ABSTRACT This study investigates the influence of closely spaced surface‐flaw interaction on failure assessment diagrams (FADs) used in fitness‐for‐service evaluations, addressing the conservatism introduced when standards such as BS 7910 and API 579 recharacterize interacting flaws as a single bounding crack. Finite element analyses were conducted on tension‐loaded ASTM A516 Gr.70 plates containing two semielliptical surface flaws, with systematic variation of aspect ratio, depth ratio, and coplanar spacing. Elastic and elastic–plastic J‐integrals were employed to construct advanced FADs and quantify interaction effects. Results indicate that decreasing coplanar spacing significantly reduces the FAD safe region due to increased plastic contributions, with depth ratio identified as the dominant parameter controlling crack‐driving force amplification. Although the bounding flaw methodology ensures safety, it can be excessively conservative, particularly for deep, closely spaced flaws, leading to allowable stress reductions exceeding 80%. A refined approach based on the deepest flaw is proposed, maintaining conservatism while substantially improving prediction accuracy.
Coêlho et al. (2026) studied this question.