Analysis of factors influencing ultimate and fatigue capacity of grade 12.9 root bolts for wind turbine blades
To reveal the limit and fatigue load bearing characteristics of grade 12.9 high-strength root bolts for wind turbine blades,a comprehensive finite element model(FEM)for the connection of pre-embedded bolt sleeves with root bolts was established,and the influence of bolt design parameters such as necking and the preload force at different yield ratios on the ultimate bearing capacity of the blade root was investigated.Subsequently,the stress range of bolts under different design parameters was investigated,and a fatigue test platform for grade 12.9 blade root bolts was constructed to assess the fatigue rating of bolts,broke through existing standards on fatigue level determination,additionally,the S/N curve of bolts was studied under various parameter values.Then,the correlation between fatigue loading of the blade root bolts under different S/N curves and various design parameters was investigated based on the Markov matrix.The research findings indicate that when applying the same preload force with a consistent yield ratio,greater necking results in higher ultimate bearing capacity.When subjected to excessive fatigue loading,the appearance of openings near the bolt contact surface leads to nonlinear increases in bending moment,under such conditions,necking fails to reduce the stress range of the bolts.The higher the exponent value of the bolt's S/N curve,the smaller the fatigue damage.If it′s possible to exceed the value of the fatigue grade,the constraint of bolt design parameters on fatigue damage will be significantly reduced.
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