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The effects of excessive prestrain in tension or in compression upon very low-cycle fatigue life were investigated for two mild steels and two high strength steels as an aid in the analysis of a ship failure in service, in which the bow structure was broken off due to local buckling and by low-cycle fatigue in extremely heavy sea conditions. The test series in the present investigation was: (1) uni-directional tension test after prestraining, (2) completely reversed strain cycling tests of non-prestrained material with several strain ranges, (3) tensile or compressive straining directly followed by completely reversed strain cycling, and (4) completely reversed strain cycling tests of specimens which were machined out of previously prestrained large diameter specimens.
The residual static fracture ductility decreases as a function of the amount of prestrain, showing much more loss of ductility in the case of compressive prestrain. Fairly good agreements on strain range and fatigue life were found between experimental data and estimated values calculated after Manson's and Iida's formulae. The failure life of a specimen subjected to prestrain showed a remarkable decrease from the failure life of the original material depending on three parameters: amount of prestrain, direction of prestrain, and specimen surface conditions. The worst case in the range of the present tests is that of tensile prestraining directly followed by strain cycling. In this case the low-cycle fatigue failure life of a prestrained specimen is, as an example, reduced by 93% of the failure life of the original material, when the amount of the tensile prestrain is 60% of the original static fracture ductility based on area reduction to failure.
fatigue (materials), fatigue failure life, fatigue life estimation, high strength steel, mild steel, prestrain, residual static fracture ductility, service failure, ship, strain range estimation, very low-cycle fatigue, visible crack initiation life
Professor, University of Tokyo, Tokyo,