The purpose of this research is to compare both British standard BS 7910 (2013) and American standard API 579/ASME FFS-1 (2016) stress intensity factor (SIF) solutions by considering a series of semielliptical surface cracks located in the external surface of a pressurized hollow cylinder in the axial direction. Finite element analysis was used as a comparison basis for both standards’ SIF results. The solution from the British standard provided consistent results compared to Finite Element (FE) results for crack depth not much higher than half the thickness in the deepest and surface-breaking points. Above those limits, the British standard’s solutions diverged quite a lot from the American standard, whose results followed FE values for every crack depth/thickness ratio tested with a maximum percentage difference of 1.83%.

Fracture toughness determination is crucial for the design phase of pressure vessels, and, although ASTM E1820 and ISO 12135 fracture toughness standards have existed for some time, some differences have been reported in the determination of this property. This study investigates the ductile fracture behavior of ASTM A516 Gr.70 pressure vessel steel and assesses the differences in estimating both standards. The steel’s tensile properties and initiation fracture toughness (J_IC) were evaluated, taking into account the parallel and perpendicular orientations to the rolling direction. The results reveal the properties’ dependence on the rolling direction, mainly attributed to perlite banding. Additionally, as for the J_IC determination, the differences were associated with the different blunting line slope estimations on each standard, reinforcing the necessity of a work-hardening-based blunting line for each material assessed.

In real industrial conditions, it’s common to witness the interaction of multiple cracks such that their stress fields and crack driving forces are disturbed. For coplanar cracks, a greater hazard is expected because of the amplification phenomenon. FFS standards deal with this by proposing interaction rules and idealizing a single bonding flaw for the FAD methodology to be used. The goal of this paper is to analyze the effectiveness of this standards methodology considering twin surface semielliptical cracks on a plate under mode I loading on the interaction range using FE analyses. Results confirm that the amplification phenomena due to the interaction are higher on the interacting crack tip and progressively higher as the coplanar horizontal distance decreases. The loss of constraint was observed to decrease as the coplanar horizontal distance decreased, but little change was observed regarding its parametric angular position. A higher amplification was found at the coplanar horizontal distance on which crack interaction is to be considered meaningful, which indicates inconsistency regarding the interaction criteria used on FFS standards. To conclude, the engineering critical assessment of the bonding flaw proved to be over-conservative as the remaining operational life was observed on the assessment of the interacting flaws.

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.

The operational safety assessment of structures and mechanical components containing crack-like flaws remains among the most challenging aspects of engineering practice. Fitness-for-service standards, such as BS 7910 and API 579/ASME FFS-1, incorporate state-of-the-art methodologies for the structural integrity assessment of flawed structures, with the Failure Assessment Diagram (FAD) approach being the most widely adopted for the evaluation of individual cracks. Nevertheless, in many cases, multiple cracks are observed in close proximity, wherein the stress fields associated with each crack interact, potentially resulting in an amplification of the driving forces for crack propagation. The methodologies outlined in fitness-for-service standards recommend recharacterizing such defects as a single flaw – referred to as a bounding flaw – and subsequently conducting the assessment of this idealized defect using the FAD methodology. Recent findings, however, have indicated that this approach may be excessively conservative, thereby leading to the premature retirement of structures or components. Accordingly, the present study aims to compare the conventional bounding flaw approach with a more refined analysis of surface cracks under interaction regimes. The influence of geometric parameters, such as aspect ratio, depth ratio, and coplanar distance, was evaluated both in the construction of FADs and in the assessment of the effectiveness of the bounding flaw methodology. To this end, a series of finite element simulations was conducted. The analysis of the failure assessment diagrams revealed a reduction in the area beneath the curve – indicative of safe operating conditions – as the flaws approached one another, which was correlated with an increase in the plastic component of the J-integral induced by their interaction. However, as the depth ratio of one of the flaws increased, the influence of the coplanar distance was observed to diminish, as the relative amplification decreased in comparison to the other flaw. Ultimately, the bounding flaw methodology was deemed effective in ensuring operational safety, as it consistently exhibited greater severity than the highest amplification levels observed among the interacting flaw pairs. Nevertheless, conservative – and, in certain instances, overly conservative – scenarios were identified. Furthermore, it was observed that this excessive conservatism could be mitigated by considering the flaw with the greater depth ratio for the estimation of the reference stress and, consequently, for the determination of the load ratio coordinate.