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1、BRAKE TEST OF SiCp/A356 BRAKE DISK AND INTERPRETATION OF EXPERIMENTAL RESULTS*Abstract: Material properties are obvious different between aluminum matrix composites and iron and steel materials. After the brake disk braked at the same speed, the average temperature of the aluminum brake disk is I.S
2、times as high as one of iron and steel brake disk, the thermal expansion value of the aluminum brake disk is 2 times as big as one of iron and steel brake disk. Mechanical property of the material decreases with the temperature increasing generally during braking, on the other hand, the big thermal
3、stress in the brake disk happens becai/ie it material expansion is constrained. Firstly, the reasons of the thermal stress generation and the T-actur: failure of brake disks during braking are analyzed qualitatively by virtue of three-bar stress frame and sandwich deformation principles in physic, a
4、nd then the five constraints which cause the thermal stress are summarized. On the base of the experimental results on the 1:1 emergency brake test, the thermal stress and temperature fields are simulated; The behavior of the fracture failure is interpreted semi-quantitatively by finite element anal
5、ysis. There is the coincident forecast for the fraction position in term of the two methods. In the end, in the light of the analysis and calculation results, it is the general principles observed by the structure design and assembly of the brake disk that are summarized. Key words: Brake disk Brake
6、 test Thermal stress Heat flux Finite element analysis0 INTRODUCTIONAfter 21st century, regarding of the higher speed requirement, the light weight of trains has to be met. The brake disk is the critical part of the brake and a large number of brake disks are used in the whole train, which is assemb
7、led on the axles of the train, shown as Fig. 1. So, the light weight of the brake disk is active to the high-speed and light weight of the trains1. Internationally, the study of the loss of weight of brake disk is being done by changing the material of it in German, Japan, French and American, etc.
8、The SiCp/A356 brake disk has been developed in Beijing Jiaotong University and Hunan University in the country, and the 200 km/h brake test has been succeeding on the 1:1 braking bench.In order to forecast for the use reliability of brake disks, the temperature and thermal stress fields inside brake
9、 disks have to be understood at the different time and on the different work conditions, but actually it is difficult to acquire these information by the direct test. Generally, these data are taken by means of the mechanical analysis or numerical simulation with FEM. Up to now, the simulations of t
10、he temperature and thermal stress fields are preformed with ANSYS and MSC marc code, there were two kinds of simulation methods, namely, the direct coupling with contact3 and the indirect coupling by flux load4. But the mechanical analysis for brake disks during braking are preformed hardly any. Tak
11、ing the brake disk as research object, which was designed and fabricated autonomously, the qualitative mechanical analysis and the semi-quantitative simulation analysis of the temperature and stress fields for brake disks were performed separately to interpret the fracture behavior1 BRAKE TESTBrake
12、tests at the 180, 190 and 200 km/h initial speed were preformed respectively on the 1:1 braking bench at Longchang brake center, Nanjing and at the brake center of China Academy of Railway Sciences, Beijing. The work situation and behavior of the fracture failure of brake disk were studied on the co
13、ndition of the emergency braking.After the brake test at the 180 km/h initial speed, there were not injury and abnormal wear on the surface of the brake disk, seen in Fig. 2, and the mean friction coefficient was 0.289, the braking length 1 482 m met the requirement of the braking length.Because of
14、the improved structure and changed mode of assembly of the brake disk and the semi-metallic pads applied, after the brake test at the 200 km/h initial speed done, the mean friction coefficient was 0.438, the braking length 1 104.7 m met the requirement of the braking length, there was little low inj
15、ury on the surface of the brake disk, but not the crack on the side of the boss, shown in Fig. 4.2 INTERPRETATION OF EXPERIMENTAL RESULTS2.1 Mechanical analysisThere are kinds of stresses during the brake process including the stress caused by the centrifugal force and the friction torque; The stres
16、s caused by the restraints of the different parts each other because of the different linear expansion factor of the materials and the stress caused by the self-restraint of the structure during distorting. The last two kinds of stresses are named for the intrinsic thermal stress and structure therm
17、al stress respectively. They are more than the other stresses and the prime force of the fracture failure of the brake disk51. 2.1.1 Self-restraint of brake diskThe model was simplified in order to analyze, and regarding the friction faces as laminar bodies, that is, they were divided into two layer
18、s such as L and L2, seen in Fig. 5a. Because of the conductivity of the material, the temperature difference between layer L and layer L2 was caused during applying the heat loads. If the deformations of the layers are freedom, the deformation of the layer L at arbitrary directions can be calculated
19、 by the equationLikely the deformation of the layer L at arbitrary directions can be calculated by the equationActually the deformations of the layers were restrained, the stress happened6. It was the structure thermal stress caused by the different expansion increment during the temperature increas
20、ed. The detail analysis was as follows.During braking there was 7 T2, so A/, A/2, if the different layers deformation was freedom, seen in Fig. 5b, there was not the stress in the layer L and layer L2, but it didnt so actually. There were the restraints among the layers each other, the stress happen
21、ed, the deformation would cause the compression stress in the layer Lt and the tensile stress in the layer L2, when it was less than cr02. Taking the fixation of the fins into account, the deformation of the friction faces would happen like Fig. 5c. The deformation caused the compression stress in a
22、rea A and area D and tensile stress in area B and area C, seen in Fig. 5a. When the stress was more than tensile strength crb of the material at the same temperature, the fracture failure would happen in the layer L2. The maximum tensile stress took place in the adjoiner of layer L2 and the fins and
23、 it was possible that the fracture appeared in the area.When the temperature was higher than the temperature at which the plastic deformation of the material happened, the deformation of the friction faces would happen like Fig. 5d. Layer L and layer L2 expanded to the same length / . But it is hard
24、ly possible that the situation happened during braking.Regardless of the distortion of the friction face, the simplified model can be seen in Fig. 6. When the heat flux load was applied, the temperature of fins increased; There was a difference between the fins, so the deformation was coordination,
25、and then the maximum stress would happen in the adjoiner between the friction face and the fins. If the deformations of fins were freedom, the deformation increment of the little volume fin isBecause there was 7, Th2, it was A/i A/T. Taking the coordinated deformation into account, the distortion of
26、 them happened like Fig. 6b, and the common increment was A/i. The deformation caused the compression stress in area A and area B and tensile stress in area C and area D, shown as Fig. 6a.Subsequently the radial deformation of the friction faces of the brake disk would be interpreted, the deformatio
27、n caused the circumferential thermal stress. It was assumed the simplified model was divided into several layers along the radial direction, shown in Fig. 7a. When the heat flux was applied on the friction faces, the temperature increased. If the temperature increment of the friction faces was unifo
28、rm, supposed it was AT, the linear strain of the different radius was identical, that was e = aATBut actually it was different, because of the materials volume and assembly, the temperature of the inner of the friction faces was obviously lower than the temperature of the outer, and the friction fac
29、es would deform like Fig. 7c. Assumed that the temperature increment of the outer was AT, and one of the inner was 1ST, there was tsT AT; The linear strain of the outer could be indicated as one of the inners could be calculated byThere was also aT ar (In general, the heat expansion factor of the ma
30、terials increases with the temperature increasing), the circumferential increment of the inner was.where Ac is the circumferential increment, while the temperature of the friction faces was uniform, the circumferential increment of the inner was.There was Ac Ac obviously, consequently the tensile st
31、ress happen at the inner. When the tensile stress value was more than erb of the materials, the fracture failure would happen at the inner of the friction faces, especially the sides of the boss, and the fracture direction was vertical with the circumferential direction.After the different material
32、components were assembled by bolts, due to the different linear expansion factors, the brake disk would loose or be compacted with the temperature increasing or decreasing. The pressure value was decided by the linear expansion factor, so the stress caused by the pressure was named for the material
33、intrinsic thermal stress.The assembly relationship among the brake disk, hub, clamping ring and bolts created the axial mechanical restrain of the brake disk. After the heat load was applied, due to the material conductivity, their temperature increased. On the ideal condition, that is, their materi
34、al and temperature was the same, the jointer between the brake disk and the boss would deform like Fig. 8a, and the assumed deformation had been remarked by the red dash. But actually their material and temperature were different, the pressure would increase with the temperature increasing. On the o
35、ther hand, considering the bolt restraint, the expansion of the boss of the brake disk was limited; The actual deformation can be seen in Fig. 8b, remarked by the dot-and-dash lines. So the intrinsic thermal stress took place at the root of the boss, its direction was along the axial direction of th
36、e brake disk2.1.2 Radial mechanical restraint of the brake diskDuring the heat load applied, the temperature of brake disk increased, if it was assumed that the temperature change was uniform, the expansion of the brake disk isnt restrained; The distortion was shown in Fig. 9a. The dash line indicat
37、es the situation after expanding. But as the matter of fact that the shift of bosses was restricted due to the assembly among the brake disk, hub, clamping ring and bolts, the actual deformation was shown in Fig. 9b. The position of the tensile stress was marked in the arrows, its direction was vert
38、ical to the radial. Because the stress was caused due to the restrained shift of the bosses, it was named for the mechanical restraint thermal stress.Via the mechanical analysis on the brake disk, it could be known that the heat load was the reason for causing the thermal stress, deformation and fra
39、cture. Taking the self-restraint thermal stress, the material intrinsic thermal stress and the mechanical restraint thermal stress into account comprehensively, it was the most possible for the fracture to occur at the sides of the jointer between the friction and the boss, seen in Fig. 10.2.2 Finit
40、e element analysisBased on the brake conditions listed in Table 1, the heat flux load can be calculated byWhen the brake process was simulated by the thermal-structure method178, the model of brake disk was simplified according to the cyclosymmetric structure characteristic of it. Only a perigon was
41、 used to calculate, and the interface couple of the model was used between area A and area B. Supposed that the thermal load is put uniformly into the whole friction surfaces and meets the cyclosymmetric characteristic, the computation model for simulation was established, shown as Fig. 11. The indi
42、rect thermal-structure method was adopted to perform the brake analysis; The flow diagram of calculation is shown as Fig. 12.The brake process was divided into a lot of load steps according to brake time, and then every load step was solved by nonlinear transient analysis. The results of the thermal
43、 stress were shown in Table 2. The mechanical property with temperature of the SiCp/A356 composites was listed in Table 3.From the finite element analysis results, it could be concluded that during the brake process at the 180 km/h and 200 km/h speed respectively, the value of maximum thermal stress
44、 were 204 MPa and 222 MPa less than the ultimate strength of the material at the same temperature, so the brake disk was safety in theoretically, shown as Fig. 2 and Fig. 4. On the other hand, when braking at the 190 km/h speed, the value of maximum thermal stress was 222 MPa approach to the ultimat
45、e strength of the material at the same temperature (ob=225 MPa), so the brake disk was failure theoretically, shown as Fig. 3. The conclusions were also conformance to the experimental results. Figs. 13 and 14 show the 200 km/h brake temperature field and thermal stress field respectively.3 CONCLUSI
46、ONSThe five constrains, which causes the stress during braking, were inducted. Based on the mechanical analysis above and the simulation results, the following suggestions were put forward to decrease the stress caused by braking.(1) The uniformization of full temperature field for the full brake di
47、sk. There are three methods to realize the aim. First, by the increase of the number and volume of fins, the stress was decreased obviously. Second, changing the material distribution along radial direction, such as the different thickness of the friction faces. Besides, the same aim could be achiev
48、ed after adding the thermal protective coating when assembled.(2) The uniformization of local temperature field for the brake disk. By the increase of the number of the contacted cylinders or the decrease of the volume difference between the fins, the temperature difference between them could be reduced, and the thermal stress could be reduced finally.(3) The plug-type assembly between the brake disk and the hub. By the assembly method, the circumferential relative shift was limited strictly, but the radial and axial relative shifts were released.