2726 ボルトの引張特性に及ぼす遊びねじ部の影響(S58-2 ねじ(2),S58 機械要素の設計・製造・応用技術)

Abstract

The mechanical properties of bolts such as the tensile strength R_m, the yield strength R_<eL> or R_<p0.2>, and the percent elongation after fracture A are specified in ISO 898-1 for each property class. Of these properties, R_m shall be tested by the tensile testing for full-size bolt for bolts with d>3 and l&ge;2.5d, and by the tensile test for machined test piece for bolts with d&le;3 or l<2.5d. So the stress area A_s to calculate the stress on the free-threaded part of a bolt sometimes becomes the issue to be discussed in terms of consistency of R_m values. Furthermore, the tensile testing for full-size bolt to obtain the yield strength R_<pf> and the elongation after fracture A_f will be introduced into the next edition of ISO 898-1 (now at the stage of CD). In the new testing method, the free-threaded length l_<th> shall be fixed to 1.2d, and the yield strength R_<pf> is defined as the stress at 0.0048d(0.4%) non-proportional elongation. However, the appropriateness of the testing method has not fully been considered. The aim of this paper is to clarify the effect of the free-threaded part of bolt on its tensile property, and to give a rational basis for the tensile test for full-size bolt. The results from tensile test and FEM analysis show that the tensile property of a bolt can be simulated by elastic-plastic FEM analysis using axisymmetric model, and that the loadability of free-threaded part is strongly affected by permanent strain distribution due to its geometry.