PSI - Issue 56

Radim Halamaa et al. / Procedia Structural Integrity 56 (2024) 111–119 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

113

3

2.1. Main idea In a standard uniaxial strain-controlled fatigue test, the amplitude of the axial strain and the mean strain in the gauge part are kept constant throughout the fatigue test. However, in the rounded part, the cross-sectional area of the specimen is larger and that is why there are lower stress and strain magnitudes. The history of strain can be measured optically using the digital image correlation (DIC) method. A simplified illustration of this procedure is shown in Figure 1(a). Figure 1(b) shows two hysteresis loops for two selected points on the curved part of the sample. For evaluation, the axial stress is considered as a nominal stress in the given cross section and the axial strain corresponds directly to the Y-component of the Cauchy strain tensor. The upper hysteresis loop is an example of an extreme condition: the signal noise is significant; the stresses are low, and the plastic deformation does not develop significantly. The bottom hysteresis loop shows the result of material behaviour close to the gauge part of the specimen (the strain amplitude measured by an extensometer in the gauge part is 1.5%). By capturing the strain history and evaluating the peaks together with the stress, one can obtain a cyclic stress-strain curve of the material. However, some corrections are necessary to achieve good correlation with the conventional cyclic stress-strain curve, as will be shown within this paper.

Fig. 1. Example of DIC measurement evaluation: (a) representation of several selected points on the curved part of the specimen; (b) each point corresponds to one hysteresis loop.

2.2. Procedure In this work, Alpha software of the X-Sight company was used. However, any DIC software can be applied that enables capturing the history of the strain components. The basic procedure consists of the following steps: 1) Determine the amplitude of the load in the middle of the life of the specimen. 2) Create a network of points in the DIC software at which the components of the strain tensor will be calculated (marked as DIC points in the following text). 3) Determine the positions of the DIC points on the curved part of the specimen in the given coordinate system and find the border between the gauge (cylindrical) part and the curved part (Fig. 2).