**1. Introduction**

In recent years, with the increasing application of new materials such as aluminum alloy, high strength steel and composite materials in automobiles as well as the continuous development of multi-material hybrid design concept in the automobile industry, the traditional mechanical connection technology (such as welding and riveting) cannot meet the connection requirements between different materials [1,2]. As a new connection method, bonding technology has the advantages of uniform stress distribution, fatigue resistance, light weight, etc. [3–5]. In this case, the connection needs of dissimilar materials can be effectively realized. Therefore, compared with other connection technologies, increasing attention has been paid to bonding technology. As a tough adhesive, polyurethane adhesive is gradually being widely used in automobiles, since it not only has high tear strength, good impact resistance and excellent toughness, but also provides relatively uniform stress distribution due to its low elastic modulus.

However, as a kind of polymer material, adhesive relies on temperature to some extent. The change of temperature will directly affect the mechanical properties of the material, and its failure strength and failure form change with different temperatures [6]. In the process of service, the ambient temperature range of adhesive structure is large. During the process of vehicle operation, the adhesive structure needs to provide enough strength in the service temperature range. The performance of the bonding structure is closely related to the service temperature, and the bonding structure significantly affects the overall strength and fatigue characteristics of the car body. Therefore, the research on the influence of the whole service temperature field on the performance of the body

**Citation:** Liu, H.; Fan, Y.; Peng, H. Effect of Full Temperature Field Environment on Bonding Strength of Aluminum Alloy. *Crystals* **2021**, *11*, 657. https://doi.org/10.3390/ cryst11060657

Academic Editors: Yifeng Ling, Chuanqing Fu, Peng Zhang, Peter Taylor and Sergio Brutti

Received: 10 May 2021 Accepted: 5 June 2021 Published: 9 June 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

bonding structure is the technical guarantee to realize the lightweight design of the body structure. Scholars at home and abroad have carried out relevant studies on the static performance and strength-checking criteria of temperature bonding structures [7,8].

Temperature is the main factor affecting the performance of the adhesive, and the mechanical properties of the adhesive will change in different temperature ranges. The bonding strength, strain and fracture toughness show temperature sensitivity [9]. The joint strength of the bonding structure is determined by the performance change of the adhesive and the influence of thermal stress [10]. The effect of temperature on the properties of the bonding structure is obvious, especially when the temperature is close to the glass transition temperature (Tg) of the material [11,12]. In addition, when the temperature is higher than Tg, the adhesive is featured with high elasticity, and its failure strength and elastic modulus decrease rapidly, while the elongation increases; however, when the temperature is lower than Tg, its performance is opposite [13]. Adams et al. [14] tested the single lap joint at different temperatures and also compared and analyzed the influence of thermal stress caused by the difference of thermal expansion coefficients and shrinkage stress caused by curing on the joint performance, which lead to the change concerning the stress state of the lap joint; the stress/strain performance of polymer adhesive also changes with the change of temperature. Na et al. [15,16] studied the effect of temperature on the mechanical properties of basalt fiber-reinforced composite/aluminum alloy bonded joints and found that with the increase in temperature, the Young's modulus and tensile strength of the joint decreased, while the tensile strain increased. The closer the temperature to Tg, the more significant the change in mechanical properties. Silva et al. [17] conducted a test on the mechanical properties of the single lap joint at low temperature and high temperature and revealed that the adhesive was brittle at low temperature and ductile at high temperature. They also analyzed the effect of porosity on failure. Banea et al. [12] investigated the stress-strain properties of polyurethane and epoxy adhesives at −40 ◦C, room temperature and 80 ◦C. It was found that with an increase in temperature, the failure strength and the Young's modulus of epoxy adhesives decreased, while the failure strain increased, which resulted from the increase of adhesive toughness at high temperature. Zhang et al. [18] conducted a study on the tensile properties of double lap joints in the temperature range of −35~60 ◦C and found that the load-elongation response was mainly affected by the thermomechanical properties of the adhesive, while it was less affected by the adhesive base material. When the temperature was higher than Tg, the strength and stiffness of the joint decreased, while the elongation increased dramatically, and the failure mechanism changed with the increase in temperature. To be specific, crack growth rate is higher at low temperature. In addition, the critical strain energy release rate for crack initiation and propagation increases continuously with increasing temperature.

Adhesion technology provides technical support for mixed material body design, but it also brings some problems. The service temperature of the bonding structure used in vehicles varies to a great extent in practical application. As a macromolecule material, the performance of the bonding structure is greatly affected by temperature, which causes the mechanical properties of the bonding structure to change with temperature. In order to achieve the safety design of the vehicle, the bonding structure must ensure the reliability of the connection within the full temperature field of the vehicle service. Therefore, temperature is one of the important factors that must be considered in the design of a bonded structure. It is of great significance to study the changing rules of bonded joint performance at different temperatures and propose the failure prediction method of bonded joints under the full temperature field for guiding the design of bonded structures.

### **2. Material Selection and Specimen Design**

### *2.1. Adhesive and Substrate*

The experimental selective adhesive was a modified silane polyurethane adhesive widely used in the window bonding of cars, trucks and trains. ISR-7008 is produced by Bostik China Co., Ltd. The mechanical parameters of the adhesive and adhesive substrate are shown in Table 1. The working temperature range provided in the technical manual is −40 ◦C~−90 ◦C. A permanent elastomer is formed by reaction with moisture in the air. 6005A aluminum alloy was selected as the adhesive substrate; it is widely used in automotive body structures. Table 2 shows the main performance parameters of ISR-7008 adhesive (provided by suppliers).

**Table 1.** Mechanical property parameters of adhesive and adhesive substrate.


**Table 2.** Technical performance parameters of ISR-7008 adhesive.


## *2.2. Design and Processing of Specimens*

To investigate the durability of adhesive joints under different stress states, the single lap joint (SLJ), the scarf joint (SJ) and the butt joint (BJ) were selected. When adherends are isotropic metallic and when the bondline thickness is very thin, the stress of the adhesive is assumed to be uniform and equal to the average values [8,19]. Thus, the SLJ and the BJ represent the shear stress and normal stress, respectively, while the SJ refers to the combined shear and normal stress. Furthermore, the normal and shear stress components with an infinitesimal block of adhesive within the central region of the scarf joints are calculated by assuming the coordinate and stress system as shown in Figure 1. The ratio between the tensile force and shear force of the adhesive layer can be changed by changing the angle *α* between the adhesive interface and the axis of specimens. It can be seen from the force decomposition that *F* represents the tensile force on both ends of the specimen. *F* sin *α* is the tensile force component of *F* on the bonding interface, and *F* cos *α* indicates the shear force component of *F* on the bonding interface. The normal *σ* and shear stress *τ* components are given by Equation (1), where *F* is the uniaxial failure load, *A* denotes the bonding area and *α* refers to the scarf angle.

$$
\sigma = \frac{F \sin \alpha}{A}, \; \tau = \frac{F \cos \alpha}{A} \tag{1}
$$

The docking and lap specimens were designed to study the mechanical properties of the adhesive joints under tensile stress and shear stress, separately. The overall size of the butt joint is 201 <sup>×</sup> <sup>25</sup> <sup>×</sup> 25 mm<sup>3</sup> , and the adhesive area is 25 <sup>×</sup> 25 mm<sup>2</sup> , while the overall size of the lap joint is 175 <sup>×</sup> <sup>25</sup> <sup>×</sup> 11 mm<sup>3</sup> , and the adhesive area is 25 <sup>×</sup> 25 mm<sup>2</sup> . The butt joint and the shear joint are shown in Figure 2, where the thickness of the adhesive layer is 1 mm.

*Crystals* **2021**, *11*, x FOR PEER REVIEW 4 of 20

*Crystals* **2021**, *11*, x FOR PEER REVIEW 4 of 20

**Figure 2.** Schematic diagram of docking and lap joints (mm). **Figure 2.** Schematic diagram of docking and lap joints (mm). **Figure 2.** Schematic diagram of docking and lap joints (mm).

**Figure 2.** Schematic diagram of docking and lap joints (mm). In the actual service process, the adhesive structure is often affected by tensile stress In the actual service process, the adhesive structure is often affected by tensile stress and shear stress; thus, it is of great significance to study the failure behavior of adhesive joints under the coupling of tensile stress and shear stress through reasonable joint design, thus being conducive to establishing the failure prediction of adhesive joints under complex stress. In order to study the failure behavior of the adhesive layer under different stress conditions, scarf joints with adhesive angles of 15°, 30°, 45°, 60°and 75° were designed and processed (as shown in Figure 3). The adhesive thickness of all specimens is unified as 1 mm. In the actual service process, the adhesive structure is often affected by tensile stress and shear stress; thus, it is of great significance to study the failure behavior of adhesive joints under the coupling of tensile stress and shear stress through reasonable joint design, thus being conducive to establishing the failure prediction of adhesive joints under complex stress. In order to study the failure behavior of the adhesive layer under different stress conditions, scarf joints with adhesive angles of 15◦ , 30◦ , 45◦ , 60◦and 75◦ were designed and processed (as shown in Figure 3). The adhesive thickness of all specimens is unified as 1 mm. In the actual service process, the adhesive structure is often affected by tensile stress and shear stress; thus, it is of great significance to study the failure behavior of adhesive joints under the coupling of tensile stress and shear stress through reasonable joint design, thus being conducive to establishing the failure prediction of adhesive joints under complex stress. In order to study the failure behavior of the adhesive layer under different stress conditions, scarf joints with adhesive angles of 15°, 30°, 45°, 60°and 75° were designed and processed (as shown in Figure 3). The adhesive thickness of all specimens is unified as 1 mm.

and shear stress; thus, it is of great significance to study the failure behavior of adhesive

**Figure 3.** Schematic diagram of 15◦ , 30◦ , 45◦ , 60◦ and 75◦ scarf joints (mm).

### *2.3. Dumbbell Sample*

ISR-7008 is a kind of flexible polyurethane adhesive. As the completely cured adhesive sheet is extremely soft, it is not feasible to use machined tensile test samples. Therefore, molding technology is adopted to process the sample for further avoiding the scratch

problem in the cutting process and ensuring that the sample is obtained without defects. Apart from that, for achieving the above purpose, the related metal abrasive tool is designed as shown in Figure 4. The grinding tool consists of three parts: the lower part is the base to play a supporting role; the middle is the template to determine the sample size and shape; the upper part is the compression plate and bolts. problem in the cutting process and ensuring that the sample is obtained without defects. Apart from that, for achieving the above purpose, the related metal abrasive tool is designed as shown in Figure 4. The grinding tool consists of three parts: the lower part is the base to play a supporting role; the middle is the template to determine the sample size and shape; the upper part is the compression plate and bolts.

ISR-7008 is a kind of flexible polyurethane adhesive. As the completely cured adhesive sheet is extremely soft, it is not feasible to use machined tensile test samples. Therefore, molding technology is adopted to process the sample for further avoiding the scratch

*Crystals* **2021**, *11*, x FOR PEER REVIEW 5 of 20

*2.3. Dumbbell Sample* 

**Figure 3.** Schematic diagram of 15°, 30°, 45°, 60° and 75° scarf joints (mm).

**Figure 4.** (**a**) Forming mold. (**b**) Dumbbell sample. **Figure 4.** (**a**) Forming mold. (**b**) Dumbbell sample.

During the process of making samples, in order to prevent adhesive from sticking to the mold, a layer of polytetrafluoroethylene (PTFE) material is spread on the upper surface of the base and the lower surface of the pressure plate, and a layer of release agent is applied on all the surfaces of the template. After the die groove is coated with adhesive, the pressure plate on the cover is pressed with bolts. ISR-7008 adhesive is moisture curing. In order to ensure complete curing, it is solidified in 25 °C/50%RH environment for 7 days to remove the pressure plate (the reference manufacturer provides curing conditions), as shown in Figure 4, and then curing continues for 21 days. The geometric dimensions of During the process of making samples, in order to prevent adhesive from sticking to the mold, a layer of polytetrafluoroethylene (PTFE) material is spread on the upper surface of the base and the lower surface of the pressure plate, and a layer of release agent is applied on all the surfaces of the template. After the die groove is coated with adhesive, the pressure plate on the cover is pressed with bolts. ISR-7008 adhesive is moisture curing. In order to ensure complete curing, it is solidified in 25 ◦C/50%RH environment for 7 days to remove the pressure plate (the reference manufacturer provides curing conditions), as shown in Figure 4, and then curing continues for 21 days. The geometric dimensions of the dumbbell stretch samples used refer to the NF ISO 527-2 standards.

the dumbbell stretch samples used refer to the NF ISO 527-2 standards.

### *2.4. Design and Manufacture of Fixture*

*2.4. Design and Manufacture of Fixture* There are two main difficulties to be overcome in the adhesive process of docking and scarf specimens: the accurate control of neutral and adhesive thickness of the upper and lower square test rods of the specimen. To address these, it is necessary to first design and make the corresponding adhesive fixture. While the fixture is being made, the upper and lower grooves of the fixture are milled with one knife, thus ensuring the neutrality of the upper and lower test rods in the adhesive process of the specimen. At the same time, there is a calibration line next to the groove on one side of the fixture, and it is employed to control the thickness of the adhesive layer. The metal strip in the upper part of the groove is used to fix the adhesive specimen, and the knob on one side is adopted to push There are two main difficulties to be overcome in the adhesive process of docking and scarf specimens: the accurate control of neutral and adhesive thickness of the upper and lower square test rods of the specimen. To address these, it is necessary to first design and make the corresponding adhesive fixture. While the fixture is being made, the upper and lower grooves of the fixture are milled with one knife, thus ensuring the neutrality of the upper and lower test rods in the adhesive process of the specimen. At the same time, there is a calibration line next to the groove on one side of the fixture, and it is employed to control the thickness of the adhesive layer. The metal strip in the upper part of the groove is used to fix the adhesive specimen, and the knob on one side is adopted to push the adhesive test rod to the bond, as shown in Figure 5. *Crystals* **2021**, *11*, x FOR PEER REVIEW 6 of 20

> There are also some difficulties in the adhesive process of lap specimens; the main ones are the parallelism of the two adhesive test rods and the warping of the upper and

All specimens are adhesive in a clean and stable environment (temperature: 25 ± 3

1. An 80 mesh sandpaper is used for the cross grind concerning the adhesive surface of aluminum alloy along the 45° direction to increase the surface roughness and facili-

2. Acetone is used to clean the specimen, thus removing the oil pollution and dust produced in processing. Wiping paper is dipped in acetone, and the adhesive surface is wiped in one direction until the surface of the tissue is clean. The surface is allowed

3. Surface pretreatment coating agent Primer M is used to clean the adhesive surface

4. As for adhesive ISR-7008, on its surface, the corresponding fixture described above is used to complete the specific adhesive work, and then it is solidified for a period

The cured fully adhesive joints are placed in the hot and humid environment box as shown in Figure 7 and are subject to the test temperature for two hours. To be specific, the temperature range is −40 °C ~ 150 °C, the humidity range is 20% RH ~ 99% RH, the temperature fluctuation is ±0.1 °C and the humidity fluctuation is ±1%. For each test joint, the

including the lateral fastening fixture and the gantry tightening fixture, are designed as shown in Figure 6. The parallelism of the adhesive specimen is guaranteed by the screw rotation clamping of the lateral fastening fixture, and the metal block between the two rods is used to control the lap width of the shear specimen, while the flatness of the upper and lower planes of the adhesive is ensured by the screw compression of the downward

**Figure 5.** Adhesion fixture for docking and scarf specimens. **Figure 5.** Adhesion fixture for docking and scarf specimens.

rotating gantry compression fixture.

*2.5. Bonding Process* 

tate the adhesive.

to dry for 10 min.

**Figure 6.** Diagram of lateral fastening jig (**left**) and gantry clamping jig (**right**).

°C; relative humidity: 50 ± 5%). The preparation process is as follows:

again. The surface is again allowed to dry for 10 min.

*2.6. Testing of Strength under the Condition of the Full Temperature Field* 

of 4 weeks in the experimental environment.

There are also some difficulties in the adhesive process of lap specimens; the main ones are the parallelism of the two adhesive test rods and the warping of the upper and lower surfaces. In order to solve these problems, in this paper, the corresponding fixtures, including the lateral fastening fixture and the gantry tightening fixture, are designed as shown in Figure 6. The parallelism of the adhesive specimen is guaranteed by the screw rotation clamping of the lateral fastening fixture, and the metal block between the two rods is used to control the lap width of the shear specimen, while the flatness of the upper and lower planes of the adhesive is ensured by the screw compression of the downward rotating gantry compression fixture. There are also some difficulties in the adhesive process of lap specimens; the main ones are the parallelism of the two adhesive test rods and the warping of the upper and lower surfaces. In order to solve these problems, in this paper, the corresponding fixtures, including the lateral fastening fixture and the gantry tightening fixture, are designed as shown in Figure 6. The parallelism of the adhesive specimen is guaranteed by the screw rotation clamping of the lateral fastening fixture, and the metal block between the two rods is used to control the lap width of the shear specimen, while the flatness of the upper and lower planes of the adhesive is ensured by the screw compression of the downward rotating gantry compression fixture.

*Crystals* **2021**, *11*, x FOR PEER REVIEW 6 of 20

**Figure 5.** Adhesion fixture for docking and scarf specimens.

**Figure 6.** Diagram of lateral fastening jig (**left**) and gantry clamping jig (**right**). **Figure 6.** Diagram of lateral fastening jig (**left**) and gantry clamping jig (**right**).

### *2.5. Bonding Process 2.5. Bonding Process*

All specimens are adhesive in a clean and stable environment (temperature: 25 ± 3 °C; relative humidity: 50 ± 5%). The preparation process is as follows: All specimens are adhesive in a clean and stable environment (temperature: 25 ± 3 ◦C; relative humidity: 50 ± 5%). The preparation process is as follows:

