Experimental study on transverse fretting wear of flange bolts
In order to study the loosening mechanism of flange connection bolts in transverse vibration, the transverse fretting wear test of fasteners was carried out by means of transverse vibration fatigue testing machine, and then the decline process of preload and preload torque of bolt connection under different transverse loads and frequencies was observed, and the wear amount and surface damage were measured. The results show that the decrease of bolt preload can be divided into two stages; At the beginning of the experiment, the decrease of preload is in the plastic deformation stage and the decrease of preload is in the fretting wear stage. The decrease of preload can be divided into three stages: in the first stage, the decrease of preload is not obvious due to the high local contact stress; In the second stage, due to the expansion of plastic deformation, the preloading moment decreases rapidly; In the third stage, the flange bolts enter the fretting wear stage, and the pre tightening torque begins to decrease slowly. Under low frequency vibration, the mass change of bolt is not sensitive to small frequency change. The results show that the surface wear of bolt and fixture increases with the increase of alternating amplitude load; It decreases with the increase of frequency.
Bolt connection is widely used in various mechanical structures because of its simple structure, convenient disassembly and inspection. However, due to the uncertain working conditions of bolts, it is easy to reduce the preload under the action of external load, resulting in joint failure, causing major safety accidents. Therefore, the research on threaded connection structure is a hot spot of engineering research at home and abroad. The results show that transverse vibration load is more likely to cause bolt loosening than axial vibration load. Nassarsa et al.  pointed out that the clamping force of bolted connection decreased rapidly under alternating load, which was mainly due to irreversible plastic deformation. Basavas et al. [9-10] showed that the decrease of clamping force was related to amplitude, vibration frequency and wear of contact thread when bolt vibrated, and pointed out that fretting wear was one of the causes of thread structure failure.
The purpose of this paper is to study the plastic deformation and fretting wear of the bolts on the main sealing flange of the oil tank under the transverse vibration condition caused by the magnetic inrush current of the transformer, and to quantitatively analyze the relationship between the amplitude, frequency and cycle times of the external load and the amount of wear and wear, as well as the influence of the above factors on the bolt looseness.
Transverse vibration test
The transverse vibration test of bolts is carried out by mtslandmark 37025 hydraulic fatigue testing machine, as shown in Fig. 1a. The testing machine can control the vibration frequency and external load force and record the vibration curve. Figure 1b shows the fixture of experimental design. A pressure sensor is set between the screw connected clamping plate and the nut to monitor the change of the preload in real time. At the end of the test, the change of pre tightening torque was measured with torque wrench. The specific test parameters are shown in Table 1.
The loss of material quality caused by wear is called wear amount, which is mainly measured by weighing method, that is, the change of sample quality before and after the test, and the difference is the wear amount. The wear condition was observed by length measurement and contour method.
- (1) The length measurement method is to measure the size of the wear mark on the friction surface before and after the wear test with a high-precision length measuring device, and the change is the wear condition;
- (2) The profile method is to record the profile fluctuation curve of the same part on the friction surface before and after the test, that is to measure the wear condition of the same part profile before and after the test.
The quality, surface change and profile curve of bolts were compared before and after the test, and the relationship between them and preload and preload torque was analyzed. In order to make the test accurate and reliable, this test selects the same test conditions and external environment (normal temperature and atmospheric pressure), and the joints are in dry friction state. A total of 90 M16 bolts with strength grade of 4.8 are selected × 5 bolts, divided into 10 groups, 9 bolts in each group. The average value method is used to process data. Under the same measurement conditions, the results of multiple measurements of a certain physical quantity will not be exactly the same. The arithmetic mean value of multiple measurements is used as the measurement result.
Decline trend of bolt pre tightening force and pre tightening torque
Figure 2 and table 2 are the decline trend and data table of pre tightening force and pre tightening torque. The ordinate in Figure 2 is the decline degree of preload, that is, the ratio of the preload P measured by the sensor in real time to the initial preload P 0; Abscissa is the number of cycles of alternating load amplitude. Under different alternating load amplitude A0 and frequency f, after the same cycle, it can be seen that the degree of bolt looseness increases with the increase of alternating load amplitude; With the increase of frequency, the looseness of bolts decreases. As shown in Table 2, when the frequency is 3Hz and the alternating amplitude load A0 is 17.5kn and 22.5kn respectively, after 105 cycles, the preload decreases by 17% and 24% respectively; When the alternating amplitude load A0 is 17.5kn, and the frequency f increases from 3Hz to 5Hz, after 105 cycles, the preload decreases from 17% to 15%.
No matter how the alternating load amplitude A0 and frequency f change, the decline degree of preload can be divided into two stages, and the decay rate of the first stage is much higher than that of the second stage. As shown in Table 2, when the frequency is 3Hz and the alternating load amplitude A0 is 17.5kn, the bolt preload decreases by 12% after 102 cycles, and only 6% after 102 to 105 cycles. When the frequency is constant and the amplitude of alternating load A0 increases to 22.5kn, the bolt preload decreases by 19% after 102 cycles, but only 6% after 102 to 105 cycles.
It can be seen from the test data of different loads and different frequencies that the number of cycles entering the slow descent phase is different. It can be seen from Figure 2 that when the frequency is 3Hz and the amplitude of alternating load A0 is 17.5kn and 27.5kn respectively, the number of cycles entering the stable state is 1000 and 300 respectively; When the amplitude of alternating load A0 is 27.5kn and the frequency increases from 3Hz to 7Hz, the number of cycles entering the stable state changes from 300 to 500.
It can be seen that in the case of lateral vibration, the initial preload decreases rapidly, and then decreases slowly. The wear of bolts can be divided into two stages: the rapid decline stage of preload and the stable decline stage. At the initial stage of transverse vibration, the bolt plastic deformation occurs under the action of external load, and the preload decreases rapidly; As the experiment continues, due to the ratchet effect of the material, the plastic deformation of the bolt enters the stable stage. At this time, the material loss occurs due to the wear between the contact surfaces. Because the thread contact surface can not have the overall sliding phenomenon, only partial sliding which belongs to fretting wear can occur. It can be seen that the slow decline of the preload in this stage is the result of fretting wear.
In view of the fact that in engineering application, the control of pre tightening force mainly depends on the torque value when installing bolts, this paper measures the pre tightening torque and screw out torque before and after the test. At the end of the test, the maximum torque measured by the torque wrench is the torque Mt. The ratio of tightening torque MT to pre tightening torque M0 is defined as the decline degree of pre tightening torque. Fig. 3 shows the decreasing trend of bolt pre tightening torque under different frequencies and loads. It can be seen that the pre tightening torque will decrease about 20% after 105 cycles. With the test, it can be found that the initial torque and screwing out torque have little change between 100 times and 300 times, which is due to the high preload applied to the bolt connection structure at the beginning of the test, the uneven contact surface between nut and bolt and fixture, the phenomenon of adhesion or cold welding, and the large static friction coefficient. However, as the deformation continues to occur, the bolt adhesion or cold welding phenomenon decreases, but the static friction coefficient decreases, and the pre tightening torque begins to decline rapidly. When the frequency is 7Hz and the amplitude of alternating load A0 is 27.5kn, the preload torque decreases slowly after 500 cycles, which is basically consistent with the number of cycles when the preload enters the stable state. Therefore, the loosening of preload can be divided into three stages: in the first stage, due to the high local contact stress, the decrease of preload is not obvious; In the second stage, due to the expansion of plastic deformation, the preloading moment decreases rapidly; In the third stage, the bolts enter the fretting wear stage, and the pre tightening torque begins to decrease slowly.
As shown in Fig. 2 and Fig. 3, under low frequency vibration, the small change of frequency has little effect on the preload and preload moment, and tends to be stable soon. This is because under low frequency vibration, the plastic deformation stage of the bolt ends quickly and begins to enter the fretting wear stage.
Wear of bolts
The mass of the bolt is measured before and after the test, and the difference is the amount of wear. The phenomenon mentioned above in the initial stage of transverse vibration is mainly due to the rapid decrease of preload caused by the plastic deformation of the material, while the later stage is due to the slow decrease of preload caused by fretting wear. After 104 external load cycles, the preload began to decrease slowly, and the bolt began to enter the fretting wear stage. Therefore, this experiment recorded the thread mass of 104 to 105 external load cycles, and calculated the wear amount of bolt mass as shown in Table 3 and table 4.
During the test, after 104 cycles, the quality of individual bolts changed, or the quality increased. It can be seen from Figure 4 that this is due to the abrasion and adhesion of the contact surface between the clamp and the bolt, and part of the clamp material is transferred to the bolt.
As shown in Table 3 and table 4, the mass change increased significantly when the number of cycles increased from 104 to 105. When the amplitude of alternating load is 22.5kn, the mass change after 105 cycles is larger than that after 104 cycles; Moreover, the amount of increase was about 10mg at three different frequencies. This is because the mass change is not sensitive to the small change of frequency under low frequency vibration.
Wear surface analysis of bolt fixture contact surface
As shown in Figure 5, before and after the test, the wear of the top of the thread and the contact surface of the fixture were observed and measured with vhx-s90f optical microscope. Fig. 5A shows the contact surface between the top of the screw thread and the fixture before the test, and Fig. 5B shows that after 105 cycles of alternating load with frequency of 5Hz and amplitude of 27.5kn, the width of the contact surface between the bolt and the fixture, that is, the top of the screw thread, changes from 457 μM becomes 864 μ m。 The increase of thread top width under different load amplitude and frequency is shown in Table 5.
It can be concluded from table 5 that the wear of thread top width increases with the increase of alternating amplitude load; It decreases with the increase of frequency. When the frequency is 3 Hz, the wear rate is about 100 as the amplitude of alternating load increases from 17.5 kn to 22.5 kn and finally to 27.5 kn μ M / 5kn. It can be seen that the effect of alternating load on wear is very significant, and the same conclusion can be obtained from the friction theory. When the amplitude of alternating load is 22.5kn, with the frequency increasing from 3Hz to 5Hz and finally to 7Hz, the average wear rate is 100 μ The trend of M / 2Hz decreases steadily. This is mainly because under the same load, the single contact time of low frequency is longer, and the wear is more serious under the same number of cycles.
Deformation analysis of bolt profile
Before and after the test, GOM optical detector was used to compare the changes of the contour surface, as shown in Figure 6, the bolt tail offset. As shown in Figure 7, after 104 cycles, the tail offset of the bolt tends to be stable, and the specific data are shown in Table 6. The larger the amplitude of alternating load is, the larger the offset of bolt tail is; The lower the frequency is, the larger the offset of bolt tail is. As shown in Fig. 7a, the bolt offset increases from 0.1167mm to 0.4564mm as the amplitude of alternating load increases from 17.5kn to 27.5kn at 3Hz; When the amplitude of alternating load is 27.5kn, the bolt offset decreases from 0.4564mm to 0.3027mm as the frequency increases from 3Hz to 7Hz. In conclusion, it is further verified that the change of preload at the initial stage of the test is caused by the plastic deformation of the material. As shown in Figure 7, the plastic deformation of the bolt is large at the beginning, and after 104 cycles, the plastic deformation of the material gradually tends to be stable. Combined with Fig. 2 and table 2, it can be seen that the plastic deformation of the pre cyclic material is the main reason for the change of preload at the beginning of the test.
In this paper, the effects of different loads and frequencies on bolt preload, preload torque and wear are studied by experiments. The causes of bolt looseness are discussed:
- 1) The decrease of preload can be divided into two stages: the plastic deformation stage in which the preload decreases rapidly at the beginning of the experiment and the fretting wear stage in which the preload decreases slowly. The decrease of preload can be divided into three stages: in the first stage, the decrease of preload is not obvious due to the high local contact stress; In the second stage, due to the expansion of plastic deformation, the preloading moment decreases rapidly; In the third stage, the bolts enter the fretting wear stage, and the pre tightening torque begins to decrease slowly.
- 2) Under low frequency vibration, the mass change of bolt is not sensitive to the small change of frequency, but increases obviously with the increase of alternating load amplitude. Under the same load, the single contact time of low frequency vibration between bolt and fixture is longer, so the wear condition is more serious under the same number of cycles; With the decrease of alternating load amplitude, the wear condition is improved. The larger the amplitude of alternating load, the larger the offset of bolt tail; The lower the frequency is, the larger the offset of bolt tail is.
On the basis of fretting wear mechanism, the loosening process of bolts is further analyzed, which provides important experimental data for its application in practical engineering.
Author: Zhao Jing, Xu Xiao
Source: China Bolts Supplier: www.epowermetals.com
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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