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

*2.6. Testing of Strength under the Condition of the Full Temperature Field*  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 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 load and displacement curves are acquired by the tensile test. It should be noticed that all the joints are tested at 25 ◦C/50%RH to obtain the residual strength of the adhesive joint.

**Figure 7.** High- and low-temperature hygrothermal cycle experimental chamber. **Figure 7.** High- and low-temperature hygrothermal cycle experimental chamber.

## **3. Experimental Data Analysis**

**3. Experimental Data Analysis**  *3.1. Dumbbell Sample Test*

*3.1. Dumbbell Sample Test*  Typical temperature conditions are selected according to the temperature range (−40 °C ~ 80 °C) in body service environment, and typical temperature points of −40 °C, RT and 80 °C are chosen. The dumbbell specimens manufactured are placed in the environmental test box according to the temperature conditions for 6 h to achieve full uniformity of the joint temperature. Immediately after taking out the dumbbell samples and loading them into the electronic universal testing machine, the dumbbell-type adhesive specimens are subjected to quasi-static tensile tests at −40 °C, RT and 80 °C, and the bonded joints are tested at a constant speed of 5 mm/min until destroyed. The stress and strain curves of dumbbell samples are recorded. The temperature required for the test is provided by a high-low temperature environment chamber; a high temperature environment can be pro-Typical temperature conditions are selected according to the temperature range (−40 ◦C~80 ◦C) in body service environment, and typical temperature points of −40 ◦C, RT and 80 ◦C are chosen. The dumbbell specimens manufactured are placed in the environmental test box according to the temperature conditions for 6 h to achieve full uniformity of the joint temperature. Immediately after taking out the dumbbell samples and loading them into the electronic universal testing machine, the dumbbell-type adhesive specimens are subjected to quasi-static tensile tests at −40 ◦C, RT and 80 ◦C, and the bonded joints are tested at a constant speed of 5 mm/min until destroyed. The stress and strain curves of dumbbell samples are recorded. The temperature required for the test is provided by a high-low temperature environment chamber; a high temperature environment can be provided by resistance wire heating, while a low temperature environment can be achieved by liquid nitrogen cooling. Apart from this, temperature changes can be accurately controlled by a temperature controller.

load and displacement curves are acquired by the tensile test. It should be noticed that all the joints are tested at 25 °C/50%RH to obtain the residual strength of the adhesive joint.

vided by resistance wire heating, while a low temperature environment can be achieved by liquid nitrogen cooling. Apart from this, temperature changes can be accurately controlled by a temperature controller. To accurately measure the strain of dumbbell specimens during tension, a non-contact full-field strain measurement system (VIC-3D, Correlated Solutions, Inc.) is adopted as shown in Figure 8. The whole-field displacement and strain measurements are carried out by the system, based on three-dimensional digital image correlation technology. The To accurately measure the strain of dumbbell specimens during tension, a non-contact full-field strain measurement system (VIC-3D, Correlated Solutions, Inc.) is adopted as shown in Figure 8. The whole-field displacement and strain measurements are carried out by the system, based on three-dimensional digital image correlation technology. The test procedure is as follows: the dumbbell specimen is set to 20 mm in test length and is fixed on the universal testing machine; then, two CCD cameras are installed and calibrated. The strain of the specimen is obtained by analyzing the images collected in the tensile test. Each test is repeated three times to ensure the validity of the data, taking the average as the result.

test procedure is as follows: the dumbbell specimen is set to 20 mm in test length and is fixed on the universal testing machine; then, two CCD cameras are installed and calibrated. The strain of the specimen is obtained by analyzing the images collected in the tensile test. Each test is repeated three times to ensure the validity of the data, taking the

average as the result.

40

**Figure 8.** VIC-3D measurement system. **Figure 8.** VIC-3D measurement system.

### *3.2. Mechanics Performance Testing 3.2. Mechanics Performance Testing*

Table 3 shows the experimental test data concerning seven groups of adhesive specimens from different angles. It can be seen from the data in the table that there is a certain degree of dispersion between the experimental test data of the same group of adhesive specimens. In order to ensure that the failure strength of each adhesive specimen is more accurate and reasonable, the experimental data shown in Table 3 are screened and extracted, and two experimental data extraction methods—the section method and the statistical method—are adopted. Table 3 shows the experimental test data concerning seven groups of adhesive specimens from different angles. It can be seen from the data in the table that there is a certain degree of dispersion between the experimental test data of the same group of adhesive specimens. In order to ensure that the failure strength of each adhesive specimen is more accurate and reasonable, the experimental data shown in Table 3 are screened and extracted, and two experimental data extraction methods—the section method and the statistical method—are adopted.


**Table 3.** Experimental test data of seven groups of adhesive specimens at different angles. **Table 3.** Experimental test data of seven groups of adhesive specimens at different angles.

60 2.60 2.61 2.57 2.36 2.46 2.52 10.75 4.26 CF 45 2.42 2.35 2.56 2.66 2.61 2.52 13.08 5.18 CF 30 2.51 2.44 2.60 2.38 2.47 2.48 8.21 3.31 CF 15 2.36 2.48 2.58 2.56 2.77 2.55 15.03 5.89 CF 0 2.64 2.53 2.66 2.47 2.70 2.60 9.62 3.70 CF


**Table 3.** *Cont.*

In the statistical method, the confidence interval in statistics is used to extract the experimental data. The confidence interval refers to the estimated interval of the overall parameters constructed by sample statistics. In this paper, the 95% confidence interval, which is commonly used in engineering data statistics, is selected to screen the experimental data falling into the confidence interval. At the same time, the failure section of the adhesive joint after static tension is analyzed and the test results of cohesion failure are selected as the effective data. Tables 4–10 show the failure strength values concerning seven groups of test specimens at different angles.

**Table 4.** Failure strength values of seven groups of adhesive specimens at different angles at −40 ◦C.


**Table 5.** Failure strength values of seven groups of adhesive specimens at different angles at −20 ◦C.


**Table 6.** Failure strength values of seven groups of adhesive specimens at different angles at 0 ◦C.



**Table 7.** Failure strength values of seven groups of adhesive specimens at different angles at 25 ◦C.

**Table 8.** Failure strength values of seven groups of adhesive specimens at different angles at 40 ◦C.


**Table 9.** Failure strength values of seven groups of adhesive specimens at different angles at 60 ◦C.


**Table 10.** Failure strength values of seven groups of adhesive specimens at different angles at 80 ◦C.


### **4. Analysis and Discussion of Experimental Results**

### *4.1. Dumbbell Sample Test*

The stress-strain curves at the high temperature of 80 ◦C, room temperature and the low temperature of −40 ◦C, as shown in Figure 9, show that the Young's modulus and tensile strength of the adhesive decrease with an increase in temperature, which is contrary to the increase of failure strain of epoxy adhesive with the increase of temperature due to the difference of glass transition temperature between the two kinds of adhesive [20]. The glass transition temperature of epoxy adhesive is higher, and the toughness of the material is enhanced with the increase in temperature. However, for polyurethane adhesive ISR-7008, the glass transition temperature is low (Tg = −59 ◦C), and the toughness is better at low temperature. The variation of the Young's modulus, tensile strength and failure strain of adhesive ISR-7008 with temperature is displayed in Table 11, and all show a downward trend with an increase in temperature.

a downward trend with an increase in temperature.

**Figure 9.** Stress-strain curve of adhesive. **Figure 9.** Stress-strain curve of adhesive.

**Table 11.** Young's modulus, tensile strength and failure strain of adhesives at −40 °C, RT and 80 °C. **Table 11.** Young's modulus, tensile strength and failure strain of adhesives at −40 ◦C, RT and 80 ◦C.

adhesive ISR-7008, the glass transition temperature is low (Tg = −59 °C), and the toughness is better at low temperature. The variation of the Young's modulus, tensile strength and failure strain of adhesive ISR-7008 with temperature is displayed in Table 11, and all show


The failure strength and the Young's modulus of the adhesive vary greatly at different temperatures. The failure strength of the adhesive decreases by 47.69% and 68.15% at RT and 80 °C, respectively, compared with −40 °C, while the Young's modulus of the adhesive decreases by 57.63% and 75.42% at RT and 80 °C, respectively. The increase in temperature results in a declining degree of failure strength and Young's modulus. The closer the glass transition temperature of the adhesive is, the more obvious the change in the properties of the adhesive [14]. For the failure strain of the adhesive, it decreases with increasing temperature, but the decrease is slight. When the temperature rises from a low level, the ductility of the adhesive clearly changes. With the increase in temperature, the change range of ductility decreases, and that of failure strength is smaller. The adhesive fracture occurs before reaching large deformation, which leads to the decrease in failure strain of dumbbell samples with the increase in temperature. Banea et al. [21] studied room temperature silicone sulfide adhesives and similarly found that the failure displacement of the adhesives decreases with increasing temperature. The failure strength and the Young's modulus of the adhesive vary greatly at different temperatures. The failure strength of the adhesive decreases by 47.69% and 68.15% at RT and 80 ◦C, respectively, compared with −40 ◦C, while the Young's modulus of the adhesive decreases by 57.63% and 75.42% at RT and 80 ◦C, respectively. The increase in temperature results in a declining degree of failure strength and Young's modulus. The closer the glass transition temperature of the adhesive is, the more obvious the change in the properties of the adhesive [14]. For the failure strain of the adhesive, it decreases with increasing temperature, but the decrease is slight. When the temperature rises from a low level, the ductility of the adhesive clearly changes. With the increase in temperature, the change range of ductility decreases, and that of failure strength is smaller. The adhesive fracture occurs before reaching large deformation, which leads to the decrease in failure strain of dumbbell samples with the increase in temperature. Banea et al. [21] studied room temperature silicone sulfide adhesives and similarly found that the failure displacement of the adhesives decreases with increasing temperature.

### *4.2. BJ and SLJ Tests 4.2. BJ and SLJ Tests*

The average lap shear strength and tensile strength for joints tested at seven temperature points were obtained. In addition, the variation curves of the average lap shear and tensile strength of joints as a function of temperature are presented in Figure 10. The average lap shear strength and tensile strength for joints tested at seven temperature points were obtained. In addition, the variation curves of the average lap shear and tensile strength of joints as a function of temperature are presented in Figure 10.

It was found that the adhesive strength of SLJs and BJs decreased gradually with an increase in temperature: the higher the temperature, the lower the adhesive strength. The highest adhesive strength appeared at the low temperature of −40 ◦C, and the lowest adhesive strength appeared at the high temperature of 80 ◦C. For SLJ adhesive strength, the adhesive strengths at −40 ◦C and −20 ◦C are 28.68% and 18.52%, respectively; 8.33% higher than that of room temperature. The adhesive strengths at high temperatures of 80 ◦C, 60 ◦C and 40 ◦C are 26.92%, 15.73% and 9.09%, respectively, lower than that at room temperature. In addition, for the adhesive strength of BJs, the adhesive strengths at high temperatures of 80 ◦C, 60 ◦C and 40 ◦C are 20.00%, 14.75% and 7.19% lower than that at room temperature, respectively.

**Figure 10.** Average SLJ and BJ strength as a function of temperature. **Figure 10.** Average SLJ and BJ strength as a function of temperature.

It was found that the adhesive strength of SLJs and BJs decreased gradually with an increase in temperature: the higher the temperature, the lower the adhesive strength. The highest adhesive strength appeared at the low temperature of −40 °C, and the lowest adhesive strength appeared at the high temperature of 80 °C. For SLJ adhesive strength, the adhesive strengths at −40 °C and −20 °C are 28.68% and 18.52%, respectively; 8.33% higher than that of room temperature. The adhesive strengths at high temperatures of 80 °C, 60 °C and 40 °C are 26.92%, 15.73% and 9.09%, respectively, lower than that at room temperature. In addition, for the adhesive strength of BJs, the adhesive strengths at high temperatures of 80 °C, 60 °C and 40 °C are 20.00%, 14.75% and 7.19% lower than that at room temperature, respectively. The adhesive strengths at low temperatures of −40 ◦C, −20 ◦C and 0 ◦C are 27.60%, 8.25% and 4.47% higher than that at room temperature, respectively, which is explained by the fact that polyurethane adhesive has low glass transition temperatures (Tg = −60 ◦C for ISR-7008, provided by the supplier). It remains ductile, and its strength increases at low temperatures, which leads to a higher joint strength. With regard to polyurethane flexible adhesive, the adhesive strength of the joint depends not only on the strength of the adhesive but also on the toughness of the material. By comparing the changing trend concerning tensile strength and shear strength of adhesive joints, it can be found that the shear strength increases more than the tensile strength at low temperature, and it decays more than the tensile strength at high temperature, which indicates that the adhesive strength of the single lap joint is more sensitive to changes in temperature.

The adhesive strengths at low temperatures of −40 °C, −20 °C and 0 °C are 27.60%,

### 8.25% and 4.47% higher than that at room temperature, respectively, which is explained *4.3. SJ Tests*

by the fact that polyurethane adhesive has low glass transition temperatures (Tg = −60 °C for ISR-7008, provided by the supplier). It remains ductile, and its strength increases at low temperatures, which leads to a higher joint strength. With regard to polyurethane flexible adhesive, the adhesive strength of the joint depends not only on the strength of the adhesive but also on the toughness of the material. By comparing the changing trend concerning tensile strength and shear strength of adhesive joints, it can be found that the shear strength increases more than the tensile strength at low temperature, and it decays more than the tensile strength at high temperature, which indicates that the adhesive strength of the single lap joint is more sensitive to changes in temperature. In practical engineering applications, most stress forms of adhesive structures are basically tensile shear interaction. The adhesive strength of lap joints and butt joints decreases with the increase in temperature, while that of scarf joints under the action of tensile shear need further study. The same quasi-static tests were performed on the 15◦ , 30◦ , 45◦ , 60◦ and 75◦ SJs at −40 ◦C, −20 ◦C, 0 ◦C, 25 ◦C, 40 ◦C, 60 ◦C and 80 ◦C. In order to ensure the credibility of the experimental data, each temperature point has three samples. The average value of failure strength of the three samples is called adhesive strength. By observing the failure forms of all joints, it is found that the failure modes of scarf joints are cohesion failure, and the adhesive strength of 15◦ , 30◦ , 45◦ , 60◦ and 75◦ SJs changes with temperature, as shown in Figure 11.

*4.3. SJ Tests*  In practical engineering applications, most stress forms of adhesive structures are basically tensile shear interaction. The adhesive strength of lap joints and butt joints decreases with the increase in temperature, while that of scarf joints under the action of tensile shear need further study. The same quasi-static tests were performed on the 15°, 30°, 45°, 60° and 75° SJs at −40 °C, −20 °C, 0 °C, 25 °C, 40 °C, 60 °C and 80 °C. In order to ensure the credibility of the experimental data, each temperature point has three samples. The average value of failure strength of the three samples is called adhesive strength. By observing the failure forms of all joints, it is found that the failure modes of scarf joints are cohesion failure, and the adhesive strength of 15°, 30°, 45°, 60° and 75° SJs changes with temperature, as shown in Figure 11. As can be seen from Figure 11, with the decrease of adhesive strength with increased temperature, the change trend of adhesive strength is similar to that of the single tensile joint and the single shear joint. First, the 15◦ and 30◦ scarf joints are analyzed. They have the same characteristics: the joint is subjected mainly to shear action, and the joint strength measured at room temperature (RT) is taken as a reference, while the strength of 15◦ joints decreased by 25.79%, 16.96% and 89.89% with an increase in temperature to 40 ◦C, 60 ◦C and 80 ◦C, respectively. When the temperature drops to 0 ◦C, −20 ◦C and −40 ◦C, the strength of the joint increases by about 29.60%, 14.50% and 8.41%, respectively. When the temperature increases from room temperature (RT) to 40 ◦C, 60 ◦C and 80 ◦C, the adhesive strength of 30◦ joints decreases by 9.82%, 16.72% and 25.79%, respectively. When the temperature decreases from room temperature (RT) to 0◦C, −20 ◦C and −40◦C, the joint strength increases by 9.54%, 14.60% and 30.90%, respectively.

**Figure 11.** Average 15°, 30°, 45°, 60°, 75° SJ strength as a function of temperature. **Figure 11.** Average 15◦ , 30◦ , 45◦ , 60◦ , 75◦ SJ strength as a function of temperature.

As can be seen from Figure 11, with the decrease of adhesive strength with increased temperature, the change trend of adhesive strength is similar to that of the single tensile joint and the single shear joint. First, the 15° and 30° scarf joints are analyzed. They have the same characteristics: the joint is subjected mainly to shear action, and the joint strength measured at room temperature (RT) is taken as a reference, while the strength of 15° joints decreased by 25.79%, 16.96% and 89.89% with an increase in temperature to 40 °C, 60 °C and 80 °C, respectively. When the temperature drops to 0 °C, −20 °C and −40 °C, the strength of the joint increases by about 29.60%, 14.50% and 8.41%, respectively. When the temperature increases from room temperature (RT) to 40 °C, 60 °C and 80 °C, the adhesive strength of 30° joints decreases by 9.82%, 16.72% and 25.79%, respectively. When the temperature decreases from room temperature (RT) to 0°C, −20 °C and −40°C, the joint strength increases by 9.54%, 14.60% and 30.90%, respectively. The 60° and 75° joints possess the same characteristics: the joint is subjected mainly to tensile action. Taking the joint strength tested at room temperature as a reference, when the temperature reaches 80 °C, the strength of 60° and 75° joints decreases by 21.13% and The 60◦ and 75◦ joints possess the same characteristics: the joint is subjected mainly to tensile action. Taking the joint strength tested at room temperature as a reference, when the temperature reaches 80 ◦C, the strength of 60◦ and 75◦ joints decreases by 21.13% and 20.50%, respectively; when the temperature decreases to −40 ◦C, the joint strength increases by 31.70% and 29.50%, respectively. The 45◦ scarf joint is subjected to the same tensile and shear loads. The joint strength tested at room temperature (RT) is taken as a reference. With the temperature rising to 40 ◦C, 60 ◦C and 80 ◦C, the joint strength decreases by 8.03%, 16.42% and 23.72%, respectively. When the temperature decreases to 0 ◦C, −20◦C and −40 ◦C, the adhesive strength increases by 10.16%, 19.88% and 31.70%, respectively. By analyzing the variation trend regarding the adhesive strength of several kinds of scarf joints and combining the strength variation rule of single lap joints and butt joints, it is found that when the temperature rises to the highest level (80 ◦C) and decreases to the lowest (−40 ◦C), the attenuation and increase of adhesive strength of the butt joint are at least 20.00% and 27.60%, respectively. It is speculated that the docking joint is the least sensitive to temperature change compared with the lap joint and the butt joint.

### 20.50%, respectively; when the temperature decreases to −40 °C, the joint strength in-*4.4. Force-Displacement Curve Comparison*

creases by 31.70% and 29.50%, respectively. The 45° scarf joint is subjected to the same tensile and shear loads. The joint strength tested at room temperature (RT) is taken as a reference. With the temperature rising to 40 °C, 60 °C and 80 °C, the joint strength decreases by 8.03%, 16.42% and 23.72%, respectively. When the temperature decreases to 0°C, −20°C and −40 °C, the adhesive strength increases by 10.16%, 19.88% and 31.70%, respectively. By analyzing the variation trend regarding the adhesive strength of several kinds of scarf joints and combining the strength variation rule of single lap joints and butt joints, it is found that when the temperature rises to the highest level (80 °C) and decreases to the lowest (−40 °C), the attenuation and increase of adhesive strength of the butt joint are at least 20.00% and 27.60%, respectively. It is speculated that the docking joint is the least sensitive to temperature change compared with the lap joint and the butt joint. *4.4. Force-Displacement Curve Comparison* Representative load-displacement curves of SLJs, 15°, 30°, 45°, 60° and 75° scarf joints and BJs as a function of temperature are shown in Figure 12. By comparing the mechanical properties of adhesive and adhesive substrate, it can be found that the elastic modulus of aluminum alloy substrate is 1.6E4 times that of adhesive. In this case, it can be considered Representative load-displacement curves of SLJs, 15◦ , 30◦ , 45◦ , 60◦ and 75◦ scarf joints and BJs as a function of temperature are shown in Figure 12. By comparing the mechanical properties of adhesive and adhesive substrate, it can be found that the elastic modulus of aluminum alloy substrate is 1.6E4 times that of adhesive. In this case, it can be considered that the deformation of adhesive joints is mainly that of the adhesive. The force-displacement curves of seven kinds of joints with different stress forms are nonlinear at all test temperature points. Furthermore, it should be noted that similar findings were obtained by I. Lubowiecka et al. [22–24] for flexible adhesive Terostat MS 9360. From the force-displacement curve, it can be found that the failure displacement of single lap joints, butt joints and scarf joints gradually decreases with the increase in temperature when the temperature changes from the low temperature of −40 ◦C to the high temperature of 80 ◦C. The lower the temperature, the larger the failure displacement, and vice versa. Similar findings were obtained by Mariana et al. [25] for Sikaflex-552 and Banea et al. [26] for SikaForce 7888. In addition, the slope of the force-displacement curve is weighed as the stiffness of the joint; it is found that the stiffness of the joint varies slightly with the increase in temperature, which indicates that the change of temperature affects the stiffness of the joint.

that the deformation of adhesive joints is mainly that of the adhesive. The force-displacement curves of seven kinds of joints with different stress forms are nonlinear at all test temperature points. Furthermore, it should be noted that similar findings were obtained by I. Lubowiecka et al. [22–24] for flexible adhesive Terostat MS 9360. From the force-

*Crystals* **2021**, *11*, x FOR PEER REVIEW 15 of 20

**Figure 12.** Load-displacement curves :(**a**) SLJ; (**b**) 15° SJ; (**c**) 30° SJ; (**d**) 45° SJ;(**e**) 60° SJ; (**f**) 75° SJ;(**g**) **Figure 12.** Load-displacement curves: (**a**) SLJ; (**b**) 15◦ SJ; (**c**) 30◦ SJ; (**d**) 45◦ SJ; (**e**) 60◦ SJ; (**f**) 75◦ SJ; (**g**) BJ.

*4.5. Joint Stiffness*  The effect of temperature on the properties of adhesives is evaluated through the change of adhesive joint stiffness. The stiffness of SLJs, 15°, 30°, 45°, 60°, 75° scarf joints and BJs varies with temperature, as shown in Figure 13. It can be clearly seen from the diagram that temperature has a significant effect on the stiffness of the joint. With the increase in temperature, the stiffness of the joint decreases gradually, and vice versa. For SLJs, when the temperature increases from RT to 80 °C, the stiffness of the joint decreases by 8.73%, and when the temperature of the joint decreases from RT to −40°C, the stiffness of the joint increases by 3.83%. For 15°, 30°, 45°, 60° and 75° scarf joints, temperature increases from RT to 80 °C, and the joint stiffness decreases by 10.48%, 30.45%, 19.98%, 9.70% The cutoff point is 45◦ scarf joints with equal tensile and shear loads on the adhesive surface. Based on the SLJs and 15◦ and 30◦ scarf joints with shear load on the adhesive surface, the curve of tensile force displacement change was observed. Therefore, it was found that the curve of tensile force displacement basically presented three consecutive stages in both high and low temperature environments: I. near-linear increase; II. nonlinear variation; III. the failure phase (shown in Figure 12a). To be specific, stage I corresponds to the linear elastic behavior, because the elastic modulus of ISR-7008 adhesive is relatively low, and all the curves deviate slightly from a straight line, which is mainly due to the characteristics of the polymer itself. Polyurethane adhesive is featured with nonlinear elastic behavior and low elastic modulus, which is obviously different from the linear elasticity of the epoxy adhesive curve with high elastic modulus [12,27].

BJ.

In stage II, the joint stiffness is reduced due to the yield of the adhesive, and the slight rotation of SLJs may also affect the joint stiffness during the tensile process: the change from stage I to stage II is obviously dependent on the characteristics of the joint itself, and stage III is mainly the failure and fracture of the joint, which is very important for material characterization. Because of the difference in the characteristics of the joint, the tension-displacement curve of 60◦ and 75◦ scarf joints and BJs is different from that of SLJs and 15◦ and 30◦ scarf joints, and stage I is closer to a straight line. In addition, the tensile displacement curve concerning the 75◦ scarf joints and BJs at the low temperature of −40 ◦C has a yield point that becomes less and less obvious with the increase in temperature, as shown in Figure 12f–g.
