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Effect of nanostructured reinforcement of adhesive on thermal cycling performance of a single-lap joint with composite adherends
Composites Part B 175 (2019) 107106
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Effect of nanostructured reinforcement of adhesive on thermal cycling performance of a single-lap joint with composite adherends Salih Akpinar a, *, Iclal Avinc Akpinar b a b
Dept. of Mechanical Eng., Erzurum Technical University, 25050, Erzurum, Turkey Independent Researcher, 25050, Erzurum, Turkey
A R T I C L E I N F O
A B S T R A C T
Keywords: Nanostructures Composites Adhesive Strength Mechanical testing Thermal cycle Joints
Nanostructure of adhesive increased the failure load of bonded joints. Adhesively bonded joints are often exposed to alternating warm and cold temperatures in many applications. Hence investigation of their thermal cycle performance is very important. In the present study, the tensile strength of adhesively bonded single lap com posite joints (SLCJs) was experimentally investigated under ambient temperature and thermal cycling condi tions. In the study, carbon fiber fabric reinforced (CFFR) composites (0/90� ) with Plain Weave (PW) were used as the adherend; DP460 tough and DP125 flexible adhesive types were used as the adhesive and 1 wt% GrapheneCOOH, Carbon Nanotube-COOH and Fullerene C60 were used as the nanostructures. Six different thermal cycles were applied to SLCJs and their tensile failure loads and failure surfaces were investigated. As a result, when the joints obtained through nanostructure non-reinforced adhesives are exposed to thermal cycle, a significant decrease appears in the failure load of the joint. Yet, adding nanostructure to the adhesive enhances the thermal cycle performance of the joint. This enhancement in the thermal cycle resistance changes depending on the structural features of the adhesive, the type of nanostructure and thermal cycle condition.
1. Introduction Adhesively bonded joints are often preferred in the aerospace and automotive industries due to their advantages such as formation of uniform stress distributions, ability to join different materials, high fa tigue resistance and impermeability [1,2]. The strength of adhesive joints depends on a number of parameters, such as the adhesive, the adherend materials, lap length, and the thickness of adherends. How ever, in the last few years, studies have been done with the aim of increasing the strength of the adhesive and adherends to improve the performance of adhesively bonded joints. Nanostructures such as Car bon Nanotubes (CNTs), Graphene, Fullerene and Organo clays are added to the adhesive to increase its strength. When studies in the literature relating to joints created by adding nanostructures into the adhesive were reviewed, it was seen that adding compatible nanostructures to the adhesive increased the failure load of the bond [3–12]. In the study performed by Avinc Akpinar [3], tensile failure loads of single lap-joints using nanocomposite adhesives – obtained by adding nanostructure to the adhesive – were experimentally examined to in crease the failure load of adhesively bonded joints. According to the results of this study, adding 1% Graphene-COOH to the rigid adhesive
increases the failure load of the joint by 109%; adding 2% Graphene-COOH increases the failure load of the joint by 276%. How ever, adding Fullerene to a rigid adhesive does not cause a significant increase or decrease in the failure load of the joint. A study, carried out by Garrett et al. [4], emphasized how carbon nanotube reinforcement in epoxy adhesive affects the Mod II rupture strength of steel composite and composite-composite bonding applica tions. It was observed that in all samples, the distribution of the Carbon Nanotubes in the adhesive had an impact on the rupture features. It was found that the nanotube’s features of carboxyl group, dispersion, structure, length and diameter all play an important role in determining whether the nanotube reinforcement strengthens or weakens the joint, and as the result of the tests, the optimum nanotube reinforcement should be around 1%. In the composite-composite adhesively bonded single-lap joint, adding 1–5% multiple walled CNT to the adhesive in creases the shear strength at a rate of about 46% [5]. Furthermore, in the adhesively bonded joints, adding different nanostructures to the adhesive (nano clay, nano Al2O3, nano CaCO3, nano SiO2 etc.) affects the failure load of the joint in the same way as adding Carbon Nanotubes to the adhesive does [13–16]. Whether the adherend is a metal or a composite significantly affects
* Corresponding author. E-mail address: [email protected] (S. Akpinar). https://doi.org/10.1016/j.compositesb.2019.107106 Received 21 January 2019; Received in revised form 1 June 2019; Accepted 4 July 2019 Available online 5 July 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
S. Akpinar and I.A. Akpinar
Composites Part B 175 (2019) 107106
the failure load of the joint [17–20]. The study performed by Ozel [20] examined the failure load of several different types of joint materials, such as aluminum–aluminum, composite–composite and aluminum-composite. According to the results of this study, the failure loads of composite joints increased by 50–80% compared to aluminum joints. Adhesively bonded joints used in aerospace applications are subject to thermal loading. For example, it is known that when an airplane is airborne (10000 feet) the temperature is approximately 60 � C, whereas while it is on the ground the temperature is approximately 35 � C (in summer). Therefore, it is critical to know the thermal behavior of adhesively bonded joints. In the literature, research on adhesively bonded joints subjected to thermal loading has been summarized below [21–26]. In a study carried out by Park [21], the lap shear strength of the adhesively bonded joints composed of glass/epoxy composite was investigated at room and cryogenic temperatures. According to the re sults of the study, at the cryogenic temperature of 150 � C, the lap shear strength of the joint was increased by about 27%, when the carbon black content was 1.0 wt%. Reinforcing the adhesive with the carbon black was less effective at the cryogenic temperature since both the tensile strength and failure strain of carbon black reinforced epoxy decreased compared to that of the neat epoxy. In a study performed by Freitas [22], the failure of a Fiber Metal Laminate (FML) skin adhesively bonded to a Carbon Fiber Reinforced Polymer (CFRP) stiffener were analyzed at different environmental temperatures ( 55 � C, Room Temperature (RT) and 100 � C). In the analysis, the maximum load and the corresponding displacement significantly increase with temperature. At 100 � C the maximum load increases approximately 15%–30% when compared to RT. For the displacement the difference is even greater with more than twice the value for 100 � C when compared to RT. At 55 � C the maximum load and correspondent displacement decrease approximately 50%–60% when compared to RT. In a study performed by Khosravani [23], to understand the effect of loading and ageing on the adhesively bonded joints, quasistatic tension experiments were performed with the original T-joints and with two sets of artificially aged specimen. In the study, accelerated aging was per formed by applying a thermal cycle in the range of 35 � C–70 � C. Additionally the data provided were confirmed by three-dimensional finite element analyses using a cohesive element technique. The re sults showed that in the specimens which experienced 25 and 100 aging cycles, the elastic strength is reduced by 2% and 40%, respectively. The effect of ageing on the strength is mainly caused by the exposure time and to a lesser degree by the temperature range. Also; in a study con ducted by Kim [25], an intelligent curing cycle was developed with sudden cooling and hardening at room temperature to eliminate thermal residual stress and to produce adequate interfacial wetting. The results of the study showed that the tensile load capability of the adhesively bonded joint fabricated by the smart cure cycle was greatly enhanced because the thermal residual stress was reduced, and sufficient interfa cial wetting between the adhesive and adherend was achieved. Adhesively bonded joints are often exposed to alternating warm and cold temperatures in many applications. According to the literature re searches, it has been observed that studies on increasing the perfor mance of adhesive bonded connections under different thermal cycles are very limited. In this study, the tensile strength of adhesively bonded single lap composite joints (SLCJs) was experimentally investigated under ambient temperature and thermal cycling conditions. In the study, carbon fiber fabric reinforced (CFFR) composites (0/90� ) with Plain Weave (PW) were used as the adherend; DP460 tough and DP125 flex ible adhesive types were used as the adhesive and 1 wt% GrapheneCOOH, Carbon Nanotube-COOH and Fullerene C60 were used as the nanostructures. Six different thermal cycles were applied to SLCJs and their tensile failure loads and failure modes were investigated. These
failure modes (adhesive, cohesive, interphase failure etc.) were dis cussed based on observation of failure location and nature of surfaces. 2. Experimental details In the test of this study, a two-part paste epoxy (DP460 toughened adhesive and DP125 flexible adhesive type, produced by 3 M Company, St. Paul, MN, USA) was used as adhesive. Twelve-laminate carbon fiber fabric reinforced (CFFR) composite was used as the adherend. Carbon fibers are plain weave and the ratio of carbon fiber is 245 g m2 3k (3000 filaments per fiber). CFFR composite was produced by vacuum infusion method and it was kept at 100 � C for 1 h for curing by Fibermak, Turkey. It comprises twelve laminations and its overall thickness is 3�0.2 mm. In a study by Akpinar [3], 0.25, 0.5, 1, 2 and 3% by weight of Graphene-COOH, Carbon Nanotube-COOH and Fullerene C60 nano structures were added to the DP460 adhesive and the damage load of the joint was investigated. According to the results of this study, it was found that the best nanostructures added to the adhesive were 1% by weight. Therefore, 1% by weight of Graphene-COOH (thickness 5–7 nm, diameter 5 μm, surface area 120–150 m2/g), Carbon Nanotubes-COOH (diameter 10–20 nm, length 10–30 mm, purity 95%, surface area 200 m2/g and COOH coverage 2% by weight) and Fullerene C60 (purity 99%) were added to the adhesive. Mechanical properties for the adhe sives and adherend used in experimental studies are given in Tables 1 and 2. Mechanical properties of composite material’s technical data was taken by Fibermak, Turkey [27]. Structural adhesives are subjected to curing depending on the tem perature and time. Curing conditions and adhesive mixing ratios are given in Table 3 [27]. To produce single lap composite joints, CFFR composite plates were cut by water jet to produce 144 CFFR composite specimens of 125 � 25 mm. Surface preparation was necessary for adhesively bonded joints to reveal high performance. To eliminate dirt and roughness on the surface of the adherend materials (CFFR), they were ground by using 1000 grade SiC emery paper so that a smooth surface was obtained. After grinding, specimens were washed with powder cleaner under tap water and then the surfaces to be bonded were held in acetone for 2 min. Surface treatment procedures were completed by drying the samples in an oven at 60 � C for 30 min. Surface treatment procedures were per formed in the same way for all samples [27]. The most important point in the preparation of the nanostructurereinforced adhesives is to distribute the carbon nanostructures homo genously within the adhesive to prevent flocculation between nano structures. In the present study, the method used by Akpinar et al. [28] was selected to add nanostructures into an adhesive. This method was developed by the Chemistry Department and briefly is as follows: 1% by weight of graphene is added to the amount of adhesive and approximately 10 g of acetone was added by using a precision balance in a clean and empty glass, and was then mixed for 10 min by the ultrasonic mixer at 30 KHz frequency (Fig. 1a). Then, after adding the required amount of epoxy, it was stirred for 30 min by the ultrasonic mixer at 30 KHz frequency. The acetone was evaporated by keeping the acetone/ nanostructure/epoxy mixture at a temperature below the curing tem perature – at 50 � C – in a drying oven. The complete evaporation of the Table 1 Material properties of the adhesives [27]. E (MPa)
ν σt (MPa) εt (%)
DP 460
DP 125
1984 �43 0.37 38.4 �1.1 4.7
25.1�2 0.35 12.7�0.4 78.5
E: Young’s modulus; ν: Poisson’s ratio; σt: Ultimate tensile strength; εt: Ultimate tensile strain. 2
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Composites Part B 175 (2019) 107106
0.16 mm, see Fig. 3 b. Since a mechanical thermal cycle would reduce the reliability of the results, it was applied by a fully automated and highly precise device in this study (Fig. 4). In experimental work six different thermal cycle was used. These are;
Table 2 Material properties of the adherend (Carbon Fiber Fabric Reinforced) [27]. CFFR E1 (GPa) E2 (GPa) G12 (GPa)
72�2.8 72�2.8 5�03 0.1 650�28 90�4.5 0.25�0.05
ν12 σ12 (MPa) τ12 (MPa)
Lamina thickness
� For one cycle, the sample is held at 60 � C for 15 min/21 � C for 5 min (ambient) and 50 � C for 15 min. The total thermal cycling opera tion comprises five cycles. � For one cycle, the sample is held at 40 � C for 15 min/21 � C for 5 min (ambient) and 50 � C for 15 min. The total thermal cycling opera tion comprises five cycles. � For one cycle, the sample is held at 30 � C for 15 min/21 � C for 5 min (ambient) and 50 � C for 15 min. The total thermal cycling opera tion comprises five cycles. � For one cycle, the sample is held at 60 � C for 30 min/21 � C for 10 min (ambient) and 50 � C for 30 min. The total thermal cycling opera tion comprises five cycles. � For one cycle, the sample is held at 40 � C for 30 min/21 � C for 10 min (ambient) and 50 � C for 30 min. The total thermal cycling opera tion comprises five cycles. � For one cycle, the sample is held at 30 � C for 30 min/21 � C for 10 min (ambient) and 50 � C for 30 min. The total thermal cycling opera tion comprises five cycles.
E: Young’s modulus; ν: Poisson’s ratio; σ: Tensile strength; τ: Shear strength. Table 3 Curing conditions and adhesive mixing ratios [27]. Adhesive
Mixing Ratio (Epoxy:A/Hardener:B)
Curing temperature/time
3 M™ DP-460 3 M™ DP-125
A:B ¼ 2:1 A:B ¼ 2:1
60 � C/120 min 70 � C/120 min
In the study, samples were designed in two main groups (DP460 nanostructure reinforced and DP125 nanostructure reinforced) and the experimental parameters of these samples are given in Table 4. One hundred forty-four single lap composite joint samples were prepared by preparing three samples for each parameter given in Table 4. All the tensile experiments were carried out using a computercontrolled Instron-5982-100 kN (USA) universal tensile device at 20 � C and 34% humidity rate, with a tensile speed of 1 mm/min. The boundary conditions and loading used are shown in Fig. 5. The overlap length and the thickness of the adhesive layer of each sample was measured and recorded before the test. At the same time, the maximum load that the samples could carry was recorded.
Fig. 1. a) Mixing by ultrasonic mixer b) Mixing by hand.
3. Results and discussions
acetone was controlled by measuring the amount of epoxy and nano structure by using a precision balance. Then it was stirred for 10 min by hand, adding the accelerator according to the ratio of epoxy and accelerator compound (Fig. 1b). The adherend material used in the single lap composite joints (SLCJs) was carbon fiber fabric reinforced composites, and the SLCJ samples’ geometry and dimensions are shown in Fig. 2 [27]. The hot press was used so that the adhesives used in the study can be cured under pressure and temperature. To protect the positions of the adherend material, to manage the thickness of the adhesive layer and to apply the proper pressure, a well-designed mold was used (Fig. 3 a). In order to obtain an adhesive layer thickness of 0.16 mm after curing, metal shims were placed into the mold. After the curing of SLCJ samples, adhesive was removed from the edges of the overlap zones to prepare the joint sam ples for testing and thicknesses of adhesive layers for all SLCJs samples were separately measured. The mean thickness value was found to be
The average failure loads of single lap composite joints (SLCJs) bonded with DP460 tough adhesive, with and without nanostructure additives under ambient conditions, are given in Table 5. These average failure load values were taken from the study by Avinc Akpinar et al. [27]. When the standard deviations given in Table 5 are examined, it is seen that the standard deviation is about 1–2%. Table 6 presents the average failure loads of SLCJs bonded with DP460 tough adhesive, with and without nanostructure additives, under six different thermal cycling conditions. The experimental results of SLCJs bonded with DP460 tough adhe sive exposed to thermal cycling at 60 � C for 15 min/at ambient tem perature for 5 min/at 50 � C for 15 min (Type-I) are shown in Table 6. The experimental results of SLCJs bonded with DP460 tough adhesive exposed to ambient temperature conditions are shown in Table 5.
Fig. 2. SLCJ geometry used in the experimental investigation. 3
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Composites Part B 175 (2019) 107106
Fig. 3. a) The manufacturing mold of the joint samples b) The joint samples after curing.
for 120 min, so if the joints at the first and fourth thermal cycling con ditions are exposed to þ60 � C five times, post-curing occurs. When the standard deviations given in Table 6 are examined, they are at a minimum, which indicates that the nanostructures were uni formly distributed within the adhesive. Table 7 shows the average failure loads of the nanostructure-free and nanostructure-added SLCJs bonded with DP125 tough adhesive under ambient conditions. These average failure loads are taken from Avinc Akpinar et al. [27]. When the standard deviations given in Table 5 are examined, it is seen that the standard deviation is about 1–1.5%. Table 8 shows average failure loads of nanostructure-added and nanostructure-free SLCJs bonded with DP125 flexible adhesive under six different thermal cycling conditions. When the tensile failure loads of additive-free composite joints bonded with DP125 adhesive at ambient temperature and under thermal cycling conditions are compared, the failure load of the joint (Type–II–1) exposed to first thermal cycle decreased by 9%, the failure load of the joint (Type–II–2) exposed to second thermal cycle decreased by 4%, the failure load of the joint (Type–II–4) exposed to fourth thermal cycle decreased by 18%, the failure load of the joint (Type–II–5) exposed to fifth thermal cycles decreased by 45% and the failure load of the joint (Type–II–6) exposed to sixth thermal cycle decreased by 9%. However, the failure load of the joint (Type–II–3) exposed to third thermal cycle increased by 8%. Nevertheless, the decrease due to the thermal cycling is between 2% and 19% in Graphene-COOH added joints, between 2% and 12% in Carbon Nanotube-COOH added joints and between 2% and 16% in Fullerene added joints. When Tables 7 and 8 are analyzed together, one of the main con clusions is that the failure load of the composite joints (Type-II) with nonanostructure additive decreases by 4%–45% when exposed to five different thermal cycles (Type–II–1,2,4,5,6). However, the failure load of the composite joints with nanostructure additive decreases by 2%– 19%. Since joints bonded with adhesives are used in aeronautics, these joints are frequently subjected to thermal cycling. According to the result obtained above, adding nanostructures to joints subjected to thermal cycling – depending on their thermal expansion coefficients and chemical properties – will improves the thermal cycle performance of the joints. This result is very important, because composite materials are used in almost all fields nowadays, and adhesives are used to bond the composite materials together. When the failure models defined in ASTM 5573 [29] are examined, adhesive failure is defined as the separation appears to be at the adhesive-adherend interface, cohesive failure is defined as the separa tion is within the adhesive and interphase failure (thin-layer cohesive failure) is defined as by a light dusting” of adhesive on one substrate surface and a thick layer of adhesive left on the other (Fig. 6).
Fig. 4. Thermal cycling device.
Failure load comparisons between these sets of results showed that the failure load of the joint without additives decreased by approximately 3%, the failure load of the joints with Graphene-COOH additive decreased by 2%, the failure load of the joints with Carbon NanotubeCOOH and Fullerene additives decreased by approximately 4%. How ever, when the results for thermal cycles applied at 60 � C for 30 min/at ambient temperature for 10 min/at 50 � C for 30 min (Type-IV) were compared with the results where no thermal cycle was applied, it was observed that the failure load of joint with no additive increased by approximately 4%, the failure load of the Graphene-COOH treated joint increased by approximately 5%, the failure load of the Carbon Nanotube-COOH treated joints increased by approximately 3.6% and the failure load of the Fullerene treated joints increased by about 2%. This increase can be accounted for by the formation of post-curing in the joint. There are two results obtained here. The first is that, when the adhesive used in the adhesively bonded composite joints is exposed to the thermal cycle at temperatures under the curing conditions, the failure load of the joint increases in contrast to the reduction of the failure load. Secondly, the addition of nanostructures into the adhesive increases the failure load of the joint was obtained from the literature [27]. However, the addition of nanostructures to the adhesive is un derstood by the data of the study on how the joint affects thermal cycling performance (increase, decrease or minimal change). In terms of the failure loads of composite joints, when the six different thermal cycle applications are compared to each other, those exposed at þ60 � C for 15 min and for 30 min (the first and fourth thermal cycling conditions), and those at þ 40 � C for 15 min and for 30 min (the second, third, fifth and sixth thermal cycling conditions) – depending on the joint types – had higher failure loads. This is because curing is achieved in DP460 adhesively bonded joints exposed to þ60 � C 4
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Composites Part B 175 (2019) 107106
Table 4 Geometric parameters used in the experimental investigation. Type
Adhesive
Reinforcement
Temperature (� C)
Time (min.)
Cycle number
Type-I-1 Type-IG-1 Type-IC-1 Type-IF-1
DP460 DP460 DP460 DP460
Non-reinforced Graphene-COOH Carbon Nanotube-COOH Fullerene
þ60/ambient/-50 þ60/ambient/-50 þ60/ambient/-50 þ60/ambient/-50
15/5/15 15/5/15 15/5/15 15/5/15
5 5 5 5
Type-I-2 Type-IG-2 Type-IC-2 Type-IF-2
DP460 DP460 DP460 DP460
Non-reinforced Graphene-COOH Carbon Nanotube-COOH Fullerene
þ40/ambient/-50 þ40/ambient/-50 þ40/ambient/-50 þ40/ambient/-50
15/5/15 15/5/15 15/5/15 15/5/15
5 5 5 5
Type-I-3 Type-IG-3 Type-IC-3 Type-IF-3
DP460 DP460 DP460 DP460
Non-reinforced Graphene-COOH Carbon Nanotube-COOH Fullerene
þ30/ambient/-50 þ30/ambient/-50 þ30/ambient/-50 þ30/ambient/-50
15/5/15 15/5/15 15/5/15 15/5/15
5 5 5 5
Type-I-4 Type-IG-4 Type-IC-4 Type-IF-4
DP460 DP460 DP460 DP460
Non-reinforced Graphene-COOH Carbon Nanotube-COOH Fullerene
þ60/ambient/-50 þ60/ambient/-50 þ60/ambient/-50 þ60/ambient/-50
30/10/30 30/10/30 30/10/30 30/10/30
5 5 5 5
Type-I-5 Type-IG-5 Type-IC-5 Type-IF-5
DP460 DP460 DP460 DP460
Non-reinforced Graphene-COOH Carbon Nanotube-COOH Fullerene
þ40/ambient/-50 þ40/ambient/-50 þ40/ambient/-50 þ40/ambient/-50
30/10/30 30/10/30 30/10/30 30/10/30
5 5 5 5
Type-I-6 Type-IG-6 Type-IC-6 Type-IF-6
DP460 DP460 DP460 DP460
Non-reinforced Graphene-COOH Carbon Nanotube-COOH Fullerene
þ30/ambient/-50 þ30/ambient/-50 þ30/ambient/-50 þ30/ambient/-50
30/10/30 30/10/30 30/10/30 30/10/30
5 5 5 5
Type-II-1 Type-IIG-1 Type-IIC-1 Type-IIF-1
DP125 DP125 DP125 DP125
Non-reinforced Graphene-COOH Carbon Nanotube-COOH Fullerene
þ60/ambient/-50 þ60/ambient/-50 þ60/ambient/-50 þ60/ambient/-50
15/5/15 15/5/15 15/5/15 15/5/15
5 5 5 5
Type-II-2 Type-IIG-2 Type-IIC-2 Type-IIF-2
DP125 DP125 DP125 DP125
Non-reinforced Graphene-COOH Carbon Nanotube-COOH Fullerene
þ40/ambient/-50 þ40/ambient/-50 þ40/ambient/-50 þ40/ambient/-50
15/5/15 15/5/15 15/5/15 15/5/15
5 5 5 5
Type-II-3 Type-IIG-3 Type-IIC-3 Type-IIF-3
DP125 DP125 DP125 DP125
Non-reinforced Graphene-COOH Carbon Nanotube-COOH Fullerene
þ30/ambient/-50 þ30/ambient/-50 þ30/ambient/-50 þ30/ambient/-50
15/5/15 15/5/15 15/5/15 15/5/15
5 5 5 5
Type-II-4 Type-IIG-4 Type-IIC-4 Type-IIF-4
DP125 DP125 DP125 DP125
Non-reinforced Graphene-COOH Carbon Nanotube-COOH Fullerene
þ60/ambient/-50 þ60/ambient/-50 þ60/ambient/-50 þ60/ambient/-50
30/10/30 30/10/30 30/10/30 30/10/30
5 5 5 5
Type-II-5 Type-IIG-5 Type-IIC-5 Type-IIF-5
DP125 DP125 DP125 DP125
Non-reinforced Graphene-COOH Carbon Nanotube-COOH Fullerene
þ40/ambient/-50 þ40/ambient/-50 þ40/ambient/-50 þ40/ambient/-50
30/10/30 30/10/30 30/10/30 30/10/30
5 5 5 5
Type-II-6 Type-IIG-6 Type-IIC-6 Type-IIF-6
DP125 DP125 DP125 DP125
Non-reinforced Graphene-COOH Carbon Nanotube-COOH Fullerene
þ30/ambient/-50 þ30/ambient/-50 þ30/ambient/-50 þ30/ambient/-50
30/10/30 30/10/30 30/10/30 30/10/30
5 5 5 5
If the failure patterns are examined by considering the types of de formations defined in ASTM 5573 [29], in the joint types bonded with DP460 adhesive exposed to þ60 � C and þ40 � C for 15 min and 30 min (first, second, fourth and fifth thermal cycle conditions) cohesive failure occurs (Table 6 and Fig. 7a). But, when the joint types bonded with DP460 adhesive exposed to þ30 � C for 15 min and 30 min (third and sixth thermal cycle conditions) interphase failure occurs (Table 6 and Fig. 7b). When the DP125 adhesive-bonded joint types were subjected to thermal cycling at 60 � C and 40 � C for 15 min (first and second thermal cycling conditions), adhesive failure was observed in the joint types bonded with un-nanostructure adhesive (Fig. 7c), cohesive failure was observed in the joint types bonded with the Graphene-COOH and Fullerene added adhesives and interphase failure was observed in the joint types bonded with the Carbon Nanotube-COOH added adhesive. (Table 8). Interphase failure occurs when the Carbon Nanotube-COOH
and Fullerene added joint types are subjected to the third, fourth, fifth and sixth thermal cycling conditions, while cohesive damage occurs in the types of joint bonded with the Graphene-COOH added adhesive. The results obtained from the failure surfaces show that post-curing happened in the joints exposed to þ60 � C and for 15 min and 30 min (the first and fourth thermal cycling conditions), and this curing accounts for the cohesive failure. This situation correlates with the result that cohe sive failure occurs in the adhesively bonded joints because their failure strength increases. 4. Conclusions In this study, the tensile strength of adhesively bonded single lap composite joints (SLCJs) was experimentally investigated under ambient temperature and thermal cycling conditions. Accordingly, the following conclusions can be drawn: 5
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Composites Part B 175 (2019) 107106
Fig. 5. Boundary conditions and loading used in the tensile test. Table 5 Failure Load of SLCJs bonded with DP460 adhesive at ambient temperature [27].
Table 7 Failure loads of SLCJs bonded with DP125 adhesive at ambient temperature conditions [27].
Joint Type
Reinforcement
Reinforcement Ratio (%)
Failure Load (N)
Joint Type
Reinforcement
Reinforcement Ratio (%)
Failure Load (N)
DP460 DP460-G DP460-K
– Graphene-COOH Carbon NanotubeCOOH Fullerene
– 1 1
16156�161 18353�184 20006�205
DP125 DP125-G DP125-K
– 1 1
9650�113 12890�171 14765�167
1
19900�197
DP125-F
– Graphene-COOH Carbon NanotubeCOOH Fullerene
1
15418�192
DP460-F
Table 6 Failure Load of SLCJs bonded with DP460 adhesive under thermal cycling conditions.
Table 8 Failure loads of SLCJs bonded with DP125 adhesive under thermal cycling conditions.
Joint Type
Failure Load (N)
Failure Mode
Joint Type
Failure Load (N)
Failure Mode
Joint Type
Failure Load (N)
Failure Mode
Joint Type
Failure Load (N)
Failure Mode
Type-I1 TypeIG-1 TypeIC-1 TypeIF-1 Type-I2 TypeIG-2 TypeIC-2 TypeIF-2 Type-I3 TypeIG-3 TypeIC-3 TypeIF-3
15632
Cohesive
Type-I-4
16812
�275
Cohesive
8817 �163
Adhesive
Type–II–4
7892
Adhesive
17937
Cohesive
Type-IG-4
19231
�236
Cohesive
11764
Cohesive
Type-IIG-4
10376 �176
Cohesive
19164
Cohesive
Type–IC–4
20735
13538
Interphase
Type–IIC–4
12183
�155
Interphase
19091
Cohesive
Type–IF–4
20251�273
Cohesive
14752
Cohesive
Type–IIF–4
13545 �147
Interphase
15224
Cohesive
Type-I-5
15887�182
Cohesive
9237 �154
Adhesive
Type–II–5
5277
Adhesive
17488
Cohesive
Type-IG-5
18382
�316
Cohesive
12143
Cohesive
Type-IIG-5
18563
Cohesive
Type–IC–5
19565
�362
13966
Interphase
18542
Cohesive
Type–IF–5
19452
�297
15044
14566
Interphase
Type-I-6
15412
�309
17074
Interphase
Type-IG-6
17518
�331
17936
Interphase
Type–IC–6
19104
�327
17691
Interphase
Type–IF–6
18814
�252
TypeII-1 TypeIIG-1 TypeIIC-1 TypeIIF-1 TypeII-2 TypeIIG-2 TypeIIC-2 TypeIIF–2 TypeII-3 TypeIIG-3 TypeIIC-3 TypeIIF-3
�191
�218
�264
�249
�215
�227
�294
�306
�128
�208
�183
�257
�224
Cohesive
Cohesive Cohesive Interphase Interphase Interphase Interphase
6
�142
�172
�137
�165
�103
11711
�127
Cohesive
Type–IIC–5
12928
�118
Interphase
Cohesive
Type–IIF–5
13079�149
Interphase
10462
Interphase
Type–II–6
8754
Interphase
14155
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Type-IIG-6
12633
13573
Interphase
Type–IIC–6
13394 �194
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13952
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Type–IIF–6
12853�111
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�175
�168
�117
�137
�124
�141
�183
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�167
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S. Akpinar and I.A. Akpinar
Composites Part B 175 (2019) 107106
Fig. 6. Failure models defined in ASTM 5573; a) adhesive failure b) cohesive failure c) interphase failure.
Fig. 7. Failure surfaces of SLCJs bonded with DP460 and DP125 adhesive; a) cohesive failure, b) interphase failure, c) adhesive failure.
� Compared to nanostructure-free SLCJs bonded with DP460 tough adhesive that were not subjected to thermal cycling, the failure loads of SLCJs subjected to thermal cycling decreased by between 3 and 4%. However, the failure loads of SLCJs with nanostructure additives increased by between 3 and 5%. � When the adhesive used in the adhesively bonded composite joints is exposed to the thermal cycle at temperatures under the curing con ditions, the failure load of the joint increases in contrast to the reduction of the failure load. � Post-curing took place in the bonded joints with the DP460 tough adhesive joints exposed to þ60 � C and þ40 � C for 15 min and 30 min (first, second, fourth and fifth thermal cycle conditions), and this curing accounts for the cohesive failure. This situation correlates to the result that cohesive failure occurs in the adhesively bonded joints because their failure strength increases. � When the tensile failure loads of additive-free composite joints bonded with DP125 adhesive at ambient temperature and under five different thermal cycling conditions (Type–II–1,2,4,5,6) are compared, one of the main conclusions is that the failure load of the composite joints decreases by 4%–45%. However, the failure load of the composite joints with nanostructure additive decreases by 2%– 19%. � When the joint types bonded with DP460 adhesive exposed to þ60 � C and þ40 � C for 15 min and 30 min (first, second, fourth and fifth thermal cycle conditions) cohesive failure occurs. But, when the joint types bonded with DP460 adhesive exposed to þ30 � C for 15 min and 30 min (third and sixth thermal cycle conditions) interphase failure occurs (Table 6 and Fig. 7b). � While interphase failure occurs in the joint types bonded with Car bon Nanotube-COOH and Fullerene added DP125 adhesives, which
are exposed to the third, fourth, fifth and sixth thermal cycling conditions, cohesive damage occurs in the types of joint bonded with the Graphene-COOH added adhesive. Acknowledgement This study was financially supported by The Scientific and Techno logical Research Council of Turkey–TUBITAK through the Project no. 114M408. References [1] Grant LDR, Adams RD, da Silva LFM. Experimental and numerical analysis of single-lap joints for the automotive industry. Int J Adhesion Adhes 2009;29: 405–13. [2] Higgins A. Adhesive bonding of aircraft structures. Int J Adhesion Adhes 2000;20: 367–76. [3] Avinc Akpinar I, Gultekin K, Akpinar S, Akbulut H, Ozel A. Experimental analysis on the single-lap joints bonded by a nanocomposite adhesives which obtained by adding nanostructures. Compos B Eng 2017;110:420–8. [4] Garrett LB, Young WK, Randall DP. Effect of carbon nanotube reinforcement on fracture strength of composite adhesive joints. J Mater Sci 2011;46:3370–7. [5] Hsiao KT, Alms J, Advani SG. Use of epoxy/multiwalled carbon nanotubes as adhesives to join graphite fibre reinforced polymer composites. Nanotechnology 2003;14:791–3. [6] Bhowmik S, Benedictus R, Poulis JA, Bonin HW, Bui VT. High performance nano adhesive bonding of titanium for aerospace and space application. Int J Adhesion Adhes 2009;29:259–67. [7] Gkikas G, Sioulas D, Lekatou A, Barkoula NM, Paipetis AS. Enhanced bonded aircraft repair using nano-modified adhesives. Mater Des 2012;41:394–402. [8] Wang Y, He X, Xing B, Deng C. Improvement in strength of adhesively bonded single-lap joints using reinforcements. J Adhes 2015;6:434–8. [9] Liu HY, Wang GT, Mai YW, Zeng Y. On fracture toughness of nano-particle modified epoxy. Compos B Eng 2011;42:2170–5. [10] Feng L, Bae DH. Joining SS3041 sheets by using nano adhesives. J Mech Sci Technol 2013;27:1943–7.
7
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Composites Part B 175 (2019) 107106 [21] Park SW, Lee DG. Strength of double lap joints bonded with CarbonnBlack reinforced adhesive under cryogenic environment. J Adhes Sci Technol 2009;23: 619–38. [22] Freitas ST, Sinke J. Failure analysis of adhesively-bonded metal-skin-to-compositestiffener: effect of temperature and cyclic loading. Compos Struct 2017;166:27–37. [23] Khosravani MR, Weinberg K. Research on strength of nanocomposite adhesively bonded composite joints. Compos Struct 2018;197:80–8. [24] Sousa JM, Correia JR, Firmo JP, Fonseca SC, Gonilha J. Effects of thermal cycles on adhesively bonded joints between pultruded GFRP adherends. Compos Struct 2018;202:518–29. [25] Kim DH, Kim HS. Smart cure cycle to improve tensile load capability of the adhesively bonded joint. J Adhes Sci Technol 2013;27:1739–54. [26] Park SW, Lee DG. Strength of double lap joints bonded with carbon black reinforced adhesive under cryogenic environment. J Adhes Sci Technol 2009;23: 619–38. [27] Avinc Akpinar I, Gultekin K, Akpinar S, Akbulut H, Ozel A. Research on strength of nanocomposite adhesively bonded composite joints. Compos B Eng 2017;126: 143–52. [28] Gultekin K, Akpinar S, Gürses A, Eroglu Z, Cam S, Akbulut H, et al. The effects of graphene nanostructure reinforcement on the adhesive method and the graphene reinforcement ratio on the failure load in adhesively bonded joints. Composites Part B 2016;67:170–8. [29] ASTM 5573-99, Standard practice for classifying failure modes in fiber-reinforcedplastic (FRP) joints.
[11] Srivastava VK. Effect of carbon nanotubes on the strength of adhesive lap joints of C/C and C/C–SiC ceramic fibre composites. Int J Adhesion Adhes 2011;31:486–9. [12] Kang MH, Choi JH, Kweon JH. Fatigue life evaluation and crack detection of the adhesive joint with carbon nanotubes. Compos Struct 2014;108:417–22. [13] Zhai LL, Ling GP, Wang YW. Effect of nano-Al2O3 on adhesion strength of epoxy adhesive and steel. Int J Adhesion Adhes 2007;28:23–8. [14] Avinc Akpinar I, Gultekin K, Akpinar S, Gürses A, Ozel A. An experimental study on composite adhesives reinforced with different types of organo-clays. J Adhes 2018; 94:124–42. [15] May M, Wang HM, Akid R. Effects of the addition of inorganic nanoparticles on the adhesive strength of a hybrid sol-gel epoxy system. Int J Adhesion Adhes 2010;30: 505–12. [16] Lee DG, Kim JK, Cho DH. Effects of adhesive fillers on the strength of tubular single lap adhesive joints. J Adhes Sci Technol 1999;13:1343–60. [17] Sassi S, Tarfaoui M, Yahia HB. An investigation of in-plane dynamic behavior of adhesively-bonded composite joints under dynamic compression at high strain rate. Compos Struct 2018;191:168–79. [18] Budhe S, Banea MD, de Barros S, da Silva LFM. An updated review of adhesively bonded joints in composite materials. Int J Adhesion Adhes 2017;72:30–42. [19] Akpinar S, Aydın MD. 3-D non-linear stress analysis on the adhesively bonded composite joint under bending moment. Int J Mech Sci 2014;81:149–57. [20] Ozel A, Yazici B, Akpinar S, Aydın MD, Temiz S. A study on the strength of adhesively bonded joints with different adherends. Composites Part B 2014;62: 167–74.
8
Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Effect of nanostructured reinforcement of adhesive on thermal cycling performance of a single-lap joint with composite adherends Salih Akpinar a, *, Iclal Avinc Akpinar b a b
Dept. of Mechanical Eng., Erzurum Technical University, 25050, Erzurum, Turkey Independent Researcher, 25050, Erzurum, Turkey
A R T I C L E I N F O
A B S T R A C T
Keywords: Nanostructures Composites Adhesive Strength Mechanical testing Thermal cycle Joints
Nanostructure of adhesive increased the failure load of bonded joints. Adhesively bonded joints are often exposed to alternating warm and cold temperatures in many applications. Hence investigation of their thermal cycle performance is very important. In the present study, the tensile strength of adhesively bonded single lap com posite joints (SLCJs) was experimentally investigated under ambient temperature and thermal cycling condi tions. In the study, carbon fiber fabric reinforced (CFFR) composites (0/90� ) with Plain Weave (PW) were used as the adherend; DP460 tough and DP125 flexible adhesive types were used as the adhesive and 1 wt% GrapheneCOOH, Carbon Nanotube-COOH and Fullerene C60 were used as the nanostructures. Six different thermal cycles were applied to SLCJs and their tensile failure loads and failure surfaces were investigated. As a result, when the joints obtained through nanostructure non-reinforced adhesives are exposed to thermal cycle, a significant decrease appears in the failure load of the joint. Yet, adding nanostructure to the adhesive enhances the thermal cycle performance of the joint. This enhancement in the thermal cycle resistance changes depending on the structural features of the adhesive, the type of nanostructure and thermal cycle condition.
1. Introduction Adhesively bonded joints are often preferred in the aerospace and automotive industries due to their advantages such as formation of uniform stress distributions, ability to join different materials, high fa tigue resistance and impermeability [1,2]. The strength of adhesive joints depends on a number of parameters, such as the adhesive, the adherend materials, lap length, and the thickness of adherends. How ever, in the last few years, studies have been done with the aim of increasing the strength of the adhesive and adherends to improve the performance of adhesively bonded joints. Nanostructures such as Car bon Nanotubes (CNTs), Graphene, Fullerene and Organo clays are added to the adhesive to increase its strength. When studies in the literature relating to joints created by adding nanostructures into the adhesive were reviewed, it was seen that adding compatible nanostructures to the adhesive increased the failure load of the bond [3–12]. In the study performed by Avinc Akpinar [3], tensile failure loads of single lap-joints using nanocomposite adhesives – obtained by adding nanostructure to the adhesive – were experimentally examined to in crease the failure load of adhesively bonded joints. According to the results of this study, adding 1% Graphene-COOH to the rigid adhesive
increases the failure load of the joint by 109%; adding 2% Graphene-COOH increases the failure load of the joint by 276%. How ever, adding Fullerene to a rigid adhesive does not cause a significant increase or decrease in the failure load of the joint. A study, carried out by Garrett et al. [4], emphasized how carbon nanotube reinforcement in epoxy adhesive affects the Mod II rupture strength of steel composite and composite-composite bonding applica tions. It was observed that in all samples, the distribution of the Carbon Nanotubes in the adhesive had an impact on the rupture features. It was found that the nanotube’s features of carboxyl group, dispersion, structure, length and diameter all play an important role in determining whether the nanotube reinforcement strengthens or weakens the joint, and as the result of the tests, the optimum nanotube reinforcement should be around 1%. In the composite-composite adhesively bonded single-lap joint, adding 1–5% multiple walled CNT to the adhesive in creases the shear strength at a rate of about 46% [5]. Furthermore, in the adhesively bonded joints, adding different nanostructures to the adhesive (nano clay, nano Al2O3, nano CaCO3, nano SiO2 etc.) affects the failure load of the joint in the same way as adding Carbon Nanotubes to the adhesive does [13–16]. Whether the adherend is a metal or a composite significantly affects
* Corresponding author. E-mail address: [email protected] (S. Akpinar). https://doi.org/10.1016/j.compositesb.2019.107106 Received 21 January 2019; Received in revised form 1 June 2019; Accepted 4 July 2019 Available online 5 July 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
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Composites Part B 175 (2019) 107106
the failure load of the joint [17–20]. The study performed by Ozel [20] examined the failure load of several different types of joint materials, such as aluminum–aluminum, composite–composite and aluminum-composite. According to the results of this study, the failure loads of composite joints increased by 50–80% compared to aluminum joints. Adhesively bonded joints used in aerospace applications are subject to thermal loading. For example, it is known that when an airplane is airborne (10000 feet) the temperature is approximately 60 � C, whereas while it is on the ground the temperature is approximately 35 � C (in summer). Therefore, it is critical to know the thermal behavior of adhesively bonded joints. In the literature, research on adhesively bonded joints subjected to thermal loading has been summarized below [21–26]. In a study carried out by Park [21], the lap shear strength of the adhesively bonded joints composed of glass/epoxy composite was investigated at room and cryogenic temperatures. According to the re sults of the study, at the cryogenic temperature of 150 � C, the lap shear strength of the joint was increased by about 27%, when the carbon black content was 1.0 wt%. Reinforcing the adhesive with the carbon black was less effective at the cryogenic temperature since both the tensile strength and failure strain of carbon black reinforced epoxy decreased compared to that of the neat epoxy. In a study performed by Freitas [22], the failure of a Fiber Metal Laminate (FML) skin adhesively bonded to a Carbon Fiber Reinforced Polymer (CFRP) stiffener were analyzed at different environmental temperatures ( 55 � C, Room Temperature (RT) and 100 � C). In the analysis, the maximum load and the corresponding displacement significantly increase with temperature. At 100 � C the maximum load increases approximately 15%–30% when compared to RT. For the displacement the difference is even greater with more than twice the value for 100 � C when compared to RT. At 55 � C the maximum load and correspondent displacement decrease approximately 50%–60% when compared to RT. In a study performed by Khosravani [23], to understand the effect of loading and ageing on the adhesively bonded joints, quasistatic tension experiments were performed with the original T-joints and with two sets of artificially aged specimen. In the study, accelerated aging was per formed by applying a thermal cycle in the range of 35 � C–70 � C. Additionally the data provided were confirmed by three-dimensional finite element analyses using a cohesive element technique. The re sults showed that in the specimens which experienced 25 and 100 aging cycles, the elastic strength is reduced by 2% and 40%, respectively. The effect of ageing on the strength is mainly caused by the exposure time and to a lesser degree by the temperature range. Also; in a study con ducted by Kim [25], an intelligent curing cycle was developed with sudden cooling and hardening at room temperature to eliminate thermal residual stress and to produce adequate interfacial wetting. The results of the study showed that the tensile load capability of the adhesively bonded joint fabricated by the smart cure cycle was greatly enhanced because the thermal residual stress was reduced, and sufficient interfa cial wetting between the adhesive and adherend was achieved. Adhesively bonded joints are often exposed to alternating warm and cold temperatures in many applications. According to the literature re searches, it has been observed that studies on increasing the perfor mance of adhesive bonded connections under different thermal cycles are very limited. In this study, the tensile strength of adhesively bonded single lap composite joints (SLCJs) was experimentally investigated under ambient temperature and thermal cycling conditions. In the study, carbon fiber fabric reinforced (CFFR) composites (0/90� ) with Plain Weave (PW) were used as the adherend; DP460 tough and DP125 flex ible adhesive types were used as the adhesive and 1 wt% GrapheneCOOH, Carbon Nanotube-COOH and Fullerene C60 were used as the nanostructures. Six different thermal cycles were applied to SLCJs and their tensile failure loads and failure modes were investigated. These
failure modes (adhesive, cohesive, interphase failure etc.) were dis cussed based on observation of failure location and nature of surfaces. 2. Experimental details In the test of this study, a two-part paste epoxy (DP460 toughened adhesive and DP125 flexible adhesive type, produced by 3 M Company, St. Paul, MN, USA) was used as adhesive. Twelve-laminate carbon fiber fabric reinforced (CFFR) composite was used as the adherend. Carbon fibers are plain weave and the ratio of carbon fiber is 245 g m2 3k (3000 filaments per fiber). CFFR composite was produced by vacuum infusion method and it was kept at 100 � C for 1 h for curing by Fibermak, Turkey. It comprises twelve laminations and its overall thickness is 3�0.2 mm. In a study by Akpinar [3], 0.25, 0.5, 1, 2 and 3% by weight of Graphene-COOH, Carbon Nanotube-COOH and Fullerene C60 nano structures were added to the DP460 adhesive and the damage load of the joint was investigated. According to the results of this study, it was found that the best nanostructures added to the adhesive were 1% by weight. Therefore, 1% by weight of Graphene-COOH (thickness 5–7 nm, diameter 5 μm, surface area 120–150 m2/g), Carbon Nanotubes-COOH (diameter 10–20 nm, length 10–30 mm, purity 95%, surface area 200 m2/g and COOH coverage 2% by weight) and Fullerene C60 (purity 99%) were added to the adhesive. Mechanical properties for the adhe sives and adherend used in experimental studies are given in Tables 1 and 2. Mechanical properties of composite material’s technical data was taken by Fibermak, Turkey [27]. Structural adhesives are subjected to curing depending on the tem perature and time. Curing conditions and adhesive mixing ratios are given in Table 3 [27]. To produce single lap composite joints, CFFR composite plates were cut by water jet to produce 144 CFFR composite specimens of 125 � 25 mm. Surface preparation was necessary for adhesively bonded joints to reveal high performance. To eliminate dirt and roughness on the surface of the adherend materials (CFFR), they were ground by using 1000 grade SiC emery paper so that a smooth surface was obtained. After grinding, specimens were washed with powder cleaner under tap water and then the surfaces to be bonded were held in acetone for 2 min. Surface treatment procedures were completed by drying the samples in an oven at 60 � C for 30 min. Surface treatment procedures were per formed in the same way for all samples [27]. The most important point in the preparation of the nanostructurereinforced adhesives is to distribute the carbon nanostructures homo genously within the adhesive to prevent flocculation between nano structures. In the present study, the method used by Akpinar et al. [28] was selected to add nanostructures into an adhesive. This method was developed by the Chemistry Department and briefly is as follows: 1% by weight of graphene is added to the amount of adhesive and approximately 10 g of acetone was added by using a precision balance in a clean and empty glass, and was then mixed for 10 min by the ultrasonic mixer at 30 KHz frequency (Fig. 1a). Then, after adding the required amount of epoxy, it was stirred for 30 min by the ultrasonic mixer at 30 KHz frequency. The acetone was evaporated by keeping the acetone/ nanostructure/epoxy mixture at a temperature below the curing tem perature – at 50 � C – in a drying oven. The complete evaporation of the Table 1 Material properties of the adhesives [27]. E (MPa)
ν σt (MPa) εt (%)
DP 460
DP 125
1984 �43 0.37 38.4 �1.1 4.7
25.1�2 0.35 12.7�0.4 78.5
E: Young’s modulus; ν: Poisson’s ratio; σt: Ultimate tensile strength; εt: Ultimate tensile strain. 2
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0.16 mm, see Fig. 3 b. Since a mechanical thermal cycle would reduce the reliability of the results, it was applied by a fully automated and highly precise device in this study (Fig. 4). In experimental work six different thermal cycle was used. These are;
Table 2 Material properties of the adherend (Carbon Fiber Fabric Reinforced) [27]. CFFR E1 (GPa) E2 (GPa) G12 (GPa)
72�2.8 72�2.8 5�03 0.1 650�28 90�4.5 0.25�0.05
ν12 σ12 (MPa) τ12 (MPa)
Lamina thickness
� For one cycle, the sample is held at 60 � C for 15 min/21 � C for 5 min (ambient) and 50 � C for 15 min. The total thermal cycling opera tion comprises five cycles. � For one cycle, the sample is held at 40 � C for 15 min/21 � C for 5 min (ambient) and 50 � C for 15 min. The total thermal cycling opera tion comprises five cycles. � For one cycle, the sample is held at 30 � C for 15 min/21 � C for 5 min (ambient) and 50 � C for 15 min. The total thermal cycling opera tion comprises five cycles. � For one cycle, the sample is held at 60 � C for 30 min/21 � C for 10 min (ambient) and 50 � C for 30 min. The total thermal cycling opera tion comprises five cycles. � For one cycle, the sample is held at 40 � C for 30 min/21 � C for 10 min (ambient) and 50 � C for 30 min. The total thermal cycling opera tion comprises five cycles. � For one cycle, the sample is held at 30 � C for 30 min/21 � C for 10 min (ambient) and 50 � C for 30 min. The total thermal cycling opera tion comprises five cycles.
E: Young’s modulus; ν: Poisson’s ratio; σ: Tensile strength; τ: Shear strength. Table 3 Curing conditions and adhesive mixing ratios [27]. Adhesive
Mixing Ratio (Epoxy:A/Hardener:B)
Curing temperature/time
3 M™ DP-460 3 M™ DP-125
A:B ¼ 2:1 A:B ¼ 2:1
60 � C/120 min 70 � C/120 min
In the study, samples were designed in two main groups (DP460 nanostructure reinforced and DP125 nanostructure reinforced) and the experimental parameters of these samples are given in Table 4. One hundred forty-four single lap composite joint samples were prepared by preparing three samples for each parameter given in Table 4. All the tensile experiments were carried out using a computercontrolled Instron-5982-100 kN (USA) universal tensile device at 20 � C and 34% humidity rate, with a tensile speed of 1 mm/min. The boundary conditions and loading used are shown in Fig. 5. The overlap length and the thickness of the adhesive layer of each sample was measured and recorded before the test. At the same time, the maximum load that the samples could carry was recorded.
Fig. 1. a) Mixing by ultrasonic mixer b) Mixing by hand.
3. Results and discussions
acetone was controlled by measuring the amount of epoxy and nano structure by using a precision balance. Then it was stirred for 10 min by hand, adding the accelerator according to the ratio of epoxy and accelerator compound (Fig. 1b). The adherend material used in the single lap composite joints (SLCJs) was carbon fiber fabric reinforced composites, and the SLCJ samples’ geometry and dimensions are shown in Fig. 2 [27]. The hot press was used so that the adhesives used in the study can be cured under pressure and temperature. To protect the positions of the adherend material, to manage the thickness of the adhesive layer and to apply the proper pressure, a well-designed mold was used (Fig. 3 a). In order to obtain an adhesive layer thickness of 0.16 mm after curing, metal shims were placed into the mold. After the curing of SLCJ samples, adhesive was removed from the edges of the overlap zones to prepare the joint sam ples for testing and thicknesses of adhesive layers for all SLCJs samples were separately measured. The mean thickness value was found to be
The average failure loads of single lap composite joints (SLCJs) bonded with DP460 tough adhesive, with and without nanostructure additives under ambient conditions, are given in Table 5. These average failure load values were taken from the study by Avinc Akpinar et al. [27]. When the standard deviations given in Table 5 are examined, it is seen that the standard deviation is about 1–2%. Table 6 presents the average failure loads of SLCJs bonded with DP460 tough adhesive, with and without nanostructure additives, under six different thermal cycling conditions. The experimental results of SLCJs bonded with DP460 tough adhe sive exposed to thermal cycling at 60 � C for 15 min/at ambient tem perature for 5 min/at 50 � C for 15 min (Type-I) are shown in Table 6. The experimental results of SLCJs bonded with DP460 tough adhesive exposed to ambient temperature conditions are shown in Table 5.
Fig. 2. SLCJ geometry used in the experimental investigation. 3
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Composites Part B 175 (2019) 107106
Fig. 3. a) The manufacturing mold of the joint samples b) The joint samples after curing.
for 120 min, so if the joints at the first and fourth thermal cycling con ditions are exposed to þ60 � C five times, post-curing occurs. When the standard deviations given in Table 6 are examined, they are at a minimum, which indicates that the nanostructures were uni formly distributed within the adhesive. Table 7 shows the average failure loads of the nanostructure-free and nanostructure-added SLCJs bonded with DP125 tough adhesive under ambient conditions. These average failure loads are taken from Avinc Akpinar et al. [27]. When the standard deviations given in Table 5 are examined, it is seen that the standard deviation is about 1–1.5%. Table 8 shows average failure loads of nanostructure-added and nanostructure-free SLCJs bonded with DP125 flexible adhesive under six different thermal cycling conditions. When the tensile failure loads of additive-free composite joints bonded with DP125 adhesive at ambient temperature and under thermal cycling conditions are compared, the failure load of the joint (Type–II–1) exposed to first thermal cycle decreased by 9%, the failure load of the joint (Type–II–2) exposed to second thermal cycle decreased by 4%, the failure load of the joint (Type–II–4) exposed to fourth thermal cycle decreased by 18%, the failure load of the joint (Type–II–5) exposed to fifth thermal cycles decreased by 45% and the failure load of the joint (Type–II–6) exposed to sixth thermal cycle decreased by 9%. However, the failure load of the joint (Type–II–3) exposed to third thermal cycle increased by 8%. Nevertheless, the decrease due to the thermal cycling is between 2% and 19% in Graphene-COOH added joints, between 2% and 12% in Carbon Nanotube-COOH added joints and between 2% and 16% in Fullerene added joints. When Tables 7 and 8 are analyzed together, one of the main con clusions is that the failure load of the composite joints (Type-II) with nonanostructure additive decreases by 4%–45% when exposed to five different thermal cycles (Type–II–1,2,4,5,6). However, the failure load of the composite joints with nanostructure additive decreases by 2%– 19%. Since joints bonded with adhesives are used in aeronautics, these joints are frequently subjected to thermal cycling. According to the result obtained above, adding nanostructures to joints subjected to thermal cycling – depending on their thermal expansion coefficients and chemical properties – will improves the thermal cycle performance of the joints. This result is very important, because composite materials are used in almost all fields nowadays, and adhesives are used to bond the composite materials together. When the failure models defined in ASTM 5573 [29] are examined, adhesive failure is defined as the separation appears to be at the adhesive-adherend interface, cohesive failure is defined as the separa tion is within the adhesive and interphase failure (thin-layer cohesive failure) is defined as by a light dusting” of adhesive on one substrate surface and a thick layer of adhesive left on the other (Fig. 6).
Fig. 4. Thermal cycling device.
Failure load comparisons between these sets of results showed that the failure load of the joint without additives decreased by approximately 3%, the failure load of the joints with Graphene-COOH additive decreased by 2%, the failure load of the joints with Carbon NanotubeCOOH and Fullerene additives decreased by approximately 4%. How ever, when the results for thermal cycles applied at 60 � C for 30 min/at ambient temperature for 10 min/at 50 � C for 30 min (Type-IV) were compared with the results where no thermal cycle was applied, it was observed that the failure load of joint with no additive increased by approximately 4%, the failure load of the Graphene-COOH treated joint increased by approximately 5%, the failure load of the Carbon Nanotube-COOH treated joints increased by approximately 3.6% and the failure load of the Fullerene treated joints increased by about 2%. This increase can be accounted for by the formation of post-curing in the joint. There are two results obtained here. The first is that, when the adhesive used in the adhesively bonded composite joints is exposed to the thermal cycle at temperatures under the curing conditions, the failure load of the joint increases in contrast to the reduction of the failure load. Secondly, the addition of nanostructures into the adhesive increases the failure load of the joint was obtained from the literature [27]. However, the addition of nanostructures to the adhesive is un derstood by the data of the study on how the joint affects thermal cycling performance (increase, decrease or minimal change). In terms of the failure loads of composite joints, when the six different thermal cycle applications are compared to each other, those exposed at þ60 � C for 15 min and for 30 min (the first and fourth thermal cycling conditions), and those at þ 40 � C for 15 min and for 30 min (the second, third, fifth and sixth thermal cycling conditions) – depending on the joint types – had higher failure loads. This is because curing is achieved in DP460 adhesively bonded joints exposed to þ60 � C 4
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Table 4 Geometric parameters used in the experimental investigation. Type
Adhesive
Reinforcement
Temperature (� C)
Time (min.)
Cycle number
Type-I-1 Type-IG-1 Type-IC-1 Type-IF-1
DP460 DP460 DP460 DP460
Non-reinforced Graphene-COOH Carbon Nanotube-COOH Fullerene
þ60/ambient/-50 þ60/ambient/-50 þ60/ambient/-50 þ60/ambient/-50
15/5/15 15/5/15 15/5/15 15/5/15
5 5 5 5
Type-I-2 Type-IG-2 Type-IC-2 Type-IF-2
DP460 DP460 DP460 DP460
Non-reinforced Graphene-COOH Carbon Nanotube-COOH Fullerene
þ40/ambient/-50 þ40/ambient/-50 þ40/ambient/-50 þ40/ambient/-50
15/5/15 15/5/15 15/5/15 15/5/15
5 5 5 5
Type-I-3 Type-IG-3 Type-IC-3 Type-IF-3
DP460 DP460 DP460 DP460
Non-reinforced Graphene-COOH Carbon Nanotube-COOH Fullerene
þ30/ambient/-50 þ30/ambient/-50 þ30/ambient/-50 þ30/ambient/-50
15/5/15 15/5/15 15/5/15 15/5/15
5 5 5 5
Type-I-4 Type-IG-4 Type-IC-4 Type-IF-4
DP460 DP460 DP460 DP460
Non-reinforced Graphene-COOH Carbon Nanotube-COOH Fullerene
þ60/ambient/-50 þ60/ambient/-50 þ60/ambient/-50 þ60/ambient/-50
30/10/30 30/10/30 30/10/30 30/10/30
5 5 5 5
Type-I-5 Type-IG-5 Type-IC-5 Type-IF-5
DP460 DP460 DP460 DP460
Non-reinforced Graphene-COOH Carbon Nanotube-COOH Fullerene
þ40/ambient/-50 þ40/ambient/-50 þ40/ambient/-50 þ40/ambient/-50
30/10/30 30/10/30 30/10/30 30/10/30
5 5 5 5
Type-I-6 Type-IG-6 Type-IC-6 Type-IF-6
DP460 DP460 DP460 DP460
Non-reinforced Graphene-COOH Carbon Nanotube-COOH Fullerene
þ30/ambient/-50 þ30/ambient/-50 þ30/ambient/-50 þ30/ambient/-50
30/10/30 30/10/30 30/10/30 30/10/30
5 5 5 5
Type-II-1 Type-IIG-1 Type-IIC-1 Type-IIF-1
DP125 DP125 DP125 DP125
Non-reinforced Graphene-COOH Carbon Nanotube-COOH Fullerene
þ60/ambient/-50 þ60/ambient/-50 þ60/ambient/-50 þ60/ambient/-50
15/5/15 15/5/15 15/5/15 15/5/15
5 5 5 5
Type-II-2 Type-IIG-2 Type-IIC-2 Type-IIF-2
DP125 DP125 DP125 DP125
Non-reinforced Graphene-COOH Carbon Nanotube-COOH Fullerene
þ40/ambient/-50 þ40/ambient/-50 þ40/ambient/-50 þ40/ambient/-50
15/5/15 15/5/15 15/5/15 15/5/15
5 5 5 5
Type-II-3 Type-IIG-3 Type-IIC-3 Type-IIF-3
DP125 DP125 DP125 DP125
Non-reinforced Graphene-COOH Carbon Nanotube-COOH Fullerene
þ30/ambient/-50 þ30/ambient/-50 þ30/ambient/-50 þ30/ambient/-50
15/5/15 15/5/15 15/5/15 15/5/15
5 5 5 5
Type-II-4 Type-IIG-4 Type-IIC-4 Type-IIF-4
DP125 DP125 DP125 DP125
Non-reinforced Graphene-COOH Carbon Nanotube-COOH Fullerene
þ60/ambient/-50 þ60/ambient/-50 þ60/ambient/-50 þ60/ambient/-50
30/10/30 30/10/30 30/10/30 30/10/30
5 5 5 5
Type-II-5 Type-IIG-5 Type-IIC-5 Type-IIF-5
DP125 DP125 DP125 DP125
Non-reinforced Graphene-COOH Carbon Nanotube-COOH Fullerene
þ40/ambient/-50 þ40/ambient/-50 þ40/ambient/-50 þ40/ambient/-50
30/10/30 30/10/30 30/10/30 30/10/30
5 5 5 5
Type-II-6 Type-IIG-6 Type-IIC-6 Type-IIF-6
DP125 DP125 DP125 DP125
Non-reinforced Graphene-COOH Carbon Nanotube-COOH Fullerene
þ30/ambient/-50 þ30/ambient/-50 þ30/ambient/-50 þ30/ambient/-50
30/10/30 30/10/30 30/10/30 30/10/30
5 5 5 5
If the failure patterns are examined by considering the types of de formations defined in ASTM 5573 [29], in the joint types bonded with DP460 adhesive exposed to þ60 � C and þ40 � C for 15 min and 30 min (first, second, fourth and fifth thermal cycle conditions) cohesive failure occurs (Table 6 and Fig. 7a). But, when the joint types bonded with DP460 adhesive exposed to þ30 � C for 15 min and 30 min (third and sixth thermal cycle conditions) interphase failure occurs (Table 6 and Fig. 7b). When the DP125 adhesive-bonded joint types were subjected to thermal cycling at 60 � C and 40 � C for 15 min (first and second thermal cycling conditions), adhesive failure was observed in the joint types bonded with un-nanostructure adhesive (Fig. 7c), cohesive failure was observed in the joint types bonded with the Graphene-COOH and Fullerene added adhesives and interphase failure was observed in the joint types bonded with the Carbon Nanotube-COOH added adhesive. (Table 8). Interphase failure occurs when the Carbon Nanotube-COOH
and Fullerene added joint types are subjected to the third, fourth, fifth and sixth thermal cycling conditions, while cohesive damage occurs in the types of joint bonded with the Graphene-COOH added adhesive. The results obtained from the failure surfaces show that post-curing happened in the joints exposed to þ60 � C and for 15 min and 30 min (the first and fourth thermal cycling conditions), and this curing accounts for the cohesive failure. This situation correlates with the result that cohe sive failure occurs in the adhesively bonded joints because their failure strength increases. 4. Conclusions In this study, the tensile strength of adhesively bonded single lap composite joints (SLCJs) was experimentally investigated under ambient temperature and thermal cycling conditions. Accordingly, the following conclusions can be drawn: 5
S. Akpinar and I.A. Akpinar
Composites Part B 175 (2019) 107106
Fig. 5. Boundary conditions and loading used in the tensile test. Table 5 Failure Load of SLCJs bonded with DP460 adhesive at ambient temperature [27].
Table 7 Failure loads of SLCJs bonded with DP125 adhesive at ambient temperature conditions [27].
Joint Type
Reinforcement
Reinforcement Ratio (%)
Failure Load (N)
Joint Type
Reinforcement
Reinforcement Ratio (%)
Failure Load (N)
DP460 DP460-G DP460-K
– Graphene-COOH Carbon NanotubeCOOH Fullerene
– 1 1
16156�161 18353�184 20006�205
DP125 DP125-G DP125-K
– 1 1
9650�113 12890�171 14765�167
1
19900�197
DP125-F
– Graphene-COOH Carbon NanotubeCOOH Fullerene
1
15418�192
DP460-F
Table 6 Failure Load of SLCJs bonded with DP460 adhesive under thermal cycling conditions.
Table 8 Failure loads of SLCJs bonded with DP125 adhesive under thermal cycling conditions.
Joint Type
Failure Load (N)
Failure Mode
Joint Type
Failure Load (N)
Failure Mode
Joint Type
Failure Load (N)
Failure Mode
Joint Type
Failure Load (N)
Failure Mode
Type-I1 TypeIG-1 TypeIC-1 TypeIF-1 Type-I2 TypeIG-2 TypeIC-2 TypeIF-2 Type-I3 TypeIG-3 TypeIC-3 TypeIF-3
15632
Cohesive
Type-I-4
16812
�275
Cohesive
8817 �163
Adhesive
Type–II–4
7892
Adhesive
17937
Cohesive
Type-IG-4
19231
�236
Cohesive
11764
Cohesive
Type-IIG-4
10376 �176
Cohesive
19164
Cohesive
Type–IC–4
20735
13538
Interphase
Type–IIC–4
12183
�155
Interphase
19091
Cohesive
Type–IF–4
20251�273
Cohesive
14752
Cohesive
Type–IIF–4
13545 �147
Interphase
15224
Cohesive
Type-I-5
15887�182
Cohesive
9237 �154
Adhesive
Type–II–5
5277
Adhesive
17488
Cohesive
Type-IG-5
18382
�316
Cohesive
12143
Cohesive
Type-IIG-5
18563
Cohesive
Type–IC–5
19565
�362
13966
Interphase
18542
Cohesive
Type–IF–5
19452
�297
15044
14566
Interphase
Type-I-6
15412
�309
17074
Interphase
Type-IG-6
17518
�331
17936
Interphase
Type–IC–6
19104
�327
17691
Interphase
Type–IF–6
18814
�252
TypeII-1 TypeIIG-1 TypeIIC-1 TypeIIF-1 TypeII-2 TypeIIG-2 TypeIIC-2 TypeIIF–2 TypeII-3 TypeIIG-3 TypeIIC-3 TypeIIF-3
�191
�218
�264
�249
�215
�227
�294
�306
�128
�208
�183
�257
�224
Cohesive
Cohesive Cohesive Interphase Interphase Interphase Interphase
6
�142
�172
�137
�165
�103
11711
�127
Cohesive
Type–IIC–5
12928
�118
Interphase
Cohesive
Type–IIF–5
13079�149
Interphase
10462
Interphase
Type–II–6
8754
Interphase
14155
Cohesive
Type-IIG-6
12633
13573
Interphase
Type–IIC–6
13394 �194
Interphase
13952
Interphase
Type–IIF–6
12853�111
Interphase
�175
�168
�117
�137
�124
�141
�183
�102
�167
Cohesive
S. Akpinar and I.A. Akpinar
Composites Part B 175 (2019) 107106
Fig. 6. Failure models defined in ASTM 5573; a) adhesive failure b) cohesive failure c) interphase failure.
Fig. 7. Failure surfaces of SLCJs bonded with DP460 and DP125 adhesive; a) cohesive failure, b) interphase failure, c) adhesive failure.
� Compared to nanostructure-free SLCJs bonded with DP460 tough adhesive that were not subjected to thermal cycling, the failure loads of SLCJs subjected to thermal cycling decreased by between 3 and 4%. However, the failure loads of SLCJs with nanostructure additives increased by between 3 and 5%. � When the adhesive used in the adhesively bonded composite joints is exposed to the thermal cycle at temperatures under the curing con ditions, the failure load of the joint increases in contrast to the reduction of the failure load. � Post-curing took place in the bonded joints with the DP460 tough adhesive joints exposed to þ60 � C and þ40 � C for 15 min and 30 min (first, second, fourth and fifth thermal cycle conditions), and this curing accounts for the cohesive failure. This situation correlates to the result that cohesive failure occurs in the adhesively bonded joints because their failure strength increases. � When the tensile failure loads of additive-free composite joints bonded with DP125 adhesive at ambient temperature and under five different thermal cycling conditions (Type–II–1,2,4,5,6) are compared, one of the main conclusions is that the failure load of the composite joints decreases by 4%–45%. However, the failure load of the composite joints with nanostructure additive decreases by 2%– 19%. � When the joint types bonded with DP460 adhesive exposed to þ60 � C and þ40 � C for 15 min and 30 min (first, second, fourth and fifth thermal cycle conditions) cohesive failure occurs. But, when the joint types bonded with DP460 adhesive exposed to þ30 � C for 15 min and 30 min (third and sixth thermal cycle conditions) interphase failure occurs (Table 6 and Fig. 7b). � While interphase failure occurs in the joint types bonded with Car bon Nanotube-COOH and Fullerene added DP125 adhesives, which
are exposed to the third, fourth, fifth and sixth thermal cycling conditions, cohesive damage occurs in the types of joint bonded with the Graphene-COOH added adhesive. Acknowledgement This study was financially supported by The Scientific and Techno logical Research Council of Turkey–TUBITAK through the Project no. 114M408. References [1] Grant LDR, Adams RD, da Silva LFM. Experimental and numerical analysis of single-lap joints for the automotive industry. Int J Adhesion Adhes 2009;29: 405–13. [2] Higgins A. Adhesive bonding of aircraft structures. Int J Adhesion Adhes 2000;20: 367–76. [3] Avinc Akpinar I, Gultekin K, Akpinar S, Akbulut H, Ozel A. Experimental analysis on the single-lap joints bonded by a nanocomposite adhesives which obtained by adding nanostructures. Compos B Eng 2017;110:420–8. [4] Garrett LB, Young WK, Randall DP. Effect of carbon nanotube reinforcement on fracture strength of composite adhesive joints. J Mater Sci 2011;46:3370–7. [5] Hsiao KT, Alms J, Advani SG. Use of epoxy/multiwalled carbon nanotubes as adhesives to join graphite fibre reinforced polymer composites. Nanotechnology 2003;14:791–3. [6] Bhowmik S, Benedictus R, Poulis JA, Bonin HW, Bui VT. High performance nano adhesive bonding of titanium for aerospace and space application. Int J Adhesion Adhes 2009;29:259–67. [7] Gkikas G, Sioulas D, Lekatou A, Barkoula NM, Paipetis AS. Enhanced bonded aircraft repair using nano-modified adhesives. Mater Des 2012;41:394–402. [8] Wang Y, He X, Xing B, Deng C. Improvement in strength of adhesively bonded single-lap joints using reinforcements. J Adhes 2015;6:434–8. [9] Liu HY, Wang GT, Mai YW, Zeng Y. On fracture toughness of nano-particle modified epoxy. Compos B Eng 2011;42:2170–5. [10] Feng L, Bae DH. Joining SS3041 sheets by using nano adhesives. J Mech Sci Technol 2013;27:1943–7.
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