FATIGUE DAMAGE FACTOR IN STRUCTURAL DESIGN

When the structure is subjected to repeated stresses with cyclic constant magnitudes, the resulting fatigue life is known to be random. In reality, structures are subjected to various levels of repeated stresses. To facilitate the design of a structure subjected to a stress history with given load repetitions at each of several stress levels, a fatigue damage factor and a damage index are introduced as design parameters. The problem is formulated with the use of the Palmgren-Miner's cumulative damage theorem, by treating the denominators in the theorem as random variables rather than constants. The design diagrams suggested can be constructed from single stress-level fatigue test data. The design approach needs only to draw parallel lines in the design diagrams in the case of structural design against multi-stress-level fatigue. To illustrate the relationship of the fatigue damage factor to cyclic stresses, number of cycles and failure probabilities, two numerical examples are also given.