Method for Quantitatively Determining Stress Intensity Factors for Complex Metal Structures by Analogy

For the strength verification of metal structures, in addition to fundamental static strength analysis, fatigue analysis is also indispensable for structures subjected to cycle fatigue loads. Notably, accurate calculation of the stress intensity factor (SIF) is a prerequisite for the quantitative evaluation of crack propagation life. For real structures such as civil aircraft or steel bridge, the complexity of geometry, component connections, and mutual constraints makes the determination of SIFs along the crack propagation path both complex and time-consuming. If the cracked structure is simplified into a two-dimensional model for analysis, the beneficial effects of structural constraints on crack opening are often neglected, leading to overly conservative life predictions, which has been confirmed in the full-scale fatigue tests. Alternatively, while finite element analysis (FEA) can be used to estimate SIFs, inaccuracies or non-convergence may arise due to improper meshing around the crack tip, particularly when singular elements are not properly incorporated. To solve these problems, an analogy method for efficient and accurate evaluation of SIFs in complex metallic structures (particularly those used in civil aircraft) is proposed in this study. The method estimates the SIF in a real structure by comparing the crack opening displacement (COD) at the crack tip with that of a reference model (an infinite plate containing a central crack) using the same local mesh refinement. Extensive validation has demonstrated that the SIFs obtained using the proposed analogy method exhibit sufficient accuracy for engineering applications, offering a practical alternative to traditional analytical or finite element-based techniques in crack propagation analysis.