by the sample reaches certain value。 For instance, the limit value of absorbable energy before

failure is W, and the total circle number before failure is N。 When the circle number is ni, the energy absorbed by the sample will be Wi。 The energy absorbed is in direct proportion to its circle

number:

Wi   = ni。 (1)

W N

If the sample bears the load of o1, o2,…, o1  under different stress levels, and its fatigue life

under  each  stress  level will be N1 , N2 ,…, N1 。  Its circle  number will  be n1 , n,…, n1 ,  and the

damage to the part is:

1

n

D = )

Ni

i=1

= 1。 (2)

Under this condition, the absorbed energy reaches the limit value W, and the fatigue   failure

occurs。

When the critical damage:

1

n

D = )

Ni

i=1

= a, (3)

where “a” stands for the safety coefficient of fatigue life。 It is a critical damage value, which is determined by materials and safety requirements for fatigue life of a structural part。 When a = 1, fatigue damages have already appeared。 Hence, “a” is a constant less than 1 in actual engineering problems。 In the manuscript, “a” was set to be 0。8 according to structure materials and life

requirements of the suspension system。

Scholars’ further research on the critical damage shows that parts of the same kind have similar damage value under similar load spectrum。 Therefore, life estimation can be carried out under similar load spectrum, i。e。, relative Miner。 Its expression is:

1

n

D = )

Ni

i=1

= Dƒ。 (4)

In the equation, Dƒ is the damage and experiment value of the parts of the same kind under

similar load spectrum [8, 9]。

3。 Suspension dynamics analysis

3。1。 Research on suspension experiment

It was necessary to test vibration displacement of vehicle wheel center on a real pavement and applied it into a multi-body dynamic model of suspension。 In this way, the suspension system can be simulated really。 As shown in Fig。 1, two wheel center sensors were mounted on vehicle wheel center of the front suspension, respectively。 Then, vehicle was travelling on the road at the speed of 20 m/s for 700 m distance。 The testing process was shown in Fig。 2。 The multi-channel data collection equipment was used to obtain the vibration displacement of wheel center, and it was then imported into test。lab to be processed。 The final testing result was shown in Fig。 3。

a) Testing equipments b) Wheel center sensor

Fig。 1。 Testing equipments of vibration displacement for vehicle wheel center

Fig。 2。 Testing process of vibration displacement and acceleration

Fig。 3。 Vibration displacement of the wheel center

It was shown in Fig。 3 that the vibration displacement fluctuated irregularly when the vehicle traveled on a real pavement。 The actual road condition was used as the excitation during the testing process。 Then, this excitation was imported into the multi-body dynamic model of the suspension system。 As a result, the research will be more realistic。 If the constant vibration excitation was applied on the suspension system, there were some problems in the subsequent researches。 Because this paper wanted to obtain the fatigue characteristic of the buffer block in the suspension system, it must require a cyclic excitation。 If a sinusoidal excitation was applied on the suspension