| Summary: | The concept of an equivalent stress intensity factor <i>K<sub>eq</sub></i> is used in the study of fatigue crack growth in mixed-mode situations. A problem seldom discussed in the research literature are the consequences of the coexistence of several alternative definitions of mixed mode <i>K<sub>eq</sub></i>, leading to rather different results associated with the alternative <i>K<sub>eq</sub></i> definitions. This note highlights the problem, considering several <i>K<sub>eq</sub></i> definitions hitherto not analyzed simultaneously. Values of <i>K<sub>eq</sub></i> calculated according to several criteria were compared through the determination of <i>K<sub>eq</sub></i>/<i>K<sub>I</sub></i> over a wide range of values of <i>K<sub>I</sub></i>/<i>K<sub>II</sub></i> or <i>K<sub>II</sub></i>/<i>K<sub>I</sub></i>. In earlier work on Al alloy AA6082 T6, the fatigue crack path and growth rate were measured in 4-point bend specimens subjected to asymmetrical loading and in compact tension specimens modified with holes. The presentation of the fatigue crack growth data was made using a Paris law based on <i>K<sub>eq</sub></i>. Important differences are found in the Paris laws, corresponding to the alternative definitions of <i>K<sub>eq</sub></i> considered, and the requirements for candidate <i>K<sub>eq</sub></i> definitions are discussed. A perspective for overcoming the shortcomings may consist in developing a data-driven modelling methodology, supported by material characterization and structure monitoring during its life cycle.
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