OVERCOMING LIMITATIONS OF THE CONVENTIONAL STRAIN-LIFE FATIGUE DAMAGEMODEL

The strain-based approach to fatigue life prediction usually relies on the conventional strain-life equation which correlates the elastic an d plastic strain to the life. The correlation is based on separate log -linear curve fits of the elastic and plastic components of the strain data versus the life. It is well known, however that these linear rel ationships may be valid only within a specific interval of stress or s train. When material behavior approaches elastic-perfectly plastic, fo r instance, it is not uncommon for the test data to deviate from linea rity at both very high and very low strains. For such materials a sepa rate fit of each curve is likely to give material constants significan tly inconsistent with the fit of the cyclic stress-strain curve, espec ially if a good local fit over a restricted interval is obtained. In t his work, some of the errors that arise as a result of this inconsiste ncy are described, and recommended methods are developed for treating these errors. Numerical concerns are also addressed, and sample result s are included.