Test is repeated for a number of similar specimens, subjecting them to different values of stress and number of stress reversals that the specimen survives before fracture are counted. Specimen subjected to constant bending moment is rotated at a very high speed due to which fibers of the specimen (except those on neutral axis) undergo repeated stress reversals (maximum tensile stress to maximum compressive stress). ![]() Fluctuating Stress 6.3 Fatigue Curve (S-N Curve) & Endurance Limitįatigue properties of the materials are obtained with the help of standard rotating beam test, in which a highly polished circular section specimen, shown in figure 6.2, is subjected to cyclic loads. Stress changes its nature and magnitude of extreme values of tensile and compressive stress is not sameĪll variable stresses can be considered to be made of two components – static component called mean stress (s m) and a variable component called stress amplitude (s a).įigure 7.1 c. Minimum value and maximum value of stress is of same nature (tensile or compressive) Stress varies from zero to certain maximum value (nature of stress does not change) Following are some important types of cyclic stresses depending upon the level of minimum and maximum stress between which the stress fluctuates:Įxtreme values of stress are of equal magnitude and opposite nature with mean equal to zero Therefore, sine wave can be conveniently used to represent any kind of variation of stress between the minimum and maximum values of stress. Also for design purpose, only maximum and minimum value of stress is important and not the wave form. This crack propagates due to fluctuating stresses, grows continually and finally sudden fracture takes place.Ī number of different regular and irregular patterns are followed by cyclic stresses in machinery but generally it follows sinusoidal pattern because of the nature of some rotating machinery. Fatigue failure begins with a microscopic crack that occurs due to some discontinuity (oil holes, keyways, screw threads etc.), surface irregularities due to machining (scratches, stamp marks, inspection marks etc.) or material defects. The fracture surfaces are flat and perpendicular to the stress axis. Its sudden occurrence, without any noticeable warning, makes it dangerous. Such failure is known as fatigue failure as it occurs after a large number of stress cycles.Ī fatigue failure resembles to a brittle fracture and occurs without any noticeable plastic deformation or necking. But it has been observed that machine members subjected to repeated or fluctuating stresses often fail at a maximum value of induced stress well below the yield or ultimate tensile strength of the material. ![]() It was discussed that the failure of members, subjected to static load, occurs when the induced stress reaches yield strength or ultimate tensile strength. For example, a particular fiber on the surface of a rotating shaft, subjected to bending load, undergoes both tension and compression for each revolution of the shaft. But there are a number of machine members which are subjected to cyclic loads resulting in variable stresses that fluctuate between different levels. In case of static loading, load is gradually applied and remains stable after reaching the maximum value giving a steady value of stress. Cyclic load and impact load are types of dynamic loads. ![]() Load which changes in magnitude or direction or both with respect to time is known as dynamic load.
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