| The grid structure has quickly developed in our country. It has sprung up a heated fatigue discussion by engineering and academic field in and abroad that rose from the widely used grid structure with bolts sphere joints under suspending crane and the key to its problem is the fatigue life. Sponsored by National Scientific and Natural Funds and academic tutor, the nine high sphere bolts in M20 and twelve in M30 were experimented. The questions will be answered as follows:1 Throughout analyzing the whole grid structure model, stress ratio range and stress diffusion by using structure software Ansys and 3D3S, whether the fatigue of grid structure exists or not is answered.2 From applying reliability theory to grid structure joints, the reliably data of static and fatigue are calculated.3 The gotten high strength bolts data in M20 and M30 are used in different cumulative damage formula to get damage value range, the convergence and reason for discreteness is analyzed. Meanwhile the cumulative damage formula suitable to engineering is advised.4 When the fracture mechanics applied to the fatigue life calculation, the Forman Life Formula is deduced by definite integral and point out that fracture toughness property and stress ratio are main influenced factor to it. By calculating the high strength bolts data in M20 and M30 and comparing them with Paris Life Formula, the fracture toughness property plays an important role in fatigue life is testified.
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