The simulation and assessment of fatigue damage of metallic materials are strongly dependent on the definition of critical damage initiation, i.e. the formation of a crack after a certain number of cycles under fatigue loading. To get this described in an appropriate way classical fatigue analysis needs to be combined with fracture mechanics to obtain a realistic size of an initiating macro crack in a component in the end. Ultimately, this size depends on the procedure of how fatigue life curves are generated from the fatigue tests performed. In this paper strain-controlled fatigue tests and fractographic results have been combined with a computational assessment to better understand the relationship between a traditional load drop criterion characterizing the size of a crack being initiated. It is stated that the location of an initiating crack versus the location of an extensometer applied to the specimen can have an influence on the crack size being initiated and thus the fatigue life for a load drop criterion being fixed. A relationship is proposed to link a crack size dependent on the fatigue test evaluation scheme with the fatigue life, which can be used in engineering assessments. The relevance of practical application cases is discussed.

金属材料疲劳损伤的模拟和评估在很大程度上取决于临界损伤引发的定义，即疲劳载荷下一定次数循环后裂纹的形成。为了以适当的方式对此进行描述，需要将经典疲劳分析与断裂力学相结合，以最终获得组件中引发的宏观裂纹的实际大小。最终，该尺寸取决于执行疲劳测试时如何生成疲劳寿命曲线的过程。在本文中，将应变控制的疲劳测试和分形结果与计算评估相结合，以更好地理解表征裂纹尺寸的传统载荷下降准则之间的关系。据指出，起始裂纹的位置与施加在试样上的引伸计的位置可能对起始裂纹的大小有影响，因此，对于载荷下降标准的疲劳寿命是固定的。提出了一种关系，将取决于疲劳测试评估方案的裂纹尺寸与疲劳寿命联系起来，可以将其用于工程评估中。讨论了实际应用案例的相关性。