao, Full-range behavior of FRP-toconcrete bonded joints, Eng. Struct. 26 (2004) 553–565.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Influence of corrosion on the bond behavior in CFRP-steel single lap joints Shanhua Xu a,b,c, Han Li a,⇑, Youde Wang a,b,c,⇑, Yujiao Wang a, Yan Wang a a
School of Civil Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China Key Lab of Engineering Structural Safety and Durability (XAUAT), Xi’an 710055, China c State Key Laboratory of Green Building in Western China, Xi’an 710055, China b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The irregular corroded steel surface
can change the failure mode near the loaded end. Only the moderately rough surface can maximize the CFRP-steel bonding behavior. Corrosion can improve the load transfer efficiency between CFRP and steel.
a r t i c l e
i n f o
Article history: Received 19 July 2019 Received in revised form 9 November 2019 Accepted 12 November 2019
Keywords: Bond behavior Corrosion characteristics Carbon fiber reinforced polymer Corroded steel
a b s t r a c t The purpose of this paper is to investigate the corrosion influence on the bond behavior of carbon fiber reinforced polymer (CFRP) strengthened steel joints. The corrosion was characterized by the mass loss ratio n, the average corrosion pit aspect ratio l and the developed interfacial area ratio Sdr. Through the shear test of CFRP-steel single lap joints, the bond behavior was obtained and its relationships with corrosion parameters were established. Results show that the corroded specimens have a higher ultimate load and a general smaller effective bond length than uncorroded specimens. As the corrosion level increases, the failure mode near the loaded end changes from the steel-adhesive adhesion failure to the cohesive failure; the ultimate load and the interfacial fracture energy also increase. In addition, when the l and the Sdr take the intermediate value, the ultimate load and the interfacial fracture energy reach the maximum, which indicates that only the proper degree of surface irregularity can maximize the bond behavior. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Carbon fiber reinforced polymer (CFRP) materials have shown a growth in popularity in the steel strengthening field. Due to its ⇑ Corresponding authors at: School of Civil Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China (H. Li and Y. Wang). E-mail addresses: [email protected] (H. Li), [email protected] (Y. Wang). https://doi.org/10.1016/j.conbuildmat.2019.117607 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
high strength to weight ratio and the excellent resistance to fatigue and corrosion [1], CFRP materials can be used in the bending and fatigue reinforcement [2–4], buckling reinforcement [5] and durability reinforcement. Existing studies indicate that through CFRP bonding method, the load bearing capacity and the durability of steel members have been improved with a minimal dead weight increment, the stress concentration effects are also weakened. However, it has been found that the debonding of CFRP from steel
2
S. Xu et al. / Construction and Building Materials 236 (2020) 117607
substrate is one of the main failure modes of these strengthening systems, which demonstrates the importance of studying the bond behavior. There exist many literatures focusing on the factors affecting the bonding properties between CFRP and steel. Both Xia [6] and Yu [7] studied the influence of adhesive thickness on the bond behavior through a similar single shear test. They found that the specimens with thin adhesive layer are prone to the cohesive failure while those with thick adhesive layer are tend to the delamination of CFRP plates. Yu also found an increment of the bonding strength with the CFRP plate rigidity. Through a butt-joint tensile test and a single-lap shear test, Teng [8] studied the effects of the surface treatment and claimed that the adhesion failure between steel and adhesive could be avoid through grit-blasting. Li [9] conducted a tensile test of CFRP-steel specimens and found a close relationship between the bond behavior and the mechanical properties of adhesive and CFRP plate. It is noticed that the steel substrates adopted in most of the current researches are uncorroded. However, in actual conditions, CFRP materials are mainly applied on the existing structures which are inevitably eroded by environment. In literature [10], the volume loss caused by corrosion was divided into uniform and non-uniform volume loss, the former causes the cross-section weakening while the later results in the irregular surface topography. Due to the negligible relationship between cross-section and the bonding interface, only the corrosion topography should be considered to affect the bonding properties. In the study of Chotickai [11], corrosion was preinstalled on the steel plates and the ultimate load of CFRP-steel bonded joints was increased. Li [12] conducted a more systematic and comprehensive research and found that the failure mode, the ultimate load and the effective bond length are all influenced by corrosion duration when the adhesive layer takes a critical thickness. This paper focused on the corrosion effects on the bond behavior of CFRP strengthened steel joints. The corroded steel plates were cut from the web of U channels served in a steel plant and suffers aggressive internal environment. Based on the corrosion topography obtained by a photographic 3D scanner, the corrosion parameters such as the mass loss ratio n, the average corrosion pit aspect ratio l and the developed interfacial area ratio Sdr were obtained. In addition, the CFRP-steel single lap joint was chosen to investigate the bond behavior such as the failure mode, the ultimate load, the effective bond length and the interfacial fracture energy. Finally, the relationships between the corrosion parameters and the bonding properties were established.
2. Experimental study In order to investigate the influence of corrosion on the bond behavior between CFRP plate and steel, a surface scanning test and a single shear test was conducted respectively. The former intends to acquire the corrosion topography while the later aims to obtain the bond behavior of CFRP-steel single lap joints. 2.1. Materials In the current study, three materials were chosen to manufacture the specimens: corroded steel, adhesive and CFRP plates. The steel adopted in this study was taken from a steel mill located in northeastern China, the web of U-shaped steel on both sides of the aisle beam in the steel slag warehouse (as shown in Fig. 1). It has been subjected to high temperature, high humidity – and high concentration of SO24 and NO3 during its over ten-year service period. The mechanical properties of steel were acquired by tensile test and are listed in Table 1. A total of 6 steel plates with dimensions of 280 mm 80 mm 5 mm (uncorroded thickness) were then taken according to the appearance (shown in Fig. 2) under different corrosion level. These steel plates were named after ‘‘CL X”, where X is a number indicating the corrosion level. The adhesive utilized in this study was an elastic two-part epoxy adhesive. In addition, the CFRP plate was a unidirectional plate produced with pultrusion technology, the width of which was 30 mm and the thickness was 1.4 mm. The mechanical properties of adhesive and CFRP plates were provided by manufacturer and are listed in Table 1. 2.2. Specimens The CFRP-steel single lap joints were chosen as the specimen in this study. Compared with double lap joints, it has only one path for debonding which allows easy monitoring and inspection [6].
Table 1 Mechanical properties of materials.
Elastic modulus (GPa) Shear strength (MPa) Tensile strength (MPa) Yield stress (MPa) Ultimate elongation (%)
Fig. 1. The source of the steel and the aggressive environment.
Steel
Adhesive
CFRP plate
200 – 359 250 22.08
3.2 14 40 – –
168 60.7 2454 – 1.66
S. Xu et al. / Construction and Building Materials 236 (2020) 117607
3
Fig. 2. Appearances of steel plates under different corrosion level (CL).
Fig. 3. (a) CFRP-steel single lap joint, (b) device for topography measurement, (c) scanning parameters and (d) loading instrument and sensors.
Prior to the fabrication, the corrosion production was removed by mechanical polishing and the steel plates were cleaned by pure ethanol to remove contaminations. Fig. 3(a) depicts the schematics of the specimen, a 700 mm 30 mm piece of CFRP plate was adhesively bonded to a 240 mm 30 mm rectangle area on the steel plate. The bond length was chosen as 240 mm which is larger than the effective bond length obtained from other literatures [6–7]. After the fabrication, the specimens were cured at room temperature for 2 weeks according to the product manual. In order to reduce the experimental discreteness, each steel plate was used for 3 times. In other words, after the specimen was loaded to destruction, the steel plate was taken out and used to make another two specimens after the surface was cleaned. Finally, a total of 18 CFRP-steel single lap joints were manufactured. The specimens were named with ‘‘CL” and two numbers, the former refers to the corrosion level while the later represents the batch. For example, CL2-3 indicates the third specimen made of the steel plate CL2.
2.3. Corrosion topography measurement Prior to the fabrication of specimens, the surface topography of steel plate was measured by a non-contact 3D scanning system (as shown in Fig. 3(b)). The mechanism and scanning parameters are shown in Fig. 3(c), the grating projection device projects a plurality of structured lights onto the steel plate, through the analysis of the phase difference of these images captured by multiple cameras, the three-dimensional point cloud data can be acquired. The measurement region was identical to the bonding area (240 mm 30 mm), both the horizontal and vertical resolution was 0.15 mm. 2.4. Instrument and loading procedure The single shear test was implemented by a loading device designed for this study. As Fig. 3(d) shows, the specimen was bolted to the base to prevent the out-of-plane bending. The CFRP plate was clamped by two plates and connected to the slider by
4
S. Xu et al. / Construction and Building Materials 236 (2020) 117607
a tension sensor. Through the pression of a hydraulic jack, the slider moved along the track and applied tension to the CFRP plate. The loading increments was 0.5 kN when the load–displacement curve was in the linear phase and reduced to 0.25 kN while it entered the nonlinear phase. To acquire the strain distribution along the interface, a total of 13 strain gauges at intervals of 20 mm were attached to the CFRP surface, in which two strain gauges were arranged in the first row near the loaded end. Both the strain and the load data were collected by a data logger. 2.5. Digital image Correlation (DIC) method The relative displacement between CFRP and steel plate at loaded end was measured using Digital Image Correlation (DIC) technology. As shown in Fig. 4(a), the measured process includes three phases: speckle production, image calibration and image processing. Before the loading procedure, the speckle on the specimen was produced by painting, then the HD camera was adjusted to make sure that the loaded end and the calibration plate was in the view. The calibration was performed by establishing the relationship between the pixel and the actual size. During the loading procedure, the deformation images were captured by HD camera. After the single shear test, those images were analyzed by software and the displacement field was obtained. In order to verify the reliability, the slip of the specimen CL4-3 at x = 10 mm obtained by DIC method was compared with that calculated by Equation (11), as shown in Fig. 4(b). It can be seen that the two results are basically consistent, which indicates that the DIC method is reliable. 3. Corrosion characteristics of steel plates For an CFRP-steel bonded interface, adhesion failure is more likely to occur at the steel-adhesive interface than at the CFRPadhesive interface, which makes the steel surface an important factor affecting the bond behavior. This view has been confirmed by several studies which found that the shear strength of the steeladhesive interface increases with the roughness parameter Ra [13–14]. Based on the above points, this paper focuses on the corrosion effects on the steel surface. The surface topography of steel plates is shown in Fig. 5. In order to quantify the corrosion degree, the steel plates were weighed and the mass loss ratio n was calculated by:
n¼
W W0 100% W0
ð1Þ
where W and W0 refer to the weight of the corroded and uncorroded steel plate, respectively. The results are listed in Table 2.
3.1. The statistics of corrosion pits Unlike sandblasted or mechanically worn surfaces, corroded surfaces are more complex due to the presence of pits with varying sizes. In order to evaluate the irregularity of these corroded surfaces, the pit identification and the aspect ratio (depth to width ratio) statistics were performed through programming. It is worth noting that due to the neglectable effect of the profiles perpendicular to the loading direction on the bonding strength, only the profiles along the loading direction were concerned. As shown in Fig. 6(a), the surface topography was divided into profiles along the loading direction, in which the minimum points were regarded as the bottom of pits while the two adjacent maximum points were considered as the top points. On the other words, the corrosion pit was approximated by a triangle containing three extreme points, the distance between the two maximum points was defined as the width and the height difference between the larger maximum and the minimum point was the depth. The statistical results of the pit aspect ratio on corroded surfaces are expressed in the form of distribution histogram, and CL2 is taken as an example in Fig. 6(b). By fitting the distribution histograms, the authors found that they all agree well with the following function: ðxlÞ2 A y ¼ y0 þ pffiffiffiffiffiffiffi e 2r2 2pr
ð2Þ
where l is the average value of the aspect ratio, r refers to the standard deviation, the y0 represents the baseline and A represents the amplitude. The mean value l and the standard deviation r of the pit aspect ratio are shown in Fig. 6(c) as a function of mass loss ratio n and are listed in Table 2. As it can be seen, both l and r show an increasing–decreasing tendency which can be explained by the evolution of corrosion pits. In the early corrosion stage, the pits are mainly caused by corrosion and initial defects, they are shallow and close in size (Profile 1 and 2 in Fig. 6(c)), resulting in a small l and r. As corrosion progresses, the pits mainly expand toward the depth direction and become deep and narrow (Profile 3 and 4 in Fig. 6(c)). Meanwhile, the size difference become larger due to the unequal expansion degree. As a result, both l and r increase in this period. In the late stage of corrosion, the pits become shallow and wide due to the coalescence of the neighboring pits (Profile 5 and 6 in Fig. 6(c)), which lead to a decrease in l. It is worth noting that in [15–16], the aspect ratio decreased monotonically with corrosion degree, which may be due to the fact that the uncorroded specimens were not considered.
Fig. 4. (a) The process of DIC method and (b) the comparison of the slip of CL4-3 at x = 10 mm between DIC readings and strain calculations.
5
S. Xu et al. / Construction and Building Materials 236 (2020) 117607
Fig. 5. The surface topography of corroded steel plates.
Table 2 Corrosion characteristics and the failure mode near the loaded end. Specimen
CL0-1 CL0-2 CL0-3 CL1-1 CL1-2 CL1-3 CL2-1 CL2-2 CL2-3 CL3-1 CL3-2 CL3-3 CL4-1 CL4-2 CL4-3 CL5-1 CL5-2 CL5-3
Corrosion Level
n/%
Aspect ratio
l
r
Sdr/%
Failure mode
(b) (a) (b) (a) (a) (a) (c) (a) (b) (a) (a) (c) (a) (b) (b) (b) (b) (b)
0
0
0.0250
0.0119
0.2776
1
12.48
0.0228
0.0116
0.2000
2
19.10
0.0412
0.0246
0.7233
3
25.72
0.0380
0.0243
0.7202
4
27.10
0.0341
0.0190
0.4427
5
28.74
0.0336
0.0175
0.4605
6
S. Xu et al. / Construction and Building Materials 236 (2020) 117607
Fig. 6. (a) Corrosion pits identification, (b) the statistics of corrosion pits on CL2 and (c) the aspect ratio curve of corrosion pits.
ðS AÞ A
3.2. Roughness parameters
Sdr ¼
There are over thirty surface roughness parameters suggested in ISO 25178-2 [17] and each of them could only describe the characteristics of a certain aspect. Therefore, only the parameter that may be related to the bond behavior was selected in this paper. From the perspective of bonding theory, the adhesion between steel and adhesive is related to the physical and chemical bonding, the mechanical interlocking effect and the bonding area [18–19]. Since the physical and chemical bonding is primarily related to the materials make up the interface which are the same in this study (steel and adhesive), this part is not considered. The mechanical interlocking effect is mainly affected by the shape of the interface, which has been evaluated by the pit statistics in section 3.1. In order to reveal the influence of the bonding area, the developed interfacial area ratio Sdr was adopted. As Fig. 7(a) shows, the physical meaning of Sdr refers to the ratio of the increment of the surface area within the definition area over the definition area, which can be calculated by Eq. (3).
Both the acreage of the corroded surface S and the definition area A were calculated by a surface treatment software and then Sdr was obtained (shown in Fig. 7(b) and listed in Table 2). A general law of increase–decrease was observed and was also related to the corrosion evolution.
Sdr ¼
1 A
" RR A
ffi sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ! # 2 2 @zðx;yÞ @zðx;yÞ 1 þ @x 1 dxdy þ @y
ð3Þ
In this study, since the geometric model of the corroded surface was composed of discrete points, it was difficult to find the expression of the surface z (x, y) and calculate the partial derivative. Moreover, as the surface area could be easily obtained by software, a simplified calculation Eq. (4) was proposed by authors based on the the definition of Sdr.
ð4Þ
4. Results and discussion 4.1. Corrosion effects on failure mode It has been demonstrated that the steel surface topography would affect the failure mode of CFRP-steel single or double lap joints. For the CFRP bonded steel systems, there are 6 typical failures [1]: (a) adhesion failure between steel and adhesive; (b) cohesive failure of adhesive layer; (c) adhesion failure between CFRP and adhesive; (d) CFRP delamination; (e) CFRP rapture; (f) steel yielding. Fig. 8 shows the typical failure modes of the specimens in this study. As it can be seen, the failure mode near the loaded end is quite different from that near the free end. At the loaded end, a total of 3 types of failure modes (a) (b) and (c) are observed. However, there is only one failure mode combined with (c) and (d) near the free end. This difference is mainly related to the failure process of the specimens. As will be discussed in 4.2, when the load reaches the ultimate load, the failure expands from the loaded end to the free end until the specimen sudden break. During the damage expansion stage, the failure near the loaded end arise and the bonding interface is mainly affected by the shear stress; with
S. Xu et al. / Construction and Building Materials 236 (2020) 117607
7
Fig. 7. (a) The calculation of Sdr and (b) the Sdr curve with n.
Fig. 8. Typical failure modes of specimens.
the expansion of damage area, the interfacial peeling effect increases significantly and finally leads to the sudden break. Since the corroded surface has little effect on the peeling failure, it can be inferred that only the failure near the loaded end could reflect the bond behavior under the influence of corrosion. Therefore, only the failure mode near the loaded end is studied in this paper.
It can be seen from Table 2 that as the corrosion level increases, the failure mode near the loaded end changes from adhesion failure to cohesive failure. For the specimens with lower corrosion levels (0 and 1), the l, r and the Sdr are small, which means that the effects of pits can be neglected and the contact area is small, resulting in an weak interface between steel and adhesive and lead
8
S. Xu et al. / Construction and Building Materials 236 (2020) 117607
to adhesion failure there. As the corrosion level increases (2 and 3), the l, r and the Sdr are the largest but the main failure mode is still the adhesion failure between steel and adhesive. The reason for this phenomenon may be that the excessive aspect ratio and the Sdr would cause an extremely uneven adhesive layer which enlarge the peeling effect of the interface between steel and adhesive. For the specimens with higher corrosion level (4 and 5), the l is still large and the r is reduced, which means that the pits can provide sufficient mechanical interlocking without excessively enlarging the interface peeling effect. As a result, the strength of the steeladhesive interface is greater than that of the adhesive layer, which lead to the cohesive failure. In summary, extreme value of the l, r and the Sdr, whether it was too big or small, would make the steeladhesive interface weaker than adhesive layer and lead to adhesion failure. 4.2. Corrosion effects on load–displacement behavior The relationships between load and the displacement at the loaded end are shown in Fig. 9. It is similar to [6] that the load–displacement curves can be divided into three parts. Initially, the curves are in the elastic stage, the load increases linearly with displacement. As the load is close to the ultimate load, the curves enter the softening stage and become nonlinear due to the softening of adhesive layer. Finally, when the load reaches the ultimate load, the failure occurs at the loaded end and expands toward the free end until the sudden break with a limitedly varying load and a continuously increasing displacement, resulting in a plateau in the curves. It is noted that no plateau is observed in the curves of specimens with corrosion levels 0, 1, 2 and 3. As mentioned before, uniform or extremely uneven surfaces may lead to the adhesion failure on the steel-adhesive interface, which is brittle and does not fully exert the ductility of the specimens. The ultimate loads of specimens are listed in Table 3. In order to eliminate the difference in results, the average ultimate load is adopted. Considering the ignorable influence of corrosion on the
specimens with adhesion failure between CFRP and adhesive (CL2-1 and CL3-3), their ultimate loads are not counted in the average value. Fig. 10 depicts the effect of corrosion on the average ultimate load and the following points can be summarized: (1) The average ultimate load of corroded specimens is generally greater than that of the uncorroded specimens, which is similar to [12] which indicates the positive impact of corrosion on the ultimate load. As can be seen from Fig. 10(a), with the increase of the n, the average ultimate load decreases slightly and then increases. In addition, it can be drawn that only when the corrosion is serious (n > 15%), the steel surface will have an obvious impact on the average ultimate load. Compared with the uncorroded specimens, the average ultimate load of specimens with a 12.48% mass loss ratio only decreases by 0.39%, while that with a 28.74% mass loss ratio increases by 10.45%. By fitting the curve, the Eq. (5) can be obtained with a good coincidence (R2 = 0.94):
F u ¼ 0:0191 en=5:213 þ 40:672
ð5Þ
(2) Fig. 10 (b)(c) depict the relationships between the average ultimate load and the corrosion parameters l and Sdr. It is worth noting that the average ultimate load shows an increasing–decreasing tendency whether the argument is l or Sdr, indicating that only the appropriate surface irregularity can provide the maximum reinforcement to the ultimate load. In the results of Teng and Fernando [8], with the increase of the fracture energy and fractal dimension of the steel surface, the ultimate load increased firstly and then stabilized, no decline was observed because the grinding and sand blasting treatments adopted in their studies were not enough to enlarge the peeling effect like corrosion effect. The fitting results are shown in Eqs. (6) and (7) (R2 = 0.99 and 0.84, respectively):
F u ¼ 4:338 e
ðl0:0337Þ2 2:723105
þ 40:687
F u ¼ 54:476S2dr þ 54:527Sdr þ 31:200
Fig. 9. Load-displacement curves of specimens.
ð6Þ ð7Þ
9
S. Xu et al. / Construction and Building Materials 236 (2020) 117607 Table 3 Bond behavior of specimens. specimen
CL0-1 CL0-2 CL0-3 CL1-1 CL1-2 CL1-3 CL2-1 CL2-2 CL2-3 CL3-1 CL3-2 CL3-3 CL4-1 CL4-2 CL4-3 CL5-1 CL5-2 CL5-3
Ultimate load (kN)
Effective bond length (mm)
Interfacial fracture energy (N/mm)
Fu
Average Fu
Le
Average Le
G
Average G
37.13 39.01 46.41 35.41 39.54 47.11 35.17* 39.24 43.71 44.4 41.11 40.25* 43.3 43.5 48.02 45.95 42.76 46.65
40.85
110 110 110 90 130 100 90 90 130 130 90 90 130 50 90 90 90 70
110.00
2.89 1.94 2.49 2.04 2.64 1.89 0.63* 0.52 2.77 3.05 1.78 1.42* 6.3 8.79 10.62 9.32 1.20# 11.21
2.44
40.69
41.48
42.76
44.94
45.12
106.67
103.33
103.33
90.00
83.33
2.19
1.65
2.42
8.57
10.26
* indicates the specimens with failure mode (c) near the loaded end and # indicates the specimens having problems with the strain gauges. These specimens were not counted in this study.
Fig. 10. Relationships between the average ultimate load ¯Fu and the corrosion parameters: (a) the mass loss ratio n, (b) the average aspect ratio l and (c) the developed interfacial area ratio Sdr.
4.3. Corrosion effects on shear stress distribution The average interfacial shear stress between two adjacent strain gauges can be calculated by dividing the force difference by the bonding area by Eq. (8) [20]:
siþ1=2 ¼ ðei eiþ1 ÞEp tp =Dl
ð8Þ
Where ei and ei+1 are the strain values at the ith and i + 1th strain gauges from the loaded end (i = 1 at the loaded end), Ep and tp are the elastic modulus and thickness of the CFRP plate respectively, Dl is the spacing of the two strain gauges, si+ 1/2 is the average interfacial shear stress between the two strain gauges. Fig. 11 depicts the interfacial shear stress distributions during the loading procedure. Since the results of different batches of specimens are similar, only the third batch is shown. It can be seen that the interfacial shear stress distribution of corroded specimens is different from that of uncorroded specimens. Compared with the gentle transition of uncorroded specimens, the shear stress of the corroded specimens exhibits a concussive reduction toward the free end. The reason for this phenomenon may be that the irregular surface changes the interfacial stress from shear stress to a composite stress, which interferes the stress distribution along the bonding area. It is notable that the shear stress is only distributed over a certain length beyond which the shear stress can be ignorable, this
length is generally called the effective bond length Le [21] and is listed in Table 3. The Le refers to the load transfer efficiency between CFRP plate and steel, which is mainly affected by the elastic modulus and the thickness of CFRP plate and steel, the width of CFRP plate, the shear modulus and the extreme shear stress of adhesive [22–23]. Since the materials and the size of specimens are the same in this study, the variation of Le can be attributed to the uneven thickness of steel and the stress state transformation of the adhesive caused by interlocking effect. Fig. 12(a) and (b) depict the relationships between the average effective bond length and corrosion parameters. Since there is no obvious law in the average Le-n curve, only the average Le-l and the average Le-Sdr curves are shown. As can be seen from Fig. 12 (a), the average effective bond length decreases with the l and the decreasing rate increases, which indicates that the sharper corrosion pits can improve the load transfer efficiency between CFRP plate and steel. In Fig. 12(b), a similar trend between the average effective bond length and the Sdr can be observed and a negative impact of bonding area on the effective bond length can be drawn. Through the fitting method, the quantitative relationships between the average effective bond length and the corrosion parameters Eqs. (9) and (10) can be obtained (the R2 of both equations are 0.96):
Le ¼ 0:0558 el=0:0066 þ 111:028
ð9Þ
10
S. Xu et al. / Construction and Building Materials 236 (2020) 117607
Fig. 11. Interfacial shear stress distribution of (a) CL0-3 (b) CL1-3 (c) CL2-3 (d) CL3-3 (e) CL4-3 (f) CL5-3.
Fig. 12. Relationships between the average effective bond length ¯Le and corrosion parameters: (a) the average aspect ratio l and (b) the developed interfacial area ratio Sdr.
Le ¼ 1:837 eSdr =0:271 þ 112:940
ð10Þ
4.4. Corrosion effects on bond-slip behavior The slip (relative displacement of CFRP plate and streel) in the middle of two strain gauges was obtained based on Eq. (11):
Siþ1=2 ¼ S0
i1 X 1 1 ðem þ emþ1 Þ Dl ðei þ eiþ1 Þ Dl 2 4 m¼1
ð11Þ
where Si+1/2 is the slip in the middle of the ith and i + 1th strain gauges, S0 is the slip at the loaded end, ei and Dl has the same meaning as in Equation (8). The relationships between interfacial shear stress and slip, i.e., the bond-slip curves are shown in Fig. 13. It can be seen that only the specimens with corrosion level 4 and 5 have a complete ascending-descending bond-slip curve like other studies [6–7]. As for specimens with corrosion level 0,1,2 and 3, however, no drop is observed in the bond-slip curves. This difference is mainly caused by the different adhesive properties. Compared with other
studies, the adhesive used in this paper has a higher ultimate strength and a lower elastic modulus (see Table 1), resulting in better load bearing and deformation ability. Therefore, unless the corroded surface can provide sufficient reinforcement to the interface, the specimens are more prone to the steel-adhesive adhesion failure than cohesive failure, which does not fully exert the bond-slip properties of the adhesive layer. The key parameter of the bond-slip curve, the interfacial fracture energy G which indicates the area enclosed by the bond-slip curve and the slip axis, is listed in Table 3. In order to eliminate the difference, the average value ¯G is adopted to investigate the relationship with corrosion parameters, as shown in Fig. 14. As can be seen in Fig. 14(a), the average G decreases first and then increases with n, and then increases sharply when n reaches 27.1%. As explained before, in the beginning of corrosion, the peeling effect caused the decrease in the average G; while in the later stage, the corroded surfaces could provide sufficient reinforcement to the interface, resulting in an increase in the average G; when the interface strength exceeded the cohesive strength, the cohesive failure occurred and the average G increased sharply.
S. Xu et al. / Construction and Building Materials 236 (2020) 117607
11
Fig. 13. Bond-slip curves of specimens.
Fig. 14. Relationships between the average interfacial fracture energy ¯G and corrosion parameters: (a) the mass loss ratio n, (b) the average aspect ratio l and (c) the developed interfacial area ratio Sdr.
In Fig. 14 (b) and (c), the average G exhibits a sharp law of increase–decrease as the independent variable increases. It is worth noting that the average G reached the maximum value when the l and the Sdr take the intermediate value, which implies that only the proper surface irregularity can maximize the bond-slip behavior. 5. Conclusions In the present study, the influence of corrosion on the bond behavior of CFRP-steel single lap joints was investigated. The corrosion parameters which consist of the mass loss ratio n, the average pit aspect ratio l and the developed interfacial area ratio Sdr were proposed to evaluate the bond behavior such as the failure mode, the ultimate load, the effective bond length and the interfacial fracture energy. Based on the experimental results, the following conclusions can be made: (1) Only the failure near the loaded end is affected by corrosion. With the increase of the n, the failure mode near the loaded end transforms from the adhesion failure between steel and
adhesive to the cohesive failure on adhesive layer. When the l and the Sdr take the intermediate value, the failure mode near the loaded end is the cohesive failure; when the l and the Sdr take the extreme value, whether it was too big or small, the failure mode is the adhesion failure between steel and adhesive. (2) The ultimate load of corroded specimens is larger than that of the uncorroded specimens. The average ultimate load exhibits an exponential increase with the n. An increasing– decreasing tendency of the average ultimate load is observed and reached the maximum value when the the l and the Sdr take the intermediate value, which indicates that only the proper degree of surface irregularity can maximize the bonding strength. (3) The interfacial shear stress distribution of the uncorroded specimens is relatively gentle compared with that of the corroded specimens. The average effective bond length decreases with the l and the Sdr, indicating that both the sharper corrosion pits and the larger bonding area can improve the load transfer efficiency between CFRP plate and steel.
12
S. Xu et al. / Construction and Building Materials 236 (2020) 117607
(4) The average interfacial fracture energy decreases first and then increases with n. As for the l and the Sdr, the average G increases first and then decreases. Similar to the average ultimate load, the average G reaches the maximum value as the l and the Sdr take the intermediate value, which implies that the moderately rough surface can maximize the bond-slip behavior. CRediT authorship contribution statement Shanhua Xu: Conceptualization, Methodology, Funding acquisition, Project administration. Han Li: Methodology, Software, Validation, Formal analysis, Investigation, Resources. : . Youde Wang: Software, Writing - review & editing, Supervision. Yujiao Wang: Writing - original draft. Yan Wang: Data curation. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors are grateful to the National Natural Science Foundation of China (Grant Nos. 51678477, 51908455) and the Scientific Research Project of Shaanxi Provincial Department of Education Key Laboratory (Grant No. 17JS061) for their financial supports. The authors would also like to express gratitude to the viewers for their comments. References [1] X.L. Zhao, L. Zhang, State-of-the-art review on FRP strengthened steel structures, Eng. Struct. 29 (2007) 1808–1823. [2] J. Deng, M.M.K. Lee, Behaviour under static loading of metallic beams reinforced with a bonded CFRP plate, Compos. Struct. 78 (2007) 232–242.
[3] J. Deng, M.M.K. Lee, Fatigue performance of metallic beam strengthened with a bonded CFRP plate, Compos. Struct. 78 (2007) 222–231. [4] P. Colombi, C. Poggi, An experimental, analytical and numerical study of the static behavior of steel beams reinforced by pultruded CFRP strips, Compos. Part B Eng. 37 (2006) 64–73. [5] J. Haedir, X.L. Zhao, Design of short CFRP-reinforced steel tubular columns, J. Constr. Steel Res. 67 (2011) 497–509. [6] S.H. Xia, J.G. Teng, Behavior of FRP-to-steel bond joints, Proceedings of the international symposium on bond behaviour of FRP in structures (BBFS 2005), Hong Kong, China, 2005. [7] T. Yu, D. Fernando, J.G. Teng, X.L. Zhao, Experimental study on CFRP-to-steel bonded interfaces, Compos. Part B Eng. 43 (2012) 2279–2289. [8] J.G. Teng, D. Fernando, T. Yu, X.L Zhao, Treatment of steel surfaces for effective adhesive bonding, in: The 5th International Conference on FRP Composites in Civil Engineering (CICE 2010), Beijing, China, 2010. [9] C. Li, L. Ke, J. He, Z. Chen, Y. Jiao, Effects of mechanical properties of adhesive and CFRP on the bond behavior in CFRP-strengthened steel structures, Compos Struct 211 (2019) 163–174. [10] Y. Wang, S. Xu, H. Wang, A. Li, Predicting the residual strength and deformability of corroded steel plate based on the corrosion morphology, Constr. Build. Mater. 152 (2017) 777–793. [11] P. Chotickai, Effect of pre-installed corrosion on steel plate-CFRP bond characteristics, Int. J. Adhes. Adhes. 84 (2018) 431–437. [12] A. Li, S. Xu, H. Wang, H. Zhang, Y. Wang, Bond behaviour between CFRP plates and corroded steel plates, Compos. Struct. 220 (2019) 221–235. [13] M. Shahid, S.A. Hashim, Effect of surface roughness on the strength of cleavage joints, Int. J. Adhes. Adhes. 22 (2002) 235–244. [14] W. Zielecki, P. Pawlus, R. Perłowski, A. Dzierwa, Surface topography effect on strength of lap adhesive joints after mechanical pre-treatment, Arch. Civ. Mech. Eng. 13 (2013) 175–185. [15] S. Xu, Y. Wang, Estimating the effects of corrosion pits on the fatigue life of steel plate based on the 3D profile, Int. J. Fatigue 72 (2015) 27–41. [16] K. van der Walde, B.M. Hillberry, Characterization of pitting damage and prediction of remaining fatigue life, Int. J. Fatigue 30 (2008) 106–118. [17] ISO, 25178–2, Geometrical product specifications (GPS)-surface texture: areal-part 2: terms, definitions and surface texture parameters, International Organization of Standardization, Geneva, 2012. [18] A.J. Kinloch, Adhesion and adhesives: science and technology, Chapman and Hall, London, 1987. [19] W.C. Wake, Adhesion and the formulation of adhesives, Applied Science Publishers, London and New York, 1976. [20] H.B. Pham, R. Al-Mahaidi, Modelling of CFRP-concrete shear-lap tests, Constr. Build. Mater. 21 (2007) 727–735. [21] X.L. Zhao, FRP-Strengthened Metallic Structures, CRC Press, New York, 2014. [22] L.J. Hart-Smith, Adhesive-bonded double-lap joints, Technical Report NASA CR-112235, Long Beach, Douglas Aircraft Company, CA, 1973. [23] H. Yuan, J.G. Teng, R. Seracino, Z.S. Wu, J. Yao, Full-range behavior of FRP-toconcrete bonded joints, Eng. Struct. 26 (2004) 553–565.