LINEAR STRAIN THEORY AND THE SMITH METHOD FOR PREDICTING FATIGUE LIFE OF STRUCTURES FOR SPECTRUM TYPE LOADING

Abstract
Theoretical and test data are presented for two methods of predicting fatigue life. The first method requires preknowledge of stress concentration and nominal stress; the second requires preknowledge of neither. Both employ smooth specimen S-N curves and consider effects of stresses at a concentration. The first method, the linear strain theory, assumes that strain at the concentration is proportional to load within practical limits. Stress at the concentration, including residual stress, can therefore be obtained from conventional stress-strain curves, and these stresses are then used in conjunction with S-N curves to evaluate fatigue life. The second method, the Smith method, requires the life of a given structure in terms of a loading that is sufficiently high to ensure local yielding, which corresponds to a lifetime of less than 10,000 cycles for 7075-T6 and 40,000 cycles for 2024-T3. The unique feature of the latter method is that it does not require knowledge of either nominal stresses or stress at the concentration, stress at the concentration being obtained directly from smooth specimen test data for the material from which the structure is fabricated.

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