Deformation and Fracture of Mild Steel Under Cyclic Stresses in Relation to Crystalline Structure

Abstract

The paper describes an extension of a recent research into the characteristics of the deformation and fracture of metals under static and fatigue stressing as revealed by precise methods of X-ray diffraction. The four series of fatigue tests were designed specially to take into consideration the cases where the upper stress of the cycle is in the immediate neighbourhood, first, of the ultimate tensile strength, and second, of the static yield point of the material. Accurate records have been made throughout of the changes in shape of the specimen. The structure has been examined by the X-ray method at the end of the test in every case, and also, in certain cases, at various interrupted stages. The results have shown that, in general, the observed changes in structure, even under the most critical conditions of stress, are similar in nature to those found in the previous work. It has also been established that, under a safe range of stress, the damage to the structure is equivalent to that caused by a single sustained application of a static stress equal in amount to that of the maximum stress of the cycle. Further, the additional and progressive damage caused by cycles of an unsafe range of stress leads to a complete fragmentation of the whole or part of the specimen. Finally, the results of the complete series of alternating and pulsating direct stresses have been reviewed in relation to the deformation in the specimen; the conclusion is reached that, though complicating factors are present, the average stress of the cycle appears to have only a small effect on the fatigue resistance of the material.