Some structures are subjected to repeated loads that vary in magnitude and direction. If the resulting stresses are sufficiently large and are repeated frequently, the members may fail because of fatigue at a stress smaller than the yield point of the material (Art. 3.8). Test results on smooth, polished specimens of structural steel indicate that, with complete reversal, there is no strength reduction if the number of the repetitions of load is less than about 10,000 cycles. The strength, however, begins to decrease at 10,000 cycles and continues to decrease up to about 10 million cycles. Beyond this, strength remains constant. The stress at, this stage is called the endurance, or fatigue, limit. For steel subjected to bending with complete stress reversal, the endurance limit is on the order of 50% of the tensile strength. The endurance limit for direct stress is somewhat lower than for bending stress.
The fatigue strength of actual structural members is typically much lower than that of test specimens because of the influences of surface roughness, connection details, and attachments (see Arts. 1.13 and 6.22).
