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1、CHINESE JOURNAL OF MECHANICAL ENGINEERING74Vol.20, No. 5, 2007YANGZhiyong HAN JianminLI WeijingWANGJinhuaSchool of Mechanical, Electronicand Control Engineering,Beijing Jiaotong University,Beijing 100044, ChinaBRAKE TEST OF SiCp/A356 BRAKE DISK AND INTERPRETATION OF EXPERIMENTAL RESULTS*Abstract: Ma
2、terial 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 times as high as one of iron and steel brake disk, the thermal expansion value of the alum
3、inum 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 stress in the brake disk happens becai/ie it material expansion is constrained. Firstly, t
4、he 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, and then the five constraints which cause the thermal stress are summarized. On the base of
5、 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 analysis. There is the coincident forecast for the fraction position in term of the two method
6、s. 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 test Thermal stress Heat flux Finite element analysis0 INTRODUCTIONAfter 21st century, re
7、garding 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 assembled on the axles of the train, shown as Fig. 1. So, the light weight of the brake disk is
8、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. The SiCp/A356 brake disk has been developed in Beijing Jiaotong University and Hunan Unive
9、rsity in the country, and the 200 km/h brake test has been succeeding on the 1:1 braking bench2.temperature and stress fields for brake disks were performed separately to interpret the fracture behavior.1 BRAKE TESTBrake tests at the 180, 190 and 200 km/h initial speed were preformed respectively on
10、 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 condition of the emergency braking.After the brake test at the 180 km/h initial s
11、peed, 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.Photo after braking at 180 km/h speedFig. 1 Braking system of SiCp/A356 composite brake di
12、skIn order to forecast for the use reliability of brake disks, the temperature and thermal stress fields inside brake 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
13、 data are taken by means of the mechanical analysis or numerical simulation with FEM. Up to now, the simulations of the 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 indi
14、rect coupling by flux load4. But the mechanical analysis for brake disks during braking are preformed hardly any. Taking the brake disk as research object, which was designed and fabricated autonomously, the qualitative mechanical analysis and the semi-quantitative simulation analysis of the This pr
15、oject is supported by National Hi-tech Research and Development Program of China (863 Program, No. 2003AA33I190). Received November 28, 2006; received in revised form July 2, 2007; accepted July 16, 2007After the brake test at the 190 km/h initial speed done, the mean friction coefficient was 0.274,
16、 the braking length 1 739.3 m met the requirement of the braking length, but there were low injury on the surface of the brake disk and the crack on the side of the boss, seen in Fig. 3.CrackFig. 3 Photo after braking at 190 km/h speed 1994-2008CHINESE JOURNAL OF MECHANICAL ENGINEERING75*Because of
17、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
18、ury on the surface of the brake disk, but not the crack on the side of the boss, shown in Fig. 4.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 expan
19、ded to the same length / . But it is hardly possible that the situation happened during braking.Fig. 4 Photo after braking at 200 km/h speed2 INTERPRETATION OF EXPERIMENTAL RESULTS2.1 Mechanical analysisThere are kinds of stresses during the brake process including the stress caused by the centrifug
20、al force and the friction torque; The stress 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 i
21、ntrinsic thermal stress and structure thermal 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
22、, that is, they were divided into two layers 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
23、L at arbitrary directions can be calculated by the equationA/,=a(7i-7i)/Likely the deformation of the layer L at arbitrary directions can be calculated by the equationM2=a(T2-T0)lActually the deformations of the layers were restrained, the stress happened6. It was the structure thermal stress caused
24、 by the different expansion increment during the temperature increased. 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. Ther
25、e were the restraints among the layers each other, the stress happened, 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 hap
26、pen like Fig. 5c. The deformation caused the compression stress in area 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 te
27、nsile stress took place in the adjoiner of layer L2 and the fins and it was possible that the fracture appeared in the area.$-#9s&#%#(a)A/I,J/ti ihC(b)(O /A/ii -+1A/(d) Fig. 5 Deformation when the thermal loads applied 1994-20076YANG Zhiyong, et al: Brake test of SiCp/A356 brake disk and interpretat
28、ion of experimental resultsRegardless 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, and then the maximum stress
29、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 isM = a(TM-T0)h And the deformation increment of the big volume fin is M = a(Th2-T0)ho(a)11t, Th2, it was A/i A/T. Taking the coordinated
30、deformation into account, the distortion of 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 brak
31、e disk would be interpreted, the deformation 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 temperatur
32、e increment of the friction faces was uniform, supposed it was AT, the linear strain of the different radius was identical, that was e = aATwhere aLinear expansion factorThe deformed model could be shown in Fig. 7b.Art, = ab.TR, i = 1,2,3,4But actually it was different, because of the materials volu
33、me and assembly, the temperature of the inner of the friction faces was obviously lower than the temperature of the outer, and the friction faces 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 o
34、f the outer could be indicated asFig. 7 Radial deformation of friction faceThere was also aT ar (In general, the heat expansion factor of the materials increases with the temperature increasing), the circumferential increment of the inner wasAc = (27iarA7-i)fl,where Ac is the circumferential increme
35、nt, while the temperature of the friction faces was uniform, the circumferential increment of the inner wasAc = (27ia7.Ar-l)/J,There was Ac Ac obviously, consequently the tensile stress happen at the inner. When the tensile stress value was more than erb of the materials, the fracture failure would
36、happen at the inner of the friction faces, especially the sides of the boss, and the 1994 20 8ACHINESE JOURNAL OF MECHANICAL ENGINEERING77*(a) Nc-restKint eypsnsior(b) Expansion restrained on the bosses Fig. 9 Deformation caused by mechanical restraintfracture direction was vertical with the circumf
37、erential direction.After the different material 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 ca
38、used by the pressure was named for the material 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 increase
39、d. On the ideal condition, that is, their material 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 in
40、crease with the temperature increasing. On the other 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, i
41、ts direction was along the axial direction of the brake disk.(a)(b)Fig. 8 Bosss deformation when the thermal loads applied2.1.2 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 unifor
42、m, the expansion of the brake disk isnt restrained; The distortion was shown in Fig. 9a. The dash line indicates 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 deforma
43、tion was shown in Fig. 9b. The position of the tensile stress was marked in the arrows, its direction was vertical 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
44、 disk, it could be known that the heat load was the reason for causing the thermal stress, deformation and fracture. 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
45、the fracture to occur at the sides of the jointer between the friction and the boss, seen in Fig. 10.2.2 Finite element analysisBased on the brake conditions listed in Table 1, the heat flux load can be calculated by(1)q(.t) = FTa(v0+at)/(.2nA)F-a-vr A-t-nn-where-Gross rail load on axle -Deceleratio
46、n-Initial speed -Friction area -Brake time -Number/axle -Heat coefficientFig. 10 Position of cracks forecastedTable 1 Brake conditions at 180,190 and 200 km/hInitial speed vo/(km If1)AxleBrakingBrakeBrakeFrictionDiskmass m/kgpressure p/kNtimeitsdistance SimcoefficientnumberN18015 4001560.614820.2893
47、19015 4001566.11 739.30.274320015 4001739.61 104.70.4383When 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 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 character
