Some Studies on the Rotating Bending Fatigue of Large Mild Steel Specimens : Part 4, Effect of Size on Fatigue Strength of Cracked Specimen and Propagation Rate of Fatigue Crack

Abstract

This paper is succeeded by No.2 and No.3 reports of the same title in which propagation of fatigue caracks on large smooth surface and fillet specimens has been dealt with. In this paper, the author has studied the following two problems : (1) The fatigue strength of large smooth specimens having a diameter of 130 mm with fine cracks artificially produced beforehand by sea water corrosion. (2) The influence of size on the fatigue strength of cracked specimens and propagation speed of fatigue cracks. The test results have revealed that the fatigue limit of steel specimens, 130 mm in diameter with fine corrosion cracks on the surface is about ±11.0 kg/mm². This limit coincides with the result assumed by studying the crack propagation of specimens, 120 mm in diameter. Analytical investigation of the propagation speed of fatigue cracks of 60 φmm and 20 φmm specimens has shown that the fatigue limit of these specimens are assumed to be about ±12.0 kg/mm² and ±15.0 kg/mm² respectively, when fine cracks may previously exist on their surfaces. From the above results, it may be stated that the fatigue limit of smooth specimens having fine cracks on their surface will tend to increase as the diameter decreases. On the other hand, from the analytical studies on the shape of shelly patterns on the fracture surface, the propagation of fatigue crack may be given approximately by the following formulae in connection with the diameter of specimen : σ_o≤ν_bσ_n<σ_o+σ_s, [numerical formula] where : υ_c is propagation speed of fatigue crack μ/c, ν_bσ_n : stress on the edge line of fracture surface kg/mm², σ_n : stress on the specimen having no crack, ν_b : stress magnifying factor due to crack, σ_o : fatigue limit of cracked specimen kg/mm², σ_s : yielding point of the material kg/mm², K and k : constants varying according to the properties of material and range of ν_bσ_n, d : diameter of specimen mm.