In general, the studies of fatigue property of plastics have been carried out experimentally by rotary bending or plane bending fatigue tests and tension-compression fatigue tests in the axial direction, but hardly by torsional fatigue tests. In this article, the torsional fatigue properties of various engineering plastics (POM, PC and Epoxy resin) were investigated experimentally, and were compared with those by the rotary and bending fatigue tests.
The results obtained are as follows:
(1) The torsional fatigue failure of POM and PC, in which the static tensile strength σ
t is larger than the shear strength τ
0, occurred by shearing stress in the first stage before the transition point B, at which the fracture mode changes from shearing to tension, and by tensile stress in the latter stage after the point B, whereas the Epoxy resin was always broken by tensile stress because its tensile strength is smaller than the shear strength. Therefore, there is no transition point B in the
Sa-N curve of the Epoxy resin.
(2) The torsional fatigue strength τ
f of POM, PC and Epoxy resin were 2.3, 0.9 and 1.7kgf/mm
2, respectively, and the torsional fatigue strength ratios α(=τ
f/σ
t) became 0.34, 0.14 and 0.34, respectively, and the another ratios γ(=τ
t/τ
0) become 0.38, 0.15 and 0.24, respectively.
(3) The torsional fatigue strength of POM and PC were nearly equal to that of the plane bending, but were about 25-30% smaller than that of the rotary bending, and the torsional fatigue and rotary bending fatigue strengths of the Epoxy resin were quantitatively nearly equal.
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