Time-dependent relaxation of stress in metal held in a strained state is a well known phenomenon. There also exists a cycle-dependent relaxation of stress which occurs under conditions of repeated straining. To investigate this type of behavior, the mean strain and amplitude of alternating strain is held constant in axial fatigue tests of SAE 4340, and the mean stress is measured as a function of the number of strain repetitions. An analysis utilizing an assumed stress-microstrain relationship is employed to study the relaxation in terms of the condition of the material, the initial and control conditions of the test, and the cycles of strain repetition. Push-pull fatigue data is also reported in which the mean strain rather than the mean stress is held constant throughout the fatigue test. The practical significance of these findings is discussed, especially as relates to the fading of residual stresses in members subjected to fatigue loading.
In the low-temperature range, the engineering yield strength of polycrystalline bcc metals can change by a factor of 10 or more with serious consequences appearing in the form of catastrophic brittle fracture. Engineering variables known to have an important effect on the yield behavior are state of stress, temperature, loading or strain rate, composition, and microstructure. For iron, chromium, molybdenum, and tungsten, it is shown that yield behavior can be represented by a single-valued relation between two dimensionless parameters.
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