引張および圧縮予歪が極低サイクル疲労寿命に及ぼす影響

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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 N_f/N_<f0>, that is the ratio of failure life of prestrained specimen to failure life of un-prestrained specimen, fell into a scatter band against _<pre>/_f, 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 10mm diameter and 4 MHz frequency, which was attached to an end edge of a specimen. In the surface removed series, N_f/N_<f0> ratio showed a gradually decreasing tendency with increasing amount of _<pre>/_f after keeping N_f/N_<f0>1 up to _<pre>/_f0.5, and N_f/N_<f0> value at _<pre>/_f=0.6 was observed 0.87 for the tension prestrained specimen and 0.53 for the compression prestrained specimen. In the surface remained series, N_f/N_<f0> ratio began its steep decreasing behaviour at about 0.2 in _<pre>/_f ratio, and N_f/N_<f0> ratio at _<pre>/_f=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 _<fR>, no difference between the surface removed condition and the surface remained condition was observed at any value of _<pre>/_f. By applying Coffin's formula to an analysis of progressive ductility loss in the course of prestraining followed by static tension, _<fR> was expressed as a function of _f and _<pre> respectively for tensile prestrain and compressive prestrain. The derived formulae showed good agreement with experimental results. The N_f/N_<f0> ratio was theoretically analysed on the basis of the retained static fracture ductility and the linear damage hypothesis in low cycle fatigue. The derived formulae were found to show quantitatively fair agreement with experimental results. As the next step, empirical formulae expressing N_f/N_<f0> as a function of _<pre>/_f were derived for the whole range of _<pre>/_f, from nought to unity, as follows : [numerical formula] where k_1=4.695, k_2=1.459 and k_1=3.868, k_2=4.211 for tensile and compressive prestrains of surface removed test series, and k_1=3.174, k_2=12.863 and k_1=2.319, k_2=3.097 for tens
社団法人日本船舶海洋工学会の論文

引張および圧縮予歪が極低サイクル疲労寿命に及ぼす影響

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