It is important to evaluate the safety of bolted joint under a load eccentric to a bolt axis. We examined tapped thread joints, with which a clamped plate is tightened with bolts to a base body, by applying eccentric loads to the bolts. The structural interface opening between the clamped plate and base body occurs due to the eccentric load based on the principle of leverage. During the growth of the interface opening, nonlinearity noticeably appears on the tensional and bending components of bolt stress, and these stress components become larger than expected in an early phase before a fatal bolt pull-out occurs. However, an evaluation method taking into account nonlinearity has not been investigated.

We propose a normalized-bolt-stress evaluation method for tapped thread joints that takes into account the effect of the nonlinearity of bolt stress during interface opening. We conducted numerical calculations, experiments, and finite element analyses to quantitatively validate the stress under the following conditions: (i) tapped thread joints with the clamped plate thinner than the bolt diameter and (ii) load eccentric to the bolt axis. We confirmed that the bolt preload and lever ratio should be fixed at an initial phase, in which has no interface openings for the appropriate normalization. By using the normalized-bolt-stress evaluation method, strength evaluation becomes easily applicable to layout changes of bolted joints as a similarity rule.

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