A combination rule for multiple surface cracks based on fatigue crack growth life

A plate under cyclic loading, containing two coplanar surface flaws with both identical and dissimilar sizes, is considered in the present study. By conducting detailed step-by-step finite element analyses, the conservatism contained in different combination rules for multiple coplanar flaws provided by fitness-for-service codes (ASME, BS7910, API579 and GB/T19624) have been quantitatively assessed for the fatigue failure mode. The findings show that the re-characterization guideline provided by ASME and BS7910 may cause non-conservative estimations when two crack sizes are similar, whereas API579 and GB/T19624 lead to excessively pessimistic predictions for almost all the cases. Based on the fatigue crack growth life, we suggest a new combination rule and conclude that it always yields a reasonable estimation with necessary conservatism, for various initial crack depths, material constants and relative sizes of two cracks.