13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- Fatigue analysis should according to the actual working condition, which means every balance designer should find out the actual load curve of the balance. It is useful to predict the true life of a balance by analysis the balance in a real loading condition and real working frequency. It is more suitable to use the Miner assumption when the curse contains various fatigue stress breadth. Miner on the assumption that: if the cycle stress were a serials breadth denote as S1,S2,S3,….and corresponding destroy life of this material were Nf1,Nf2,Nf3,… then, the life of every stress breadth will be express as the quotient of the actual cycles Ni and life Nfi. We can assumption that the damage cumulate as above method, see Eq. (1) 3 1 2 1 2 3 ... 1 i f f f fi N N N N N N N N + + + = , (1) Life will come to the end while the cycles up to the failure times Nfi under a specified stress breadth. It has many reproaches when we regard the Miner assumption as a effectively designing rule for the testing results are very decentralization. It is widely used in many fields for its simpleness and other strongpoint. 2.3. process of fatigue life prediction The process of fatigue life prediction listed as follows: (1) create the frame of fatigue life prediction, define the characteristic of material, set up the parameter of analysis； (2) define the mode of contact; (3) using solution combination or specify special stress curve。 (4) solve this problem； 3. example of the fatigue Life Prediction of wind tunnel balance This section analysis the fatigue life of the B630Q wind tunnel balance which was used in the supersonic wind tunnel. B630Q wind tunnel balance is a usual balance used for the force and moment measurement. The rated load of this balance listed in the table 1. Table 1. rated load of B630Q B630Q wind tunnel balance connected with the model by a structure which looked like a cone, which means the contact region is a cone. The finite element model which contains the load equipment showed in the figure 2. Figure 2. finite element analysis model My （N.M） Z （N） Mz （N.M） Y （N） Mx （N.M） Q（N） Rated load 50 600 100 2000 30 600
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