1 Introduction Due to its simple structure, convenient disassembly and low cost, the threaded joint is widely used in the connection of various engineering structures. The slack of the threaded connection seriously affects the reliability of the steam year operation, and even causes serious accidents of vehicle crash. Therefore, the mechanism of thread coupling torque attenuation is a hot topic in domestic and foreign engineering fields. Nassar et al. studied the stress relaxation of bolts under cyclic lateral loads by model and test. Sakai research shows that under axial vibration, the preload will decrease, but the nut does not move relative to the bolt. Yu Yan et al. analyzed the main causes of engine cylinder head bolt breakage and proposed corresponding improvement measures. Hou Shiyuan outlines the analysis of the loose connection analysis and research status. Hou Mingren analyzed the reasons for the looseness of the wheel bolts and proposed measures to prevent loosening. Teng Zhimin studied the calculation of the reasonable value of bolt pre-tightening torque. In the above studies, there has been little research on the torque attenuation of the body bolts during the manufacturing process. Based on the actuality of the body engineering, based on the pre-tightening force theory calculation, combined with the actual situation of the back door connecting bolt of a certain type of vehicle, the torque attenuation test of the back door mounting bolt is designed. Through theoretical and experimental analysis, the waterproof gasket is revealed. And the influence of the mounting nut on the torque attenuation of the back door mounting bolt provides theoretical support and engineering guidance for the bolt anti-loose technology in the body development. 2. The problem is raised The back door assembly is bolted to the body side by a hinge. Figure 1 is a cross-sectional view of a vehicle rear door bolt mounted on a vehicle body. The bolts of the hinge are coupled to the outer cover of the top cover, the reinforcing plate and the inner cover of the top cover. The rear door assembly of this model has a bolt tightening torque of 23+2 Nm mounted on the vehicle body side. After painting, the model found that the torque was reduced to approximately 13.5 Nm and the attenuation rate was as high as 47%, as shown in Table 1. Figure 1 Thread connection cross-section Table 1 Change of back door torque Torque attenuation may cause the back door to sag and the appearance gap is too flat, thus affecting the appearance of the back door. When the situation is serious, quality accidents are likely to occur, resulting in the back door not being able to open and close normally. 3. Theoretical calculation and analysis of bolts 3.1 Torque calculation The tightening of the threaded joint refers to the assembly process of tightening the nut with a wrench, so that the tightly clamped part is subjected to the pre-tightening force. The purpose of the threaded joint is to achieve contact between two or more parts to ensure the relative relationship between the parts. . In this paper, the function of the bolt is to connect the back door assembly with the body top cover. The bolt not only has to fix the position of the back door and the top cover, but also has certain waterproof performance. The size of the pre-tightening force indicates that the bolt is tightened. Generally, the tightening torque is used to overcome the relative rotational resistance torque T1 of the thread pair and the friction torque T2 of the nut support surface: Where F is the preload; d is the nominal diameter of the thread; λ is the angle of the thread; Ï is the equivalent friction angle of the thread; f is the coefficient of friction of the nut support surface; r is the friction radius of the support. Equation (1) can be simplified to the empirical equation: Where k is the torque factor; d is the nominal diameter of the thread. Thread material, surface treatment and surface friction are the main factors affecting the torque factor. The K torque factor is recommended to range from 0.2 to 0.25. Normally, the pre-tightening stress of the joint of the thread after tightening shall not exceed 80% of the yield point of the material. According to the yield limit of the material, it is generally recommended that the preload force be calculated as follows: Where σs is the yield stress; As is the nominal stress cross-sectional area. The back door is generally M8 thread, the bolt strength grade is 8.8, which is 640 MPa, As is 36.6 mm. The substitution type (3) can be used to calculate the pre-tightening force range F=11712～14054 N, according to formula (2) Tmin = 18.7 Nm, Tmax = 28.1 Nm. The pre-tightening torque recommended for the manufacturing process is 18 to 25 Nm. 3.2 Torque attenuation analysis It can be known from formula (2) that since the nominal diameter of the thread is constant, the main influencing factors of the tightening torque are the pre-tightening force and the torque coefficient. The main influencing factors of the k-factor are the thread friction coefficient and the friction coefficient of the support surface. The thread surface of the threaded coupling pair, the relative sliding support surface state (machining roughness, cover type and lubricant, etc.) and the supporting surface affect the friction coefficient, which affects the torque coefficient. 3.2.1 Preload force change In the tightened state, the screw connection can be considered to be in a sub-stable state, and the internal stress is constantly changing. After the tightening, the normal bolt connection stress tends to be released, that is, the stress is continuously reduced. If relevant influencing factors such as vibration, temperature, axial load, etc. occur, the thread connection stress will accelerate the slack. During the painting process, the thread temperature changes a few times, causing the thread stress to be released. When the bolt is not rotated, the joint stress of the thread may be slack and the bolt pre-tightening force may decrease. 3.2.2 Torque coefficient change To prevent water leakage from the hinge mounting surface, a waterproof gasket is added between the back door hinge and the body top cover. First, the waterproof gasket causes a large change in the coefficient of friction between the nut support surface and the friction coefficient between the skimmers. During the painting process, physical changes in the waterproof gasket can also cause changes in the coefficient of friction. Secondly, due to the softness of the waterproof gasket material, the annular collapse is more likely to occur under the stress of the joint surface. Finally, the hardness of the waterproof gasket changes after multiple soaking and baking of the coating solution. A decrease in the bolt joint stiffness results in a reduction in the torsion factor. 4. Test design and analysis 4.1 Test design The hinges of the back are generally composed of a movable sheet, a rigid bracket, a pin, a bolt and a waterproof gasket. The page board and bracket are generally made of SPHC, SAPH370 or QSTE420 materials. The waterproof gasket adopts a red steel plate paper with high strength and good water absorption performance. The surface is smooth and flat, and has toughness. It is suitable for waterproof sealing of various door hinges. The compression performance is 350g/cm3, which can be compressed by 10% to 20%, and can recover at least 55% after pressure removal. The bolts are made of embossed bolts, the embossed bolts have a strength class of 8.8, the nuts are coded Q32008, and the surface is galvanized and passivated. In order to verify the above theoretical analysis of the torque attenuation, the simulation of the post-back door installation and the body after the painting conditions, designed the torque attenuation verification test shown in Figure 2. A DC06 sheet with a thickness of 1.0 mm was selected as the connected piece. In order to simulate the actual situation, each sample uses a 3-layer plate connection, the same hinge and the same tightening torque. The juice-free variables in the test are the thickness of the waterproof gasket and the nut type. The specific verification method is as follows: the nut model uses 932008 (hex flange nut) and Q32208 (hex nut and tapered elastic washer assembly); the thickness of the waterproof gasket is 0.5 mm, 1.0 mm and 2.0 mm. In order to ensure the accuracy and accuracy of the test, each group of tests was divided into 3 samples, totaling 24 pieces. Each sample piece was fastened with a torque of 23 N·m. 4.2 Test results and analysis After the coating electrophoresis, the tightening torque of the sample was tested, and the results are shown in Table 2. It was found by the test group having a thickness of 0 that the sample after painting did not attenuate the same moment. Therefore, the hinge structure, the type of nut, the initial value of the preload force, and the temperature are not the main causes of torque decay. During the painting process, the test piece was subjected to multiple soaking and drying, and the surface of the sample was covered with a coating thickness of >12 μm. The coating increases the friction coefficient of the contact and the coating has a certain adhesion, which increases the torque of the thread. Due to the low temperature of the coating (generally 100 ° C ~ 150 ° C), the short time (about 20 min), the temperature has little effect on the stress relaxation of the thread. When the sample is added with waterproof gasket, the tightening torque of the bolt produces different degrees of attenuation. The test with or without the waterproof mat can prove that the existence of the waterproof gasket is the most important reason for the torque decay of the coupling bolt. Figure 2 bolt torque attenuation test design Table 2 Torque attenuation test results First, the waterproof gasket material allows 20% deformation. In order to study the immersion of the waterproof gasket in the solvent, the simple design test is as follows: Take 5 waterproof gaskets with a theoretical thickness of 0.5 mm and soak them in water at 50 °C. At regular intervals, the thickness of the gasket was measured with a vernier caliper, and the measurement results are shown in Fig. 3. It can be seen from Fig. 3 that the waterproof gasket expands rapidly within 15 minutes after soaking, and the thickness is increased by about 0.2 mm, which is increased by 40%. After 15 minutes, the waterproof gasket no longer expands and the thickness remains stable. If the initial thickness of the waterproof gasket increases, the amount of expansion of the thickness will be greater. During the coating process, the sample is immersed in the solvent for about 20 minutes. After the water shims are immersed several times, the thickness is continuously changed, resulting in a decrease in the connection rigidity of the hinge. When there is a soft waterproof gasket in the threaded joint, the joint changes from a hard joint to a soft joint. The drop in connection stiffness is the most important cause of torque decay. Secondly, since the hardness of the waterproof gasket is smaller than that of the sheet metal and the strong water absorption property, the contact panel is liable to collapse after the nut is tightened. The waterproof gasket on the support surface sinks, causing the preload to be attenuated. Finally, after the waterproof gasket is coated, the friction coefficient of the gasket support surface will also change significantly. The change of the friction coefficient is also one of the causes of the torque attenuation. Figure 3 Relationship between gasket thickness and soaking time 4.2.1 Waterproof gasket thickness As the thickness of the waterproof gasket increases, the attenuation value of the thread torque gradually increases. The torque attenuation evaluation values ​​of the two nuts vary as the thickness changes as shown in 4. Taking Q32008 as an example, when the thickness of the waterproof gasket is 1.0, the tightening torque value is only 7.5 Nm, which is reduced by 15.5 Nm, and the attenuation is half-to-approximately 67.39%. The attenuated torque value cannot meet the relevant requirements. As the thickness of the gasket increases, the amount of expansion of the gasket during the coating process is greater. The greater the variation of the gasket, the more severe the torque reduction. Figure 4 Relationship between bolt torque value and gasket thickness 4.2.2 Nut category When there is no waterproof gasket, the Q32008 torque moment increases to 37Nm on average, and the Q3220S torque increases to 31 Nm on average. Under 3 different thickness waterproof gaskets. The torque attenuation values ​​of Q32008 are both more severe than the torque attenuation values ​​of Q3220s. Through experiments, it is found that the torque variation range of Q32008 is 4.5-37 Nm, and the torque variation range of Q32208 is 7.5-31 Nm. Q32208 has a smaller torque variation range than Q32008 during the test, and the torque is more stable. Since the contact surface of Q3220s and the connecting material is larger than Q32008, the friction coefficient of the supporting surface is large. Based on the above theoretical analysis, the torque of Q32208 is more stable than the torque of Q32008 after painting. 4.3 Improvement measures According to the above theory and experimental analysis, combined with the specific joint working conditions, the following improvement measures are proposed: 4.3.1 Develop a reasonable check torque range The pre-tightening force is adjusted from 23±2 Nm to 26±2Nm, perfecting the torque check specification. Real-time inspection and monitoring of torque during production. 4.3.2 Tightening of the torque after painting After the painting is carried out, the tightening torque of the actual vehicle is repeatedly tightened. 4.3.3 Changing the thickness of the waterproof gasket According to the test, the torque sag decreases as the thickness of the waterproof gasket decreases. Reducing the waterproof gasket or replacing the sealing material is an important measure to improve the torque attenuation. In view of the sudden change in the flatness of the back door caused by the change in thickness, the model is temporarily taken to take this measure. For it, in the early stage of development, it is recommended to use a waterproof gasket with a thickness of 0.5 mm. 4.3.4 Renewing the nut According to the test of two types of nuts, Q32208 has better resistance to moment attenuation than Q32008. This model was changed from ordinary Q32008 to Q32208. For other models, a combination nut with a washer is recommended in the early stage of development. In response to the above improvement measures, combined with the specific situation of the vehicle, the improvement plan is as follows: the same torque is changed to 26±2 Nm, the nut is replaced by Q32208, and the actual vehicle verification shows that the torque is attenuated from 26 Nm to 22~26 Nm, which is consistent with company request. 5. Conclusion Based on the theoretical analysis and test data, the reasons for the attenuation of the bolts on the back door of the model are studied in detail, and the following conclusions are obtained: a. If there is no waterproof gasket, the same torque will not be attenuated, and the torque will increase; b. Waterproof gaskets are the leading cause of stress relaxation. As the thickness of the waterproof gasket increases, the stress relaxation phenomenon is severe.
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