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aluminum alloy joints for automobiles
International Journal of Adhesion and Adhesives 95 (2019) 102439
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International Journal of Adhesion and Adhesives journal homepage: www.elsevier.com/locate/ijadhadh
Effect of thermal cycling on the degradation of adhesively bonded CFRP/ aluminum alloy joints for automobiles
T
Guofeng Qina,b,∗, Jingxin Nab, Wenlong Mub, Wei Tanb a b
Teachers College for Vocational and Technical Education, Guangxi Normal University, Guilin, 541004, PR China State Key Laboratory of Automotive Simulation and Control, Jilin University, Changchun, 130021, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Epoxy adhesive CFRP Thermal cycling Chemical analysis Mechanical testing
Thermal cycling is one of the representative service environments of automobiles. As epoxy adhesive and resin matrix of CFRP (Carbon Fiber Reinforced Plastics) composites are polymer materials, their properties may change during long-term thermal cycling and thus affect the joint strength of adhesively bonded composites joints. To investigate the degradation mechanism of adhesively bonded CFRP/aluminum alloy joints subjected to thermal cycling, firstly, the chemical transformations of adhesive and CFRP were analyzed by FTIR (Fourier Transform Infrared Spectroscopy), DSC (Differential Scanning Calorimetry) and TG/DTG (Thermal Gravimetric/ Differential Thermal Gravimetric), and changes in mechanical properties of adhesive and CFRP were also tested by quasi-static tests. Then the variations of failure strength and failure modes of adhesively bonded CFRP/ aluminum alloy shear joints and butt joints after different aging time were investigated. Results show that the Tg (glass transition temperature), thermal stability, failure strength and Young's modulus of adhesive improved after thermal cycling because of post-curing. The Tg and thermal stability of the CFRP decreased due to surface oxidation, resulting in the decline of surface adhesiveness of CFRP and the appearance of interface failure. Besides, the mechanical performance of the fiber/matrix interface also declined after thermal cycling, which was verified by SEM (Scanning Electron Microscope). The failure strength of shear and butt joints dropped by more than 40% after thermal cycling for 30 days. The degradation of shear joints was mainly caused by the combined effect of thermal stress and post-curing of adhesive as well as interface failure, while the butt joints were also affected by the fiber tear of CFRP.
1. Introduction Two major challenges of automotive industry are energy consumption and exhaust emissions, which force researchers and engineers to find solutions. Weight reduction is one of the most efficient ways to reduce fuel demand and corresponding emissions. In order to achieve that goal, composites and lighter alloys with high modulus and strength are applied to replace the traditional steel components, such as body panels and load-bearing parts [1]. Besides, multi-material applications according to their performance characteristics not only significantly reduce the weight, but also control costs effectively [2]. Therefore, joining technology for all kinds of materials is becoming the key to manufacturing lightweight cars. Traditional joining methods (such as bolts and rivets) always destroy the integrity of the structures, resulting in the reduction of load capacity and early failure, especially for the CFRP composites. Adhesive joints are widely used for connecting composite structures because they preserve the structural integrity and
∗
Corresponding author. NO.15Yucai Road, Guilin, Guangxi, 541004, PR China. E-mail address: [email protected] (G. Qin).
https://doi.org/10.1016/j.ijadhadh.2019.102439
Available online 18 September 2019 0143-7496/ © 2019 Elsevier Ltd. All rights reserved.
a uniform stress distribution, and also join these complex shaped structures [3]. Both the adhesive and resin matrix of CFRP in adhesively bonded CFRP/aluminum alloy joints are polymer materials, the mechanical performance of which are affected significantly by variable temperatures [4]. Besides, the physical and chemical properties of adhesive and resin matrix of CFRP may degrade at different temperatures during a long-time service. The typical characteristic of physical aging may be the changes in molecular conformation of material without changing the structural integrity of the molecules [5]. While chemical aging may be the changes of molecular structure caused by chain scission, oxidation, depolymerization and changes in crosslink density [6,7]. Thus the physical aging is reversible and chemical aging is irreversible, but the two types of aging occur simultaneously in most cases. Carbas [8] studied the effects of post-curing and cure temperature on the glass transition temperature Tg, and the mechanical properties of epoxy adhesives, and found that Tg and mechanical properties depend on the
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degree of crosslinking and post-cure temperatures. Anderson [6] investigated the thermal degradation of two high-temperature epoxy adhesives at 195–250 °C and results show that the adhesive strength and weight decrease with the increase of temperature and aging time, indicating the scission of chemical bonds in adhesive when exposed to high temperature. Wolfrum [7] exposed two commercially available CFRP to heat above the maximum operating temperature at various durations. Mass loss, loss of mechanical strength, and changes in fracture behavior were observed and the decrease of adhesion strength of fiber/matrix interface was also found. Therefore, apart from the resin matrix of CFRP, the fiber/matrix interface is sensitive to environmental temperature because the heterogeneous nature and the very dissimilar expansion/contraction behavior between fiber and matrix can easily cause thermal stress. Thermal cycling is one of the typical service conditions of automobiles because the service temperature changes periodically as a result of the day-night cycle and climatic variation. Due to the strong differences in the thermal expansion coefficient of adhesive and substrate, the adhesive joint subjected to thermal cycling undergoes cycles of high stress at low temperature and low stress at high temperature. The residual stresses in the polymeric layer are generated over time as a result of viscoelastic response, which leads to damage and debonding, thus reducing bond durability [9]. The durability of adhesive joints in thermal cycling condition applied to the automotive industry has become a great concern and some related studies have been carried out by researchers. For instance, Hu [10]studied the degradation process of single lap steel/aluminum alloy joints subjected to cyclic-temperature (−30–80 °C) environment and a numerical approach was proposed to predict the joint mechanical behavior after environmental exposure. Li [11] investigated the long-term cyclic temperature exposure on adhesively bonded T-joints of steel and aluminum alloy and found that the ultimate load degraded gradually. Zhang [12] also studied the durability of adhesively-bonded single lap–shear joints with galvanized steel and aluminum alloy in a cyclic temperature environment. In summary, most scholars focused on the adhesive joints of steel and aluminum alloy subjected to thermal cycling, and little research was found in the degradation of composite adhesive joints. The bonding structures for automotive applications often have complex shapes and always bear a wide variety of loads, such as tension, torsion and bend caused by flexural deformation and vibration. Besides, CFRP composites are anisotropic materials with complex failure modes when subjected to external loads, such as fiber tear, delamination, matrix cracking and mixed failure. Therefore, the types of loads have to be taken into consideration in the failure mechanism study of CFRP adhesive joints. The physical, chemical and mechanical properties of adhesive and CFRP may change in the cycling thermal environment and their performance changes can affect the failure of CFRP adhesive joints. This means that the degree of degradation of CFRP adhesive joints is determined collectively by adhesive and CFRP. In this study, the degradation mechanism of adhesively bonded CFRP/ aluminum alloy joints subjected to thermal cycling was investigated. The chemical changes of adhesive and CFRP were tested by FTIR, DSC and TG/DTG to show the effect of thermal cycling on the composition, Tg and thermal stability. Mechanical tests were also used to investigate the performance changes of adhesive and CFRP caused by thermal cycling. The failure strength and failure modes of adhesively bonded CFRP/aluminum joints under different stress states before and after thermal cycling were analyzed. The degradation mechanism of adhesively bonded CFRP/aluminum joints was revealed according to the comparative analysis of adhesive, CFRP and CFRP/aluminum joints.
Table 1 Material properties of 6005A aluminum alloy [14]. Material
Young's modulus (MPa)
Poisson's ratio
Density(kg/m3)
6005A
72000
0.32
2700
Table 2 Properties of the adhesive Araldite® 2015 [13]. Property Young's modulus, E [GPa] Poisson's ratio, ν Tensile failure strength, σf [MPa] Tensile failure strain, εf [%]
1.85 ± 0.21 0.33 21.63 ± 1.61
Shear modulus, G [GPa] Shear failure strength, τf [MPa] Shear failure strain, γf [%]
0.56 ± 0.21 17.9 ± 1.8
4.77 ± 0.15
43.9 ± 3.4
this study, aluminum alloy 6005A, CFRP and adhesive Araldite® 2015 were selected to fabricate adhesively bonded CFRP/aluminum alloy shear joints and butt joints. The material properties of aluminum alloy 6005A are shown in Table 1. Adhesive Araldite® 2015 is a two component epoxy paste adhesive with excellent performances, such as good impact resistance, high shear and peel strength, so it has been used in practical engineering and research [10,13]. The mechanical properties of the adhesive are shown in Table 2. The CFRP was provided by Dongguan LiWeiSheng Carbon Fiber Products Technology Co., LTD. According to the supplier, the CFRP laminate was made from twill weave and unidirectional pre-preg, the fiber and epoxy resin of which were T300 and YPH-23 respectively. The hot pressing process was used and the process parameters were as follows, curing temperature 130 °C, pressure magnitude 0.1 MPa and curing time 100 min. The thickness of the woven fabric and the unidirectional prepreg was approximately 0.5 mm and 0.25 mm respectively and the whole thickness of CFRP laminate was about 2 mm. The outer layers of CFRP laminate were woven fabrics while the internal layers were unidirectional prepregs and the layup was [(0/90)/0/90/0/90(0/90)]. Material properties of unidirectional-CFRP and twill weave-CFRP are shown in Table 3.
2.2. Specimens manufacture 2.2.1. Bulk specimens Bulk specimens of adhesive Araldite® 2015 was fabricated to investigate the mechanical performances of adhesive according to NF ISO 527-2 standards. The shapes and dimensions of bulk specimens are shown in Fig. 1 and the section was 2 × 10 mm2 A stainless steel mould was designed and manufactured, shown in Fig. 2 (a and b). The mould consists of three parts: (1) the base is used to support the other parts, (2) the middle is the core that determines the shapes and dimensions of bulk specimens, (3) the top is applied to close the mould with screws. The bulk specimens were manufactured in the condition of 25 ± 2 °C/ 50% RH and the process is as follows: a type of mould release was applied to the inner surface of the mould and then cleaned by acetone. A static mixer nozzle (Fig. 2 c) was used to dispense and mix the two components of adhesive, and apply the mixed adhesive to the mould. After that the three parts were fitted together with screws. The bulk specimens cured in the mould at room temperature for 24 h, and then Table 3 Material properties of unidirectional-CFRP and twill weave-CFRP.
2. Experimental work 2.1. Material
Unidirectional-CFRP Twill weave-CFRP
CFRP and Aluminum alloy are increasingly used in automobiles. In 2
Ex (GPa)
Ey (GPa)
Gxy (GPa)
νxy
125 ± 12 55 ± 5
10 ± 2 55 ± 5
7 ± 0.6 4 ± 0.5
0.07 0.14
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Fig. 1. Geometry and dimensions of a bulk specimen. Fig. 3. Geometry and dimensions of butt joints.
were removed from the mould and cured at 80 °C for 2 h in an electrothermostatic fan oven.
2.2.2. Adhesive joints Butt and shear joints were designed to investigate the degradation mechanism of adhesively bonded CFRP/aluminum alloy under normal and shear stress state respectively. The butt joints are shown in Fig. 3. There are two layers of adhesive on both sides of the CFRP plate. Two holes are set in the two sides of the butt joint to connect to the universal tensile testing machine. The bonding area is 25 × 25mm2 and bondline thickness is 0.2 mm. Both length and width of the CFRP plate (35 × 35 mm2) are larger than the bonding area (25 × 25 mm2) to avoid fiber tear from the edges. Therefore, the whole dimension of the butt joint is 202.4 × 35 × 35 mm3. The experimental schematic diagram of a butt joint under normal stress is shown in Fig. 4(a). To simplify the adhesive joints fabrication, a butt joint was also used to test the failure of adhesive joints under shear stress with a stainless steel auxiliary testing equipment as shown in Fig. 4(b). The assembled butt joint and auxiliary testing equipment formed a shear joint. Although the stress state of adhesive joints is combined shear and normal near free edges, and there also exist significant stress concentrations, simulation analysis found that the main stress in the adhesive layer of shear and butt joints is shear and normal stress respectively [15]. Therefore, shear and butt joints are assumed to under pure shear and pure normal stress respectively in engineering. As shown in Fig. 5(a), a bonding fixture was designed to manufacture adhesive joints. The bonding fixture is composed of three main parts: (a) a base with two concentric grooves used to support adhesive joints, and ensure the concentricity of the two aluminum alloy rods; (b) battens that fix the adhesive joints with screw nuts; (c) a bolt knob positioned on the right side of the base used to supply pressure and control the thickness of the adhesive layer via 0.2-mm-diameter glass balls. The manufacturing process is as follows: First, the adhesive surface of aluminum alloy rods was treated by sand blasting with 80# white fused alumina to achieve elegant appearance. Then the adhesive surface of aluminum alloy rods and CFRP were cleaned by acetone. Araldite® 2015 adhesive was applied to the substrate's surface and adhesive joints were bonded using the bonding fixtures. After that almost all spew fillets of adhesive were cleared before the adhesive was fully cured. Finally, the adhesive joints (shown in Fig. 5b) were disassembled from the bonding fixture after curing at room temperature
Fig. 4. Experimental schematic diagrams of (a) butt joints and (c) shear joints.
for 24 h and then were cured at 80 °C for 2 h in the electro-thermostatic fan oven. 2.3. Experimental tests 2.3.1. Thermal cycling exposure To simulate the thermal cycling environment in the service condition of automobiles, a spectrum of temperature variation shown in Fig. 6 was designed based on the standard “VW PV 1200 Climate test – Alternating climate temperature test with controlled humidity”. There are four stages in one thermal cycling: (I) heating stage, the temperature rises from −40 °C to 80 °C in 2 h; (II) high temperature stage, the temperature is 80 °C for 4 h; (III) cooling stage, the temperature
Fig. 2. (a) Schematic diagram and (b) real object of the mould, (c) the static mixer nozzle. 3
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Fig. 5. (a) The bonding fixture and (b) the adhesive joint.
cured bulk specimens of adhesive were exposed in the thermal cycling for 30 days and then tested at room temperature, as shown in Fig. 7(a). The CFRP plates (35 × 35 × 2 mm3) was degraded first in the thermal cycling for 30 days and then the degraded CFRP were bonded with aluminum alloy and adhesive Araldite® 2015 to manufacture adhesive joints, after that the adhesive joints were tested at room temperature. The effect of degraded CFRP on adhesive joints was obtained from the test shown in Fig. 7(b) because only the CFRP was degraded. From Fig. 7(c), the adhesively bonded CFRP/aluminum alloy joints were exposed in the thermal cycling for 10, 20 and 30 days and then were tested at room temperature. 2.3.2. Fourier transform infrared (FTIR) analysis FTIR was conducted by using a VERTEX70-Bruker spectrometer to investigate the compositional variation of adhesive Araldite® 2015 and CFRP after thermal cycling. Four samples were taken for each test and the weight was about 10 ± 0.2 mg. The spectrum was obtained by ATR (Attenuated Total Reflection) using 128 scans and spectra from the surface of adhesive and CFRP sheets were scanned from 4000-600cm−1 with a resolution of 4 cm−1.
Fig. 6. Spectrum of temperature variation in one thermal cycling.
declines from 80 °C to −40 °C in 2 h; (IV) low temperature stage, the temperature is −40 °C for 4 h; It takes 12 h to complete one thermal cycling and the relative humidity is below 20% during the whole aging process to minimize moisture influence on the experimental results. The thermal cycling was conducted in an environmental chamber WSHW080BF provided by Jiaxing WEISS Experiment Equipment CO., Ltd (Jiaxing, China). Both the adhesive and CFRP in the adhesive joints experienced thermal cycling and their performance changes had effect on the adhesive joints. To investigate the degradation mechanism of adhesively bonded joints, the bulk specimens of adhesive, CFRP plates and adhesively bonded joints were exposed in the thermal cycling environment and the corresponding schematic diagrams were shown in Fig. 7. The
2.3.3. Differential scanning calorimetry (DSC) analysis Tg of adhesive Araldite® 2015 and CFRP before and after thermal cycling was measured using a PerkinElmer DSC7. The DSC measurements were performed with standard aluminum pans and lids under nitrogen atmosphere at a 5 °C/min heating/cooling rate. The samples of adhesive and CFRP were taken from the bulk specimens and CFRP plates respectively. Four samples were taken for each test and all the samples included the surface and core. The sample weight was approximately 20 ± 0.2 mg. The first heating run was carried out to remove the thermal history of the samples because Tg is influenced significantly by the thermal history, such as post-cure temperature and its duration [16]. Tg was obtained From the second heating run.
Fig. 7. Schematic diagrams: effect of thermal cycling on (a) bulk specimens, (b) CFRP, (c) adhesive joints. 4
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Fig. 8. The VIC-3D™ measurement system.
2.3.4. Thermal gravimetric analysis (TG)/Differential thermal gravimetric (DTG) TG/DTG was carried out on a TGA/DSC1 METTLER TOLEDO STARe System by heating adhesive Araldite® 2015 and CFRP contained in alumina pans at a rate of 10 °C/min from 25 to 700 °C in a nitrogen atmosphere to investigate the changes of thermal stability. Four samples were taken for each test and the weights were about 20 ± 0.2 mg. The instrument records the weight and the weight loss rate with the temperature increment. 2.3.5. Tensile tests of bulk specimens The bulk specimens of Araldite® 2015 before and after 30 days’ thermal cycling were tested at room temperature by an electronic universal testing machine at a constant speed of 2 mm/min. The VIC3D™ Measurement System shown in Fig. 8 was applied to measure the displacement and strain of the bulk specimens during the quasi-static tests. The system serves as a non-contact measurement tool for measuring full-field displacement and strain via utilizing Digital Image Correlation (DIC), which mainly includes lights, cameras and software system. The system supports a field of view (FOV) of between 0.8 and 7 mm with strain measurement levels ranging from 0.005% (50 microstrain) to 2000%. The system had to be used cooperatively with an electronic universal testing machine and the test of bulk specimens was conducted as follows: the gauge length of the bulk specimen was set to 50 mm and was speckled with an idler wheel; then it was fixed into the universal testing machine. Two cameras were then installed and calibrated to better visualize the bulk and pictures were collected during the quasi-static tests. Finally, the displacements and strain of the bulk were obtained after analyzing the pictures via the software system. Four specimens were tested for each test to ensure data validity and the results were averaged.
Fig. 9. Representative FTIR spectra of adhesive Araldite® 2015. Table 4 Tentative assignment of major bands in the spectrum of adhesive [18,19].
2.3.6. Tensile tests of adhesive joints The same electronic universal testing machine mentioned above was used to conduct quasi-static tests of shear and butt joints. To eliminate the non-axial forces, both ends of the specimens were connected to the testing machine through the cross universal joints. Four specimens were tested for each test and the failure loads were obtained from the load-displacement curves recorded by the electronic universal testing machine.
Positions (cm−1)
Assignment
3328 2923,2850 1736 1648 1604 1509 1452 1293 1237 1098, 1072, 1035 827
-OH, –NH stretching -CH3, and –CH2stretching C=O N–H bending Quadrant stretching of the benzene ring Semicircle stretching of p-disubstituted benzene C–H bending of aliphatic groups Twisting mode of –CH2- groups Stretching mode for aromatic ether Stretching of the trans forms of the ether linkage Stretching C–O–C of oxirane group
for the main absorption that appears in the spectrum and changes in intensities or positions of absorption peaks can provide information on specific interactions involved during thermal cycling. The broad peak at 3328 cm−1 is most likely related to O–H and N–H stretching and intermolecular hydrogen bonds. Peaks in the region from 2923 to 2850 cm−1 are related to C–H stretching of CH3 and CH2 groups. Another feature of the adhesive is the appearance of peaks from 1035 to 1098 cm−1 and 1736 cm−1associated to the stretching of C–O–C and C]O respectively, which are useful for identification of epoxy adhesive. From Fig. 9, it can be seen that the absorption intensity of some bonds obviously changed. Although most of the absorption intensities fluctuated before and after thermal cycling, the major difference was the obvious increase in the intensity signal of ether at 1072 cm−1 of the aged adhesive. It can be inferred that the adhesive Araldite® 2015 may cure again in the thermal cycling environment. The curing process of epoxy adhesive is the set of chemical reactions that leads to the
3. Experimental results and discussions 3.1. Effect of thermal cycling on bulk specimens 3.1.1. FTIR results of the adhesive The unaged and aged bulk specimens were analyzed by FTIR to understand the degradation mechanism of adhesive. FTIR spectra of the unaged and aged adhesive are shown in Fig. 9 and some absorption peaks were observed in the range between 4000 cm−1 and 600 cm−1. The spectral subtraction in Fig. 9 was obtained by removing the unaged spectrum from the aged spectrum. Table 4 gives a tentative assignment 5
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Fig. 10. (a) Representative DSC results of adhesive Araldite® 2015 and (b) variations of Tg.
formation of a highly cross-linked network and during the chemical reaction, both molecular weight and polydispersity increase until one single macromolecule is formed. As a result of the curing reaction, the epoxide (oxirane) ring decreases, while the hydroxyl and ether (C–O–C stretch) band increase. Therefore, the rate of polymerization of epoxy adhesive can be estimated by the loss of epoxide or the increase in hydroxyl and ether absorptions. Unlike the bands mentioned above that take part in the chemical reaction, phenyl bands at 1508 cm−1 are present and remain constant [17]. Thus the absorption peak at 1508 cm−1 is used as a reference to normalize the FTIR spectra of the adhesive before and after thermal cycling [17]. The results presented in Fig. 9 are the spectra after normalization. According to the curing reaction and the increase in the ether at 1072 cm−1, it is concluded that the post-curing of the adhesive Araldite® 2015 occurred during the thermal cycling.
four phases, but the first peak of DTG curve at 174.1 °C disappeared after thermal cycling and there were only three phases in the aged adhesive. The temperature value of the first and second peak of aged DTG curve is almost the same with the second and third peak of the unaged DTG curve, but the temperature of the third peak of the aged adhesive was obviously higher than the fourth peak of the unaged. The disappearance of the first peak and the lag of the fourth peak of the DTG curve after thermal cycling indicated that the thermal stability of adhesive Araldite® 2015 improved, especially at low and high temperature phase. From the TG curves, it was obvious that the weight loss of the aged adhesive was lower than the unaged at any temperature between 25 °C and 800 °C, suggesting that the formation of chemical bonds when subjected to thermal cycling. All the test results of chemical analysis (FTIR, DSC, TG/DTG) illustrated that the post-curing of adhesive occurred in the thermal cycling.
3.1.2. DSC results of the adhesive The DSC thermograms of adhesive Araldite® 2015 before and after thermal cycling are shown in Fig. 10. To determine the value of Tg, straight lines were extended along the left- and right-hand branches of the heat flow curve. Tg was obtained from the point of intersection between the bisecting line of the angle and the measured curve. It is found that the Tg of adhesive Araldite® 2015 increased from 65.8 °C to 68.3 °C after thermal cycling, indicating that the curing degree improved. Generally speaking when heating above Tg, the curing reaction is reactivated and Tg increases again, which is due to the fact that vitrification is a reversible process. The temperature in the high temperature stage of thermal cycling was 80 °C, which was higher than the Tg(65.8 °C), resulting in the reactivation of post-curing and improvement of Tg to 68.3 °C.
3.1.4. Test results of bulk specimens To investigate the effect of thermal cycling on the mechanical performances of adhesive, the bulk specimens before and after 30 days' thermal cycling were tested and the stress-strain curves are shown in Fig. 12(a). The failure strength, Young's modulus and failure strain of bulk specimens were calculated from the stress-strain curves. For the unaged adhesive, the failure strength, Young's modulus and failure strain were 22.3 MPa, 1826 MPa and 4.77%, respectively, which were almost equivalent with the mechanical performances found in Ref. [13]. It suggested that the test method of the VIC-3D™ Measurement System in this study was able to satisfy the measuring accuracy. Compared with the stress-strain curve of the unaged bulk specimen, the failure strength and Young's modulus improved, while the failure strain decreased after thermal cycling. To clearly show the performance changes of the adhesive before and after thermal cycling, the mechanical performances of the bulk specimens were normalized by the unaged, as shown in Fig. 12(b). Compared with the unaged bulk specimens, the failure strength of the aged increased by 25.4% and Young's modulus increased by 21.6%, the failure strain decreased by 29.8%, indicating the extent of these changes was at roughly equal levels. The changes in mechanical properties of bulk specimens demonstrated the post-curing of adhesive.
3.1.3. TG/DTG results of the adhesive TG/DTG analysis has been an important measurement to determine material degradation kinetics under accelerated thermal aging, which can be applied to estimate the adhesive life because degradation of the thermoset network occurs through the scission of chemical bonds. As chemical bonds are broken, volatile species are released to cause a loss of mass. Thermal stability of adhesive Araldite® 2015 was analyzed by TG/DTG and the weight loss curve (TG) and the weight loss derivative curve (DTG) were recorded as a function of temperature, which is shown in Fig. 11. The process of weight loss is divided into several phases according to the peak number of weight loss derivative curve. It can be seen that the weight loss of the unaged adhesive occurred via
3.2. Effect of thermal exposure on CFRP 3.2.1. FTIR results of CFRP To investigate the effect of thermal cycling on the composition of 6
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Fig. 11. Representative (a) TG and (b) DTG thermograms of the adhesive Araldite® 2015.
CFRP, the unaged and aged CFRP were analyzed by FTIR and the result is shown in Fig. 13. The spectral subtraction in Fig. 13 was obtained by removing the unaged spectrum from the aged spectrum. The absorption peaks of CFRP were almost the same with adhesive Araldite® 2015 because they were epoxy and have similar bonds. The specific absorption peak of CFRP is the appearance of 2167 cm−1 related to nitrile group. Most of the bond intensities decreased after thermal cycling except the absorption peak at 1736 cm−1, which was caused by the oxidation of CFRP with oxygen in the air when subjected to the hightemperature stage in the thermal cycling [20]. The oxidizing reaction of CFRP means the bond scission and the degradation and may influence other chemical and mechanical performances. Compared with the adhesive Araldite® 2015, the variation degree of bond intensities of CFRP was low, which suggested that the effect of thermal cycling on the CFRP is less obvious. This is due to the fact that the curing temperature of CFRP was greater than 80 °C.
3.2.2. DSC results of CFRP The DSC thermograms of CFRP before and after thermal cycling are shown in Fig. 14. The Tg of the unaged and aged CFRP was 122.4 and 121.8 °C, indicating that the Tg of CFRP decreased a little by thermal cycling. This was caused by the oxidizing reaction of CFRP. The Tg of CFRP decreased by about 0.6 °C, while the Tg of adhesive Araldite® 2015 increased by about 2.5 °C. It can be concluded that the Tg of adhesive Araldite® 2015 was more sensitive to thermal cycling than CFRP, which was due to the fact that the high temperature stage (80 °C) in the thermal cycling was higher than the Tg of adhesive Araldite® 2015, but it was lower than the Tg of CFRP. The varying degree of Tg is in
Fig. 13. Representative FTIR spectra of CFRP.
accordance with FTIR results. 3.2.3. TG/DTG results of CFRP TG/DTG thermograms of CFRP are shown in Fig. 15. There was only one peak in the DTG curves whether the CFRP was aged or not, and the temperature position and height of the two peaks were almost the same. From the TG curves, the weight loss of the aged and unaged CFRP showed no obvious difference at temperatures below 450 °C, but the
Fig. 12. Effect of thermal cycling on the bulk specimens of adhesive: (a) representative stress-strain curves, (b) normalized mechanical performances. 7
International Journal of Adhesion and Adhesives 95 (2019) 102439
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Fig. 14. (a) Representative DSC results of CFRP and (b) variations of Tg.
Fig. 15. Representative (a) TG and (b) DTG thermograms of CFRP.
weight loss of the aged CFRP was higher when the temperature exceeded 450 °C. The thermal degradation of aged and unaged CFRP took place through one step, but the thermal stability decreased after thermal cycling. As with the FTIR and DSC results, the effect of thermal cycling on the stability of adhesive was more obvious than CFRP.
3.2.4. Effect of aged CFRP on the failure strength To investigate the performance changes of CFRP after thermal cycling, the CFRP plates were exposed to thermal cycling first and then were bonded with aluminum alloy and adhesive Araldite® 2015 to fabricate shear and butt joints. The failure strength of shear and butt joints as shown in Fig. 16 were called shear strength and butt tensile strength. In consideration of the surface oxidation of CFRP that may affect the adhesion characteristic, the aged CFRP were treated by two types of surface treatment methods, namely no abrading and abrading with #80 sandpaper [20]. The shear strength of aged CFRP without abrading decreased by about 16.5%, but it almost recovered to the level of unaged CFRP when the aged CFRP was abraded by #80 sandpaper. The butt tensile strength of aged CFRP without abrading had a similar decrease degree, but it did not recover after abrading with #80 sandpaper. It is concluded that the failure strength of aged CFRP without abrading decreased to the same extent, but the surface treatment had a different effect on the adhesive joints under different stress states.
Fig. 16. Failure strength of shear and butt joints bonded with unaged and aged CFRP.
3.2.5. Effect of aged CFRP on the failure surfaces To investigate the failure mechanism of shear and butt joints bonded with aged CFRP, the failure modes were analyzed. The representative fracture surfaces of shear and butt joints were shown in Fig. 17 and Fig. 18 respectively. The failure mode of shear joints with unaged CFRP was a cohesive 8
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Fig. 17. Representative fracture surfaces of shear joints:(a) unaged, (b) aged CFRP without abrading, (c) aged CFRP abraded by #80 sandpaper.
fiber/matrix interface of CFRP degraded in the thermal cycling environment.
failure, but the failure mode of aged CFRP without abrading changed to mixed failure of cohesive and interface failure. The interface failure was located on the surface of CFRP as shown in the red box of Fig. 17 (b), which was the main cause for the decrease of failure strength. It is found that the thermal cycling reduced the surface adhesiveness of CFRP, which was caused by the oxidation of CFRP surface. When the aged CFRP was abraded by #80 sandpaper, the failure mode recovered to cohesive failure as shown in Fig. 17 (c). The appearance of cohesive failure suggested that the inside of CFRP hadn't been oxidized because it was not in direct contact with air. The surface oxidation layer of aged CFRP was removed by abrading and then the failure strength recovered to the level of the unaged. As for the butt joints, the failure mode of unaged CFRP was mixed failure of cohesive and fiber tear, as shown in Fig. 18(a). In contrast with the cohesive failure of shear joints, the fiber tear of butt joints was caused by normal stress. Cohesive failure, fiber tear and interface failure were shown in the fracture surfaces of aged CFRP without abrading. The interface failure was shown in the red box of Fig. 18(b) and the area of fiber tear decreased when compared with the unaged. This is due to the fact that the interface failure of aged CFRP without abrading reduced the failure strength of butt joints, which was unable to cause more fiber tear. From Fig. 18(c), it was found that the interface failure disappeared and the area of fiber tear increased after the abrading of aged CFRP. The area of fiber tear of aged CFRP abraded by #80 sandpaper was larger than that of the unaged because the mechanical performance of the fiber/matrix interface decreased in the thermal cycling due to the difference of thermal expansion between fiber and matrix. Although the abrading eliminated the interface failure of aged CFRP to improve the failure strength, the larger area of fiber tear reduced the failure strength sharply. To further investigate the effect of thermal cycling on the fiber/ matrix interface, SEM was applied to inspect the fiber tear before and after thermal cycling, as shown in Fig. 19(a) and (b). The fiber surface of unaged CFRP was covered with resin matrix as shown in the red box of Fig. 19(a), while there was no resin matrix on the fiber surface of aged CFRP as shown in the red box of Fig. 19(b). Besides, the space between fibers of unaged CFRP was filled with resin matrix and the fracture surface was rough as shown in the green box of Fig. 19(a), but the resin matrix was relatively less between the fibers, and the fracture surface was relatively smooth as shown in the green box of Fig. 19(b). All the changes in the fiber surface and resin matrix suggested that the fiber tear of unaged CFRP was mainly caused by the fracture of the resin matrix, but after thermal cycling, the fiber tear was mainly attributed to the debonding of fiber/matrix interface. It was concluded that the
3.3. Effect of thermal exposure on adhesive joints 3.3.1. Failure strength of adhesive joints The peak loads from the load-displacement curves were defined as the failure loads of adhesive joints and then were divided by the bonding area to obtain the failure strength, which was shown in Fig. 20. The failure strength of shear joints decreased by about 18.5% after 10 days' thermal cycling, but the decrease slowed down in the second 10day and it only decreased by 24.1% after 20 days’ aging. During the third 10-day, the failure strength showed a sharp decline by about 43.3%. The failure strength of butt joints in the thermal cycling had the similar downtrend with shear joints, namely the first and third 10-day significantly affected the failure strength, but the extent of decrease of butt joints was higher than shear joints, especially in the first and second 10-day. It can also be seen that the data discreteness of failure strength increased after thermal cycling. 3.3.2. Failure surfaces of adhesive joints The representative fracture surfaces of shear and butt joints were shown in Fig. 21 and Fig. 22 respectively. As shown in Fig. 21(a–c), the failure modes of unaged shear joints and after 10, 20 days' thermal cycling were a cohesive failure, indicating that the thermal cycling didn't change the failure modes. During the thermal cycling, the postcuing of adhesive improved the failure strength, but the thermal stress caused by the difference of thermal expansion coefficient between adhesive and adherend reduced the failure strength. The failure strength of shear joints decreased with the increase of aging time, which suggested that the effect of thermal stress was much more obvious than the post-cuing. When the shear joints were exposed to thermal cycling for 30 days, the mixed failure of cohesive and interface was seen in the fracture surfaces. The interface failure was located in the edges of the bonding area as shown in the red box of Fig. 21(d), which may be caused by the long-term impact of thermal stress. The appearance of interface failure exacerbated the decline of failure strength, resulting in the sharp fall in the third 10-day. Although the failure mode of unaged butt joints and after exposure in the thermal cycling for 10 and 20 days remained mixed failure of cohesive and fiber tear, the area of fiber tear decreased after 10 days and then increased after 20 days, as shown in the Fig. 22(a–c). From the above analysis, it was found that the failure strength of adhesive and fiber/matrix interface of CFRP decreased in thermal cycling. The
Fig. 18. Representative fracture surfaces of butt joints:(a) unaged, (b) aged CFRP without abrading, (c) aged CFRP abraded by #80 sandpaper. 9
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Fig. 19. Fiber tear from SEM: (a) unaged CFRP, (b) aged CFRP abraded by #80 sandpaper.
failure strength of fiber/matrix interface of CFRP declined more quickly. The effect of fiber tear on butt joints may be the reason why the failure strength of butt joints reduced faster than shear joints. The failure mode of butt joints changed to the mixed failure of cohesive and interface after 30 days’ thermal cycling and the characteristic was similar to shear joints. In conclusion, the performance changes of CFRP caused by thermal cycling almost did not affect the failure of shear joints, but it had an obvious effect on the failure of butt joints. In other words, the shear joints subjected to thermal cycling was mainly influenced by the thermal stress and post-curing of adhesive and interface failure, while the butt joints were also affected by the fiber/matrix interface of CFRP. 4. Conclusions Chemical analysis (FTIR, DSC and TG/DTG) and mechanical tests were applied to investigate the effect of thermal cycling on the thermal behavior of adhesive, CFRP and adhesively bonded CFRP/aluminum alloy joints. The degradation mechanism of CFRP/aluminum alloy joints was analyzed based on the performance changes of adhesive and CFRP. The following conclusions are drawn:
Fig. 20. Failure strength of shear and butt joints after different aging time.
decrease of fiber tear after 10 days suggested that the failure strength of adhesive fell more rapidly than fiber/matrix interface of CFRP, while the subsequent increase of fiber tear after 20 days indicated that the
(a) The changes of FTIR spectra and the improvements of Tg and thermal stability showed that the thermal behavior of adhesive was caused by a post-curing process. As a result, the failure strength and
Fig. 21. Representative fracture surfaces of shear joints after different aging time: (a) unaged, (b) 10 days, (b) 20 days, (b) 30 days. 10
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Fig. 22. Representative fracture surfaces of butt joints after different aging time: (a) unaged, (b) 10 days, (b) 20 days, (b) 30 days.
Young's modulus of the bulk adhesive increased by more than 20% after 30 days' thermal cycling, while the failure strain decreased by about 30%. (b) The CFRP surface was oxidized, and Tg and thermal stability decreased in the thermal cycling. Besides, the effect of thermal cycling on the chemical properties of CFRP was less than adhesive. The surface oxidation of CFRP can cause interface failure and the decrease of mechanical performances of the fiber/matrix interface resulted in the larger area of fiber tear. (c) The failure strength of shear joints subjected to thermal cycling for 30 days decreased by about 43.3%, which was mainly caused by the combined effect of thermal stress, post-curing and interface failure. The failure strength of butt joints had the similar downtrend with shear joints, but the extent of decrease was higher than shear joints, which was caused by the thermal stress, post-curing, interface failure and fiber tear of CFRP. (d) The failure mode of shear joints changed from cohesive failure to mixed failure of cohesive and interface after thermal cycling, while the failure mode of butt joints changed from mix failure of cohesive and fiber tear to mixed failure of cohesive and interface. The performance changes of CFRP caused by thermal cycling almost did not affect the failure of shear joints, but it had an obvious effect on the failure of butt joints.
Berlin, Heidelberg: Springer Berlin Heidelberg; 2018. [4] Avendaño R, Carbas RJC, Marques EAS, et al. Effect of temperature and strain rate on single lap joints with dissimilar lightweight adherends bonded with an acrylic adhesive. Compos Struct 2016;152:34–44. [5] Odegard GM, Bandyopadhyay A. Physical aging of epoxy polymers and their composites. J Polym Sci B Polym Phys 2011;49(24):1695–716. [6] Anderson BJ. Thermal stability of high temperature epoxy adhesives by thermogravimetric and adhesive strength measurements. Polym Degrad Stab 2011;96(10):1874–81. [7] Wolfrum J, Eibl S, Lietch L. Rapid evaluation of long-term thermal degradation of carbon fibre epoxy composites. Compos Sci Technol 2009;69(3–4):523–30. [8] Carbas RJC, Da Silva LFM, Marques EAS, et al. Effect of post-cure on the glass transition temperature and mechanical properties of epoxy adhesives. J Adhes Sci Technol 2013;27(23):2542–57. [9] Robert Humfeld Jr. G, Dillard DA. Residual stress development in adhesive joints subjected to thermal cycling. J Adhes 1998;65(1–4):277–306. [10] Hu P, Han X, Da Silva LFM, et al. Strength prediction of adhesively bonded joints under cyclic thermal loading using a cohesive zone model. Int J Adhesion Adhes 2013;41:6–15. [11] Li W, Pang B, Han X, et al. Predicting the strength of adhesively bonded T-joints under cyclic temperature using a cohesive zone model. J Adhes 2016;92(11):892–907. [12] Zhang F, Yang X, Wang HP, et al. Durability of adhesively-bonded single lap–shear joints in accelerated hygrothermal exposure for automotive applications. Int J Adhesion Adhes 2013;44:130–7. [13] da Silvaa LFM, da SILVAA RAM, Chousala JAG, et al. Alternative methods to measure the adhesive shear displacement in the thick adherend shear test. J Adhes Sci Technol 2008;22(1):15–29. [14] Simar A, Nielsen KL, de Meester B, et al. Micro-mechanical modelling of ductile failure in 6005A aluminium using a physics based strain hardening law including stage IV. Eng Fract Mech 2010;77(13):2491–503. [15] Qin G, Na J, Tan W, et al. Failure prediction of adhesively bonded CFRP-Aluminum alloy joints using cohesive zone model with consideration of temperature effect. J Adhes 2019;95(8):723–46. [16] Zhang Y, Adams RD, da Silva LFM. Effects of curing cycle and thermal history on the glass transition temperature of adhesives. J Adhes 2014;90(4):327–45. [17] Smith RE, Larsen FN, Long CL. Epoxy resin cure. II. FTIR analysis. J Appl Polym Sci 1984;29(12):3713–26. [18] Barbosa APC, Fulco APP, Guerra ESS, et al. Accelerated aging effects on carbon fiber/epoxy composites. Compos B Eng 2017;110:298–306. [19] González MG, Cabanelas JC, Baselga J. Applications of FTIR on epoxy resinsidentification, monitoring the curing process, phase separation and water uptake. Infrared Spectrosc. Materials Science, Engineering and Technology. IntechOpen; 2012. [20] Buch X, Shanahan MER. Influence of the gaseous environment on the thermal degradation of a structural epoxy adhesive. J Appl Polym Sci 2000;76(7):987–92.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No.51775230). References [1] Friedrich K, Almajid AA. Manufacturing aspects of advanced polymer composites for automotive applications. Appl Compos Mater 2013;20(2):107–28. [2] Girubha J, Vinodh S, Kek V. Application of interpretative structural modelling integrated multi criteria decision making methods for sustainable supplier selection. J Model Manag 2016;11(2):358–88. [3] da Silva LFM, Chsner AO, Adams RD. Handbook of adhesion technology. second ed.
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Contents lists available at ScienceDirect
International Journal of Adhesion and Adhesives journal homepage: www.elsevier.com/locate/ijadhadh
Effect of thermal cycling on the degradation of adhesively bonded CFRP/ aluminum alloy joints for automobiles
T
Guofeng Qina,b,∗, Jingxin Nab, Wenlong Mub, Wei Tanb a b
Teachers College for Vocational and Technical Education, Guangxi Normal University, Guilin, 541004, PR China State Key Laboratory of Automotive Simulation and Control, Jilin University, Changchun, 130021, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Epoxy adhesive CFRP Thermal cycling Chemical analysis Mechanical testing
Thermal cycling is one of the representative service environments of automobiles. As epoxy adhesive and resin matrix of CFRP (Carbon Fiber Reinforced Plastics) composites are polymer materials, their properties may change during long-term thermal cycling and thus affect the joint strength of adhesively bonded composites joints. To investigate the degradation mechanism of adhesively bonded CFRP/aluminum alloy joints subjected to thermal cycling, firstly, the chemical transformations of adhesive and CFRP were analyzed by FTIR (Fourier Transform Infrared Spectroscopy), DSC (Differential Scanning Calorimetry) and TG/DTG (Thermal Gravimetric/ Differential Thermal Gravimetric), and changes in mechanical properties of adhesive and CFRP were also tested by quasi-static tests. Then the variations of failure strength and failure modes of adhesively bonded CFRP/ aluminum alloy shear joints and butt joints after different aging time were investigated. Results show that the Tg (glass transition temperature), thermal stability, failure strength and Young's modulus of adhesive improved after thermal cycling because of post-curing. The Tg and thermal stability of the CFRP decreased due to surface oxidation, resulting in the decline of surface adhesiveness of CFRP and the appearance of interface failure. Besides, the mechanical performance of the fiber/matrix interface also declined after thermal cycling, which was verified by SEM (Scanning Electron Microscope). The failure strength of shear and butt joints dropped by more than 40% after thermal cycling for 30 days. The degradation of shear joints was mainly caused by the combined effect of thermal stress and post-curing of adhesive as well as interface failure, while the butt joints were also affected by the fiber tear of CFRP.
1. Introduction Two major challenges of automotive industry are energy consumption and exhaust emissions, which force researchers and engineers to find solutions. Weight reduction is one of the most efficient ways to reduce fuel demand and corresponding emissions. In order to achieve that goal, composites and lighter alloys with high modulus and strength are applied to replace the traditional steel components, such as body panels and load-bearing parts [1]. Besides, multi-material applications according to their performance characteristics not only significantly reduce the weight, but also control costs effectively [2]. Therefore, joining technology for all kinds of materials is becoming the key to manufacturing lightweight cars. Traditional joining methods (such as bolts and rivets) always destroy the integrity of the structures, resulting in the reduction of load capacity and early failure, especially for the CFRP composites. Adhesive joints are widely used for connecting composite structures because they preserve the structural integrity and
∗
Corresponding author. NO.15Yucai Road, Guilin, Guangxi, 541004, PR China. E-mail address: [email protected] (G. Qin).
https://doi.org/10.1016/j.ijadhadh.2019.102439
Available online 18 September 2019 0143-7496/ © 2019 Elsevier Ltd. All rights reserved.
a uniform stress distribution, and also join these complex shaped structures [3]. Both the adhesive and resin matrix of CFRP in adhesively bonded CFRP/aluminum alloy joints are polymer materials, the mechanical performance of which are affected significantly by variable temperatures [4]. Besides, the physical and chemical properties of adhesive and resin matrix of CFRP may degrade at different temperatures during a long-time service. The typical characteristic of physical aging may be the changes in molecular conformation of material without changing the structural integrity of the molecules [5]. While chemical aging may be the changes of molecular structure caused by chain scission, oxidation, depolymerization and changes in crosslink density [6,7]. Thus the physical aging is reversible and chemical aging is irreversible, but the two types of aging occur simultaneously in most cases. Carbas [8] studied the effects of post-curing and cure temperature on the glass transition temperature Tg, and the mechanical properties of epoxy adhesives, and found that Tg and mechanical properties depend on the
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degree of crosslinking and post-cure temperatures. Anderson [6] investigated the thermal degradation of two high-temperature epoxy adhesives at 195–250 °C and results show that the adhesive strength and weight decrease with the increase of temperature and aging time, indicating the scission of chemical bonds in adhesive when exposed to high temperature. Wolfrum [7] exposed two commercially available CFRP to heat above the maximum operating temperature at various durations. Mass loss, loss of mechanical strength, and changes in fracture behavior were observed and the decrease of adhesion strength of fiber/matrix interface was also found. Therefore, apart from the resin matrix of CFRP, the fiber/matrix interface is sensitive to environmental temperature because the heterogeneous nature and the very dissimilar expansion/contraction behavior between fiber and matrix can easily cause thermal stress. Thermal cycling is one of the typical service conditions of automobiles because the service temperature changes periodically as a result of the day-night cycle and climatic variation. Due to the strong differences in the thermal expansion coefficient of adhesive and substrate, the adhesive joint subjected to thermal cycling undergoes cycles of high stress at low temperature and low stress at high temperature. The residual stresses in the polymeric layer are generated over time as a result of viscoelastic response, which leads to damage and debonding, thus reducing bond durability [9]. The durability of adhesive joints in thermal cycling condition applied to the automotive industry has become a great concern and some related studies have been carried out by researchers. For instance, Hu [10]studied the degradation process of single lap steel/aluminum alloy joints subjected to cyclic-temperature (−30–80 °C) environment and a numerical approach was proposed to predict the joint mechanical behavior after environmental exposure. Li [11] investigated the long-term cyclic temperature exposure on adhesively bonded T-joints of steel and aluminum alloy and found that the ultimate load degraded gradually. Zhang [12] also studied the durability of adhesively-bonded single lap–shear joints with galvanized steel and aluminum alloy in a cyclic temperature environment. In summary, most scholars focused on the adhesive joints of steel and aluminum alloy subjected to thermal cycling, and little research was found in the degradation of composite adhesive joints. The bonding structures for automotive applications often have complex shapes and always bear a wide variety of loads, such as tension, torsion and bend caused by flexural deformation and vibration. Besides, CFRP composites are anisotropic materials with complex failure modes when subjected to external loads, such as fiber tear, delamination, matrix cracking and mixed failure. Therefore, the types of loads have to be taken into consideration in the failure mechanism study of CFRP adhesive joints. The physical, chemical and mechanical properties of adhesive and CFRP may change in the cycling thermal environment and their performance changes can affect the failure of CFRP adhesive joints. This means that the degree of degradation of CFRP adhesive joints is determined collectively by adhesive and CFRP. In this study, the degradation mechanism of adhesively bonded CFRP/ aluminum alloy joints subjected to thermal cycling was investigated. The chemical changes of adhesive and CFRP were tested by FTIR, DSC and TG/DTG to show the effect of thermal cycling on the composition, Tg and thermal stability. Mechanical tests were also used to investigate the performance changes of adhesive and CFRP caused by thermal cycling. The failure strength and failure modes of adhesively bonded CFRP/aluminum joints under different stress states before and after thermal cycling were analyzed. The degradation mechanism of adhesively bonded CFRP/aluminum joints was revealed according to the comparative analysis of adhesive, CFRP and CFRP/aluminum joints.
Table 1 Material properties of 6005A aluminum alloy [14]. Material
Young's modulus (MPa)
Poisson's ratio
Density(kg/m3)
6005A
72000
0.32
2700
Table 2 Properties of the adhesive Araldite® 2015 [13]. Property Young's modulus, E [GPa] Poisson's ratio, ν Tensile failure strength, σf [MPa] Tensile failure strain, εf [%]
1.85 ± 0.21 0.33 21.63 ± 1.61
Shear modulus, G [GPa] Shear failure strength, τf [MPa] Shear failure strain, γf [%]
0.56 ± 0.21 17.9 ± 1.8
4.77 ± 0.15
43.9 ± 3.4
this study, aluminum alloy 6005A, CFRP and adhesive Araldite® 2015 were selected to fabricate adhesively bonded CFRP/aluminum alloy shear joints and butt joints. The material properties of aluminum alloy 6005A are shown in Table 1. Adhesive Araldite® 2015 is a two component epoxy paste adhesive with excellent performances, such as good impact resistance, high shear and peel strength, so it has been used in practical engineering and research [10,13]. The mechanical properties of the adhesive are shown in Table 2. The CFRP was provided by Dongguan LiWeiSheng Carbon Fiber Products Technology Co., LTD. According to the supplier, the CFRP laminate was made from twill weave and unidirectional pre-preg, the fiber and epoxy resin of which were T300 and YPH-23 respectively. The hot pressing process was used and the process parameters were as follows, curing temperature 130 °C, pressure magnitude 0.1 MPa and curing time 100 min. The thickness of the woven fabric and the unidirectional prepreg was approximately 0.5 mm and 0.25 mm respectively and the whole thickness of CFRP laminate was about 2 mm. The outer layers of CFRP laminate were woven fabrics while the internal layers were unidirectional prepregs and the layup was [(0/90)/0/90/0/90(0/90)]. Material properties of unidirectional-CFRP and twill weave-CFRP are shown in Table 3.
2.2. Specimens manufacture 2.2.1. Bulk specimens Bulk specimens of adhesive Araldite® 2015 was fabricated to investigate the mechanical performances of adhesive according to NF ISO 527-2 standards. The shapes and dimensions of bulk specimens are shown in Fig. 1 and the section was 2 × 10 mm2 A stainless steel mould was designed and manufactured, shown in Fig. 2 (a and b). The mould consists of three parts: (1) the base is used to support the other parts, (2) the middle is the core that determines the shapes and dimensions of bulk specimens, (3) the top is applied to close the mould with screws. The bulk specimens were manufactured in the condition of 25 ± 2 °C/ 50% RH and the process is as follows: a type of mould release was applied to the inner surface of the mould and then cleaned by acetone. A static mixer nozzle (Fig. 2 c) was used to dispense and mix the two components of adhesive, and apply the mixed adhesive to the mould. After that the three parts were fitted together with screws. The bulk specimens cured in the mould at room temperature for 24 h, and then Table 3 Material properties of unidirectional-CFRP and twill weave-CFRP.
2. Experimental work 2.1. Material
Unidirectional-CFRP Twill weave-CFRP
CFRP and Aluminum alloy are increasingly used in automobiles. In 2
Ex (GPa)
Ey (GPa)
Gxy (GPa)
νxy
125 ± 12 55 ± 5
10 ± 2 55 ± 5
7 ± 0.6 4 ± 0.5
0.07 0.14
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Fig. 1. Geometry and dimensions of a bulk specimen. Fig. 3. Geometry and dimensions of butt joints.
were removed from the mould and cured at 80 °C for 2 h in an electrothermostatic fan oven.
2.2.2. Adhesive joints Butt and shear joints were designed to investigate the degradation mechanism of adhesively bonded CFRP/aluminum alloy under normal and shear stress state respectively. The butt joints are shown in Fig. 3. There are two layers of adhesive on both sides of the CFRP plate. Two holes are set in the two sides of the butt joint to connect to the universal tensile testing machine. The bonding area is 25 × 25mm2 and bondline thickness is 0.2 mm. Both length and width of the CFRP plate (35 × 35 mm2) are larger than the bonding area (25 × 25 mm2) to avoid fiber tear from the edges. Therefore, the whole dimension of the butt joint is 202.4 × 35 × 35 mm3. The experimental schematic diagram of a butt joint under normal stress is shown in Fig. 4(a). To simplify the adhesive joints fabrication, a butt joint was also used to test the failure of adhesive joints under shear stress with a stainless steel auxiliary testing equipment as shown in Fig. 4(b). The assembled butt joint and auxiliary testing equipment formed a shear joint. Although the stress state of adhesive joints is combined shear and normal near free edges, and there also exist significant stress concentrations, simulation analysis found that the main stress in the adhesive layer of shear and butt joints is shear and normal stress respectively [15]. Therefore, shear and butt joints are assumed to under pure shear and pure normal stress respectively in engineering. As shown in Fig. 5(a), a bonding fixture was designed to manufacture adhesive joints. The bonding fixture is composed of three main parts: (a) a base with two concentric grooves used to support adhesive joints, and ensure the concentricity of the two aluminum alloy rods; (b) battens that fix the adhesive joints with screw nuts; (c) a bolt knob positioned on the right side of the base used to supply pressure and control the thickness of the adhesive layer via 0.2-mm-diameter glass balls. The manufacturing process is as follows: First, the adhesive surface of aluminum alloy rods was treated by sand blasting with 80# white fused alumina to achieve elegant appearance. Then the adhesive surface of aluminum alloy rods and CFRP were cleaned by acetone. Araldite® 2015 adhesive was applied to the substrate's surface and adhesive joints were bonded using the bonding fixtures. After that almost all spew fillets of adhesive were cleared before the adhesive was fully cured. Finally, the adhesive joints (shown in Fig. 5b) were disassembled from the bonding fixture after curing at room temperature
Fig. 4. Experimental schematic diagrams of (a) butt joints and (c) shear joints.
for 24 h and then were cured at 80 °C for 2 h in the electro-thermostatic fan oven. 2.3. Experimental tests 2.3.1. Thermal cycling exposure To simulate the thermal cycling environment in the service condition of automobiles, a spectrum of temperature variation shown in Fig. 6 was designed based on the standard “VW PV 1200 Climate test – Alternating climate temperature test with controlled humidity”. There are four stages in one thermal cycling: (I) heating stage, the temperature rises from −40 °C to 80 °C in 2 h; (II) high temperature stage, the temperature is 80 °C for 4 h; (III) cooling stage, the temperature
Fig. 2. (a) Schematic diagram and (b) real object of the mould, (c) the static mixer nozzle. 3
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Fig. 5. (a) The bonding fixture and (b) the adhesive joint.
cured bulk specimens of adhesive were exposed in the thermal cycling for 30 days and then tested at room temperature, as shown in Fig. 7(a). The CFRP plates (35 × 35 × 2 mm3) was degraded first in the thermal cycling for 30 days and then the degraded CFRP were bonded with aluminum alloy and adhesive Araldite® 2015 to manufacture adhesive joints, after that the adhesive joints were tested at room temperature. The effect of degraded CFRP on adhesive joints was obtained from the test shown in Fig. 7(b) because only the CFRP was degraded. From Fig. 7(c), the adhesively bonded CFRP/aluminum alloy joints were exposed in the thermal cycling for 10, 20 and 30 days and then were tested at room temperature. 2.3.2. Fourier transform infrared (FTIR) analysis FTIR was conducted by using a VERTEX70-Bruker spectrometer to investigate the compositional variation of adhesive Araldite® 2015 and CFRP after thermal cycling. Four samples were taken for each test and the weight was about 10 ± 0.2 mg. The spectrum was obtained by ATR (Attenuated Total Reflection) using 128 scans and spectra from the surface of adhesive and CFRP sheets were scanned from 4000-600cm−1 with a resolution of 4 cm−1.
Fig. 6. Spectrum of temperature variation in one thermal cycling.
declines from 80 °C to −40 °C in 2 h; (IV) low temperature stage, the temperature is −40 °C for 4 h; It takes 12 h to complete one thermal cycling and the relative humidity is below 20% during the whole aging process to minimize moisture influence on the experimental results. The thermal cycling was conducted in an environmental chamber WSHW080BF provided by Jiaxing WEISS Experiment Equipment CO., Ltd (Jiaxing, China). Both the adhesive and CFRP in the adhesive joints experienced thermal cycling and their performance changes had effect on the adhesive joints. To investigate the degradation mechanism of adhesively bonded joints, the bulk specimens of adhesive, CFRP plates and adhesively bonded joints were exposed in the thermal cycling environment and the corresponding schematic diagrams were shown in Fig. 7. The
2.3.3. Differential scanning calorimetry (DSC) analysis Tg of adhesive Araldite® 2015 and CFRP before and after thermal cycling was measured using a PerkinElmer DSC7. The DSC measurements were performed with standard aluminum pans and lids under nitrogen atmosphere at a 5 °C/min heating/cooling rate. The samples of adhesive and CFRP were taken from the bulk specimens and CFRP plates respectively. Four samples were taken for each test and all the samples included the surface and core. The sample weight was approximately 20 ± 0.2 mg. The first heating run was carried out to remove the thermal history of the samples because Tg is influenced significantly by the thermal history, such as post-cure temperature and its duration [16]. Tg was obtained From the second heating run.
Fig. 7. Schematic diagrams: effect of thermal cycling on (a) bulk specimens, (b) CFRP, (c) adhesive joints. 4
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Fig. 8. The VIC-3D™ measurement system.
2.3.4. Thermal gravimetric analysis (TG)/Differential thermal gravimetric (DTG) TG/DTG was carried out on a TGA/DSC1 METTLER TOLEDO STARe System by heating adhesive Araldite® 2015 and CFRP contained in alumina pans at a rate of 10 °C/min from 25 to 700 °C in a nitrogen atmosphere to investigate the changes of thermal stability. Four samples were taken for each test and the weights were about 20 ± 0.2 mg. The instrument records the weight and the weight loss rate with the temperature increment. 2.3.5. Tensile tests of bulk specimens The bulk specimens of Araldite® 2015 before and after 30 days’ thermal cycling were tested at room temperature by an electronic universal testing machine at a constant speed of 2 mm/min. The VIC3D™ Measurement System shown in Fig. 8 was applied to measure the displacement and strain of the bulk specimens during the quasi-static tests. The system serves as a non-contact measurement tool for measuring full-field displacement and strain via utilizing Digital Image Correlation (DIC), which mainly includes lights, cameras and software system. The system supports a field of view (FOV) of between 0.8 and 7 mm with strain measurement levels ranging from 0.005% (50 microstrain) to 2000%. The system had to be used cooperatively with an electronic universal testing machine and the test of bulk specimens was conducted as follows: the gauge length of the bulk specimen was set to 50 mm and was speckled with an idler wheel; then it was fixed into the universal testing machine. Two cameras were then installed and calibrated to better visualize the bulk and pictures were collected during the quasi-static tests. Finally, the displacements and strain of the bulk were obtained after analyzing the pictures via the software system. Four specimens were tested for each test to ensure data validity and the results were averaged.
Fig. 9. Representative FTIR spectra of adhesive Araldite® 2015. Table 4 Tentative assignment of major bands in the spectrum of adhesive [18,19].
2.3.6. Tensile tests of adhesive joints The same electronic universal testing machine mentioned above was used to conduct quasi-static tests of shear and butt joints. To eliminate the non-axial forces, both ends of the specimens were connected to the testing machine through the cross universal joints. Four specimens were tested for each test and the failure loads were obtained from the load-displacement curves recorded by the electronic universal testing machine.
Positions (cm−1)
Assignment
3328 2923,2850 1736 1648 1604 1509 1452 1293 1237 1098, 1072, 1035 827
-OH, –NH stretching -CH3, and –CH2stretching C=O N–H bending Quadrant stretching of the benzene ring Semicircle stretching of p-disubstituted benzene C–H bending of aliphatic groups Twisting mode of –CH2- groups Stretching mode for aromatic ether Stretching of the trans forms of the ether linkage Stretching C–O–C of oxirane group
for the main absorption that appears in the spectrum and changes in intensities or positions of absorption peaks can provide information on specific interactions involved during thermal cycling. The broad peak at 3328 cm−1 is most likely related to O–H and N–H stretching and intermolecular hydrogen bonds. Peaks in the region from 2923 to 2850 cm−1 are related to C–H stretching of CH3 and CH2 groups. Another feature of the adhesive is the appearance of peaks from 1035 to 1098 cm−1 and 1736 cm−1associated to the stretching of C–O–C and C]O respectively, which are useful for identification of epoxy adhesive. From Fig. 9, it can be seen that the absorption intensity of some bonds obviously changed. Although most of the absorption intensities fluctuated before and after thermal cycling, the major difference was the obvious increase in the intensity signal of ether at 1072 cm−1 of the aged adhesive. It can be inferred that the adhesive Araldite® 2015 may cure again in the thermal cycling environment. The curing process of epoxy adhesive is the set of chemical reactions that leads to the
3. Experimental results and discussions 3.1. Effect of thermal cycling on bulk specimens 3.1.1. FTIR results of the adhesive The unaged and aged bulk specimens were analyzed by FTIR to understand the degradation mechanism of adhesive. FTIR spectra of the unaged and aged adhesive are shown in Fig. 9 and some absorption peaks were observed in the range between 4000 cm−1 and 600 cm−1. The spectral subtraction in Fig. 9 was obtained by removing the unaged spectrum from the aged spectrum. Table 4 gives a tentative assignment 5
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Fig. 10. (a) Representative DSC results of adhesive Araldite® 2015 and (b) variations of Tg.
formation of a highly cross-linked network and during the chemical reaction, both molecular weight and polydispersity increase until one single macromolecule is formed. As a result of the curing reaction, the epoxide (oxirane) ring decreases, while the hydroxyl and ether (C–O–C stretch) band increase. Therefore, the rate of polymerization of epoxy adhesive can be estimated by the loss of epoxide or the increase in hydroxyl and ether absorptions. Unlike the bands mentioned above that take part in the chemical reaction, phenyl bands at 1508 cm−1 are present and remain constant [17]. Thus the absorption peak at 1508 cm−1 is used as a reference to normalize the FTIR spectra of the adhesive before and after thermal cycling [17]. The results presented in Fig. 9 are the spectra after normalization. According to the curing reaction and the increase in the ether at 1072 cm−1, it is concluded that the post-curing of the adhesive Araldite® 2015 occurred during the thermal cycling.
four phases, but the first peak of DTG curve at 174.1 °C disappeared after thermal cycling and there were only three phases in the aged adhesive. The temperature value of the first and second peak of aged DTG curve is almost the same with the second and third peak of the unaged DTG curve, but the temperature of the third peak of the aged adhesive was obviously higher than the fourth peak of the unaged. The disappearance of the first peak and the lag of the fourth peak of the DTG curve after thermal cycling indicated that the thermal stability of adhesive Araldite® 2015 improved, especially at low and high temperature phase. From the TG curves, it was obvious that the weight loss of the aged adhesive was lower than the unaged at any temperature between 25 °C and 800 °C, suggesting that the formation of chemical bonds when subjected to thermal cycling. All the test results of chemical analysis (FTIR, DSC, TG/DTG) illustrated that the post-curing of adhesive occurred in the thermal cycling.
3.1.2. DSC results of the adhesive The DSC thermograms of adhesive Araldite® 2015 before and after thermal cycling are shown in Fig. 10. To determine the value of Tg, straight lines were extended along the left- and right-hand branches of the heat flow curve. Tg was obtained from the point of intersection between the bisecting line of the angle and the measured curve. It is found that the Tg of adhesive Araldite® 2015 increased from 65.8 °C to 68.3 °C after thermal cycling, indicating that the curing degree improved. Generally speaking when heating above Tg, the curing reaction is reactivated and Tg increases again, which is due to the fact that vitrification is a reversible process. The temperature in the high temperature stage of thermal cycling was 80 °C, which was higher than the Tg(65.8 °C), resulting in the reactivation of post-curing and improvement of Tg to 68.3 °C.
3.1.4. Test results of bulk specimens To investigate the effect of thermal cycling on the mechanical performances of adhesive, the bulk specimens before and after 30 days' thermal cycling were tested and the stress-strain curves are shown in Fig. 12(a). The failure strength, Young's modulus and failure strain of bulk specimens were calculated from the stress-strain curves. For the unaged adhesive, the failure strength, Young's modulus and failure strain were 22.3 MPa, 1826 MPa and 4.77%, respectively, which were almost equivalent with the mechanical performances found in Ref. [13]. It suggested that the test method of the VIC-3D™ Measurement System in this study was able to satisfy the measuring accuracy. Compared with the stress-strain curve of the unaged bulk specimen, the failure strength and Young's modulus improved, while the failure strain decreased after thermal cycling. To clearly show the performance changes of the adhesive before and after thermal cycling, the mechanical performances of the bulk specimens were normalized by the unaged, as shown in Fig. 12(b). Compared with the unaged bulk specimens, the failure strength of the aged increased by 25.4% and Young's modulus increased by 21.6%, the failure strain decreased by 29.8%, indicating the extent of these changes was at roughly equal levels. The changes in mechanical properties of bulk specimens demonstrated the post-curing of adhesive.
3.1.3. TG/DTG results of the adhesive TG/DTG analysis has been an important measurement to determine material degradation kinetics under accelerated thermal aging, which can be applied to estimate the adhesive life because degradation of the thermoset network occurs through the scission of chemical bonds. As chemical bonds are broken, volatile species are released to cause a loss of mass. Thermal stability of adhesive Araldite® 2015 was analyzed by TG/DTG and the weight loss curve (TG) and the weight loss derivative curve (DTG) were recorded as a function of temperature, which is shown in Fig. 11. The process of weight loss is divided into several phases according to the peak number of weight loss derivative curve. It can be seen that the weight loss of the unaged adhesive occurred via
3.2. Effect of thermal exposure on CFRP 3.2.1. FTIR results of CFRP To investigate the effect of thermal cycling on the composition of 6
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Fig. 11. Representative (a) TG and (b) DTG thermograms of the adhesive Araldite® 2015.
CFRP, the unaged and aged CFRP were analyzed by FTIR and the result is shown in Fig. 13. The spectral subtraction in Fig. 13 was obtained by removing the unaged spectrum from the aged spectrum. The absorption peaks of CFRP were almost the same with adhesive Araldite® 2015 because they were epoxy and have similar bonds. The specific absorption peak of CFRP is the appearance of 2167 cm−1 related to nitrile group. Most of the bond intensities decreased after thermal cycling except the absorption peak at 1736 cm−1, which was caused by the oxidation of CFRP with oxygen in the air when subjected to the hightemperature stage in the thermal cycling [20]. The oxidizing reaction of CFRP means the bond scission and the degradation and may influence other chemical and mechanical performances. Compared with the adhesive Araldite® 2015, the variation degree of bond intensities of CFRP was low, which suggested that the effect of thermal cycling on the CFRP is less obvious. This is due to the fact that the curing temperature of CFRP was greater than 80 °C.
3.2.2. DSC results of CFRP The DSC thermograms of CFRP before and after thermal cycling are shown in Fig. 14. The Tg of the unaged and aged CFRP was 122.4 and 121.8 °C, indicating that the Tg of CFRP decreased a little by thermal cycling. This was caused by the oxidizing reaction of CFRP. The Tg of CFRP decreased by about 0.6 °C, while the Tg of adhesive Araldite® 2015 increased by about 2.5 °C. It can be concluded that the Tg of adhesive Araldite® 2015 was more sensitive to thermal cycling than CFRP, which was due to the fact that the high temperature stage (80 °C) in the thermal cycling was higher than the Tg of adhesive Araldite® 2015, but it was lower than the Tg of CFRP. The varying degree of Tg is in
Fig. 13. Representative FTIR spectra of CFRP.
accordance with FTIR results. 3.2.3. TG/DTG results of CFRP TG/DTG thermograms of CFRP are shown in Fig. 15. There was only one peak in the DTG curves whether the CFRP was aged or not, and the temperature position and height of the two peaks were almost the same. From the TG curves, the weight loss of the aged and unaged CFRP showed no obvious difference at temperatures below 450 °C, but the
Fig. 12. Effect of thermal cycling on the bulk specimens of adhesive: (a) representative stress-strain curves, (b) normalized mechanical performances. 7
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Fig. 14. (a) Representative DSC results of CFRP and (b) variations of Tg.
Fig. 15. Representative (a) TG and (b) DTG thermograms of CFRP.
weight loss of the aged CFRP was higher when the temperature exceeded 450 °C. The thermal degradation of aged and unaged CFRP took place through one step, but the thermal stability decreased after thermal cycling. As with the FTIR and DSC results, the effect of thermal cycling on the stability of adhesive was more obvious than CFRP.
3.2.4. Effect of aged CFRP on the failure strength To investigate the performance changes of CFRP after thermal cycling, the CFRP plates were exposed to thermal cycling first and then were bonded with aluminum alloy and adhesive Araldite® 2015 to fabricate shear and butt joints. The failure strength of shear and butt joints as shown in Fig. 16 were called shear strength and butt tensile strength. In consideration of the surface oxidation of CFRP that may affect the adhesion characteristic, the aged CFRP were treated by two types of surface treatment methods, namely no abrading and abrading with #80 sandpaper [20]. The shear strength of aged CFRP without abrading decreased by about 16.5%, but it almost recovered to the level of unaged CFRP when the aged CFRP was abraded by #80 sandpaper. The butt tensile strength of aged CFRP without abrading had a similar decrease degree, but it did not recover after abrading with #80 sandpaper. It is concluded that the failure strength of aged CFRP without abrading decreased to the same extent, but the surface treatment had a different effect on the adhesive joints under different stress states.
Fig. 16. Failure strength of shear and butt joints bonded with unaged and aged CFRP.
3.2.5. Effect of aged CFRP on the failure surfaces To investigate the failure mechanism of shear and butt joints bonded with aged CFRP, the failure modes were analyzed. The representative fracture surfaces of shear and butt joints were shown in Fig. 17 and Fig. 18 respectively. The failure mode of shear joints with unaged CFRP was a cohesive 8
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Fig. 17. Representative fracture surfaces of shear joints:(a) unaged, (b) aged CFRP without abrading, (c) aged CFRP abraded by #80 sandpaper.
fiber/matrix interface of CFRP degraded in the thermal cycling environment.
failure, but the failure mode of aged CFRP without abrading changed to mixed failure of cohesive and interface failure. The interface failure was located on the surface of CFRP as shown in the red box of Fig. 17 (b), which was the main cause for the decrease of failure strength. It is found that the thermal cycling reduced the surface adhesiveness of CFRP, which was caused by the oxidation of CFRP surface. When the aged CFRP was abraded by #80 sandpaper, the failure mode recovered to cohesive failure as shown in Fig. 17 (c). The appearance of cohesive failure suggested that the inside of CFRP hadn't been oxidized because it was not in direct contact with air. The surface oxidation layer of aged CFRP was removed by abrading and then the failure strength recovered to the level of the unaged. As for the butt joints, the failure mode of unaged CFRP was mixed failure of cohesive and fiber tear, as shown in Fig. 18(a). In contrast with the cohesive failure of shear joints, the fiber tear of butt joints was caused by normal stress. Cohesive failure, fiber tear and interface failure were shown in the fracture surfaces of aged CFRP without abrading. The interface failure was shown in the red box of Fig. 18(b) and the area of fiber tear decreased when compared with the unaged. This is due to the fact that the interface failure of aged CFRP without abrading reduced the failure strength of butt joints, which was unable to cause more fiber tear. From Fig. 18(c), it was found that the interface failure disappeared and the area of fiber tear increased after the abrading of aged CFRP. The area of fiber tear of aged CFRP abraded by #80 sandpaper was larger than that of the unaged because the mechanical performance of the fiber/matrix interface decreased in the thermal cycling due to the difference of thermal expansion between fiber and matrix. Although the abrading eliminated the interface failure of aged CFRP to improve the failure strength, the larger area of fiber tear reduced the failure strength sharply. To further investigate the effect of thermal cycling on the fiber/ matrix interface, SEM was applied to inspect the fiber tear before and after thermal cycling, as shown in Fig. 19(a) and (b). The fiber surface of unaged CFRP was covered with resin matrix as shown in the red box of Fig. 19(a), while there was no resin matrix on the fiber surface of aged CFRP as shown in the red box of Fig. 19(b). Besides, the space between fibers of unaged CFRP was filled with resin matrix and the fracture surface was rough as shown in the green box of Fig. 19(a), but the resin matrix was relatively less between the fibers, and the fracture surface was relatively smooth as shown in the green box of Fig. 19(b). All the changes in the fiber surface and resin matrix suggested that the fiber tear of unaged CFRP was mainly caused by the fracture of the resin matrix, but after thermal cycling, the fiber tear was mainly attributed to the debonding of fiber/matrix interface. It was concluded that the
3.3. Effect of thermal exposure on adhesive joints 3.3.1. Failure strength of adhesive joints The peak loads from the load-displacement curves were defined as the failure loads of adhesive joints and then were divided by the bonding area to obtain the failure strength, which was shown in Fig. 20. The failure strength of shear joints decreased by about 18.5% after 10 days' thermal cycling, but the decrease slowed down in the second 10day and it only decreased by 24.1% after 20 days’ aging. During the third 10-day, the failure strength showed a sharp decline by about 43.3%. The failure strength of butt joints in the thermal cycling had the similar downtrend with shear joints, namely the first and third 10-day significantly affected the failure strength, but the extent of decrease of butt joints was higher than shear joints, especially in the first and second 10-day. It can also be seen that the data discreteness of failure strength increased after thermal cycling. 3.3.2. Failure surfaces of adhesive joints The representative fracture surfaces of shear and butt joints were shown in Fig. 21 and Fig. 22 respectively. As shown in Fig. 21(a–c), the failure modes of unaged shear joints and after 10, 20 days' thermal cycling were a cohesive failure, indicating that the thermal cycling didn't change the failure modes. During the thermal cycling, the postcuing of adhesive improved the failure strength, but the thermal stress caused by the difference of thermal expansion coefficient between adhesive and adherend reduced the failure strength. The failure strength of shear joints decreased with the increase of aging time, which suggested that the effect of thermal stress was much more obvious than the post-cuing. When the shear joints were exposed to thermal cycling for 30 days, the mixed failure of cohesive and interface was seen in the fracture surfaces. The interface failure was located in the edges of the bonding area as shown in the red box of Fig. 21(d), which may be caused by the long-term impact of thermal stress. The appearance of interface failure exacerbated the decline of failure strength, resulting in the sharp fall in the third 10-day. Although the failure mode of unaged butt joints and after exposure in the thermal cycling for 10 and 20 days remained mixed failure of cohesive and fiber tear, the area of fiber tear decreased after 10 days and then increased after 20 days, as shown in the Fig. 22(a–c). From the above analysis, it was found that the failure strength of adhesive and fiber/matrix interface of CFRP decreased in thermal cycling. The
Fig. 18. Representative fracture surfaces of butt joints:(a) unaged, (b) aged CFRP without abrading, (c) aged CFRP abraded by #80 sandpaper. 9
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Fig. 19. Fiber tear from SEM: (a) unaged CFRP, (b) aged CFRP abraded by #80 sandpaper.
failure strength of fiber/matrix interface of CFRP declined more quickly. The effect of fiber tear on butt joints may be the reason why the failure strength of butt joints reduced faster than shear joints. The failure mode of butt joints changed to the mixed failure of cohesive and interface after 30 days’ thermal cycling and the characteristic was similar to shear joints. In conclusion, the performance changes of CFRP caused by thermal cycling almost did not affect the failure of shear joints, but it had an obvious effect on the failure of butt joints. In other words, the shear joints subjected to thermal cycling was mainly influenced by the thermal stress and post-curing of adhesive and interface failure, while the butt joints were also affected by the fiber/matrix interface of CFRP. 4. Conclusions Chemical analysis (FTIR, DSC and TG/DTG) and mechanical tests were applied to investigate the effect of thermal cycling on the thermal behavior of adhesive, CFRP and adhesively bonded CFRP/aluminum alloy joints. The degradation mechanism of CFRP/aluminum alloy joints was analyzed based on the performance changes of adhesive and CFRP. The following conclusions are drawn:
Fig. 20. Failure strength of shear and butt joints after different aging time.
decrease of fiber tear after 10 days suggested that the failure strength of adhesive fell more rapidly than fiber/matrix interface of CFRP, while the subsequent increase of fiber tear after 20 days indicated that the
(a) The changes of FTIR spectra and the improvements of Tg and thermal stability showed that the thermal behavior of adhesive was caused by a post-curing process. As a result, the failure strength and
Fig. 21. Representative fracture surfaces of shear joints after different aging time: (a) unaged, (b) 10 days, (b) 20 days, (b) 30 days. 10
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Fig. 22. Representative fracture surfaces of butt joints after different aging time: (a) unaged, (b) 10 days, (b) 20 days, (b) 30 days.
Young's modulus of the bulk adhesive increased by more than 20% after 30 days' thermal cycling, while the failure strain decreased by about 30%. (b) The CFRP surface was oxidized, and Tg and thermal stability decreased in the thermal cycling. Besides, the effect of thermal cycling on the chemical properties of CFRP was less than adhesive. The surface oxidation of CFRP can cause interface failure and the decrease of mechanical performances of the fiber/matrix interface resulted in the larger area of fiber tear. (c) The failure strength of shear joints subjected to thermal cycling for 30 days decreased by about 43.3%, which was mainly caused by the combined effect of thermal stress, post-curing and interface failure. The failure strength of butt joints had the similar downtrend with shear joints, but the extent of decrease was higher than shear joints, which was caused by the thermal stress, post-curing, interface failure and fiber tear of CFRP. (d) The failure mode of shear joints changed from cohesive failure to mixed failure of cohesive and interface after thermal cycling, while the failure mode of butt joints changed from mix failure of cohesive and fiber tear to mixed failure of cohesive and interface. The performance changes of CFRP caused by thermal cycling almost did not affect the failure of shear joints, but it had an obvious effect on the failure of butt joints.
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Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No.51775230). References [1] Friedrich K, Almajid AA. Manufacturing aspects of advanced polymer composites for automotive applications. Appl Compos Mater 2013;20(2):107–28. [2] Girubha J, Vinodh S, Kek V. Application of interpretative structural modelling integrated multi criteria decision making methods for sustainable supplier selection. J Model Manag 2016;11(2):358–88. [3] da Silva LFM, Chsner AO, Adams RD. Handbook of adhesion technology. second ed.
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