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epoxy composites with excellent shear properties for construction adhesives
Accepted Manuscript Graphene nanoplatelets/epoxy composites with excellent shear properties for construction adhesives Zhenyu Wang, Zhemin Jia, Xiaoping Feng, Yun Zou PII:
S1359-8368(18)32529-0
DOI:
10.1016/j.compositesb.2018.08.113
Reference:
JCOMB 5943
To appear in:
Composites Part B
Received Date: 9 August 2018 Accepted Date: 23 August 2018
Please cite this article as: Wang Z, Jia Z, Feng X, Zou Y, Graphene nanoplatelets/epoxy composites with excellent shear properties for construction adhesives, Composites Part B (2018), doi: 10.1016/ j.compositesb.2018.08.113. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Graphene nanoplatelets/epoxy composites with excellent shear properties for construction adhesives Zhenyu Wang1, Zhemin Jia2*, Xiaoping Feng2, Yun Zou2
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1. Jiangsu Key Laboratory of Advanced Food Manufacturing Equipment and Technology, School of Mechanical Engineering, Jiangnan University, Wuxi, China
2. School of Environment and Civil Engineering, Jiangnan University, Wuxi, Jiangsu, 214122;
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*Corresponding author: Zhemin Jia: [email protected]
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Abstract
The properties and bonding capabilities of construction adhesives have attracted tremendous interests in the past decades. This paper conducts an experimental study on the shear properties of epoxy construction adhesive reinforced with graphene
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nanoplatelets (GNPs) through thick adherend shear test (TAST). The experimental results show that the shear strength of nanocomposites increases with the increased graphene content. It is worth noting that the shear strength of nanocomposites at a
EP
graphene content of only 0.75 wt% exhibit a 102% enhancement compared with neat
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epoxy adhesive. Other shear properties, including shear modulus, shear strain at failure and toughness also deliver much better performances compared with neat epoxy, indicating the effectiveness of graphene on shear properties improvement. The mechanical behavior of the TAST specimens with different nanocomposites adhesive are predicted using 3D finite elements analysis (FEA). The shear properties of nanocomposites obtained from the experimental results are used as cohesive zone model parameters in FEA. The prediction agree very well with the experimental
ACCEPTED MANUSCRIPT results. Keywords: graphene nanoplatelets； ；epoxy adhesives; shear properties; numerical simulations
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1. Introduction Construction adhesives have attracted tremendous attentions in the last few decades, and have been extensively utilized in strengthening metal components,
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concrete structures, as well as bonding fiber-reinforced composites onto existing
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structures [1–3]. As a commonly used construction adhesive, epoxy adhesives exhibit many advantages, including excellent stress transfer properties between the substrate and bonded components, lightweight, low-cost, in the meanwhile avoiding the use of mechanical fasteners, which are generally susceptible to corrosion [4–6]. As a
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fundamental mechanical property, shear properties are an important criterion for evaluating the quality of adhesives [7,8]. Although great efforts have been made to significantly improve the shear properties of epoxy adhesives, most current products
EP
cannot fulfill the increasingly higher requirements in civil engineering, aerospace, and
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automobile industries.
In order to enhance the mechanical properties of epoxy adhesives, one of the
most efficient methods is incorporating fillers into the polymer matrix to make composites, and polymer based nanocomposites have become one of the most attractive topics for the past decades [9–11]. The commonly used reinforcing fillers including 0 dimensional metal particles [12,13], 1 dimensional carbon nanofibers (CNFs) [14,15], carbon nanotubes (CNTs) [16,17], metal nanowires [18], and 2
ACCEPTED MANUSCRIPT dimensional graphene [19,20]. For example, Tee et al. [13] utilized Ag nanoparticles as reinforcing fillers to make epoxy composites. To improve the dispersion state and create bonding with the epoxy matrix, the Ag particles were functionalized using
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silane. The flexural modulus and strength of the composites increased by ~25% and 22%, respectively, compared to the neat epoxy. Mecklenburg et al. [16] developed aligned CNTs using a chemical vapor deposition (CVD) process, which was then
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incorporated into epoxy to make composites. The as-produced composites solved the
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issue of dispersion, and presented a high Young’s modulus of ~36 GPa at a filler content of 68 wt%. Compared with the aforementioned 0 and 1 dimensional fillers, graphene, as a one-atomic thick, two dimensional carbon material, has delivered superior performances over their metal and carbon filler counterparts, due to the
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exceptional characteristics of graphene, such as large specific surface area, and high modulus and strength. Therefore, it has been made into polymer based composites for outstanding mechanical and other functional properties [21–23].
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So far, there has appeared many types of graphene fillers, including graphene
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nanoplatelets (GNPs) [20], reduced graphene oxide (rGO) [19], and three dimensional graphene [24]. They have been incorporated into epoxy matrix to make composites, and delivered excellent mechanical properties. For instance, Tang et al. [19] studied the effect of rGO dispersion on the mechanical properties of composites in detail. It is found that the composites with a better dispersion state showed higher mechanical strength. The flexural strength and fracture toughness of the resultant rGO/epoxy composites increased by 15% and 52%, respectively. The recently developed 3D
ACCEPTED MANUSCRIPT graphene materials, such as graphene foam [25] and graphene aerogel [24], were also used as reinforcing filler to make epoxy composites. Take graphene aerogel as an example, Wang et al. [24] fabricated graphene aerogel/epoxy composites through a
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vacuum infiltration method, the flexural modulus and fracture toughness of the resultant composites showed 12% and 47% improvement. Among the above mentioned graphene, both rGO and 3D graphene suffered from the high cost. Besides,
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3D graphene materials are difficult to be used in a liquid form. Therefore, GNPs
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based composites are considered to be particularly useful in construction adhesives with low-cost and high performance. For example, Chandrasekaran, et al. [20] fabricated GNPs/epoxy using two different dispersing techniques, namely three-roll milling, and sonication combined with high speed shear mixing. The GNPs were
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uniformly dispersed into the epoxy matrix, and the composites delivered excellent mechanical, electrical, and thermal properties. It is interesting to find that the storage modulus of composites containing 2 wt% GNPs present 14% enhancement, together
EP
with a 36% increase of fracture toughness, compared to the pure epoxy. Although the
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mechanical properties of GNPs/epoxy composites, including tensile, flexural, and fracture properties, have been extensively studied, the shear properties of GNPs/epoxy composites and their potential applications in construction adhesives are still rarely studied.
Herein, GNPs have been used to reinforce epoxy adhesive. Nanocomposites with different graphene contents, including 0.25 wt%, 0.5 wt%, and 0.75 wt% were fabricated. The thick adherend shear test (TAST) specimens were used to test shear
ACCEPTED MANUSCRIPT properties of nanocomposites, including shear strength, shear modulus and shear strain at failure. The variation trend of shear properties with the graphene content were obtained. The numerical analysis conducted by ABAQUS was also carried out to
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simulate TAST specimens with different nanocomposites. 2 Experiment 2.1 Materials
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The epoxy adhesive used in this paper was manufactured by Kangda Company in Shanghai, a two-component adhesive which contains component A for epoxy and
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component B for curing agent. This adhesive has been widely used in the construction area. The cure condition for this adhesive is 23 ºC for 72 h according to the manufacturer of adhesive.
GNPs are fabricated from natural graphite (supplied by Asbury Graphite Mills)
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[21]. Briefly, 1 g of natural graphite flakes are mixed with 30 ml H2SO4 and stirred in a flask at 200 rpm. After that, 10 ml of fuming HNO3 is added into the mixture, and
EP
kept at room temperature (RT), followed by stirring for 24 h. 40 ml of de-ionized (DI) water is subsequently poured slowly into the mixture, followed by water washing.
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After centrifuging, and drying at 80 ºC for 24 h, the graphite intercalated compound (GIC) is obtained. The as-fabricated dry GIC powder is thermally expanded at 1050 ºC for 30 s to obtain GNPs. 2.2 Preparation of GNP/epoxy composites GNPs were dispersed in acetone at a graphene concentration of 2mg/ml, followed by sonication for 6 h in a bath sonicator. Then the GNP/acetone dispersions were mixed with a certain amount of epoxy (component A of the adhesive), depending
ACCEPTED MANUSCRIPT on the graphene content of the final composites. After that, GNPs and epoxy adhesive were pre-mixed using a magnetic stirrer at 2000 rpm for 3h to evaporate acetone at RT. The temperature of the mixture was subsequently elevated to 100 °C for full
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evaporation of acetone. After cooling down to RT, curing agent (component B of the adhesive) at a stoichiometric ratio (weight fraction epoxy/ hardener = 3/1) was added into the mixture and mixed using a planetary mixer (ZYMC-180V, ZYE Technology
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Co., Ltd) at 2000 rpm for 3 min to obtain the final GNP/epoxy composites.
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Nanocomposites containing three different graphene contents, including 0.25 wt%, 0.5 wt% and 0.75 wt% were prepared. 2.3 Sample fabrication
TAST specimens were used to test the shear properties of GNP/epoxy under
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quasi-static conditions. The specimens were fabricated according to standard ISO 11003-2, and the detailed dimensions were shown in Fig. 1. Stainless steel with the overlap width of 25mm and the overlap length of 5mm
EP
was chosen as adherends. The surface of the bonded part was first scrubbed with
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acetone to erase the metal oxide and oil stain, followed by sanded with #80 sandpaper manually in the direction of ±45° to increase the roughness. The rubber spacers were inserted in the overlapping gap to control the adhesive thickness and the overlap length. The adhesive thickness is 0.5 mm. The adhesive was carefully applied to the bonded surface to avoid excessive mixing of air bubbles. The TAST specimens were cured at RT for 72 h. After curing, scrape the excess adhesive on the side and pull out the spacer to complete the preparation of the TAST specimens.
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EP
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(a)
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(b)
Fig. 1 (a) schematic of TAST specimens, and (b) dimensions of TAST specimens (Unit: mm).
2.4 Test procedure and data analysis method The TAST specimens with GNP/epoxy adhesive and neat epoxy adhesive were tested using electronic universal testing machine as shown in Fig. 2. The maximum
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mm/min. At least four valid duplicate data were taken for each material
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Fig. 2 Experimental set up for TAST specimens.
According to the requirements of standard ISO 11003-2, the shear stress-strain curves of the TAST specimens were obtained by the below equations.
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EP
τ=
F BL
(1)
ds t
(2)
τ(ta -t) Ga
(3)
γ= ds =d-
where τ is the shear stress; F is the force value obtained by the force sensor, B is the overlap width; t is the thickness of adhesive; L is the overlap length. The shear strain γ is obtained by dividing the adhesive shear displacement ds by the thickness of adhesive. The shear displacement of adhesive is obtained through Equation (3) by
ACCEPTED MANUSCRIPT subtracting the displacement of adherends from the measured displacement d, where Ga is the shear modulus of adherend, and ta is the total thickness of TAST specimens
which is 12 mm in this paper. The equation (3) according to the ISO standard neglects
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the tensile strain of adherends between the extensometer. However, the measured crosshead displacement is used as “d” in this paper. The shear strain obtained in this paper is used as reference data to compare with each other in this paper.
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3. Results and discussion
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3.1 Experimental study on shear properties of neat adhesive and GNP/epoxy adhesive
To investigate the effect of graphene content on shear properties of epoxy adhesive, tests on TAST specimens with neat epoxy and different content of
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GNP/epoxy adhesive were carried out. Fig. 3 presents typical shear stress-strain curves of neat adhesive and GNP/epoxy nanocomposites with three different graphene contents. It can be observed that all three nanocomposites showed higher shear
EP
strength than the neat epoxy adhesive, which increased with the increased GNP
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content. The shear strength variation of the composites as a function of the GNP content was given in Fig. 4. It can be seen from Fig. 4 that the enhancement of shear strength was not obvious when the GNP content is below 0.5 wt%. The nanocomposite reinforced at a graphene content of 0.75 wt% delivered almost 102% increase in shear strength compared with the neat epoxy, which was the highest improvement among all the materials tested in the paper. It is also found that shear modulus and shear strain at failure follow a similar trend as the shear strength, i.e.,
ACCEPTED MANUSCRIPT they increased after the addition of GNP, although the enhancement is not as obvious. Detailed shear properties data of neat epoxy and nanocomposites were given in Table 1. The areas under the shear stress-strain curves were also listed as an estimation of
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the toughness for nanocomposites.
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Fig. 3 Typical shear stress-strain curves of neat epoxy and different content of
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GNP/epoxy nanocomposites.
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Fig. 4 Shear strength of nanocomposites with a function of graphene content.
Table 1 Shear Properties of Neat Epoxy and GNP/Epoxy Nanocomposites. Shear Strength
Content (w.t.%)
(MPa)
Shear strain at
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Graphene
Shear Modulus
Toughness
(MPa)
(MJ/m3)
failure
0
EP
(%)
11.2±0.72
1.52±0.2
395±25
0.063±0.05
12.62±1.23
1.58±0.16
644±27
0.103±0.04
0.5
14.8±1.68
2±0.3
435±50
0.11±0.08
0.75
22.7±2
1.67±0.15
946±35
0.16±0.04
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0.25
3.2 Numerical Simulation Details and results Commercial finite element analysis software ABAQUS was used to analyze the shear behavior of TAST specimens with the nanocomposites containing different
ACCEPTED MANUSCRIPT graphene contents. Fig. 5 shows the meshed model of TAST specimens. The solid elements were employed for steel adherends and the three-dimensional 8-node cohesive elements (COH3D8) were used to simulate the adhesive.
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The steel adherends were divided into 15 elements in the thickness direction, and the mesh was refined close to the overlap area. The adhesive was divided into one element in the thickness direction due to the instinctive property of cohesive elements,
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and in the overlap length direction the cohesive elements were three times refined
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than the solid elements of adherend. The boundary conditions were applied according to the experiment, one end of the TAST specimens was given a clamped boundary condition, meaning all degrees of freedom of the end were fixed and the displacement was applied on the other side.
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Elastic-plastic material model was used for adherend steel and the parameters were shown in Table 2. A bilinear cohesive zone model (CZM) was used for neat adhesive and nanocomposites with different GNP content. The shear strength value
EP
was used as mode II cohesive shear strength. The mode II critical strain energy release
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rate of the adhesive was obtained by integration of shear stress-strain curves, and the cohesive stiffness was obtained through the shear modulus divided by the thickness of adhesive. Reaction force and the displacement curves were obtained through FEA results. To compare with the experimental stress-strain curves, Equation (1-3) were used to translate force-displacement curves into shear stress-strain curves.
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Fig. 5 Meshed model for TAST specimens. Table 2 List of material properties of the adherends. Plastic strain
400
0
420
2.0e-2
500
20.2e-2
600
50.0e-2
EP
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Yield stress /MPa
The comparisons between the shear-strain curves obtained from the FEA and
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experimental curves were shown in Fig. 6. The peak load and maximum displacement of TAST specimens agreed well the experimental data, while the stiffness of the curves showed some deviations. The reasons to cause such disagreement mainly because the mode II critical strain energy release rate should take the mode II fracture toughness obtained through End-Notched Flexure (ENF) specimens instead of the rough estimation by integrating the shear stress-strain curves.
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Fig. 6 Comparison of the experimental and numerical results of TAST specimens with nanocomposites containing different GNP contents.
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4. Conclusion
This paper conducted experiments on shear properties of the nanocomposites
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with different graphene content. The results show that shear strength of nanocomposites
increased
with
graphene
content.
The shear
strength
of
nanocomposites reinforced by 0.75 wt% graphene increased by 102% compared with neat epoxy adhesive, which was the highest improvement among all the nanocomposites. In addition to shear strength, shear modulus, shear strain at failure and toughness also showed increasing trend compared with neat epoxy, which demonstrated the effectiveness of graphene on improving shear properties of epoxy
ACCEPTED MANUSCRIPT adhesive. The mechanical behavior of the TAST specimens with different nanocomposites adhesive were predicted by FEA. The shear strength of the adhesive was used as the mode II cohesive strength in cohesive zone models. The prediction
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agreed well with experimental curves while the prediction on the stiffness still need improvement. The reason to cause the deviation was mainly because a rough estimation of cohesive stiffness and mode II critical strain energy release in cohesive
Acknowledgement
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zone models were used.
This Project is supported by the National Science Foundation for Young Scientists of
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China (Grant No. 51808261).
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S1359-8368(18)32529-0
DOI:
10.1016/j.compositesb.2018.08.113
Reference:
JCOMB 5943
To appear in:
Composites Part B
Received Date: 9 August 2018 Accepted Date: 23 August 2018
Please cite this article as: Wang Z, Jia Z, Feng X, Zou Y, Graphene nanoplatelets/epoxy composites with excellent shear properties for construction adhesives, Composites Part B (2018), doi: 10.1016/ j.compositesb.2018.08.113. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Graphene nanoplatelets/epoxy composites with excellent shear properties for construction adhesives Zhenyu Wang1, Zhemin Jia2*, Xiaoping Feng2, Yun Zou2
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1. Jiangsu Key Laboratory of Advanced Food Manufacturing Equipment and Technology, School of Mechanical Engineering, Jiangnan University, Wuxi, China
2. School of Environment and Civil Engineering, Jiangnan University, Wuxi, Jiangsu, 214122;
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*Corresponding author: Zhemin Jia: [email protected]
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Abstract
The properties and bonding capabilities of construction adhesives have attracted tremendous interests in the past decades. This paper conducts an experimental study on the shear properties of epoxy construction adhesive reinforced with graphene
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nanoplatelets (GNPs) through thick adherend shear test (TAST). The experimental results show that the shear strength of nanocomposites increases with the increased graphene content. It is worth noting that the shear strength of nanocomposites at a
EP
graphene content of only 0.75 wt% exhibit a 102% enhancement compared with neat
AC C
epoxy adhesive. Other shear properties, including shear modulus, shear strain at failure and toughness also deliver much better performances compared with neat epoxy, indicating the effectiveness of graphene on shear properties improvement. The mechanical behavior of the TAST specimens with different nanocomposites adhesive are predicted using 3D finite elements analysis (FEA). The shear properties of nanocomposites obtained from the experimental results are used as cohesive zone model parameters in FEA. The prediction agree very well with the experimental
ACCEPTED MANUSCRIPT results. Keywords: graphene nanoplatelets； ；epoxy adhesives; shear properties; numerical simulations
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1. Introduction Construction adhesives have attracted tremendous attentions in the last few decades, and have been extensively utilized in strengthening metal components,
SC
concrete structures, as well as bonding fiber-reinforced composites onto existing
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structures [1–3]. As a commonly used construction adhesive, epoxy adhesives exhibit many advantages, including excellent stress transfer properties between the substrate and bonded components, lightweight, low-cost, in the meanwhile avoiding the use of mechanical fasteners, which are generally susceptible to corrosion [4–6]. As a
TE D
fundamental mechanical property, shear properties are an important criterion for evaluating the quality of adhesives [7,8]. Although great efforts have been made to significantly improve the shear properties of epoxy adhesives, most current products
EP
cannot fulfill the increasingly higher requirements in civil engineering, aerospace, and
AC C
automobile industries.
In order to enhance the mechanical properties of epoxy adhesives, one of the
most efficient methods is incorporating fillers into the polymer matrix to make composites, and polymer based nanocomposites have become one of the most attractive topics for the past decades [9–11]. The commonly used reinforcing fillers including 0 dimensional metal particles [12,13], 1 dimensional carbon nanofibers (CNFs) [14,15], carbon nanotubes (CNTs) [16,17], metal nanowires [18], and 2
ACCEPTED MANUSCRIPT dimensional graphene [19,20]. For example, Tee et al. [13] utilized Ag nanoparticles as reinforcing fillers to make epoxy composites. To improve the dispersion state and create bonding with the epoxy matrix, the Ag particles were functionalized using
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silane. The flexural modulus and strength of the composites increased by ~25% and 22%, respectively, compared to the neat epoxy. Mecklenburg et al. [16] developed aligned CNTs using a chemical vapor deposition (CVD) process, which was then
SC
incorporated into epoxy to make composites. The as-produced composites solved the
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issue of dispersion, and presented a high Young’s modulus of ~36 GPa at a filler content of 68 wt%. Compared with the aforementioned 0 and 1 dimensional fillers, graphene, as a one-atomic thick, two dimensional carbon material, has delivered superior performances over their metal and carbon filler counterparts, due to the
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exceptional characteristics of graphene, such as large specific surface area, and high modulus and strength. Therefore, it has been made into polymer based composites for outstanding mechanical and other functional properties [21–23].
EP
So far, there has appeared many types of graphene fillers, including graphene
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nanoplatelets (GNPs) [20], reduced graphene oxide (rGO) [19], and three dimensional graphene [24]. They have been incorporated into epoxy matrix to make composites, and delivered excellent mechanical properties. For instance, Tang et al. [19] studied the effect of rGO dispersion on the mechanical properties of composites in detail. It is found that the composites with a better dispersion state showed higher mechanical strength. The flexural strength and fracture toughness of the resultant rGO/epoxy composites increased by 15% and 52%, respectively. The recently developed 3D
ACCEPTED MANUSCRIPT graphene materials, such as graphene foam [25] and graphene aerogel [24], were also used as reinforcing filler to make epoxy composites. Take graphene aerogel as an example, Wang et al. [24] fabricated graphene aerogel/epoxy composites through a
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vacuum infiltration method, the flexural modulus and fracture toughness of the resultant composites showed 12% and 47% improvement. Among the above mentioned graphene, both rGO and 3D graphene suffered from the high cost. Besides,
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3D graphene materials are difficult to be used in a liquid form. Therefore, GNPs
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based composites are considered to be particularly useful in construction adhesives with low-cost and high performance. For example, Chandrasekaran, et al. [20] fabricated GNPs/epoxy using two different dispersing techniques, namely three-roll milling, and sonication combined with high speed shear mixing. The GNPs were
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uniformly dispersed into the epoxy matrix, and the composites delivered excellent mechanical, electrical, and thermal properties. It is interesting to find that the storage modulus of composites containing 2 wt% GNPs present 14% enhancement, together
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with a 36% increase of fracture toughness, compared to the pure epoxy. Although the
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mechanical properties of GNPs/epoxy composites, including tensile, flexural, and fracture properties, have been extensively studied, the shear properties of GNPs/epoxy composites and their potential applications in construction adhesives are still rarely studied.
Herein, GNPs have been used to reinforce epoxy adhesive. Nanocomposites with different graphene contents, including 0.25 wt%, 0.5 wt%, and 0.75 wt% were fabricated. The thick adherend shear test (TAST) specimens were used to test shear
ACCEPTED MANUSCRIPT properties of nanocomposites, including shear strength, shear modulus and shear strain at failure. The variation trend of shear properties with the graphene content were obtained. The numerical analysis conducted by ABAQUS was also carried out to
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simulate TAST specimens with different nanocomposites. 2 Experiment 2.1 Materials
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The epoxy adhesive used in this paper was manufactured by Kangda Company in Shanghai, a two-component adhesive which contains component A for epoxy and
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component B for curing agent. This adhesive has been widely used in the construction area. The cure condition for this adhesive is 23 ºC for 72 h according to the manufacturer of adhesive.
GNPs are fabricated from natural graphite (supplied by Asbury Graphite Mills)
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[21]. Briefly, 1 g of natural graphite flakes are mixed with 30 ml H2SO4 and stirred in a flask at 200 rpm. After that, 10 ml of fuming HNO3 is added into the mixture, and
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kept at room temperature (RT), followed by stirring for 24 h. 40 ml of de-ionized (DI) water is subsequently poured slowly into the mixture, followed by water washing.
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After centrifuging, and drying at 80 ºC for 24 h, the graphite intercalated compound (GIC) is obtained. The as-fabricated dry GIC powder is thermally expanded at 1050 ºC for 30 s to obtain GNPs. 2.2 Preparation of GNP/epoxy composites GNPs were dispersed in acetone at a graphene concentration of 2mg/ml, followed by sonication for 6 h in a bath sonicator. Then the GNP/acetone dispersions were mixed with a certain amount of epoxy (component A of the adhesive), depending
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evaporation of acetone. After cooling down to RT, curing agent (component B of the adhesive) at a stoichiometric ratio (weight fraction epoxy/ hardener = 3/1) was added into the mixture and mixed using a planetary mixer (ZYMC-180V, ZYE Technology
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Co., Ltd) at 2000 rpm for 3 min to obtain the final GNP/epoxy composites.
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Nanocomposites containing three different graphene contents, including 0.25 wt%, 0.5 wt% and 0.75 wt% were prepared. 2.3 Sample fabrication
TAST specimens were used to test the shear properties of GNP/epoxy under
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quasi-static conditions. The specimens were fabricated according to standard ISO 11003-2, and the detailed dimensions were shown in Fig. 1. Stainless steel with the overlap width of 25mm and the overlap length of 5mm
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was chosen as adherends. The surface of the bonded part was first scrubbed with
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acetone to erase the metal oxide and oil stain, followed by sanded with #80 sandpaper manually in the direction of ±45° to increase the roughness. The rubber spacers were inserted in the overlapping gap to control the adhesive thickness and the overlap length. The adhesive thickness is 0.5 mm. The adhesive was carefully applied to the bonded surface to avoid excessive mixing of air bubbles. The TAST specimens were cured at RT for 72 h. After curing, scrape the excess adhesive on the side and pull out the spacer to complete the preparation of the TAST specimens.
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(a)
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(b)
Fig. 1 (a) schematic of TAST specimens, and (b) dimensions of TAST specimens (Unit: mm).
2.4 Test procedure and data analysis method The TAST specimens with GNP/epoxy adhesive and neat epoxy adhesive were tested using electronic universal testing machine as shown in Fig. 2. The maximum
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mm/min. At least four valid duplicate data were taken for each material
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Fig. 2 Experimental set up for TAST specimens.
According to the requirements of standard ISO 11003-2, the shear stress-strain curves of the TAST specimens were obtained by the below equations.
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τ=
F BL
(1)
ds t
(2)
τ(ta -t) Ga
(3)
γ= ds =d-
where τ is the shear stress; F is the force value obtained by the force sensor, B is the overlap width; t is the thickness of adhesive; L is the overlap length. The shear strain γ is obtained by dividing the adhesive shear displacement ds by the thickness of adhesive. The shear displacement of adhesive is obtained through Equation (3) by
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which is 12 mm in this paper. The equation (3) according to the ISO standard neglects
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the tensile strain of adherends between the extensometer. However, the measured crosshead displacement is used as “d” in this paper. The shear strain obtained in this paper is used as reference data to compare with each other in this paper.
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3. Results and discussion
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3.1 Experimental study on shear properties of neat adhesive and GNP/epoxy adhesive
To investigate the effect of graphene content on shear properties of epoxy adhesive, tests on TAST specimens with neat epoxy and different content of
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GNP/epoxy adhesive were carried out. Fig. 3 presents typical shear stress-strain curves of neat adhesive and GNP/epoxy nanocomposites with three different graphene contents. It can be observed that all three nanocomposites showed higher shear
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strength than the neat epoxy adhesive, which increased with the increased GNP
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content. The shear strength variation of the composites as a function of the GNP content was given in Fig. 4. It can be seen from Fig. 4 that the enhancement of shear strength was not obvious when the GNP content is below 0.5 wt%. The nanocomposite reinforced at a graphene content of 0.75 wt% delivered almost 102% increase in shear strength compared with the neat epoxy, which was the highest improvement among all the materials tested in the paper. It is also found that shear modulus and shear strain at failure follow a similar trend as the shear strength, i.e.,
ACCEPTED MANUSCRIPT they increased after the addition of GNP, although the enhancement is not as obvious. Detailed shear properties data of neat epoxy and nanocomposites were given in Table 1. The areas under the shear stress-strain curves were also listed as an estimation of
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the toughness for nanocomposites.
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Fig. 3 Typical shear stress-strain curves of neat epoxy and different content of
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GNP/epoxy nanocomposites.
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Fig. 4 Shear strength of nanocomposites with a function of graphene content.
Table 1 Shear Properties of Neat Epoxy and GNP/Epoxy Nanocomposites. Shear Strength
Content (w.t.%)
(MPa)
Shear strain at
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Graphene
Shear Modulus
Toughness
(MPa)
(MJ/m3)
failure
0
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(%)
11.2±0.72
1.52±0.2
395±25
0.063±0.05
12.62±1.23
1.58±0.16
644±27
0.103±0.04
0.5
14.8±1.68
2±0.3
435±50
0.11±0.08
0.75
22.7±2
1.67±0.15
946±35
0.16±0.04
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0.25
3.2 Numerical Simulation Details and results Commercial finite element analysis software ABAQUS was used to analyze the shear behavior of TAST specimens with the nanocomposites containing different
ACCEPTED MANUSCRIPT graphene contents. Fig. 5 shows the meshed model of TAST specimens. The solid elements were employed for steel adherends and the three-dimensional 8-node cohesive elements (COH3D8) were used to simulate the adhesive.
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The steel adherends were divided into 15 elements in the thickness direction, and the mesh was refined close to the overlap area. The adhesive was divided into one element in the thickness direction due to the instinctive property of cohesive elements,
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and in the overlap length direction the cohesive elements were three times refined
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than the solid elements of adherend. The boundary conditions were applied according to the experiment, one end of the TAST specimens was given a clamped boundary condition, meaning all degrees of freedom of the end were fixed and the displacement was applied on the other side.
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Elastic-plastic material model was used for adherend steel and the parameters were shown in Table 2. A bilinear cohesive zone model (CZM) was used for neat adhesive and nanocomposites with different GNP content. The shear strength value
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was used as mode II cohesive shear strength. The mode II critical strain energy release
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rate of the adhesive was obtained by integration of shear stress-strain curves, and the cohesive stiffness was obtained through the shear modulus divided by the thickness of adhesive. Reaction force and the displacement curves were obtained through FEA results. To compare with the experimental stress-strain curves, Equation (1-3) were used to translate force-displacement curves into shear stress-strain curves.
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Fig. 5 Meshed model for TAST specimens. Table 2 List of material properties of the adherends. Plastic strain
400
0
420
2.0e-2
500
20.2e-2
600
50.0e-2
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Yield stress /MPa
The comparisons between the shear-strain curves obtained from the FEA and
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experimental curves were shown in Fig. 6. The peak load and maximum displacement of TAST specimens agreed well the experimental data, while the stiffness of the curves showed some deviations. The reasons to cause such disagreement mainly because the mode II critical strain energy release rate should take the mode II fracture toughness obtained through End-Notched Flexure (ENF) specimens instead of the rough estimation by integrating the shear stress-strain curves.
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Fig. 6 Comparison of the experimental and numerical results of TAST specimens with nanocomposites containing different GNP contents.
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4. Conclusion
This paper conducted experiments on shear properties of the nanocomposites
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with different graphene content. The results show that shear strength of nanocomposites
increased
with
graphene
content.
The shear
strength
of
nanocomposites reinforced by 0.75 wt% graphene increased by 102% compared with neat epoxy adhesive, which was the highest improvement among all the nanocomposites. In addition to shear strength, shear modulus, shear strain at failure and toughness also showed increasing trend compared with neat epoxy, which demonstrated the effectiveness of graphene on improving shear properties of epoxy
ACCEPTED MANUSCRIPT adhesive. The mechanical behavior of the TAST specimens with different nanocomposites adhesive were predicted by FEA. The shear strength of the adhesive was used as the mode II cohesive strength in cohesive zone models. The prediction
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agreed well with experimental curves while the prediction on the stiffness still need improvement. The reason to cause the deviation was mainly because a rough estimation of cohesive stiffness and mode II critical strain energy release in cohesive
Acknowledgement
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zone models were used.
This Project is supported by the National Science Foundation for Young Scientists of
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China (Grant No. 51808261).
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