The field of fatigue stress-life estimation addresses the effects mentioned in the title differently. The load effect differentiates among various loading modes that lead to uneven stress distributions across the critical cross-section. The notch effect accounts for this uneven stress distribution when induced by structural irregularities. Conversely, the size effect – sometimes referred to as the “statistical” size effect – addresses the weak-link mechanism of fatigue damage, where larger components subjected to the maximum stress are more likely to initiate cracks if compared to smaller components with the same maximum stress. This effect may interact with both the load and notch effects.

The FKM Guideline identifies two additional effects to consider: (1) the mechanical deformation effect, which assesses the material’s yielding capability, and (2) fracture mechanics effect, which slows crack propagation in the presence of stress gradients. The latter effect shares similarities with both the load and notch effects.

This paper proposes an attempt to unify the notch and size effects into a critical volume effect, relating to the size of the control volume directly influenced by stress distribution within the component. The evaluation is based on a test case involving six variants of specimens manufactured from a single batch of S355J2 structural steel. These variants include one unnotched hollow configuration and five notched configurations with varying notch acuities. The specimens are subject to load control through three different fatigue loading modes: push-pull, torsion and plane bending.

Based on experimental results, the critical volume solution is developed to address this issue. The paper discusses its current limitations and provides a comparison with typical stress-gradient solutions and with the application of the theory of critical distances.