On fracture characteristics of adhesive joints with dissimilar materials – An experimental study using digital image correlation (DIC) technique

Accepted Manuscript On fracture characteristics of adhesive joints with dissimilar materials - An experimental study using digital image correlation (DIC) technique Guangyong Sun, Xinglong Liu, Gang Zheng, Zhihui Gong, Qing Li PII: DOI: Reference:

S0263-8223(18)30418-5 https://doi.org/10.1016/j.compstruct.2018.06.018 COST 9812

To appear in:

Composite Structures

Received Date: Revised Date: Accepted Date:

27 January 2018 28 April 2018 4 June 2018

Please cite this article as: Sun, G., Liu, X., Zheng, G., Gong, Z., Li, Q., On fracture characteristics of adhesive joints with dissimilar materials - An experimental study using digital image correlation (DIC) technique, Composite Structures (2018), doi: https://doi.org/10.1016/j.compstruct.2018.06.018

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On fracture characteristics of adhesive joints with dissimilar materials - An experimental study using digital image correlation (DIC) technique Guangyong Sun1, 2, Xinglong Liu1, Gang Zheng1, Zhihui Gong1, *, Qing Li2 1

State Key Laboratory of Advanced Design and Manufacture for Vehicle Body,

Hunan University, Changsha, 410082, China 2

School of Aerospace, Mechanical and Mechatronic Engineering, The University of

Sydney, Sydney, NSW 2006, Australia

Abstract With expanding use of multi-materials in vehicle components for attaining light-weighting structures, growing interests have been refilled in the adhesive bonding technique attributable to its compelling advantages to bond dissimilar materials, especially for carbon fiber reinforced plastic (CFRP) composite and metals. A significant issue related to the bonding of dissimilar materials is, nevertheless, that there are substantial differences between adherend material properties, which often lead to different structural and fracture responses. This study aimed to investigate the effects of adherend thickness and adherend material types (e.g. steel, aluminum, CFRP composite) on the fracture behavior of the joints. The adherend deformation and fracture processes of the joints were monitored by the charge couple device (CCD) cameras during the tensile tests. A digital image correlation (DIC) technique was used to capture full-field, out-of-plane deformation of the adherend, the strain distribution and strain evolution along the bondline so that the fracture process can be characterized visually. The microscopic and macroscopic studies on the fracture surface were also carried out to explore the influences of adherend thickness and material types on the failure modes of the joints quantitatively. The results divulged that the joint strength increased with increasing adherend thickness; and the sensitivity of adherend thickness on the joint strength depended on the joint material types. When the adherends underwent the elastic deformation, adherend stiffness affected the joint stiffness and joint fracture process. Adherend yield strength

* Corresponding Author: Tel: +86-(0)73188821445; Fax: +86-(0)73188822051; Email: [email protected]

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determined the joint strength, fracture state and failure modes when the failure occurred with yielding in the adherend. The fracture processes and strain evolutions in the bondline were found to be symmetric for the joints with the same adherends, while the maximum strain and crack appeared first on the lap end of the lower yield strength adherend for the joints with dissimilar materials. The study is expected to provide new insights into the design of multi-material joints with different thicknesses.

Key words: Dissimilar materials; CFRP composite; Single-lap adhesive joints; Adherend material properties; Fracture response; Digital image correlation (DIC)

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1. Introduction With growing concern in energy consumption, the demand for light-weighting is drawing rising attention in automotive industry by integrating various materials, such as advanced high strength steels (HSS), aluminum alloys, magnesium alloys and carbon fiber reinforced plastic (CFRP) composites, for structuring the vehicle body [1-5]. There has been a trend that the futuristic light-weighted vehicles would be more likely constructed using ample combination of these different materials by taking into account the values of functions and weight to cost. Thus it raises a long-lasting yet critical question on how to join these multiple materials with different chemical, physical and mechanical methods [6]. Traditional joining methods, such as welding and mechanical fastening, have been commonly used in automotive engineering. However, it may become inapplicable for joining such adherends as carbon fiber reinforced plastics (CFRP) by welding [7]. While mechanical fastening including bolts, rivets and pins are widely used to join dissimilar adherends, there exist some apparent disadvantages such as high stress concentration, low fatigue resistance and destruction of structural integrity, especially when joining CFRP components where reinforcement fibers are often cut in order to drill the fixation holes [8]. For this reason, adhesive bonding has been of particular interest to join dissimilar materials substrates for its relatively low stress concentration, high fatigue endurance, corrosion resistance, and preservation of structural integrity [9]. Numerous studies have been conducted on metal-metal bonding, composite-composite bonding and composite-metal bonding joints due to the aforementioned advantages. For example, Karachlios et al. [10, 11] investigated the single-lap joints with high strength steel and ductile steel in tension; and they revealed that for the high strength steel substrates, the fracture response was dominated by the global yielding of adhesive or local shear strain at the overlap ends; and the fracture mechanism of ductile steel substrates was dictated by the yielding of adherends. It was shown that the adhesive joints would have different mechanical behaviors when bonding different materials even if they used the same adhesive and had the

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same adherend thickness. Yang et al. [12] performed the quasi-static tensile tests for the metal-metal single-lap adhesive joints with dissimilar materials; and they divulged that the bond between the strong-strong substrates could withstand much higher load than the bond with the weak-strong substrates due to the early yield of the weak substrate. They also observed that the interfacial fracture was prone to occur on the adherend with relatively lower yield strength; and a cohesive fracture could be generated when the load-carrying capacities of the two substrates were similar. Ozel et al. [13] investigated the influence of the stacking sequence of the composite adherend in the composite-composite joints; and found that the stacking sequence affected the failure load considerably. They also confirmed that the adherend material type can affect the load carrying capacity of the joints. Seong et al. [14] conducted an experimental study on the single lap carbon composite-to-aluminum joints; and they demonstrated that greater adherend thickness led to higher joint strength and failure load because increasing thickness could delay yielding of the aluminum adherend and reduce the bending effect. They also found that substrate material types could strongly affect the overall strength of the bonded joints; and indicated that the woven fabric composite was not susceptible to delamination. Comparing with Seong’s work [14], Anyfantis et al. [15] performed an experimental and numerical study on adhesively bonded single lap joint for relatively thicker dissimilar substrates (i.e. above 5mm). They showed that the failure loads of the tested CFRP-to-steel adhesive joints were insensitive to the substrate thickness and stiffness ratio, which had negligible effect on the stiffness and strength of the joints. Ozel et al. [13] explored the effect of adherend thickness of the aluminum adherend on the behavior of composite-aluminum joints; and they found that the joints with the thicker adherends was able to withstand a higher load. Reis et al. [16] investigated the single joints with different materials, including high elastic limit steel, aluminum and CFRP adherends; and they revealed that joint strength was influenced by the adherend stiffness, and the higher joint strength were obtained using higher stiffness adherend materials. However the authors did not use the lower yield strength material and neglected the influence of the yield strength of adherend on joint strength.

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Although numerous studies on the joint with dissimilar substrates have been available in literature, it remains an open topic of research for gaining comprehensive understanding of mechanical characteristics due to the significant difference of various experimental materials such as CFRP, steel, aluminum alloys, as well as adhesives. There is limited interpretation in the effects of adherend thickness and adherend materials on the fracture characteristics of the joints. Besides, most authors had concentrated on the failure load, rather than fracture process and failure modes in a progressive fashion. Regarding joint configuration, the single lap joint has been widely studied for its simplicity and efficiency [9]. Nevertheless, single lap joint configuration results in secondary bending effect under tension due to eccentric loading path as well as concentration of peel and shear stresses in the adhesive layer [11]. It is necessary to characterize surface strain and out-of-plane deformation in these joints for monitoring the damage development and failure mechanism. Note that with use of conventional strain gauge and extensometer, it is difficult to quantify full-field strain evolution in the joint, and the measurement accuracy can be easily affected by strain gradient, material properties and test environment. Digital image correlation (DIC) technique, on the other hand, is a relatively new and more effective tool for strain measurement, which can provide full-field strain and out-of-plane displacement data for hundreds of points from one testing specimen [17, 18]. This technique has been introduced and applied to many sophisticated problems in literature to date. For example, Zhu et al. [19] used the DIC system to obtain the full-field strain data at the CFRP laminas. Sun et al. [20] captured the deformation process of lower skin of sandwich panels to explore the failure process. Wang et al. [21] used the DIC technique to study the advanced Comeld hybrid joining technique for joint of Ti6Al4V and CFRP materials, in which the full-field, in-plane strain during the tests were monitored and the development of failure process was identified. Mao et al. [22] adopted the DIC technique to scrutinize full/local strain evolution and cracking propagation in-situ for the bending tests; and they revealed the potential failure mechanisms of the C/SiC composites. Crammond et al. [23] investigated the strain development and effects of crack initiation and propagation to failure in the joints.

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Note that there have been limited attempts made to use the DIC technique for qualifying strain and/or displacement fields in the single-lap adhesively bonded joints. For example, Comer et al. [24] evaluated the evolution of deformation and strain in composite single lap bonded joints prior to failure by using three-dimensional (3D) and two-dimensional (2D) DIC, respectively. Bai et al. [25] measured the displacement field using the DIC to correlate with the finite element analysis (FEA) results. However, rather limited works have been reported in literature thus far to use the DIC technique for identifying the strain evolution of adhesive bondline and cracking process in the joints with dissimilar adherend materials. In this study, a DIC based experimental procedure was presented to systematically investigate the effects of adherend thickness and adherend material on the cracking responses of adhesive joint. Attention was paid to their effects on the fracture characteristics of the joints. The fracture processes were monitored by the CCD cameras under a tension process; and the displacement and strain fields were measured using the DIC system. The strain evolution and strain distribution in the adhesive bondline were quantified to understand the fracture process and fracture mechanism visually. Finally, the microscopic analysis of the fractured surfaces was carried out to depict the failure mode of the joints quantitatively.

2. Materials and Methods 2.1. Specimen preparation In this study, four different materials were involved for preparing the adhesive joints, namely Q235 steel, 5182 aluminum alloy and woven carbon fiber reinforced plastic (CFRP) as the adherends, and an epoxy adhesive as the adhesive agent. The CFRP composite laminates were fabricated using prepreg press molding process with T300-3K plain woven carbon fiber fabrics and epoxy matrix. The composite panels with two different thicknesses were prepared, one with 8 and another with 16 layers, i.e. around 2mm and 4mm in thickness, respectively. The mechanical properties of the laminates were in-house measured according to the ASTM standards [26, 27]; and the material properties of the CFRP are listed in Table 1. Note that such metal adherends as Q235 steel and 5182 aluminum alloy are widely used in

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automotive and aerospace engineering. They were also tested to obtain their mechanical properties in this study. The adhesive used to bond specimens was Araldite 2015 (Huntsman, USA), a relatively stiff two-component epoxy adhesive; and the typical stress-strain curve was measured from the bulk coupons [28]. In order to study the failure mode and failure mechanism, typical stress-strain curves and the material properties for the metal adherends and adhesive were given in Table 2 [29, 30].

Table 1 Mechanical properties of the CFRP laminate. Table 2 Tensile stress-strain curves and material properties of metal adherends and adhesive. Note: E:Young’s modulus, ν: Poisson’s ratio, ‘*’: referring to Marques [28].

As highlighted in the introduction, the single-lap joint configuration has been widely used to study the behavior of adhesive joints. According to the ASTM standard D1002-01 for metal-to-metal single-lap joints [31] and the ASTM standard D5868-01 for composite-to-composite single-lap joints [32], the configuration of the specimen designed for the single-lap joint test is shown in Fig. 1. The length and width of adherends, tab length and width, overlap length and adhesive thickness are 113mm, 25mm, 25mm, 25mm ,25mm and 0.2mm, respectively [33]; and the specimens were manufactured by including end tabs to make the specimen perfectly align when gripped in the testing machine. The details of the other dimensions used for the test were summarized in Table 3. In the specimen ID, (namely AA, AS, AC, SS, SC, CC-t1-t2), A, S and C denote the Al5182 aluminum, Q235 steel and CFRP, respectively; and t1 and t2 represent the thicknesses of the two adherends. Type I includes these joints that have at least one Al5182 aluminum adherend. These joints with one or two Q235 steel adherends are included in Type II. Similarly, Type III covers these joints which adherends include at one CFRP adherend. Group A represents the thinner joints with the same substrates, while Group D represents the thicker joints with the same substrates. Group B and Group C include the same joints. These joints are with dissimilar adherend materials, and the adherend thickness is equal to 2mm. Group E and Group F are corresponding to Group B and Group C, and the difference exists in the thickness 7

of adherends is 4mm.

Fig. 1. Geometry and dimensions of single lap joints.

Table 3 Dimensions of single-lap adhesive joint specimens.

Before bonding, the prismatic substrates (113mm×25mm) and tabs (25mm×25mm) were cut from the steel panels, aluminum panels and CFRP laminates; and then their surfaces were treated and prepared to the required bonding conditions. Adhesive surface (overlap surfaces and tab surfaces) of both the substrates and tabs were polished along the parallel lines to the tensile direction with water-proof abrasive paper (180 meshes) and degreased with acetone to remove surface oxide layer and greasy dirt [34, 35]. Note that the abrasion for the CFRP adherends should just remove the resin surface without exposing fibers to avoid damage to the CFRP substrates. After surface treatment and pasting the adhesive, the specimens were placed in the hot-press machine. In order to obtain an adhesive thickness of 0.2mm after curing, the same thickness adherend spacers and glass epoxy shims with a thickness of 0.2mm (calibration spacers) were placed in the machine as shown in Fig. 2. This process enabled us to keep the adhesive layers being of uniform thickness without contaminants. After fixed the spacers, the hot-press machine was kept a constant pressure of 0.5MPa at 40℃ for 2 hours, followed by a slow cooling for about 6 hours to the ambient temperature of 25℃. Three specimens for each joint configuration were prepared, for a total of thirty-six specimens (note that some in Table 3 had the same configurations, e.g. AS-2-2 and SA-2-2). After curing, the fillets of adhesive at all the edges of the specimens were removed carefully with a knife and abrasive paper; and were finely polished to eliminate the effect of adhesive fillet and also to enable a smooth speckle pattern to be placed for observing the crack development accurately in the DIC measurement. Fig. 3 showed a representative specimen of composite-to-aluminum single-lap joint after curing and the side view of the joint after removing the fillets.

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Fig. 2. Configuration utilized for the fabrication of single-lap adhesive joints. Fig. 3. Specimens used for test: (a) a typical single-lap joint after curing; (b) side view of the joint.

2.2. Tensile test The tensile tests of the joints was conducted under the quasi-static conditions in a 150 KN testing machine (Instron 5985, USA) with the displacement control mode at a speed of 1.3 mm/min based on the ASTM D1002-01 and D5868-01 standards. All the specimens were tested in the same testing machine and the deformation process was imaged with the DIC system in the ambient temperature and humidity (25C, 33%RH). Prior to the tests, the special black and white paintings were sprayed to the polished front surface and side surface by an airbrush to create speckling patterns [36]. In order to improve the performance of the DIC measurement, the speckle distribution needs to be irregular and the speckle size should be about 220-440 μm according to the area of interest. The experimental setup and the speckling patterns of the specimen were shown in Fig. 4. Fig. 4. Experiment: (a) tensile setup and DIC system; (b) speckle pattern on the specimen front surface; (c) speckle pattern on the specimen side surface.

The ARAMIS v6.3.1 (GOM mbH, Germany) and VIC-2D (CSI, America) digital image correlation systems were used to capture real time full-field surface strain in the front view of the adherends and the side view of the adhesive bondline, respectively. In the DIC system, the digital images were recorded at the specified time interval for acquiring the surface deformation and strain until the specimen was fractured completely in tension. A 2448×2050 pixels CCD camera equipped with a lens of 50 mm focal length was used to acquire the images. The sampling rate of the camera was set to be two images per second to describe the strain evolution during the quasi-static tensile test, especially recorded the deformation of the side speckle pattern. The post-processing analysis of all the measurement results was conducted with the DIC professional software (ARAMIS, Germany). The facet size and facet step were set up as per the resolution of the images and the average speckle size (diameter) to

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ensure the accuracy of computation [37]. Specifically, the facet size and facet step were set to be 11 and 8 pixels for computing the deformation of the front surface; while the facet size and facet step were set to be 9 and 5 pixels for the side view to obtain more deformation information of adhesive.

3. Results and Discussion 3.1. Strength analysis 3.1.1. Effects of adherend thickness Fig. 5. Average peak load to fracture with different adherend thicknesses.

All the joint specimens were tested and the results of average peak fracture load with error bar were graphed in Fig. 5. Note that the results for the same joints, e.g. AS-2-2 and SA-2-2, were also presented for comparison. According to the overall trends of the average peak fracture load, joint strength (note that in this study the overlap length of all specimens were the same, i.e. 25mm, thus the average peak load can be used to evaluate the joint strength) decreased as the adherend thickness decreased from 4mm to 2mm regardless of the type of joints. This means that the joint strength increased with the adherend thickness [38]. The interpretations of this phenomenon are that, on the one hand, the bending stiffness of the adherend is proportional to the cube of its thickness (It is well known that improving the bending stiffness of the adherends, the resistance of bend for adherend can improve. This would decrease the peel stress which is the major stress result in the failure of single-lap joints), while the bending moment resulted from the eccentric loading increases linearly, which makes the thicker adherends more resistant to eccentric bending under tension [14]. On the other hand, the metal adherends are easier to undergo plastic deformation when decreasing their thickness, which may lead the joint edges to developing very high localized strains, thereby reducing the joint strength considerably [39]. However, when comparing the strength of thin and thick joints further, the general increment of strength for each type joint differs, resulting in the average improvement of

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strength by 26.20%, 21.85% and 11.45% for Type I, Type II and Type III joints, respectively. First, this implies that the joint strength is not linearly proportional to the thickness of the adherends as the joint strength was not doubled as the adherend thickness is doubled. Second, this also indicates that the sensitivity of adherend thickness on the joint strength is related to the adherend material type because the average improvement of Type I (all the joints with the aluminum adherends) is the highest while that of Type III (all joints with the CFRP adherends) is the lowest with the same adhesive. As aforementioned, the adherend yielding could reduce the joint strength. Increasing thickness can delay yielding of the adherend, thereby increasing the joint strength sizably. However, if the adherend has relatively high yield strength, the adherend may not yield even with a small thickness. In this case, increasing thickness may not delay the yielding of this adherend but only increase its stiffness; and thus it could increase the joint strength only to a limited extent. Note that CFRP has been known as an elastic-brittle material, which has no obvious yielding under tension [40]. It can be however assumed that the ultimate strength of CFRP indicates the quasi-yield strength when discussing the effect of material type on the joint strength. Thus, it was found that the yield strength of CFRP is the highest, followed by the Q235 steel and AA 5182 aluminum alloy as summarized in Tables 1 and 2. Thus, it explained why the average improvement for adherend material Type III is the lowest.

3.1.2. Effects of adherend material type Not only is the material type related to the sensitivity of adherend thickness on the joint strength, but also affect the joint strength and joint stiffness, directly. Fig. 6 shows the typical load-displacement curves of different groups of joints (Groups A-F). It is worth pointing out that the displacement measured should not be taken as the accurate value because it could have included some machine/fixture deformation and slippage between the grips and adherends. But the peak load and trend of curve can still properly reflect the effects of material type on the joint strength and joint stiffness. Fig. 6(a) shows the typical load-displacement of Group A, where the adherends had the same material and thickness

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(2mm). It was found that the strength of CC joint is the highest, followed by the SS joints, whereas the AA joints the lowest. Interestingly, the SS joint has the highest initial stiffness (slope of the curve), followed by the AA joint, whereas the CC joint is slightly lower than the AA joint. The results shown in Fig. 6(b) for Group B are almost the same as those for Group A except that the SS joints have higher strengths than the CC joints. The CFRP adherends would generate the delamination while experiencing relatively higher tensile load, which made the mechanical properties of the CFRP be degraded [41], thus decreasing the strength of the CC joints. Fig. 6(c) presents the results of Group B/C composed of thinner joints with dissimilar materials. It can be seen that the strength of the CS joint was the highest, followed by the AC joints, whereas the SA joints came to the lowest. However, the difference between the joint strength and initial stiffness (seen in the enlarged insert in Fig. 6(c)) is that the SA joint had the highest initial stiffness, followed by the CS joints and the AC joint was the lowest. Similar to Group B/C, Group E/F exhibits the same characteristics. It has been well known that increase in the stiffness of materials lowers the elastic deformation in the adherend, thereby improving the initial stiffness of the joints. As shown in Tables 1 and 2, the Q235 steel has the highest stiffness, while CFRP the lowest (i.e. Q235﹥AA5182﹥CFRP). Fig. 6 compares the differences of initial stiffness of all the joints. It can be seen that the higher the stiffness of two adherends, the higher the initial stiffness of the joint regardless of the joints configuration. Further, the difference of joint strength can be elucidated from the yielding of the adherends, which may reduce bonding strength due to the higher plasticity of adherend [39]. Note that the yield strength of CFRP is higher than those of the Q235 adherend and 5182 aluminum adherend (i.e. CFRP﹥Q235﹥ AA5182). It can be also seen that the higher the yield strength of adherends, the higher the joint strength if the CFRP adherends do not have damage or delamination [42]. Fig. 7 further presents the results of the Type I (these joints that have at least one Al5182 aluminum adherend) configuration. The results of joint strength and initial stiffness of Type I

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joints are consistent with the aforementioned conclusions that the higher the stiffness of the adherends, the higher the initial stiffness of the joint; as well as the higher the yield strength of the adherends, the higher the joint strength. And Type II and Type III presented the same behaviors except that the strength of the SS-4-4 joint was higher than that of the SC-4-4 joint because the CFRP adherend with 4mm thickness was damaged in such a tension state. Fig. 6. Typical load-displacement curves for different groups joints: (a) Group A; (b) Group D; (c) Group B/C; (d) Group E/F. Fig. 7. Typical load-displacement curves for TypeⅠjoints: (a) thinner joints; (b) thicker joints.

3.2. Fracture process analysis 3.2.1. Joints with the same adherend material Fig. 8(a) presents the typical load-displacement curve of the CC-2-2 joint, in which the curve exhibits a linear trend up to failure, somewhat similar to the CFRP laminate alone. Fig. 8(b) shows the deformation pattern of the joint at the peak load. Note that the adherends could be rotated and generated a hinge due to the strong bending effect. The rotation angle α and the location of hinge p, as shown Fig. 8(b), can be uesed to descibe the extent of bending and bending location of the adherends, respectively [16]. It is clear that the rotation angles α1 and α2 are approximately equal (i.e. 3.246 on average), and the locations of hinges are almost the same at the lap ends, where the small differences were likely due to the manufacturing errors of the specimens. As shown in Fig. 8(b), two clear cracks can be identified in the lap ends with roughly the same length, which indicates that the cracks initiated and propagated symmetrically.

Fig. 8. Fracture process of the CC-2-2 joint: (a) typical load-displacement curve; (b) deformation pattern of the joint under the 100% peak load; (c) deformation patterns under different loads.

Fig. 8(c) presents a sequence of six images from the side view at several respresentative points A-E shown in Fig. 8(a) as well as the images of complete fracture. The progression of cracking images can be used to describe relation of the load history with the crack growth

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path in the joint. The picture corresponding to Point A represents the original state without loading, where the upper and bottom edges of joint are parallel to the two red horizontal lines, indicating that no deformation occurred in the joint. With increase in loading from Point A to Point E seen in Fig. 8(a), the adherends bent and formed a hinge due to the bending effect. It can be seen that the rotation angle α became larger and the joint edges ran more and more over the horizontal lines symmetrically. Meanwhile, the cracks initiated from the both ends and gradually propagated to the central region until catastrophic fracture. The complete fracture photo shows that the two adherends both returned to the original shapes with the rotation angle equals to zero with negligible deformation. It indicates that the both CFRP adherends only generated negligible deformation due to high yield strength even though withstood a substantial tension with the accessary bending [43]. Fig. 9(a) shows the fracture process of the AA-2-2 joint, in which it can be seen that the cracking paths and the location of hinge are similar to those of the CC-2-2 joint. The difference resided in a smaller deformation prior to the 50% peak load, but the difference increased with the load. The deformation increased and the rotation angle became larger than that of the CC-2-2 joint. In comparison with the complete fracture photo of the CC-2-2 joint, the adherends of AA-2-2 generated substantial plastic deformation, indicating that the adherends yielded under strong tension [44]. Compared with the AA-2-2 joint, the fracture of the SS-2-2 joint had a similar process but smaller difference in terms of elastic/plastic deformation and rotation angle, as shown in Fig. 9(b). Fig. 9. Comparison of the fracture processes of thinner joints with the same adherends.

By the comparison of these three thin joints same adherend material, it can be concluded, that when the adherends are in elastic state (e.g. the load up to the 50% peak), the greater the adherend stiffness, the smaller the deformation of bonded joints. When the joint fractures completely, the higher the yield strength of adherends, the smaller the plastic deformation of adherends for the same thickness adherend [39]. Figs. 10(a)-(c) show the fracture processes of the thicker joints, where no obvious cracks

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and hinges could be identified. The deformation of these joints was very small prior to the 75% peak load. Further up to the peak load, deformation of these joints gradually increased but was still smaller than the corresponding thinner joint counterparts, which means that the adherend thickness affected the deformation pattern of the joints. When observing the complete fracture photo of these thicker joints, the deformation of adherends is almost negligible. It can be explained that increasing adherend thickness improves the resistance to deformation. It should be pointed out however that there was also some asymmetric behavior in these processes due to presence of manufacturing defects in the bondline and the asymmetric gripping in the test machine during the experiments. Fig. 10. Comparison of the fracture processes of thicker joints with the same adherends.

3.2.2. Joints with the dissimilar adherend material When two different adherend materials were involved, the typical load-displacement curve can be characterized by a nonlinear behavior up to peak load, where any further increase in the displacement led to the decline of the load and subsequent cracking in the joint, as illustrated in Fig. 11(a). Taking the AC-2-2 joint as an example, as the load increased, the aluminum adherends yielded when reached a relatively lower yielding strength of the 5182 aluminum alloy. Fig. 11(b) shows the deformation pattern of the joint at the peak load. An evident crack can be seen in the lap end, which initiated from the aluminum adherend. Fig. 11. Fracture process of the AC-2-2 joint: (a) typical load-displacement curve; (b) deformation pattern of the joint under the 100% peak load; (c) deformation patterns under the different loads.

It is found that the rotation angle α1 (4.324) was approximately equal to α2 (4.103). Besides, the locations of the two hinges differed as seen in Fig. 11(b). The location of hinge p1 was not at the lap end, whereas that of p2 was. And the deformation patterns presented in Fig. 11(c) indicate that with increase in the load, the rotation angle α1 and α2 both increased. The location of hinge p1 formed at the root of the lap end first, and then moved toward the central region of the overlap, while p2 remained almost at the lap end. Meanwhile, an envident crack initiated from the lap end of aluminum adherend and propagated to the central region whilst 15

the other cracks initiated. It is clear that the deformation of adherends was not symmetric and the side of the aluminum adherend had a larger deformation, leading to a higher peel stress there that led to crack first. The photo of complete fracture exhibits the final failure state of the joint, which was easy to see that the CFRP adherend returned to its original shape with negligible deformation, whilst the aluminum adherend underwent a substantial plastic deformation. Similarly to the joints with the same adherends, it can be explained that the CFRP adherend generated neligible deformation while the aluminum adherend had underwent considerable yield subject to the tension.

Fig.12. Comparison of the fracture processes of the thinner joints with dissimilar adherend materials.

Fig. 12 compares the fracture processes of the different joints with the dissimilar adherend materials. Fig. 12(b) presents the fracture process of the AS-2-2 joint, in which the location of crack initiation can be found around the lap on the aluminum adherend. With the increase in the load, the hinge at the aluminum adherend moved to the center from the lap end and the rotation angle increased. These are similar to those of the AC-2-2 joint. However, the difference between the AS-2-2 and AC-2-2 joints is the adherend deformation. The Q235 steel adherend generated plastic deformation while the CFRP returned the original shape. Compared with the AC-2-2 and AS-2-2 joints, the SC-2-2 joint was of the similar behavior to the AC-2-2 joint as the crack initiated from the lap end on the metal adherend side and the hinge moved to the central overlap region from the end gradually. In addition, the trend of rotation angle and the fracture pattern of adherend are also similar to those of the AC-2-2 joint. For these abovementioned thinner joints with dissimilar adherends, it can be concluded that the crack initiated from the adherend side with a lower yield strength and the deformation became asymmetric with the increasing load; and the lower the adherend yield strength, the larger the deformation of the adherend to complete fracture.

Fig. 13. Comparison of the fracture processes of thicker joints with dissimilar adherend materials.

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Figs. 13(a)-(c) show the fracture processes of the thicker joints. It can be found that the behaviors of these joints are fairly similar. The difference existed only with a smaller deformation of adherends in comparison with the thinner joints. Interestingly, it was observed that the location of crack initiation was somewhat random and the adherends almost returned to the original state at fracture. It can be elucidated that the thick adherend generated small deformation under tension, so there was no visible difference of these joints. This provides further evidence that the adherend thickness has more substantial influence on the fracture process of the joints.

3.3. Digital Image Correlation analysis 3.3.1. Adherend displacement and strain distribution In order to explore the effect of the adherend thickness on the strength of the joints visually, the out-of-plane deformation (z-direction displacement) distributions of the overlap region were acquired using the DIC system, as shown in Fig. 14. It is found that all the joints have fairly similar displacement gradient regardless of the adherend thickness and adherend materials. Increase of adherend deformation reduced the bending moment by making the loading line closer to the neutral plane of each adherend for the single-lap joints under tension [38]. Theoretically, this means that the joints with the same adherends deformed with respect to the central line of overlap symmetrically; and the displacements reached the maximum at the extremities of the overlap region. However, none of the joints exhibits prefect symmetry due to the presence of manufacturing errors/detects; the variations in the bondline and gripping of specimens caused asymmetry. The fact that not all the contour lines for different joints were horizontal indicates that a degree of twisting occurred under tension. Hence, the displacement measured by DIC cannot be taken as an absolute value to evaluate the bending effect; instead, the displacement difference △U, defined in terms of the difference of the maximum displacement (UZmax) to the minimum displacement (UZmin) in the overlap region could be an indicator to evaluating the bending effect of the different joints. As shown in Fig. 14, it is found that the displacement differences △U of the thin joints are higher than those of

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the thicker joints, meaning that the resistance to the eccentric bending effect increases with thickness of the adherends. Fig. 14. Comparison of the displacement distributions of joints with different thicknesses when being loaded at 5000N: (a) the location of speckle; (b) the displacement distribution.

In comparison with the displacement differences △U of these three thinner joints under the same load of 5,000N, it is found that the value of the AA joint was the highest, followed by the SS joint and the CC joint the lowest. The aluminum and steel adherends experienced a certain level of yielding under this load, which led to a larger deformation. For the thicker joints, the CC joint has the highest deformation followed by the AA and SS joint under the 5,000 N load. It can be explained that the thicker joints all remained in the elastic state under 5,000 N load, and the higher stiffness resulted in the smaller deformation.

Fig. 15. The strain contours measured from the DIC System of the thick metal adherends at peak load: (a) the location of speckle; (b) strains of aluminum adherends; (c) strains of steel adherend.

In comparison with the thin adherends, the adherend of the thicker joints generated a smaller deformation. Fig. 15 presents the strain contours of the thick metal adherends at the peak load to depict the adherend deformation. From the stress-strain curves as shown in Table 2, the strains at yielding calculated for the 5182 aluminum and Q235 steel are 0.13% and 0.10%, respectively. Fig. 15(b) displayed the strain distribution of all the thick aluminum adherends. It is found that the aluminum adherends experienced yielding as shown in the strain contours. Note that the strains near the lap ends are higher than those in the other regions. It can be also explained that the formation of hinge here made the adherends experience in both tension and bending. Fig. 15(c) presents the strain distribution of the steel adherends. It can be seen that the strains of steel adherends exceed 0.1%, corresponding to the strain of steel yield. The strain of steel adherend of the AS-4-4 joint is the smallest for the corresponding lowest peak load. In

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comparison with the aluminum adherend in the AS-4-4 joints, the deformation of steel adherend is substantially lower. Fig. 16 presents the strain contours of the AC-2-2 joint measured from the DIC system for showing the difference of strain distributions in the dissimilar adherend materials. Fig. 16(a) illustrates the locations of the analysis surfaces chosen for the DIC measurement. The analysis area outlined with the blue frame is the free region of the aluminum adherend; the analysis area in the red frame is the overlapping region of the aluminum adherend; and the analysis area in the black frame is the region of CFRP adherend without lap. Figs. 16(b)-(d) show the computed distribution of the full-field longitudinal strain (εxx) at fracture in the areas with the blue, red and black frames, respectively. It is found that the magnitudes of strains in the non-lap regions are higher than those at the overlap region. The longitudinal strains (Fig. 16(b)) in the aluminum adherend are rather uniform and the values are relatively higher because the aluminum adherend had experienced yielding under the peak load. Fig. 16. The strain contours measured from the DIC System of the AC-2-2 bonded joint at the peak load: (a) surfaces for DIC analysis; (b) surface of aluminum adherend; (c) surface of aluminum adherend at lap area; (d) surface of CFRP adherend.

Compared to the aluminum adherend, the CFRP adherend experienced smaller deformation and lower strains. The strain in the CFRP adherend located in the overlap end is higher than that in the other areas due to formation of the hinge; and the surface there experienced both tension and bending as shown in Fig. 11(b). As presented in Fig. 16(c), majority of strains in the overlap region was approaching to zero, implying that there was small deformation due to the high stiffness of joints after adhesion. The blue band area represents compressive strains, which corresponds to the location of the plastic hinge as shown in Fig. 11(c). The existence of compressive strains well explains the presence of bending effect and exhibits the location of plastic hinge. It can also be shown that the plastic hinge in the aluminum adherend moved from the lap end to the center in the overlap region [45]. Such results also demonstrated that the DIC technique can be an effective tool to capture the full-field deformation and strain of the adherends in the joints.

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3.3.2. Strain analysis of adhesive bondline The DIC technique allows monitoring the strain evolution on the sampling surface in real-time, which is crucial to analyze the fracture process via the strain evolution of the adhesive bondline. Figs. 17-20 present the fracture analyses of the AC-2-2 joint by using the acquired DIC data. As aforementioned, the failure at the free edges of the adhesive layer initiated from the peel effect in tension. Fig. 17 shows the two representative peel strains (εzz) at two specific points (P1 and P2) in the ends of the joint versus the loading duration (in second). It can be seen that they have limited difference and coincide very well with increasing tensile loads prior to the crack nucleation at point P1 (see the fitted red solid line in the inset of Fig. 17), which indicates that the crack initiates from the lap end on the aluminum adherend side of the AC-2-2 joint. For the curve of point P1, when the loading time increased to over 46s, the peel strain (εzz) increased gradually first and then grew exponentially later (see the fitted purple solid line in the inset of Fig. 17). This transition process implies that a new crack may have developed. In order to determine the critical transition location and obtain the corresponding critical tensile load, the peel strain (εzz) data were linearly fitted in different stages, respectively. Note that the red and purple lines intersect at angle H, the black dashed line was plotted along the angular bisector of angle H, which intersects with the actual experimental curve of point P1 at point M. Therefore, the testing time at point M can be approximately regarded as the critical fracture time (t=56.5 s). Similarly, point N can be determined for obtaining the critical fracture time (t=62.5 s) corresponding to the crack initiation at point P2 on the right side of the joint. It can be found that point P1 at the CFRP lap end (where aluminum lap starts) initiated the crack first, which coincided with the fracture process captured by the cameras. At this critical time, the critical tensile load (7,003.33 N) and peel strain (1.24%) can be determined by using the recorded data, respectively.

Fig. 17. Determination of the critical strain corresponding to cracking formation of the AC-2-2 joint. Fig. 18. A typical loading-displacement-time curve of the AC-2-2 joint during tension test.

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Fig. 18 shows the relationships of tensile load (F), displacement (d) and test time (t). The several representative points, i.e. a-f as illustrated in Fig. 18, were used to extract the full/local strain contours on the side surface of joint, as displayed in Fig. 19. It is found that the load increased linearly with displacement until the time reached 10.5s, where the aluminum adherend started yielding. The peel strain (εzz) measured by DIC showed no apparent change in this time step from the strain contour (ⅱ), as shown in Fig. 19(a). As the displacement increased, the load increased but with a relative smaller slope and the peel strain gradually increased as seen from the strain contour (ⅲ) in Fig. 19(a). The contour also exhibits that new cracks started nucleating along the weakest region at the lap extremities (i.e. Point P1 as seen in Fig. 17, where F=6,582.96 N and t=49 s). Similarly, as load increased up to 7,003.33 N，the crack at Point P1 (seen in Fig. 17) initiated as a result of sufficient cracking nucleation as displayed in the strain contour (ⅳ) in Fig. 19(a). However, the load in this time point had not reached the peak yet. With the crack propagation, the load increased further to 7,100.07 N, indicating the peak load. The strain contour (ⅴ) presents the evident strain concentration in the lap ends. By then, the joints still had some load bearing capacity even though the crack had initiated. After point e as in Fig. 18, the joint gradually lost its load bearing capacity and the cracks propagated further (as seen from the strain contour in Fig. 19(b)). In order to analyze the whole fracture process, Fig. 19(b) shows the fracture strain contours (ⅴ-ⅵ) of the AC-2-2 joint, from the peak load to the complete fracture. It can be observed that cracks from these two lap ends propagated to the central region rapidly until the residual lap length reached a critical value to fracture catastrophically [38]. Fig. 19. Evolution of strain contours on the side view of the AC-2-2 joint: (a) the loading process up to the peak load; (b) the fracture process of catastrophic propagation.

To better illustrate the loading process, Figs. 20(a)-(d) show the computed longitudinal strain (εxx), peel strain (εzz), shear strain (εxz), and principal strain (ε1) field on the adhesive layer along line P1P2 as illustrated in Fig. 17 at several representative points a-e illustrated in Fig. 18. By observing these strain distributions under the peak load at point e, it is evident that

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all the strains are not symmetric with respect to the central line of the overlap region; and the peel, shear and principal strains located in the lap ends are higher than those in the overlap region. The values on the left (aluminum adherend side, as shown in Fig. 17) are higher than those on the right (the CFRP adherend side). However, the longitudinal strain (εxx) distribution differs from the others, where not all the maximum strains appeared at the lap ends as the maximum strain on the left side moved toward the center whereas the maximum strain on the right side still stayed in the end. These phenomena imply that the deformation of adhesive on the left is greater than that on the right; thus it explains why the crack occurred on the left first. With regard to the magnitude of these strains, the magnitudes of peel and shear strains at the lap ends were higher than those of the longitudinal strain, implying that the high peel and shear strain distributions are of significant influence on the initiation of cracks at the lap ends. Further, in comparison with the longitudinal and peel strains, the shear strain in the center region of overlap edge was high, indicating that the shear strain is of more influence in the propagation of cracks along the lap edge. However, compared to the peel and shear strains, the longitudinal strain was relatively lower, thus it had less effect on the failure of the joints. When observing the distribution of principal strain, it is found that the normal strain distribution was similar to that of the shear strain, implying that the shear strain is of more influence in the failure of single-lap joints [46]. Fig. 20. (a) longitudinal strains; (b) peel strains; (c) shear strains; (d) principle strains measured from DIC along the line P1P2 as shown in Fig.17 at several representative points a-e in Fig.18.

From Fig. 20(b), it is found that prior to the aluminum adherend yielding (point b), there was no much change in the strain distribution. With further increase in loading, the peel strain started increasing (see point c), which became compressive near the ends. It can be elucidated that the adherends bent near the ends, making the adhesive in compression. Then, when the load increasing up to the critical level (point d, corresponding to the crack initiation), the cracks initiated so that the strains rapidly increased in comparison with the strains at point c. When the load increased to the peak level (point e), the strains were getting higher quickly but the distribution did not change much, meaning that the strains changed faster when the cracks

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initiated and propagated. When considering the shear strains as seen in Fig. 20(c), it is found that prior to yielding of the aluminum adherend (point b), the shear strain on the bondline had no evident change. With increase in the load, the strains increased progressively until failure; and the strain distribution along the overlap length of point a-e curves all remained the same. The increase in the shear strain at the central of overlap region means that the adhesive here withstood more load progressively with the increasing load. In order to describe the effect of adherend thickness and adherend materials on the strain distributions of the adhesive region, Fig. 21 graphs the longitudinal, peel and shear strain distributions along the adhesive bondline for Groups A and D joints, respectively. Firstly, the strain distributions for different joints are found to be fairly similar. When considering the effect of adherend thickness, it is clear that these three strains of the thinner joints were higher than those of the thicker joints under the same load of 5,000N. This is due to the fact that the bending stiffness of adherend increased with the adherend thickness, which reduced the peeling effect and can delay the adherend to yield. It can be also observed that the change in the strains between the thinner and thicker joints depended on the adherend material. The change in the CC joint was the smallest while the AA joint was the greatest, corresponding to the sensitivity of the adherend thickness on the joint strength. For the thinner joints, the values of the longitudinal, peel and shear strains in the AA joint were the highest followed by the SS joint, whereas those in the CC joint were the lowest. For the thicker joints, all the strains of the AA joint were the highest, the CC joint the second. However, the strain difference for the thicker joints was fairly marginal. It can be thus concluded that the strains decreased with the increase in adherend yield strength when the adherend produced yielding under the same load. When looking at the strengths of Groups A and D, the higher the load-bearing capacity of joints, the lower the magnitude of adhesive strains was under the same load. Thus, the DIC system can monitor the adhesive strain for predicting the strength of joints.

Fig. 21. Comparative analysis about the joints with Groups A and D joints: (a) peak force of joints; (b) εxx; (c) εzz; (d) εxz distributions in the adhesive layer along the overlap length under the same load (5000N).

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3.4. Fracture surface and failure mode analysis In this study, four failure modes of the adhesive joints were identified, which are the adhesive failure (AF), cohesive failure (CF), light-fiber-tear failure (LFTF) and the thin-layer cohesive failure (TCF) [47]. In order to distinguish the epoxy matrix and adhesive in the thin-layer cohesive failure mode, where they could become “white” after failure, the scanning electron microscope (SEM) analysis was conducted on the sectional plane of the CFRP adherend. The CFRP adherend with the thin-layer adhesive (Area 1) and the CFRP adherend after surface preparation (sand papering in Area 2) were included, as shown in Fig. 22(a). Fig. 22(b) shows that the adherends with the thin-layer adhesive were of a graded layer with the adhesive at the top layer, which presented a better conductivity under SEM scanning. Fig. 22(c) presents the adherend with the fibers fully covered in the epoxy matrix after sandpaper preparation. In comparison with Area 3 (where the CFRP was not grinded by sandpaper as shown in Fig. 22(a)), it is evident that the fibers were not exposed even after sand papering. By comparing Fig. 22(b) with (c), it was found that the white resin region covered on the failure surface of the thin-layer cohesive failure mode was the adhesive rather than the epoxy matrix. Fig. 22. SEM analysis of the CFRP adherends: (a) the locations for the SEM analysis; (b) CFRP adherends with the thin-layer adhesive; (c) CFRP adherend after surface preparation.

Fig. 23 shows the failure modes of the metal adherend, which indicates that the failure modes of the metal adherends were adhesive failure located in the red frame and cohesive failure mode (Fig. 23(c)). In contrast to the metal adherend, the CFRP adherends were of some different failure modes as shown in Fig. 24, which contained three failure modes, namely cohesive failure, light-fiber-tear failure located in the green frames and thin-layer cohesive failure located in the yellow frames, as presented in Figs. 24(b)-(d), respectively. Fig. 23. Failure modes of the metal adherends: (a) locations of different modes; (b) adhesive failure (c) cohesive failure.

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Fig. 24. Failure modes of CFRP adherends: (a) locations of different modes; (b) cohesive failure; (c) light-fiber-tear failure; (d) thin-layer cohesive failure. Table 4 Fracture surfaces and failure mode ratios of all the adhesive joints.

Table 4 summarizes the fracture surfaces and failure mode ratio (surface area of each failure mode divided by the overlap surface area) for each type of the adhesive joints. Note that in the fracture surfaces of such three types of joints (TypeⅠ,TypeⅡ and TypeⅢ), the left adherends are the aluminum, steel and CFRP adherends, respectively. From this Table, it is easy to see that all the joints exhibit a mixed failure mode. The cohesive failure is a principal mode through calculating the surface ratio. It has been known that adhesive failure cannot take the maximum potentials of the adhesive layer, thus the more the adhesive failure, the worse the bonding performance [48]. In Group A, the adhesive failure ratio of the AA joint is the highest (33.51%), followed by the SS joint (16.31%), while the CC joint had no adhesive failure. Group D presents the ratios of adhesive failure for the thicker joints in 23.60%, 8.98% and 0% for the AA, SS and CC joints, respectively. One of the reasons is the difference of interface chemistry because the bonding processes were the same. It is known that aluminum always forms a layer of dense surface oxidation in air even within a short period of time, making it difficult for reaction to the adhesive in comparison with the steel [49]. Compared with the metallic adherends, CFRP can establish a strong interfacial bonding due to the similarity between the epoxy resin matrix and epoxy adhesive. On the other hand, the adherend thickness also affects the failure modes of the joints due to its stiffness and yield strength delaying. And it can be verified by comparing Group A with D or comparing Group B with E because the adherend materials are the same but the stiffness increased and yielding delayed due to increase in the adherend thickness. It can be seen that the ratios of adhesive failure (AF) in the thinner joints are all higher than those in the thicker joints for the metal adherends. For the CFRP adherends, the thin-layer cohesive failure ratios in the thin joints are higher than those in the thicker joints, especially for the CC-2-2 joints which generated the light-fiber-tear failure while the CC-4-4 joints mostly produced the cohesive failure, as shown in Table 4. It can be explained that the peeling effect is more

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significant when the adherends are thinner; and the interfacial strength is more sensitive to the peel stress. Fig. 25 presents the relationship between the AF ratio and adherend materials for Type I joints. When comparing the surface ratio of failure for the Type I joints with the thinner adherend, it can be seen that the adhesive failure ratio of the AA joints (33.51%) is the highest while that of the AC joints is the lowest (10.16%), indicating that the joint strength of the AA joints was lowest. The joints with the thicker adherends have the similar trend but with lower magnitudes in comparison with the thinner joints as shown in Fig. 25, meaning that the failure modes of the same adherends (such as aluminum adherends in TypeⅠ) would differ when they were bonded with different adherend materials. This is because the adherend material properties could alter the stress and strain distribution of the joints, thereby affecting the failure modes of the adherend. The results of Type II (steel adherends) are fairly similar to those of Type I (aluminum adherends). The main difference is that the average AF ratio of Type II (12.35%) is lower than that of Type I (18.38%), meaning that the joints with the Q235 steel adherends had better adhesive performance than the 5182 aluminum counterparts. However, the CFRP adherends (included in Type Ⅲ) had no adhesive failure due to its good interfacial bonding and higher yield strength; Thus there were no clearly different failure surfaces when bonding with the other materials. The adherend material properties can affect the failure modes due to alteration of stress and strain distribution in the adhesive. However, the changes in the stress and strain statuses did not make the joint generate the adhesive failure (interfacial failure) easily because the bonding strength of the adhesive with the CFRP adherends is much stronger. This implies that the material type can affect the failure modes and thus determine the joint strength when the bonding performance of adherend with adhesive is relatively poor.

Fig. 25. Effects of adherend materials on the AF ratio of Type I joints.

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4. Conclusion In this study, twelve different configurations of single-lap adhesive joints were tested to investigate the influence of adherend thickness and adherend materials on the fracture characteristics of the adhesive joints. The adherend deformation and fracture process were monitored by the CCD cameras in real time. The DIC analysis was conducted for acquiring the full-field strain information in the adherends as well as the strain evolution in the adhesive bondline during the tensile tests. The microstructural analysis of the fracture surfaces was also carried out to characterize the failure modes of the joints. Within the limitation of this study, the following conclusions can be drawn: (1). The joint strength increased with the adherend thickness regardless of the joint types; but it was not linearly proportional to the thickness of the adherends. The higher the yield strength of adherend material, the lower the sensitivity of adherend thickness to the joint strength. In other words, increase in adherend thickness for improving the joint strength needs to take into account the yield strength of adherend materials. (2). The adherend material type can affect the initial stiffness and strength of the joint. The initial stiffness increased with the material stiffness of either adherend or both adherends. The higher the yield strength of adherend materials, the higher the joint strength. It was confirmed that the degradation (e.g. damage or delamination) of CFRP performance could decrease the joint strength. (3). For the joints with the same adherends, the fracture process was symmetric in terms of the cracks initiation and propagation from the end to the center of the overlap region as well as the location of bending hinge lying on the lap end. The fracture processes of joints with dissimilar adherends were asymmetric. For a thinner joint, the crack initiated in the lap end on the adherend side whose yield strength is lower; the crack and hinge both moved toward the center of the overlap region as the load increased. (4). The out-of-plane deformation in the overlap region measured by the DIC technique showed that the resistance to bending effect increased with the adherend thickness. The strain distribution in the adhesive bondline showed that the peel and shear strains were

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the major components to affect the joint failure. The strain evolution revealed that the crack initiation of the joints with dissimilar adherends located in the lap end on the adherend side with a lower yield strength. (5). The failure modes in the metal adherend included the adhesive failure and cohesive failure; while those in the CFRP adherend involved cohesive failure, light-fiber-tear failure and thin-layer cohesive failure. The SEM analysis revealed that the white resin covered on the failure surface of the thin-layer cohesive (TLC) failure mode was the adhesive rather than the epoxy matrix. Adherend thickness and adherend material type can affect the failure modes as well as the mode ratios of the joints. For the metal adherends, the adhesive failure (AF) ratio decreased with the adherend thickness and adherend yield strength. For the CFRP adherend, the adherend would lead to the light-fiber-tear failure with the decrease in the adherend thickness.

Acknowledgments This work is supported by National Natural Science Foundation of China (51575172, 51475155) and the Open Fund of the State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body (31615001). Dr Guangyong Sun is a recipient of Australian Research Council (ARC) Discovery Early Career Researcher Award (DECRA) at the University of Sydney.

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[26] ASTM D.3039/D 3039M-07[J]. Standard test method for tensile properties of polymer matrix composite materials, 2008. [27] ASTM D. 3158/D 3258 M–94[J].Standard test method for in-plane shear response of polymer matrix composite materials by tensile test of a ±45 º Laminates,2007. [28] Marques GP, Campilho RDSG, da Silva FJG, Moreira RDF. Adhesive selection for hybrid spot-welded/bonded single-lap joints: Experimentation and numerical analysis. Composites Part B: Engineering. 2016;84:248-57. [29] Hu P, Han X, Li WD, Li L, Shao Q. Research on the static strength performance of adhesive single lap joints subjected to extreme temperature environment for automotive industry. International Journal of Adhesion and Adhesives. 2013;41:119-26. [30] Grant LDR, Adams RD, da Silva LFM. Effect of the temperature on the strength of adhesively bonded single lap and T joints for the automotive industry. International Journal of Adhesion and Adhesives. 2009;29:535-42. [31] ASTM D. 1002–01[J]Standard test method for apparent shear strength of single-lap-joint adhesively bonded metal Specimens by tension loading (Metal-to-Metal),2001. [32] ASTM D. 5868–01[J]Standard test method for lap shear adhesion for fiber reinforced plastic (FRP) Bonding,2001. [33] Naito K, Onta M, Kogo Y. The effect of adhesive thickness on tensile and shear strength of polyimide adhesive. International Journal of Adhesion and Adhesives. 2012;36:77-85. [34] Arenas JM, Alía C, Narbón JJ, Ocaña R, González C. Considerations for the industrial application of structural adhesive joints in the aluminium–composite material bonding. Composites Part B: Engineering. 2013;44:417-23. [35] da Silva LFM, Carbas RJC, Critchlow GW, Figueiredo MAV, Brown K. Effect of material, geometry, surface treatment and environment on the shear strength of single lap joints. International Journal of Adhesion and Adhesives. 2009;29:621-32. [36] Crammond G, Boyd SW, Dulieu-Barton JM. Speckle pattern quality assessment for digital image correlation. Optics and Lasers in Engineering. 2013;51:1368-78. [37] Triconnet K, Derrien K, Hild F, Baptiste D. Parameter choice for optimized digital image correlation. Optics and Lasers in Engineering. 2009;47:728-37.

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[38] Aydin M D, Özel A, Temiz Ş. The effect of adherend thickness on the failure of adhesively-bonded single-lap joints[J]. Journal of adhesion science and technology, 2005, 19(8): 705-718. [39] Kafkalidis M S, Thouless M D. The effects of geometry and material properties on the fracture of single lap-shear joints[J]. International Journal of Solids and Structures, 2002, 39(17): 4367-4383. [40] Sokolinsky VS, Indermuehle KC, Hurtado JA. Numerical simulation of the crushing process of a corrugated composite plate. Composites Part A: Applied Science and Manufacturing. 2011;42:1119-26. [41] Gao F, Boniface L, Ogin S L, et al. Damage accumulation in woven-fabric CFRP laminates under tensile loading: Part 1. Observations of damage accumulation[J]. Composites science and technology, 1999, 59(1): 123-136. [42] He J, Xian G. Debonding of CFRP-to-steel joints with CFRP delamination. Composite Structures. 2016;153:12-20. [43] Budhe S, Banea MD, de Barros S, da Silva LFM. An updated review of adhesively bonded joints in composite materials. International Journal of Adhesion and Adhesives. 2017;72:30-42. [44] Anyfantis KN, Tsouvalis NG. Loading and fracture response of CFRP-to-steel adhesively bonded joints with thick adherents – Part II: Numerical simulation. Composite Structures. 2013;96:858-68. [45] Pinto AMG, Magalhães AG, Campilho RDSG, de Moura MFSF, Baptista APM. Single-Lap Joints of Similar and Dissimilar Adherends Bonded with an Acrylic Adhesive. The Journal of Adhesion. 2009;85:351-76. [46] Ruiz P, Jumbo F, M. Huntley J, Ashcroft I, Swallowe GM. Experimental and Numerical Investigation of Strain Distributions Within the Adhesive Layer in Bonded Joints2009. [47] ASTM D. 5573/D 5573 M–99[J]. D5573-99[J]. Standard practice for classifying failure modes in fiber-reinforced-plastic (FRP) joints, 2012. [48] Baldan A. Adhesively-bonded joints and repairs in metallic alloys, polymers and composite materials: adhesives, adhesion theories and surface pretreatment[J]. Journal of materials science, 2004, 39(1): 1-49. [49] Yan C, Mao J, Nassar S, Wu X, Kazemi A. Experimental and numerical investigation of the effect of key joint variables on the static and fatigue performance of bonded metallic single-lap joints. Journal of Adhesion Science and Technology. 2014;28:2069-88.

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Table Captions Table 1 Mechanical properties of the CFRP laminate. Table 2 Tensile stress-strain curves and material properties of metal adherends and adhesive. Table 3 Dimensions of single-lap adhesive joint specimens. Table 4 Fracture surfaces and failure mode ratios of all the adhesive joints.

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Figure Captions Fig. 1. Geometry and dimensions of single lap joints. Fig. 2. Configuration utilized for the fabrication of single-lap adhesive joints. Fig. 3. Specimens used for test: (a) a typical single-lap joint after curing; (b) side view of the joint. Fig. 4. Experiment: (a) tensile setup and DIC system; (b) speckle pattern on the specimen front surface; (c) speckle pattern on the specimen side surface. Fig. 5. Average peak load to fracture with different adherend thicknesses. Fig. 6. Typical load-displacement curves for different groups joints: (a) Group A; (b) Group D; (c) Group B/C; (d) Group E/F. Fig. 7. Typical load-displacement curves for TypeⅠjoints: (a) thinner joints; (b) thicker joints Fig. 8. Fracture process of the CC-2-2 joint: (a) typical load-displacement curve; (b) deformation pattern of the joint under the 100% peak load; (c) deformation patterns under different loads. Fig. 9. Comparison of the fracture processes of thinner joints with the same adherends. Fig. 10. Comparison of the fracture processes of thicker joints with the same adherends. Fig. 11. Fracture process of the AC-2-2 joint: (a) typical load-displacement curve; (b) deformation pattern of the joint under the 100% peak load; (c) deformation patterns under the different loads. Fig. 12. Comparison of the fracture processes of the thinner joints with dissimilar adherend materials. Fig. 13. Comparison of the fracture processes of thicker joints with dissimilar adherend materials. Fig. 14. Comparison of the displacement distributions of joints with different thicknesses when being loaded at 5000N: (a) the location of speckle; (b) the displacement distribution. Fig. 15. The strain contours measured from the DIC System of the thick metal adherends at peak load: (a) the location of speckle; (b) strains of aluminum adherends; (c) strains of steel adherend. Fig. 16. The strain contours measured from the DIC System of the AC-2-2 bonded joint at the peak load: (a) surfaces for DIC analysis; (b) surface of aluminum adherend; (c) surface of aluminum adherend at lap area; (d) surface of CFRP adherend. Fig. 17. Determination of the critical strain corresponding to cracking formation of the AC-2-2 joint. Fig. 18. A typical loading-displacement-time curve of the AC-2-2 joint during tension test. Fig. 19. Evolution of strain contours on the side view of the AC-2-2 joint: (a) the loading process up to the peak load; (b) the fracture process of catastrophic propagation.

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Fig. 20. (a) longitudinal strains; (b) peel strains; (c) shear strains; (d) principle strains measured from DIC along the line P1P2 as shown in Fig.17 at several representative points a-e in Fig.18. Fig. 21. Comparative analysis about the joints with Groups A and D joints: (a) peak force of joints; (b) εxx; (c) εzz; (d) εxz distributions in the adhesive layer along the overlap length under the same load (5000N). Fig. 22. SEM analysis of the CFRP adherends: (a) the locations for the SEM analysis; (b) CFRP adherends with the thin-layer adhesive; (c) CFRP adherend after surface preparation. Fig. 23. Failure modes of the metal adherends: (a) locations of different modes; (b) adhesive failure (c) cohesive failure. Fig. 24. Failure modes of CFRP adherends: (a) locations of different modes; (b) cohesive failure; (c) light-fiber-tear failure; (d) thin-layer cohesive failure. Fig. 25. Effects of adherend materials on the AF ratio of TypeⅠjoints.

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Table 1 Mechanical properties of the CFRP laminate. Description

Variable

Longitudinal modulus (GPa)

E11

Transverse modulus (GPa)

E22

Shear modulus (GPa)

G12

Principal Poisson’s ratio

v12

Longitudinal tensile strength (MPa)

X1+

Transverse tensile strength (MPa)

Y2+

In-plane shear strength (MPa)

S

Ply-thickness (mm)

T

36

Value 57.02 57.02 8.59 0.067 679.67 592.53 68.00 0.25

Table 2 Tensile stress-strain curves and material properties of metal adherends and adhesive.

properties

materials Q235

Al5182

Araldite2015*

E(GPa)

201

72.9

1.85

ν

0.29

0.33

0.33

Yield stress (MPa)

204.33

96.48

14.60

Ultimate stress(MPa)

375.75

227.75

17.90

Note: E:Young’s modulus, ν: Poisson’s ratio, ‘*’: referring to Marques [28]

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Table 3 Dimensions of single-lap adhesive joint specimens. Group

TypeⅠ

Type Ⅲ

TypeⅡ

Specimen

thickness (mm)

Specimen

thickness (mm)

Specimen

thickness (mm)

ID

t1

t2

ID

t1

t2

ID

t1

t2

A

AA-2-2

2

2

SS-2-2

2

2

CC-2-2

2

2

B

AS-2-2

2

2

SC-2-2

2

2

CA-2-2

2

2

C

AC-2-2

2

2

SA-2-2

2

2

CS-2-2

2

2

D

AA-4-4

4

4

SS-4-4

4

4

CC-4-4

4

4

E

AS-4-4

4

4

SC-4-4

4

4

CA-4-4

4

4

F

AC-4-4

4

4

SA-4-4

4

4

CC-4-4

4

4

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Table 4 Fracture surfaces and failure mode ratios of all the adhesive joints. Group

Fracture surfaces TypeⅠ

TypeⅡ

Type Ⅲ

33.51%AF 66.49%CF

16.31%AF 83.69%CF

4.76%LFTF 31.40%TLC

18.75%AF 81.25%CF

7.92%AF 15.57%TLC

A AA-2-2 SS-2-2 CC-2-2 Failure modes

63.84%CF

B AS-2-2 SC-2-2 CA-2-2 Failure modes

10.16%AF 12.03%TLC

77.81%CF

18.75%AF 81.25%CF

7.92%AF 15.57%TLC

76.51%CF

23.60%AF 76.40%CF

8.98%AF 91.02%CF

0%AF 0.84%TLC

99.16%CF

14.80%AF 85.20%CF

7.36%AF 1.77%TLC

9.48%AF 19.81%TLC

70.71%CF

9.48%AF 19.81%TLC

14.80%AF 85.20%CF

7.36%AF 1.77%TLC

90.87%CF

76.51%CF

C AC-2-2 SA-2-2 CS-2-2 Failure modes

10.16%AF 12.03%TLC

77.81%CF

D AA-4-4 SS-4-4 CC-4-4 Failure modes E AS-4-4 SC-4-4 CA-4-4 Failure modes

90.87%CF

F AC-4-4 SA-4-4 CS-4-4 Failure modes

70.71%CF

39

Fig. 1. Geometry and dimensions of single lap joints.

40

Fig. 2. Configuration utilized for the fabrication of single-lap adhesive joints.

41

Fig. 3. Specimens used for test: (a) a typical single-lap joint after curing; (b) side view of the joint.

42

Fig. 4. Experiment: (a) tensile setup and DIC system; (b) speckle pattern on the specimen front surface; (c) speckle pattern on the specimen side surface.

43

Fig. 5. Average peak load to fracture with different adherend thicknesses.

44

Fig. 6. Typical load-displacement curves for different groups joints: (a) Group A; (b) Group D; (c) Group B/C; (d) Group E/F.

45

Fig. 7. Typical load-displacement curves for TypeⅠjoints: (a) thinner joints; (b) thicker joints.

46

Fig. 8. Fracture process of the CC-2-2 joint: (a) typical load-displacement curve; (b) deformation pattern of the joint under the 100% peak load; (c) deformation patterns under different loads.

47

Fig. 9. Comparison of the fracture processes of thinner joints with the same adherends.

48

Fig. 10. Comparison of the fracture processes of thicker joints with the same adherends.

49

Fig. 11. Fracture process of the AC-2-2 joint: (a) typical load-displacement curve; (b) deformation pattern of the joint under the 100% peak load; (c) deformation patterns under the different loads.

50

Fig. 12. Comparison of the fracture processes of the thinner joints with dissimilar adherend materials.

51

Fig. 13. Comparison of the fracture processes of thicker joints with dissimilar adherend materials.

52

Fig. 14. Comparison of the displacement distributions of joints with different thicknesses when being loaded at 5000N: (a) the location of speckle; (b) the displacement distribution.

53

Fig. 15. The strain contours measured from the DIC System of the thick metal adherends at peak load: (a) the location of speckle; (b) strains of aluminum adherends; (c) strains of steel adherend.

54

Fig. 16. The strain contours measured from the DIC System of the AC-2-2 bonded joint at the peak load: (a) surfaces for DIC analysis; (b) surface of aluminum adherend; (c) surface of aluminum adherend at lap area; (d) surface of CFRP adherend.

55

Fig. 17. Determination of the critical strain corresponding to cracking formation of the AC-2-2 joint.

56

Fig. 18. A typical loading-displacement-time curve of the AC-2-2 joint during tension test.

57

Fig. 19. Evolution of strain contours on the side view of the AC-2-2 joint: (a) the loading process up to the peak load; (b) the fracture process of catastrophic propagation.

58

Fig. 20. (a) longitudinal strains; (b) peel strains; (c) shear strains; (d) principle strains measured from DIC along the line P1P2 as shown in Fig.17 at several representative points a-e in Fig.18.

59

Fig. 21. Comparative analysis about the joints with Groups A and D joints: (a) peak force of joints; (b) εxx; (c) εzz; (d) εxz distributions in the adhesive layer along the overlap length under the same load (5000N).

60

Fig. 22. SEM analysis of the CFRP adherends: (a) the locations for the SEM analysis; (b) CFRP adherends with the thin-layer adhesive; (c) CFRP adherend after surface preparation.

61

Fig. 23. Failure modes of the metal adherends: (a) locations of different modes; (b) adhesive failure (c) cohesive failure.

62

Fig. 24. Failure modes of CFRP adherends: (a) locations of different modes; (b) cohesive failure; (c) light-fiber-tear failure; (d) thin-layer cohesive failure.

63

Fig. 25. Effects of adherend materials on the AF ratio of TypeⅠjoints.

64