PSI - Issue 59

M. Karuskevich et al. / Procedia Structural Integrity 59 (2024) 642–649 M. Karuskevich et al. / Structural Integrity Procedia 00 (2019) 000 – 000

643

2

√ 2 [( ) ( ) ( ) ] ( ) where: eq – equivalent Von Mises stress; x , y , z – normal (axial) stresses; xy , yz , zx , – shear stresses. The Von Mises criterion works well for isotropic metals. However, it is widely used in practice for anisotropic materials, inherently introducing some incorrectness in the calculation result. Constructional materials are often anisotropic. For example, for metals due to rolling, the tensile yield stress in the direction of rolling is typically 15% greater than that in the transverse direction (Socie and Marquis, 2000). It was shown by Maslak and Karuskevich (2023) by the series of fatigue experiments that the Von Mises calculation’s accuracy in fatigue damage assessment tasks can be increased by introducing a crystallographic factor. In that research, combined tension-torsion loading of the aluminum alloy specimen was considered. The stress components due to the tension and torsion were resolved to the crystallographic slip planes and slip directions. The obtained resolved stress values were introduced into the formula for Von Mises stress. Analysis of the fatigue tests’ results has shown that fatigue lives of teste d specimens do not correlate well with Von Mises stress calculated conventionally but correspond to the equivalent stresses found considering the dominant rolling texture. Most of the structural elements work at the irregular repetitive loading. It means the method for the fatigue damage summation is required. A commonly accepted method based on a linear model of damage accumulation was proposed by Palmgren and Miner (1945) many decades ago and is still in use despite the well-known drawbacks, namely: a) Miner’s rule does not consider the order in which cycles of different amplitudes occur; b) action of stresses below fatigue limit are not taken into account. This reduces the accuracy of the prediction. Despite numerous corrections to Miner’s rule (Fatemi and Yang, 1998; Hectors and De Waele, 2021; Corten & Dolan, 1956; Chen, 1996; Cheng, 1998; Blacha, 2021), the problem of prediction accuracy is still actual. In the case of multiaxial fatigue, the problem of life prediction is much more complex, for example, as it is for in-phase and out-of-phase loading, which causes a different damage effect. The complexity and lack of accuracy of the analytical methods for fatigue life prediction led to different kinds of instrumental methods. At the top of these methods is the concept of Structural Health Monitoring presented by Boller (2001), Chen & Ni (2018). The key element of the Structural Health Monitoring system is a fatigue sensor (fatigue indicator, fatigue gauge). One type of fatigue indicator is based on the possibility of monitoring surface deformation relief. Deformation relief is a three-dimensional system of extrusions and intrusions, now proposed to be called persistent slip markings (PSMs) by Man et al. (2009). There is aluminum, copper, or iron, among metals that exhibit the marks of the deformation relief visible by the simple light microscope. Detailed description and analysis of the surface relief can be found in the review devoted to the state of the art. Components of the surface relief are persistent slip bands, extrusions, and intrusions (Fig. 1).

Fig. 1. Components of the surface relief by Man et al. (2009).