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The influence of residual stress on fatigue is studied using electron microscopy on fracture surfaces of specimens tested under spectrum loading. The level of residual stress that exists at the point of stress concentration is shown to be one of the parameters affecting striation spacing. A review of numerous variable amplitude test results generated by government agencies and aerospace companies further suggests that a straightforward technique quantitatively defining the effect of continually changing residual stress patterns should provide a reliable tool for predicting cumulative damage in fatigue. The two major difficulties relating to the utilization of residual stress considerations in a cumulative damage rule are: (a) determining the level of residual stress existing at the notch root; and (b) computing its effect on N, cycles to failure. A cumulative damage rule including the developments in (a) and (b) above is presented. Essentially, the rule involves the use of conventional Σn/N computations, except the selection of a value for N is affected by the levels and variations of residual stress existing at a discontinuity. Loading parameters that affect fatigue-life predictions using the cumulative damage rule include peak load level, load ratio, block size, and load-level sequence. Typical laboratory spectrum-test results that are investigated include the NASA gust spectrum. Specimens tested to this spectrum sustained 9 blocks to failure in a low-high sequence, 26 blocks in a high-low sequence, and 20 blocks in a high-low-high-sequence. Corresponding life predictions are 10.9, 25.2, and 19.6 blocks to failure.
fatigue (materials), mechanical properties, cumulative damage, electron microscope, residual stress, stress concentration, cyclic creep, load-level sequence, reversed loading, tests, evaluation
Senior Structural Research Engineer, McDonnell-Douglas Corporation, St. Louis, Mo.