出版社：The Japan Society of Naval Architects and Ocean Engineers
摘要：The effects of excessive prestrain in tension or compression upon the very low cycle fatigue life of a mild steel were investigated on the basis of a hint received from a service failure of a ship, of which bow structure was broken off due to local buckling followed by low cycle fatigue in extremely heavy sea conditions. In order to separate two possible factors influencing the failure mechanism, micro-cracks and loss of ductility, two series of tensile and fatigue tests were carried out after one cycle prestraining.In the first test series (surface removed series), an hour-glass shaped specimen of 8 mm in test section diameter was machined out from the center of 15 mm diameter and smooth shaped specimen, that was loaded to a certain amount of prestrain, and then the 8 mm diameter specimen was subjected to static tension test for checking retained static ductility or to reversed strain cycling (R=-1) in the life regime less than 100 cycles. An hour-glass shaped specimen of 8 mm diameter was directly prestrained, and then subjected to static tension test or to reversed strain cycling, in the second test series (surface remained series). All the fatigue tests were conducted at strain range of 0.1, because a previous work showed that the normalized failure life ratio Nf/Nf0, that is the ratio of failure life of prestrained specimen to failure life of un-prestrained specimen, fell into a scatter band against εpre/εf, that is the ratio of prestrain to static fracture ductility, regardless of strain range.The amount of prestrain was varied in seven conditions in the range of 0% to 80% of the static fracture ductility. In addition to fracture tests, electron microscopic examination was carried out of surfaces and cross sections of the test section of 36 specimens, which were subjected to strain cycling up to a certain number of cycles in the range of 0% to 70% of the fatigue life of un-prestrained specimen. Inspectability of crack initiation during fatigue was also examined by UT straight beam method using a probe 10 mm diameter and 4 MHz frequency, which was attached to an end edge of a specimen.In the surface removed series, Nf/Nf0 ratio showed a gradually decreasing tendency with increasing amount of εpre/εf after keeping Nf/Nf0_??_1 up to εpre/εf_??_0.5, and Nf/Nf0 value at εpre/εf=0.6 was observed 0.87 for the tension prestrained specimen and 0.53 for the compression prestrained specimen. In the surface remained series, Nf/Nf0 ratio began its steep decreasing behaviour at about 0.2 in εpre/εf ratio, and Nf/Nf0 ratio at εpre/εf=0.6 was figured as 0.07 for the tension prestrained specimen and 0.32 for the compression prestrained specimen. As to the retained static fracture ductility of a specimen subjected to one cycle prestrain εfR, no difference between the surface removed condition and the surface remained condition was observed at any value of εpre/εf. By applying Coffin's formula to an analysis of progressive ductility loss in the course of prestraining followed by static tension, εfR was expressed as a function of εf and εpre respectively for tensile prestrain and compressive prestrain. The derived formulae showed good agreement with experimental results.
