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steel single-lap joints under different loading rates and temperatures
Composite Structures 145 (2016) 68–79
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Composite Structures journal homepage: www.elsevier.com/locate/compstruct
Experimental study on basalt FRP/steel single-lap joints under different loading rates and temperatures Mingxia Yao a, Deju Zhu a,⇑, Yiming Yao b, Huaian Zhang a, Barzin Mobasher b a b
College of Civil Engineering, Hunan University, Changsha, Hunan 410082, China School of Sustainable Engineering and Built Environment, Arizona State University, Tempe, AZ 85287, United States
a r t i c l e
i n f o
Article history: Available online 26 February 2016 Keywords: A. Single lap joints B. Mechanical properties C. Loading rates D. Temperatures E. Failure modes
a b s t r a c t In this work, basalt FRP/steel single-lap joints were tested under various temperatures (25, 0, 25, 50, 100 °C) and dynamic loading speeds (0.625, 1.25, 2.5, 5.0 m/s), using a servo-hydraulic high-rate testing machine with an environmental chamber. The deformation and failure behavior of the sample in the overlap region were captured by a Phantom v7.3 high speed digital camera at a sampling rate of 20,000 fps, and analyzed using a digital image correlation (DIC) method. Experimental results presented in this paper include average bond strength, toughness, bond strength-displacement curves and FRP strain distributions in the overlapping areas. In aspect of the loading rate effect, both the bond strength and shear stiffness increase with the increasing loading rate. Most specimens showed a mixed failure mode under dynamic tensile loads. On the other hand, the average bond strength increases in the temperature range of 25 to 50 °C, while decreases significantly from 50 to 100 °C as the glass transition temperature Tg is exceeded. The observed typical failure mode in most specimens at low temperature is debonding at adhesive-steel interface, whereas samples rupture at the adhesive-BFRP interface at elevated temperature with lower load-bearing capacity. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction The use of fiber reinforced polymer (FRP) has gained an increasing interest due to its wide applications in automobile industries, shipbuilding applications, aerospace structures and civil engineering constructions [1,2]. The common FRP materials mainly include carbon fiber reinforced polymer (CFRP), glass fiber reinforced polymer (GFRP) and aramid fiber reinforced polymer (AFRP). CFRP, which exhibits the highest strength and stiffness among common FRPs, has been extensively used for strengthening and upgrading existing structures [3–5]. However, the cost of CFRP raises a demand of more cost-effective materials for infrastructures. The development and research studies of basalt fibers and BFRP in recent years enable the application of this material in the field of engineering like glass or carbon reinforcements [6–9] as a result of many advantages. Basalt fiber, which is eco-friendly, non-toxic, non-conducting, non-combustible and common worldwide, is made from melting basalt rock. In addition, the manufacturing process of the basalt fiber is similar to that of glass fiber, but with less energy consump-
⇑ Corresponding author. Tel.: +86 0731 88823861. E-mail address: [email protected] (D. Zhu). http://dx.doi.org/10.1016/j.compstruct.2016.02.061 0263-8223/Ó 2016 Elsevier Ltd. All rights reserved.
tion and no additives, which makes it superior to glass or carbon fibers in terms of cost [6,10,11]. Besides, basalt fiber is an inorganic material with good durability, high strength and modulus, good antioxidative performance, resistance to high temperature and low temperature, improved strain to failure, good chemical resistance (especially in acid environment). Research has also shown that when the temperature exceeded 600 °C, only the basalt fiber (compared to carbon and S-glass fibers) maintained its volumetric integrity and 90% of the strength [9,12]. Sim et al. [9] aimed at the mechanical performance and the durability of the basalt fiber and performed a series of four-point bending tests on reinforced concrete beams strengthened with the basalt fiber sheet. The test results showed that the basalt fiber strengthening improved both the yielding and the ultimate strength of the beam specimen. In our landscape, damages to masonry structure may also be caused by inappropriate construction practices, aging effects, untimely maintenance or load increments. Basalt textile-reinforced mortar (BTRM) seems to be a good strengthening material for the masonry structures such as the reinforcement of stone masonry arch-shaped structures [13]. Wu et al. [14] investigated the fatigue behaviors of steel beams strengthened by four kinds of fiber-reinforced composite plates (high-modulus carbon-fiber-reinforced polymer plate, high-
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M. Yao et al. / Composite Structures 145 (2016) 68–79 Table 1 Mechanical and physical properties of test materials. Material
Young’s modulus (GPa)
Elongation (%)
Yield strength (MPa)
Tensile strength (MPa)
Coefficient of thermal expansion (°C)
Sikadur 330 Steel plate (Q235) Basalt fiber sheet (CP12640)
4.6 199 122
1.0 35.6 1.70
– 235 –
38.1 441 1452
4.5 105 1.3 105 –
Fig. 1. Schematic view of BFRP-steel single-lap joints (not to scale): (a) front view and (b) side view.
Fig. 2. Prepared typical test sample with speckle pattern.
Fig. 3. Schematic drawing of adhesive thickness controlling device.
strength CFRP plate, steel-wire basalt-fiber-reinforced polymer plate, and weld steel plate). The tests revealed that the steel-wire basalt-fiber-reinforced polymer plate is the optimal strengthening material by considering the cost-performance ratio. Li et al. [15]
proposed a new BFRP-steel composite plate, which has a stable post-yield stiffness, it could effectively design a damagecontrollable structures with good reparability. It is therefore attractive to investigate the performance of BFRP in strengthening and retrofitting the steel structures by means of widely accepted experimental methods such as the single-lap and double strap joints. Numerous experimental works were carried out to thoroughly investigate the bond behavior of CFRP/steel double lap joints under static loading [16], fatigue loading [17], low and elevated temperatures [18,19], ultraviolet radiation [20], impact loading [21]. In contrast, to date no research has been conducted on the bond between BFRP and steel plates. Dorigato and Pegoretti [22] compared the quasi static tensile properties of laminated FRPs reinforced by basalt and carbon woven fabrics with the same areal density. It was reported that the tensile strength of BFRP was comparable to CFRP or even higher and the strains at peak were about twice of CFRP. Elastic modulus of FRP is another concern in the practical applications of retrofitting the steel structures due to relative high modulus of steel [23]. Al-Mosawe et al. [24] studied the effects of CFRP properties on the bond performance. It is found that changing from low (159 GPa) to normal (203 GPa) CFRP modulus had insignificant effects on the maximum failure strain and strain distribution, though slightly lower strain values were observed for specimens with normal CFRP modulus compared to those with low CFRP modulus. Significant reduction in ductility was observed
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M. Yao et al. / Composite Structures 145 (2016) 68–79 Table 2 Summary of the test results of the BFRP-steel single-lap joints. Specimen No. 1 2 3 4 5 6 1 2 3 4 5 6 1 2 3 4 5 6
Fig. 4. MTS servo-hydraulic high-rate testing machine and specimen installation.
when ultra-high modulus (450 GPa) CFRP was used. Nevertheless, the joint load capacity was primarily affected by the tensile strength of CFRP. On the other hand, the failure modes of low and normal CFRP specimens were dominated by the debonding between steel and adhesive and failure of adhesive itself, which indicates that the mechanical properties of FRP may not govern the ultimate state. As far as the relatively low modulus of basalt compared to carbon fibers, more layers of basalt reinforcements can be added to obtain comparative stiffness of the FRP considering its lower cost. On the other hand, in the cases such as seismic loadings when high energy absorption capacity is required, the use of basal fiber may be beneficial by improving strain to failure processes. Therefore BFRP may be used as an economic alternative materials to CFRP in certain applications. According to field applications, FRP-steel composite structures may be subjected to dynamic loadings inevitably, such as explosions, ballistic projectiles, earthquakes, wind driven objects, fast moving traffic, and machine vibrations [11]. Besides, exposure to low and elevated temperatures is also common in industrial applications. As a consequence, this paper presents the effects of various temperatures and dynamic loading rates on the mechanical behavior of single lap BFRP-steel joints. The primary objective of this research is to assess the effects of different temperatures (25, 0, 25, 50, 100 °C) and loading rates (0.625, 1.25, 2.5, 5.0 m/s) on the mechanical properties and failure modes of BFRP-steel single-lap joints, utilizing a high-rate servo-hydraulic (MTS) testing machine and 2D digital image correlation (DIC). Digital images were recorded at different levels of load and then processed in a commercial DIC software (Vic-2D 2009) to obtain the surface strain distributions of BFRP in overlapping area. The mode of failure, bond strength, toughness and shear stiffness are presented and discussed. The results of this investigation will be significant for theoretical researches and practical applications of BFRP in extreme loadings and severe environmental conditions.
Temperature (°C)
25
25
25
Bond strength (MPa)
Stiffness (MPa/ mm)
Failure mode
0.625
5.42 6.92 6.05 6.63 5.65 6.33
2.04 2.52 2.58 2.43 1.87 2.06
IV IV IV IV I IV
1.25
6.57 6.68 5.63 6.38 6.55 7.63
6.83 4.67 5.57 6.23 5.47 8.04
IV IV IV IV IV IV
2.5
7.13 5.51 7.68 6.04 6.85 7.64
8.87 7.02 7.81 3.41 5.92 6.40
IV IV IV IV IV IV
5.0
7.33 9.74 6.56 7.48 7.53 6.66
9.47 9.53 8.43 6.18 6.95 8.18
III III IV IV IV IV
Loading rate (m/s)
1 2 3 4 5 6
25
1 2 3 4
25
0.625
3.77 7.52 5.69 3.32
1.09 7.68 7.05 1.05
I I I I
1 2 3
0
0.625
5.29 6.39 6.06
2.98 6.03 4.01
I I I
7.59 6.61 3.37 2.98 2.30 4.38
IV IV IV IV IV II
1.81 2.84 1.79 1.29
II II II II
1 2 3 4 5 6
50
0.625
6.77 8.82 7.29 7.07 8.60 5.75
1 2 3 4
100
0.625
5.98 5.65 5.92 4.41
2. Experimental program 2.1. Materials Hot rolled steel plates (Q235, a nominal yield stress of 235 MPa, China standard, GB/T 700), BFRP laminates and the epoxy adhesive are selected for the bond assemblies. The thickness of steel plates is 4.00 mm ± 0.02 mm. A two component, solvent free and thixotropic epoxy resin structural adhesive SikadurÒ 330 [25,26] is recommended for bonding the FRP laminates and the steel plates together. Thin unidirectional BFRP laminates were manufactured by means of the vacuum-assisted resin transfer molding (VARTM) technique [27], using the unidirectional basalt fabric (made by Nanjing Hitech Composites Co., Ltd) and epoxy resin (JN-C3P, a fiber impregnated paste adhesive). Then BFRP laminates were cut to the required size of 25 mm 80 mm (W L). It should be emphasized that the nominal thickness of BFRP (made of three layers basalt fabrics) was measured as 0.95 mm. A summary of the mechanical parameters of these materials are given in Table 1. 2.2. Specimen preparation The simplest single-lap BFRP-to-steel bonded joint was adopted in the present study which is a basic structural element and often
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Fig. 5. Experimental bond stress–displacement responses of BFRP-steel single-lap joints at different loading rates of (a) 0.625, (b) 1.25, (c) 2.5, and (d) 5.0 m/s.
used to investigate the bond between steel plates and FRP laminates. The side and front views of joint specimen in Fig. 1 provide detailed geometric information of the joint. Two end spacers (BFRP and steel) of 25 mm long, 25 mm wide were glued on each side of specimens to diminish the eccentricity of the load path. The final specimens are shown in Fig. 2. Great attention was paid to the surface preparation of steel plates because of its sensitivity to rust. It has been reported that the surface preparation has a significant effect on bonding performance between FRP and steel [17,26]. The first essential step is polishing steel plates on both sides to remove any rust within the bond area. Then acetone was applied to the steel plate to further clean the surfaces. In particular, it should be pointed out that adhesive thickness has considerable effects on the bond strength and failure mode. The experiments [28,29] showed that the bond strength of lap-joints decreased as the bond line became thicker, since more defects (voids, micro-cracks, etc.) were formed with increasing thickness. Therefore, a bond line thickness controlling device was designed and manufactured, which is shown in Fig. 3, for the sake of keeping the adhesive thickness of all the joints uniform (1.00 mm). The following preparation procedures were as follows: two steel plates were aligned in U-shaped bottom plate, and a thin layer of epoxy adhesive was smeared on the surface of the steel using a brush. Then BFRP laminate was put on the top of the adhe-
sive layer. A top plate was utilized to squeeze out the air bubbles and excessive adhesive by adjusting the bolt. All specimens were then cured at room temperature for 7 days before mechanical testing. The total thickness of the specimen is measured with a caliper at three locations marked initially on the samples, and an average value is calculated. The adhesive layer thickness (ta) can be approximated using the following equation:
ta ¼ t total t steel t BFRP
ð1Þ
where the thickness of steel plates tsteel and BFRP laminates tBFRP were measured and recorded before specimen preparation. 2.3. Dynamic test setup and technique Dynamic tensile tests were conducted using an MTS high rate servo-hydraulic testing machine operated in open-loop control. The speed of the stroke is controlled by the opening and closing of the servo-valve of hydraulic supply. By manually turning the servo-valve, the rate of flow of hydraulic fluid can be controlled, resulting in different stroke speeds. Calibration records show that the stroke can reach a maximum speed of 14 m/s with a load capacity of 25 kN. The environmental chamber can be heated up by internal electric resistance wire and cooled down by liquid nitrogen, given an operating temperature ranging from 60 to 200 °C. The actual tests were performed after approximately
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Fig. 6. Loading rate effect on the mechanical properties of BFRP-steel single-lap joints: (a) bond strength, (b) toughness, and (c) shear stiffness.
15 min of achieving the test temperature in the specimens, to ensure a steady state temperature throughout the specimen prior to testing. A high speed digital camera was utilized to record the test process at a sampling rate of 20,000 fps (frame per second, time interval = 50 ls) with resolution of 256 512 pixels such that the failure modes could be examined in detail. Fig. 4 presents the dynamic tensile testing setup. More details of the testing system can be found elsewhere [30]. 3. Results and discussion 3.1. Loading rate effect Dynamic tensile tests were carried out with four different loading velocities (0.625, 1.25, 2.5, 5.0 m/s) at room temperature (25 °C). The bond stress is given by:
s¼
F bl
ð2Þ
where F is the tensile load, b is the joint width and l is the joint overlap length. Toughness is evaluated using the area under the bond stress–displacement curve. Shear stiffness is defined as the slope of the curve in the elastic region. Table 2 lists the test design and results of BFRP/steel single lap joint at different loading conditions. Fig. 5 shows the bond stress– displacement curves under different loading rates. Quite uniform responses are observed under each loading rate, which exhibits linear elastic behavior up to peak and brittle failure. The curves at four loading rates indicate that most failure processes of specimens are quite rapid (the bond stress dropped directly to zero when it reaches the peak stress), which means that the failure mode of the joint is not a ductile failure. In the case of a loading rate of 0.625 m/s, a slight fluctuation in the ascending slope of curves is observed, before reaching the maximum bond stress, the curves exhibit nonlinearity. However, for other loading rates, the bond stress–displacement curves are approximately liner elastic before
reaching the maximum stress. The fluctuations in the curves are most likely caused by the vibration of the test device when the specimens were loaded [31] and progressive failure of specimens. The fluctuations are also observed in the bond stress–displacement curves under different temperatures as discussed in Section 3.2. Dynamic mechanical properties of bond strength, toughness and shear stiffness versus loading rates are presented in Fig. 6 (in terms of average values and standard deviations). The bond strength, toughness and shear stiffness are sensitive to the loading rates but exhibit different degrees of sensitivity. The bond strength and shear stiffness of joints all increase from 0.625 to 5.0 m/s. Specifically, for the same temperature (25 °C), the bond strength all increase from 6.16 ± 0.57 MPa at a loading rate of 0.625 m/s to 6.57 ± 0.64, 6.81 ± 0.87 and 7.55 ± 1.15 MPa at the loading rates of 1.25, 2.5 and 5.0 m/s, respectively. The reason for this increase in the bond strength enhancement of BFRP/steel single lap joints is mainly due to the fact that the entire test duration decreases as the loading speed increases such that test specimens need higher stress to satisfy the energy which the failure processing must be absorbed [32]. Similar phenomena have been reported in the Refs. [21,33]. The shear stiffness significantly increases by as much as 172% from 2.25 ± 0.29 MPa/mm to 6.14 ± 1.18 MPa/ mm as the loading rate increases from 0.625 to 1.25 m/s. When the loading rate increases from 1.25 m/s to 2.5 and 5.0 m/s, shear stiffness relatively slowly increases 7%, 32% from 6.14 ± 1.18 MPa/mm to 6.57 ± 1.86 MPa/mm and 8.12 ± 1.34 MPa/ mm. It is also interesting to note that there is an increment in the deviation of bond strength and shear stiffness which can be attributed to the larger scatter at higher dynamic loading speeds. 3.2. Temperature effect Fig. 7 shows the experimental bond stress–displacement responses of BFRP-steel single lap joints under different temperatures (25, 0, 50, 100 °C) at the same loading rate of 0.625 m/s. At lower temperatures (25 and 0 °C), it can be found that most
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Fig. 7. Experimental bond stress–displacement responses of BFRP-steel single-lap joints at different temperatures of (a) 25, (b) 0, (c) 50, and (d) 100 °C.
of curves show a linear ascending region and ultimate failures are quite rapid. However, fluctuations in the ascending regions are observed in the curves at the temperatures of 50 and 100 °C. Note that the curves show relatively large variations at the temperature of 50 °C. Fig. 8 shows the experimental observations of the influence of temperatures on the dynamic mechanical properties of BFRPsteel single lap joints. It indicates the temperature-sensitivity of the BFRP/steel adhesively bonded joints. Overall, both bond strength and toughness of joints increase from 25 to 50 °C and significantly decrease from 50 to 100 °C. The bond strength increases from 5.07 ± 1.93 MPa at a subzero temperature of 25 °C to 5.91 ± 0.57, 6.18 ± 1.19 and 7.38 ± 1.16 MPa at the temperature of 0, 25 and 50 °C, respectively. Toughness decreases from 25 to 25 °C and increases from 25 to 50 °C. The shear stiffness increases by as much as 33% from 4.21 MPa/mm to 5.60 MPa/mm with the temperature increasing from 25 to 25 °C. However, bond strength decreases 25% from 7.38 ± 1.16 MPa at the temperature of 50 °C to 5.49 ± 0.73 MPa at the temperature of 100 °C. Similarly, the toughness decreases 14% from 4.81 ± 2.54 MPamm to 4.10 ± 1.31 MPamm. It must be emphasized that the shear stiffness significantly decreases 65% from 5.60 ± 1.79 MPa/mm at the
temperature of 25 °C to 1.93 ± 0.65 MPa/mm at the temperature of 100 °C. When the test temperatures are below the glass transition temperature (Tg), the mechanical performances are improved with increasing temperature. In a certain range of the temperature, higher temperature may be advantageous to the binder molecules for further diffusion, osmosis and entanglement such that the chemical reaction of binder could be more complete, increasing the degree of curing and crosslinking. Thus both the cohesive strength of binder and the bonding strength between adhesive and steel increased. However, as the temperature is higher than Tg, the bonding performance is reduced significantly due to the softening of the adhesive and weakening of the interfaces between BFRP sheets and adhesives. At high temperature (100 °C), a ductile failure manner can be observed from high speed camera videos. This change trend in the mechanical properties of single-lap joints is similar to that observed by Banea et al. [34]. The change of the failure modes can also explain this phenomenon, which will be further discussed in Section 3.3. It should be noted that the coefficients of thermal expansion of steel, BFRP and adhesive are dissimilar, as a consequence, the thermal stress must be produced. Refs. [35–37] gave an estimation of the thermal stress in the adhesive, but the thermal stress is relatively small compared to the
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Fig. 8. Temperature effect on the mechanical properties of BFRP-steel single-lap joints: (a) bond strength, (b) toughness, and (c) shear stiffness.
Fig. 9. Fracture modes under different loading rates: (a) 0.625, (b) 1.25, (c) 2.5, and (d) 5.0 m/s.
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Fig. 10. Fracture modes under different temperatures: (a) 25, (b) 0, (c) 50, and (d) 100 °C.
Fig. 11. Basic principle of DIC: (a) speckle pattern, area of interest (AOI) and subset, (b) track of subset using cross correlation, and (c) schematic representation of the selection of AOI.
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0
1 ms
3 ms
5 ms
6 ms
0.3 ms
0.4 ms
0.3 ms
0.35 ms
(a)
0
0.1 ms
0.2 ms
(b)
0
0.1 ms
0.2 ms
(c)
0
0.1 ms
0.15 ms
0.2 ms
0.25 ms
(d) Fig. 12. Normal strain distributions of BFRP under different loading rates (a) 0.625, (b) 1.25, (c) 2.5, and (d) 5.0 m/s. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
bond stress. Therefore the main reason for strength decrease is primarily due to the degradation of the adhesive. 3.3. Failure mode The potential failure patterns of BFRP/steel single-lap joints are categorized into four modes: (I) adhesive/steel interface debonding, (II) adhesive/BFRP interface debonding, (III) BFRP delamination, (IV) mixed failure. Mixed failure is defined as some adhesives remaining on BFRP and (or) steel. In the present study, tensile failure of BFRP or steel yielding is not observed. Representative samples showing the dominant failure modes under each loading rate and temperature are depicted in Figs. 9 and 10.
The failure modes of all the specimens are listed in Table 2. It is obvious that the actual failure is different from the expected inadhesive failure, i.e., the fracture is partially in-adhesive and partially interfacial in nature, especially in the case of different loading velocities. Yet in aspect of temperature effect, it can be found that the interfacial failure is dominant. Fig. 9 shows a mixed failure in the tested specimens, it can be seen that part of adhesive adheres to BFRP or steel. Note that two samples show the failure mode of some fiber delamination at speed of 5.0 m/s, as illustrated by Table 2. This is because the shear strength of the adhesive improves when the loading rate is increased [38,39]. Concerning the temperature effect, typical samples of the primary failure modes are depicted in Fig. 10. At low temperatures (25 and 0 °C), the failure modes are mainly
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0
0.25 ms
0.5 ms
0.75 ms
1.0 ms
3.0 ms
4.0 ms
6.0 ms
7.0 ms
6.0 ms
8.0 ms
(a)
0
1.0 ms
2.0 ms
(b)
0
2.0 ms
4.0 ms
(c)
0
1.5 ms
4.0 ms
(d) Fig. 13. Normal strain distributions of BFRP under different temperatures (a) 25, (b) 0, (c) 50, and (d) 100 °C.
adhesive/steel interface debonding, as shown in Figs. 10(a) and (b). However, when the temperature reaches 50 °C, adhesives remain on both steel and BFRP, namely, the joints fail primarily as a mixed mode. When the temperature exceeds Tg, bond strength decreases rapidly and failure mode changes to adhesive/BFRP interface debonding. In other words, the BFRP-to-adhesive interface is stronger than adhesive-to-steel interface at low temperature and room temperature, similar experimental observations have been found in normal modulus CFRP/steel double strap joints [16]. While high temperature weakens the interfacial strength between steel and adhesive. It appears that the change of temperature influences the properties of adhesives, and causes the change of failure mode in single lap joints. 4. Image analysis by digital image correlation (DIC) method DIC has been extensively applied in the mechanical testing fields in the past decade. Specifically, the shear strain fields of sin-
gle lap joint interface were investigated by different researchers using DIC [40,41]. However, the determination of normal strain distributions of adherents is limited [42]. Fig. 11 shows the random speckle pattern and area of interest (AOI) selected in the present study. The displacements and strain fields are obtained by tracking the movement from reference to deformed images [43]. A commercial software Vic-2D 2009 developed by Correlated Solutions, Inc. was used to preform image analysis. DIC images of strain fields (eyy) in the overlap area along the loading direction at varying loading rates are shown in Fig. 12 using a color code with purple representing the lowest strain values and red at 0.4% strain. Similar patterns of strain filed development can be observed such that at the beginning of the test, a relative uniform distribution of strains is obtained. However, as the load increases, normal strain starts to localize at the bottom of the AOI, which is corresponding to the lower junction of BFRP and steel on the back face (see Fig. 11c). The localization of normal strain indicates the concentration of stress along the edge of the
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Fig. 14. Normal strain along loading direction (y-axis) of a specimen tested at 1.25 m/s and 25 °C.
adhesive while the far fields are still well bonded. As the tensile load increases, the localization zone continues to grow and propagate toward the top edge and a transition zone in colors of green and blue forms between the concentrated and uniform strains. The development of non-uniform strain fields implies the fact that the damage of the adhesive joint may initiate at the edges and spread out to the far fields under room temperature. It is worthy to mention that the localization of strain and stress is also expected at the top edge of the adhesive (see Fig. 11c), which is, however, not included in the AOI since the speckle pattern loses its continuity across the junction. In addition, the pattern of strain fields seems to be independent on the effect of loading rate. Fig. 13 illustrates the strain maps of the specimens tested under varying temperatures at the loading speed of 0.625 m/s. Images of specimens tested at room temperature are referred to Fig. 12(a). Similar phenomena can be observed in the specimens under 25, 0 and 25 °C, but the process is changed at elevated temperatures. As shown in Figs. 13(c) and (d) , the localization of normal strain initiates near the center of the overlap area instead of the bottom edge as demonstrated in lower temperatures. Localized strains propagate toward both sides as the load increases, but tend to concentrate at the bottom edge of AOI and exhibit a similar pattern to those of other temperatures at the final stage. The unusual behavior of strain fields with increasing load may be traced back to the altered bonding characteristics as a result of elevated temperature. The bond strength of the adhesive may be reduced by the heat and
(a)
debonding starts to take place within the overlap area instead of the junction which is caused by the stress concentration effects. From a perspective of quantification, the normal strains along the loading direction (y-direction) are extracted and plotted as a function of y-location, as shown in Fig. 14. In accordance with the strain maps shown in Fig. 12(b), the distribution is uniform at the beginning of the test. As the load increases, normal strain starts to localize at the left end of the curve (bottom edge) and subsequently propagate toward the other end. Figs. 15(a) and (b) compare the strain distributions (prior to failure) of specimens tested under various loading rates and temperatures, respectively. The behaviors at different loading rates are quite uniform. On the other hand, the shape of the curves extracted from the specimens tested under varying temperatures changes when it is above 50 °C (see Fig. 15b). The correspondence between quantitatively measured distribution of normal strain and the development of strain map further reveals that the bond properties of adhesive may be affected by the elevated temperatures.
5. Concluding remarks This paper presents an experimental study on the bonding properties between BFRP and steel plates to assess the effect of high speed tensile loads and temperatures on the bond strength, toughness, shear stiffness, FRP strain distribution in the overlapping area and failure mechanisms. Based on these results, the following conclusions can be drawn: (1) The mechanical properties of joints are sensitive to the loading rate. The bond strength and shear stiffness increase with increasing loading rates. Large increase in shear stiffness is found under the loading rate of 1.25 m/s compared to the loading rate of 0.625 m/s. (2) The average bond strength increases from 25 to 50 °C, but decreases significantly from 50 to 100 °C. Shear stiffness increases from 25 to 25 °C, while it decreases from 25 to 100 °C. The elevated temperature has pronounced effect on the mechanical performances. (3) Most specimens show a mixed failure mode at different loading rates under room temperature (25 °C). On the other hand, at low temperature (25 and 0 °C), the failure modes are mainly adhesive/steel interface debonding. When the temperature exceeds the Tg of the adhesive, bond strength decreases rapidly and the failure mode changes to
(b)
Fig. 15. Normal strain along the loading direction (y-axis) of specimens tested at varying (a) loading rates and (b) temperatures.
M. Yao et al. / Composite Structures 145 (2016) 68–79
adhesive/BFRP interface debonding, since high temperature weakens the interfacial bond between BFRP and the adhesive. (4) DIC is used to obtain the FRP strain distributions in the overlapping area at different levels of the load. Higher strain is achieved on the joint edge where the load is applied as compared to the center of the overlapping area. The ultimate failure of test specimen initiate at the joint edge. The distribution of normal strain in overlapping region at elevated temperature (100 °C) is quite different from those at lower temperature and room temperature, since the bond properties of the adhesive are affected by the elevated temperature.
Acknowledgments This work was supported by the funds from National Basic Research Program of China (973 program, Project No. 2012CB026200), the Sci-Tech Support Plan of Hunan Province (Grant No. 2014WK2026), the Interdisciplinary Research Project of Hunan University. References [1] Bakis C, Bank LC, Brown V, Cosenza E, Davalos J, Lesko J, et al. Fiber-reinforced polymer composites for construction-state-of-the-art review. J Compos Constr 2002;6:73–87. [2] Zhao X-L, Zhang L. State-of-the-art review on FRP strengthened steel structures. Eng Struct 2007;29:1808–23. [3] Colombi P, Poggi C. An experimental, analytical and numerical study of the static behavior of steel beams reinforced by pultruded CFRP strips. Compos B 2006;37:64–73. [4] Miller TC, Chajes MJ, Mertz DR, Hastings JN. Strengthening of a steel bridge girder using CFRP plates. J Bridge Eng 2001;6:514–22. [5] Zhou H, Attard TL, Wang Y, Wang J-A, Ren F. Rehabilitation of notch damaged steel beams using a carbon fiber reinforced hybrid polymeric-matrix composite. Compos Struct 2013;106:690–702. [6] Fiore V, Scalici T, Di Bella G, Valenza A. A review on basalt fibre and its composites. Compos B 2015;74:74–94. [7] Fiore V, Di Bella G, Valenza A. Glass–basalt/epoxy hybrid composites for marine applications. Mater Des 2011;32:2091–9. [8] Lopresto V, Leone C, De Iorio I. Mechanical characterisation of basalt fibre reinforced plastic. Compos B 2011;42:717–23. [9] Sim J, Park C. Characteristics of basalt fiber as a strengthening material for concrete structures. Compos B 2005;36:504–12. [10] Sarasini F, Tirillò J, Ferrante L, Valente M, Valente T, Lampani L, et al. Dropweight impact behaviour of woven hybrid basalt–carbon/epoxy composites. Compos B 2014;59:204–20. [11] Dhand V, Mittal G, Rhee KY, Park S-J, Hui D. A short review on basalt fiber reinforced polymer composites. Compos B 2015;73:166–80. [12] Rambo DAS, de Andrade Silva F, Toledo Filho RD, Gomes OdFM. Effect of elevated temperatures on the mechanical behavior of basalt textile reinforced refractory concrete. Mater Des 2015;65:24–33. [13] Garmendia L, San-José J, García D, Larrinaga P. Rehabilitation of masonry arches with compatible advanced composite material. Constr Build Mater 2011;25:4374–85. [14] Wu G, Wang H-T, Wu Z-S, Liu H-Y, Ren Y. Experimental study on the fatigue behavior of steel beams strengthened with different fiber-reinforced composite plates. J Compos Constr 2011;16:127–37. [15] Li Y, Wang Y, Ou J. Mechanical behavior of BFRP-steel composite plate under axial tension. Polymers 2014;6:1862–76. [16] Wu C, Zhao X, Duan WH, Al-Mahaidi R. Bond characteristics between ultra high modulus CFRP laminates and steel. Thin Walled Struct 2012;51:147–57.
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[17] Liu H, Zhao X, Al-Mahaidi R. Effect of fatigue loading on bond strength between CFRP sheets and steel plates. Int J Struct Stab Dyn 2010;10:1–20. [18] Al-Shawaf A, Zhao X-L. Adhesive rheology impact on wet lay-up CFRP/steel joints’ behaviour under infrastructural subzero exposures. Compos B 2013;47:207–19. [19] Nguyen T-C, Bai Y, Zhao X-L, Al-Mahaidi R. Mechanical characterization of steel/CFRP double strap joints at elevated temperatures. Compos Struct 2011;93:1604–12. [20] Nguyen T-C, Bai Y, Zhao X-L, Al-Mahaidi R. Effects of ultraviolet radiation and associated elevated temperature on mechanical performance of steel/CFRP double strap joints. Compos Struct 2012;94:3563–73. [21] Al-Zubaidy HA, Zhao X-L, Al-Mahaidi R. Dynamic bond strength between CFRP sheet and steel. Compos Struct 2012;94:3258–70. [22] Dorigato A, Pegoretti A. Fatigue resistance of basalt fibers-reinforced laminates. J Compos Mater 2012;46:1773–85. [23] Shaat A, Schnerch D, Fam A, Rizkalla S. Retrofit of steel structures using fiberreinforced polymers (FRP): state-of-the-art. In: Transportation research board (TRB) annual meeting, 2004. [24] Al-Mosawe A, Al-Mahaidi R, Zhao X-L. Effect of CFRP properties, on the bond characteristics between steel and CFRP laminate under quasi-static loading. Constr Build Mater 2015;98:489–501. [25] Colombi P, Poggi C. Strengthening of tensile steel members and bolted joints using adhesively bonded CFRP plates. Constr Build Mater 2006;20:22–33. [26] Fernando D, Teng J-G, Yu T, Zhao X-L. Preparation and characterization of steel surfaces for adhesive bonding. J Compos Constr 2013. [27] Kuentzer N, Simacek P, Advani SG, Walsh S. Correlation of void distribution to VARTM manufacturing techniques. Compos A 2007;38:802–13. [28] Kahraman R, Sunar M, Yilbas B. Influence of adhesive thickness and filler content on the mechanical performance of aluminum single-lap joints bonded with aluminum powder filled epoxy adhesive. J Mater Process Technol 2008;205:183–9. [29] da Silva LF, Rodrigues T, Figueiredo M, De Moura M, Chousal J. Effect of adhesive type and thickness on the lap shear strength. J Adhes 2006;82:1091–115. [30] Zhu D, Mobasher B, Rajan SD. Dynamic tensile testing of Kevlar 49 fabrics. J Mater Civil Eng 2010;23:230–9. [31] Zhu D, Rajan S, Mobasher B, Peled A, Mignolet M. Modal analysis of a servohydraulic high speed machine and its application to dynamic tensile testing at an intermediate strain rate. Exp Mech 2011;51:1347–63. [32] Chen X, Li Y. An experimental technique on the dynamic strength of adhesively bonded single lap joints. J Adhes Sci Technol 2010;24:291–304. [33] Galliot C, Rousseau J, Verchery G. Drop weight tensile impact testing of adhesively bonded carbon/epoxy laminate joints. Int J Adhes Adhes 2012;35:68–75. [34] Banea M, da Silva LF, Campilho RD. Effect of temperature on the shear strength of aluminium single lap bonded joints for high temperature applications. J Adhes Sci Technol 2014;28:1367–81. [35] Adams R, Coppendale J, Mallick V, Al-Hamdan H. The effect of temperature on the strength of adhesive joints. Int J Adhes Adhes 1992;12:185–90. [36] Da Silva LF, Adams R. Adhesive joints at high and low temperatures using similar and dissimilar adherends and dual adhesives. Int J Adhes Adhes 2007;27:216–26. [37] Adamvalli M, Parameswaran V. Dynamic strength of adhesive single lap joints at high temperature. Int J Adhes Adhes 2008;28:321–7. [38] Yokoyama T, Shimizu H. Evaluation of Impact shear strength of adhesive joints with the split Hopkinson bar. JSME Int J Ser A 1998;41:503–9. [39] Raykhere SL, Kumar P, Singh R, Parameswaran V. Dynamic shear strength of adhesive joints made of metallic and composite adherents. Mater Des 2010;31:2102–9. [40] Vijaya kumar R, Bhat M, Murthy C. Analysis of composite single lap joints using numerical and experimental approach. J Adhes Sci Technol 2014;28:893–914. [41] Moutrille M-P, Derrien K, Baptiste D, Balandraud X, Grédiac M. Throughthickness strain field measurement in a composite/aluminium adhesive joint. Composites Par A 2009;40:985–96. [42] Comer A, Katnam K, Stanley W, Young T. Characterising the behaviour of composite single lap bonded joints using digital image correlation. Int J Adhes Adhes 2013;40:215–23. [43] Pan B, Qian K, Xie H, Asundi A. Two-dimensional digital image correlation for in-plane displacement and strain measurement: a review. Meas Sci Technol 2009;20:062001.
Contents lists available at ScienceDirect
Composite Structures journal homepage: www.elsevier.com/locate/compstruct
Experimental study on basalt FRP/steel single-lap joints under different loading rates and temperatures Mingxia Yao a, Deju Zhu a,⇑, Yiming Yao b, Huaian Zhang a, Barzin Mobasher b a b
College of Civil Engineering, Hunan University, Changsha, Hunan 410082, China School of Sustainable Engineering and Built Environment, Arizona State University, Tempe, AZ 85287, United States
a r t i c l e
i n f o
Article history: Available online 26 February 2016 Keywords: A. Single lap joints B. Mechanical properties C. Loading rates D. Temperatures E. Failure modes
a b s t r a c t In this work, basalt FRP/steel single-lap joints were tested under various temperatures (25, 0, 25, 50, 100 °C) and dynamic loading speeds (0.625, 1.25, 2.5, 5.0 m/s), using a servo-hydraulic high-rate testing machine with an environmental chamber. The deformation and failure behavior of the sample in the overlap region were captured by a Phantom v7.3 high speed digital camera at a sampling rate of 20,000 fps, and analyzed using a digital image correlation (DIC) method. Experimental results presented in this paper include average bond strength, toughness, bond strength-displacement curves and FRP strain distributions in the overlapping areas. In aspect of the loading rate effect, both the bond strength and shear stiffness increase with the increasing loading rate. Most specimens showed a mixed failure mode under dynamic tensile loads. On the other hand, the average bond strength increases in the temperature range of 25 to 50 °C, while decreases significantly from 50 to 100 °C as the glass transition temperature Tg is exceeded. The observed typical failure mode in most specimens at low temperature is debonding at adhesive-steel interface, whereas samples rupture at the adhesive-BFRP interface at elevated temperature with lower load-bearing capacity. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction The use of fiber reinforced polymer (FRP) has gained an increasing interest due to its wide applications in automobile industries, shipbuilding applications, aerospace structures and civil engineering constructions [1,2]. The common FRP materials mainly include carbon fiber reinforced polymer (CFRP), glass fiber reinforced polymer (GFRP) and aramid fiber reinforced polymer (AFRP). CFRP, which exhibits the highest strength and stiffness among common FRPs, has been extensively used for strengthening and upgrading existing structures [3–5]. However, the cost of CFRP raises a demand of more cost-effective materials for infrastructures. The development and research studies of basalt fibers and BFRP in recent years enable the application of this material in the field of engineering like glass or carbon reinforcements [6–9] as a result of many advantages. Basalt fiber, which is eco-friendly, non-toxic, non-conducting, non-combustible and common worldwide, is made from melting basalt rock. In addition, the manufacturing process of the basalt fiber is similar to that of glass fiber, but with less energy consump-
⇑ Corresponding author. Tel.: +86 0731 88823861. E-mail address: [email protected] (D. Zhu). http://dx.doi.org/10.1016/j.compstruct.2016.02.061 0263-8223/Ó 2016 Elsevier Ltd. All rights reserved.
tion and no additives, which makes it superior to glass or carbon fibers in terms of cost [6,10,11]. Besides, basalt fiber is an inorganic material with good durability, high strength and modulus, good antioxidative performance, resistance to high temperature and low temperature, improved strain to failure, good chemical resistance (especially in acid environment). Research has also shown that when the temperature exceeded 600 °C, only the basalt fiber (compared to carbon and S-glass fibers) maintained its volumetric integrity and 90% of the strength [9,12]. Sim et al. [9] aimed at the mechanical performance and the durability of the basalt fiber and performed a series of four-point bending tests on reinforced concrete beams strengthened with the basalt fiber sheet. The test results showed that the basalt fiber strengthening improved both the yielding and the ultimate strength of the beam specimen. In our landscape, damages to masonry structure may also be caused by inappropriate construction practices, aging effects, untimely maintenance or load increments. Basalt textile-reinforced mortar (BTRM) seems to be a good strengthening material for the masonry structures such as the reinforcement of stone masonry arch-shaped structures [13]. Wu et al. [14] investigated the fatigue behaviors of steel beams strengthened by four kinds of fiber-reinforced composite plates (high-modulus carbon-fiber-reinforced polymer plate, high-
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M. Yao et al. / Composite Structures 145 (2016) 68–79 Table 1 Mechanical and physical properties of test materials. Material
Young’s modulus (GPa)
Elongation (%)
Yield strength (MPa)
Tensile strength (MPa)
Coefficient of thermal expansion (°C)
Sikadur 330 Steel plate (Q235) Basalt fiber sheet (CP12640)
4.6 199 122
1.0 35.6 1.70
– 235 –
38.1 441 1452
4.5 105 1.3 105 –
Fig. 1. Schematic view of BFRP-steel single-lap joints (not to scale): (a) front view and (b) side view.
Fig. 2. Prepared typical test sample with speckle pattern.
Fig. 3. Schematic drawing of adhesive thickness controlling device.
strength CFRP plate, steel-wire basalt-fiber-reinforced polymer plate, and weld steel plate). The tests revealed that the steel-wire basalt-fiber-reinforced polymer plate is the optimal strengthening material by considering the cost-performance ratio. Li et al. [15]
proposed a new BFRP-steel composite plate, which has a stable post-yield stiffness, it could effectively design a damagecontrollable structures with good reparability. It is therefore attractive to investigate the performance of BFRP in strengthening and retrofitting the steel structures by means of widely accepted experimental methods such as the single-lap and double strap joints. Numerous experimental works were carried out to thoroughly investigate the bond behavior of CFRP/steel double lap joints under static loading [16], fatigue loading [17], low and elevated temperatures [18,19], ultraviolet radiation [20], impact loading [21]. In contrast, to date no research has been conducted on the bond between BFRP and steel plates. Dorigato and Pegoretti [22] compared the quasi static tensile properties of laminated FRPs reinforced by basalt and carbon woven fabrics with the same areal density. It was reported that the tensile strength of BFRP was comparable to CFRP or even higher and the strains at peak were about twice of CFRP. Elastic modulus of FRP is another concern in the practical applications of retrofitting the steel structures due to relative high modulus of steel [23]. Al-Mosawe et al. [24] studied the effects of CFRP properties on the bond performance. It is found that changing from low (159 GPa) to normal (203 GPa) CFRP modulus had insignificant effects on the maximum failure strain and strain distribution, though slightly lower strain values were observed for specimens with normal CFRP modulus compared to those with low CFRP modulus. Significant reduction in ductility was observed
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M. Yao et al. / Composite Structures 145 (2016) 68–79 Table 2 Summary of the test results of the BFRP-steel single-lap joints. Specimen No. 1 2 3 4 5 6 1 2 3 4 5 6 1 2 3 4 5 6
Fig. 4. MTS servo-hydraulic high-rate testing machine and specimen installation.
when ultra-high modulus (450 GPa) CFRP was used. Nevertheless, the joint load capacity was primarily affected by the tensile strength of CFRP. On the other hand, the failure modes of low and normal CFRP specimens were dominated by the debonding between steel and adhesive and failure of adhesive itself, which indicates that the mechanical properties of FRP may not govern the ultimate state. As far as the relatively low modulus of basalt compared to carbon fibers, more layers of basalt reinforcements can be added to obtain comparative stiffness of the FRP considering its lower cost. On the other hand, in the cases such as seismic loadings when high energy absorption capacity is required, the use of basal fiber may be beneficial by improving strain to failure processes. Therefore BFRP may be used as an economic alternative materials to CFRP in certain applications. According to field applications, FRP-steel composite structures may be subjected to dynamic loadings inevitably, such as explosions, ballistic projectiles, earthquakes, wind driven objects, fast moving traffic, and machine vibrations [11]. Besides, exposure to low and elevated temperatures is also common in industrial applications. As a consequence, this paper presents the effects of various temperatures and dynamic loading rates on the mechanical behavior of single lap BFRP-steel joints. The primary objective of this research is to assess the effects of different temperatures (25, 0, 25, 50, 100 °C) and loading rates (0.625, 1.25, 2.5, 5.0 m/s) on the mechanical properties and failure modes of BFRP-steel single-lap joints, utilizing a high-rate servo-hydraulic (MTS) testing machine and 2D digital image correlation (DIC). Digital images were recorded at different levels of load and then processed in a commercial DIC software (Vic-2D 2009) to obtain the surface strain distributions of BFRP in overlapping area. The mode of failure, bond strength, toughness and shear stiffness are presented and discussed. The results of this investigation will be significant for theoretical researches and practical applications of BFRP in extreme loadings and severe environmental conditions.
Temperature (°C)
25
25
25
Bond strength (MPa)
Stiffness (MPa/ mm)
Failure mode
0.625
5.42 6.92 6.05 6.63 5.65 6.33
2.04 2.52 2.58 2.43 1.87 2.06
IV IV IV IV I IV
1.25
6.57 6.68 5.63 6.38 6.55 7.63
6.83 4.67 5.57 6.23 5.47 8.04
IV IV IV IV IV IV
2.5
7.13 5.51 7.68 6.04 6.85 7.64
8.87 7.02 7.81 3.41 5.92 6.40
IV IV IV IV IV IV
5.0
7.33 9.74 6.56 7.48 7.53 6.66
9.47 9.53 8.43 6.18 6.95 8.18
III III IV IV IV IV
Loading rate (m/s)
1 2 3 4 5 6
25
1 2 3 4
25
0.625
3.77 7.52 5.69 3.32
1.09 7.68 7.05 1.05
I I I I
1 2 3
0
0.625
5.29 6.39 6.06
2.98 6.03 4.01
I I I
7.59 6.61 3.37 2.98 2.30 4.38
IV IV IV IV IV II
1.81 2.84 1.79 1.29
II II II II
1 2 3 4 5 6
50
0.625
6.77 8.82 7.29 7.07 8.60 5.75
1 2 3 4
100
0.625
5.98 5.65 5.92 4.41
2. Experimental program 2.1. Materials Hot rolled steel plates (Q235, a nominal yield stress of 235 MPa, China standard, GB/T 700), BFRP laminates and the epoxy adhesive are selected for the bond assemblies. The thickness of steel plates is 4.00 mm ± 0.02 mm. A two component, solvent free and thixotropic epoxy resin structural adhesive SikadurÒ 330 [25,26] is recommended for bonding the FRP laminates and the steel plates together. Thin unidirectional BFRP laminates were manufactured by means of the vacuum-assisted resin transfer molding (VARTM) technique [27], using the unidirectional basalt fabric (made by Nanjing Hitech Composites Co., Ltd) and epoxy resin (JN-C3P, a fiber impregnated paste adhesive). Then BFRP laminates were cut to the required size of 25 mm 80 mm (W L). It should be emphasized that the nominal thickness of BFRP (made of three layers basalt fabrics) was measured as 0.95 mm. A summary of the mechanical parameters of these materials are given in Table 1. 2.2. Specimen preparation The simplest single-lap BFRP-to-steel bonded joint was adopted in the present study which is a basic structural element and often
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Fig. 5. Experimental bond stress–displacement responses of BFRP-steel single-lap joints at different loading rates of (a) 0.625, (b) 1.25, (c) 2.5, and (d) 5.0 m/s.
used to investigate the bond between steel plates and FRP laminates. The side and front views of joint specimen in Fig. 1 provide detailed geometric information of the joint. Two end spacers (BFRP and steel) of 25 mm long, 25 mm wide were glued on each side of specimens to diminish the eccentricity of the load path. The final specimens are shown in Fig. 2. Great attention was paid to the surface preparation of steel plates because of its sensitivity to rust. It has been reported that the surface preparation has a significant effect on bonding performance between FRP and steel [17,26]. The first essential step is polishing steel plates on both sides to remove any rust within the bond area. Then acetone was applied to the steel plate to further clean the surfaces. In particular, it should be pointed out that adhesive thickness has considerable effects on the bond strength and failure mode. The experiments [28,29] showed that the bond strength of lap-joints decreased as the bond line became thicker, since more defects (voids, micro-cracks, etc.) were formed with increasing thickness. Therefore, a bond line thickness controlling device was designed and manufactured, which is shown in Fig. 3, for the sake of keeping the adhesive thickness of all the joints uniform (1.00 mm). The following preparation procedures were as follows: two steel plates were aligned in U-shaped bottom plate, and a thin layer of epoxy adhesive was smeared on the surface of the steel using a brush. Then BFRP laminate was put on the top of the adhe-
sive layer. A top plate was utilized to squeeze out the air bubbles and excessive adhesive by adjusting the bolt. All specimens were then cured at room temperature for 7 days before mechanical testing. The total thickness of the specimen is measured with a caliper at three locations marked initially on the samples, and an average value is calculated. The adhesive layer thickness (ta) can be approximated using the following equation:
ta ¼ t total t steel t BFRP
ð1Þ
where the thickness of steel plates tsteel and BFRP laminates tBFRP were measured and recorded before specimen preparation. 2.3. Dynamic test setup and technique Dynamic tensile tests were conducted using an MTS high rate servo-hydraulic testing machine operated in open-loop control. The speed of the stroke is controlled by the opening and closing of the servo-valve of hydraulic supply. By manually turning the servo-valve, the rate of flow of hydraulic fluid can be controlled, resulting in different stroke speeds. Calibration records show that the stroke can reach a maximum speed of 14 m/s with a load capacity of 25 kN. The environmental chamber can be heated up by internal electric resistance wire and cooled down by liquid nitrogen, given an operating temperature ranging from 60 to 200 °C. The actual tests were performed after approximately
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Fig. 6. Loading rate effect on the mechanical properties of BFRP-steel single-lap joints: (a) bond strength, (b) toughness, and (c) shear stiffness.
15 min of achieving the test temperature in the specimens, to ensure a steady state temperature throughout the specimen prior to testing. A high speed digital camera was utilized to record the test process at a sampling rate of 20,000 fps (frame per second, time interval = 50 ls) with resolution of 256 512 pixels such that the failure modes could be examined in detail. Fig. 4 presents the dynamic tensile testing setup. More details of the testing system can be found elsewhere [30]. 3. Results and discussion 3.1. Loading rate effect Dynamic tensile tests were carried out with four different loading velocities (0.625, 1.25, 2.5, 5.0 m/s) at room temperature (25 °C). The bond stress is given by:
s¼
F bl
ð2Þ
where F is the tensile load, b is the joint width and l is the joint overlap length. Toughness is evaluated using the area under the bond stress–displacement curve. Shear stiffness is defined as the slope of the curve in the elastic region. Table 2 lists the test design and results of BFRP/steel single lap joint at different loading conditions. Fig. 5 shows the bond stress– displacement curves under different loading rates. Quite uniform responses are observed under each loading rate, which exhibits linear elastic behavior up to peak and brittle failure. The curves at four loading rates indicate that most failure processes of specimens are quite rapid (the bond stress dropped directly to zero when it reaches the peak stress), which means that the failure mode of the joint is not a ductile failure. In the case of a loading rate of 0.625 m/s, a slight fluctuation in the ascending slope of curves is observed, before reaching the maximum bond stress, the curves exhibit nonlinearity. However, for other loading rates, the bond stress–displacement curves are approximately liner elastic before
reaching the maximum stress. The fluctuations in the curves are most likely caused by the vibration of the test device when the specimens were loaded [31] and progressive failure of specimens. The fluctuations are also observed in the bond stress–displacement curves under different temperatures as discussed in Section 3.2. Dynamic mechanical properties of bond strength, toughness and shear stiffness versus loading rates are presented in Fig. 6 (in terms of average values and standard deviations). The bond strength, toughness and shear stiffness are sensitive to the loading rates but exhibit different degrees of sensitivity. The bond strength and shear stiffness of joints all increase from 0.625 to 5.0 m/s. Specifically, for the same temperature (25 °C), the bond strength all increase from 6.16 ± 0.57 MPa at a loading rate of 0.625 m/s to 6.57 ± 0.64, 6.81 ± 0.87 and 7.55 ± 1.15 MPa at the loading rates of 1.25, 2.5 and 5.0 m/s, respectively. The reason for this increase in the bond strength enhancement of BFRP/steel single lap joints is mainly due to the fact that the entire test duration decreases as the loading speed increases such that test specimens need higher stress to satisfy the energy which the failure processing must be absorbed [32]. Similar phenomena have been reported in the Refs. [21,33]. The shear stiffness significantly increases by as much as 172% from 2.25 ± 0.29 MPa/mm to 6.14 ± 1.18 MPa/ mm as the loading rate increases from 0.625 to 1.25 m/s. When the loading rate increases from 1.25 m/s to 2.5 and 5.0 m/s, shear stiffness relatively slowly increases 7%, 32% from 6.14 ± 1.18 MPa/mm to 6.57 ± 1.86 MPa/mm and 8.12 ± 1.34 MPa/ mm. It is also interesting to note that there is an increment in the deviation of bond strength and shear stiffness which can be attributed to the larger scatter at higher dynamic loading speeds. 3.2. Temperature effect Fig. 7 shows the experimental bond stress–displacement responses of BFRP-steel single lap joints under different temperatures (25, 0, 50, 100 °C) at the same loading rate of 0.625 m/s. At lower temperatures (25 and 0 °C), it can be found that most
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Fig. 7. Experimental bond stress–displacement responses of BFRP-steel single-lap joints at different temperatures of (a) 25, (b) 0, (c) 50, and (d) 100 °C.
of curves show a linear ascending region and ultimate failures are quite rapid. However, fluctuations in the ascending regions are observed in the curves at the temperatures of 50 and 100 °C. Note that the curves show relatively large variations at the temperature of 50 °C. Fig. 8 shows the experimental observations of the influence of temperatures on the dynamic mechanical properties of BFRPsteel single lap joints. It indicates the temperature-sensitivity of the BFRP/steel adhesively bonded joints. Overall, both bond strength and toughness of joints increase from 25 to 50 °C and significantly decrease from 50 to 100 °C. The bond strength increases from 5.07 ± 1.93 MPa at a subzero temperature of 25 °C to 5.91 ± 0.57, 6.18 ± 1.19 and 7.38 ± 1.16 MPa at the temperature of 0, 25 and 50 °C, respectively. Toughness decreases from 25 to 25 °C and increases from 25 to 50 °C. The shear stiffness increases by as much as 33% from 4.21 MPa/mm to 5.60 MPa/mm with the temperature increasing from 25 to 25 °C. However, bond strength decreases 25% from 7.38 ± 1.16 MPa at the temperature of 50 °C to 5.49 ± 0.73 MPa at the temperature of 100 °C. Similarly, the toughness decreases 14% from 4.81 ± 2.54 MPamm to 4.10 ± 1.31 MPamm. It must be emphasized that the shear stiffness significantly decreases 65% from 5.60 ± 1.79 MPa/mm at the
temperature of 25 °C to 1.93 ± 0.65 MPa/mm at the temperature of 100 °C. When the test temperatures are below the glass transition temperature (Tg), the mechanical performances are improved with increasing temperature. In a certain range of the temperature, higher temperature may be advantageous to the binder molecules for further diffusion, osmosis and entanglement such that the chemical reaction of binder could be more complete, increasing the degree of curing and crosslinking. Thus both the cohesive strength of binder and the bonding strength between adhesive and steel increased. However, as the temperature is higher than Tg, the bonding performance is reduced significantly due to the softening of the adhesive and weakening of the interfaces between BFRP sheets and adhesives. At high temperature (100 °C), a ductile failure manner can be observed from high speed camera videos. This change trend in the mechanical properties of single-lap joints is similar to that observed by Banea et al. [34]. The change of the failure modes can also explain this phenomenon, which will be further discussed in Section 3.3. It should be noted that the coefficients of thermal expansion of steel, BFRP and adhesive are dissimilar, as a consequence, the thermal stress must be produced. Refs. [35–37] gave an estimation of the thermal stress in the adhesive, but the thermal stress is relatively small compared to the
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Fig. 8. Temperature effect on the mechanical properties of BFRP-steel single-lap joints: (a) bond strength, (b) toughness, and (c) shear stiffness.
Fig. 9. Fracture modes under different loading rates: (a) 0.625, (b) 1.25, (c) 2.5, and (d) 5.0 m/s.
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Fig. 10. Fracture modes under different temperatures: (a) 25, (b) 0, (c) 50, and (d) 100 °C.
Fig. 11. Basic principle of DIC: (a) speckle pattern, area of interest (AOI) and subset, (b) track of subset using cross correlation, and (c) schematic representation of the selection of AOI.
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0
1 ms
3 ms
5 ms
6 ms
0.3 ms
0.4 ms
0.3 ms
0.35 ms
(a)
0
0.1 ms
0.2 ms
(b)
0
0.1 ms
0.2 ms
(c)
0
0.1 ms
0.15 ms
0.2 ms
0.25 ms
(d) Fig. 12. Normal strain distributions of BFRP under different loading rates (a) 0.625, (b) 1.25, (c) 2.5, and (d) 5.0 m/s. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
bond stress. Therefore the main reason for strength decrease is primarily due to the degradation of the adhesive. 3.3. Failure mode The potential failure patterns of BFRP/steel single-lap joints are categorized into four modes: (I) adhesive/steel interface debonding, (II) adhesive/BFRP interface debonding, (III) BFRP delamination, (IV) mixed failure. Mixed failure is defined as some adhesives remaining on BFRP and (or) steel. In the present study, tensile failure of BFRP or steel yielding is not observed. Representative samples showing the dominant failure modes under each loading rate and temperature are depicted in Figs. 9 and 10.
The failure modes of all the specimens are listed in Table 2. It is obvious that the actual failure is different from the expected inadhesive failure, i.e., the fracture is partially in-adhesive and partially interfacial in nature, especially in the case of different loading velocities. Yet in aspect of temperature effect, it can be found that the interfacial failure is dominant. Fig. 9 shows a mixed failure in the tested specimens, it can be seen that part of adhesive adheres to BFRP or steel. Note that two samples show the failure mode of some fiber delamination at speed of 5.0 m/s, as illustrated by Table 2. This is because the shear strength of the adhesive improves when the loading rate is increased [38,39]. Concerning the temperature effect, typical samples of the primary failure modes are depicted in Fig. 10. At low temperatures (25 and 0 °C), the failure modes are mainly
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0
0.25 ms
0.5 ms
0.75 ms
1.0 ms
3.0 ms
4.0 ms
6.0 ms
7.0 ms
6.0 ms
8.0 ms
(a)
0
1.0 ms
2.0 ms
(b)
0
2.0 ms
4.0 ms
(c)
0
1.5 ms
4.0 ms
(d) Fig. 13. Normal strain distributions of BFRP under different temperatures (a) 25, (b) 0, (c) 50, and (d) 100 °C.
adhesive/steel interface debonding, as shown in Figs. 10(a) and (b). However, when the temperature reaches 50 °C, adhesives remain on both steel and BFRP, namely, the joints fail primarily as a mixed mode. When the temperature exceeds Tg, bond strength decreases rapidly and failure mode changes to adhesive/BFRP interface debonding. In other words, the BFRP-to-adhesive interface is stronger than adhesive-to-steel interface at low temperature and room temperature, similar experimental observations have been found in normal modulus CFRP/steel double strap joints [16]. While high temperature weakens the interfacial strength between steel and adhesive. It appears that the change of temperature influences the properties of adhesives, and causes the change of failure mode in single lap joints. 4. Image analysis by digital image correlation (DIC) method DIC has been extensively applied in the mechanical testing fields in the past decade. Specifically, the shear strain fields of sin-
gle lap joint interface were investigated by different researchers using DIC [40,41]. However, the determination of normal strain distributions of adherents is limited [42]. Fig. 11 shows the random speckle pattern and area of interest (AOI) selected in the present study. The displacements and strain fields are obtained by tracking the movement from reference to deformed images [43]. A commercial software Vic-2D 2009 developed by Correlated Solutions, Inc. was used to preform image analysis. DIC images of strain fields (eyy) in the overlap area along the loading direction at varying loading rates are shown in Fig. 12 using a color code with purple representing the lowest strain values and red at 0.4% strain. Similar patterns of strain filed development can be observed such that at the beginning of the test, a relative uniform distribution of strains is obtained. However, as the load increases, normal strain starts to localize at the bottom of the AOI, which is corresponding to the lower junction of BFRP and steel on the back face (see Fig. 11c). The localization of normal strain indicates the concentration of stress along the edge of the
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Fig. 14. Normal strain along loading direction (y-axis) of a specimen tested at 1.25 m/s and 25 °C.
adhesive while the far fields are still well bonded. As the tensile load increases, the localization zone continues to grow and propagate toward the top edge and a transition zone in colors of green and blue forms between the concentrated and uniform strains. The development of non-uniform strain fields implies the fact that the damage of the adhesive joint may initiate at the edges and spread out to the far fields under room temperature. It is worthy to mention that the localization of strain and stress is also expected at the top edge of the adhesive (see Fig. 11c), which is, however, not included in the AOI since the speckle pattern loses its continuity across the junction. In addition, the pattern of strain fields seems to be independent on the effect of loading rate. Fig. 13 illustrates the strain maps of the specimens tested under varying temperatures at the loading speed of 0.625 m/s. Images of specimens tested at room temperature are referred to Fig. 12(a). Similar phenomena can be observed in the specimens under 25, 0 and 25 °C, but the process is changed at elevated temperatures. As shown in Figs. 13(c) and (d) , the localization of normal strain initiates near the center of the overlap area instead of the bottom edge as demonstrated in lower temperatures. Localized strains propagate toward both sides as the load increases, but tend to concentrate at the bottom edge of AOI and exhibit a similar pattern to those of other temperatures at the final stage. The unusual behavior of strain fields with increasing load may be traced back to the altered bonding characteristics as a result of elevated temperature. The bond strength of the adhesive may be reduced by the heat and
(a)
debonding starts to take place within the overlap area instead of the junction which is caused by the stress concentration effects. From a perspective of quantification, the normal strains along the loading direction (y-direction) are extracted and plotted as a function of y-location, as shown in Fig. 14. In accordance with the strain maps shown in Fig. 12(b), the distribution is uniform at the beginning of the test. As the load increases, normal strain starts to localize at the left end of the curve (bottom edge) and subsequently propagate toward the other end. Figs. 15(a) and (b) compare the strain distributions (prior to failure) of specimens tested under various loading rates and temperatures, respectively. The behaviors at different loading rates are quite uniform. On the other hand, the shape of the curves extracted from the specimens tested under varying temperatures changes when it is above 50 °C (see Fig. 15b). The correspondence between quantitatively measured distribution of normal strain and the development of strain map further reveals that the bond properties of adhesive may be affected by the elevated temperatures.
5. Concluding remarks This paper presents an experimental study on the bonding properties between BFRP and steel plates to assess the effect of high speed tensile loads and temperatures on the bond strength, toughness, shear stiffness, FRP strain distribution in the overlapping area and failure mechanisms. Based on these results, the following conclusions can be drawn: (1) The mechanical properties of joints are sensitive to the loading rate. The bond strength and shear stiffness increase with increasing loading rates. Large increase in shear stiffness is found under the loading rate of 1.25 m/s compared to the loading rate of 0.625 m/s. (2) The average bond strength increases from 25 to 50 °C, but decreases significantly from 50 to 100 °C. Shear stiffness increases from 25 to 25 °C, while it decreases from 25 to 100 °C. The elevated temperature has pronounced effect on the mechanical performances. (3) Most specimens show a mixed failure mode at different loading rates under room temperature (25 °C). On the other hand, at low temperature (25 and 0 °C), the failure modes are mainly adhesive/steel interface debonding. When the temperature exceeds the Tg of the adhesive, bond strength decreases rapidly and the failure mode changes to
(b)
Fig. 15. Normal strain along the loading direction (y-axis) of specimens tested at varying (a) loading rates and (b) temperatures.
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adhesive/BFRP interface debonding, since high temperature weakens the interfacial bond between BFRP and the adhesive. (4) DIC is used to obtain the FRP strain distributions in the overlapping area at different levels of the load. Higher strain is achieved on the joint edge where the load is applied as compared to the center of the overlapping area. The ultimate failure of test specimen initiate at the joint edge. The distribution of normal strain in overlapping region at elevated temperature (100 °C) is quite different from those at lower temperature and room temperature, since the bond properties of the adhesive are affected by the elevated temperature.
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