PSI - Issue 72

Levente Tatár et al. / Procedia Structural Integrity 72 (2025) 345–353

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However, both the stresses and the strains have some kind of distribution in the necking zone. The stress determined in this way is average axial stress, the strain is radial strain at the outer surface of the specimen. What we really need are equivalent strain – equivalent stress values. To obtain such values, some kind of correction is required. There are different correction methods, from which the most well-known is the Bridgman (1952) correction defined for cylindrical specimens. For specimens having other cross-sections than cylindrical, correction methods are sparse. A good review can be found in Choung Cho (2008). 2. “Traditional” measurements augmented with optical system Measurements have been done at the University of Dunaújváros on a Gleeble 3800 Thermal-Mechanical Physical Simulation System, at room temperature. Hybrid measurement was performed, where the specimens had been equipped with longitudinal type strain gauge, and concomitant an optical observation of the specimen had been done by industrial cameras and in-house developed program. To facilitate the optical measurements, specimens with rectangular cross sections have been used. We used two types of specimens: Prior to the tensile tests the coordinates of the flat specimens had been measured with a coordinate measuring machine, then a rectangular grid has been applied on the surface of the specimens with grid density of 0.5x0.5 [mmxmm]. During the test, pictures of the specimen has been taken at regular intervals in a synchronized way with the testing machine. Two cameras had been used for the specimens with 8x8 [mmxmm] nominal cross-section and only a single camera for specimens with 8 x 2 [mm x mm] cross-section. In continuation we present only a single flat specimen with nominal cross-section of 8x2 mm. The specimen is depicted in Fig. 1. 1. square cross section with nominal dimensions 8x8 [mm x mm] 2. rectangular cross section with nominal dimensions 2x8 [mm x mm]

Fig. 1. Flat type specimen

The specimens had cylindrical heads to comply with the gripping system of the Gleeble simulator. The main dimensions of the specimen measured by caliper were:  Thickness: 2.04 [mm]  Width: 7.92 [mm]  Gauge length: 25.04 [mm] Coordinates on the surface of the specimen were measured on the four plane surfaces of the middle part of the specimen, along scan lines parallel to the specimen axis. The nominal distance between measurement points along these lines is 1 mm. For the wider surfaces 3-3 lines of measurements were taken at the centerline of the specimen and at a distance of ±3 mm from it. For the narrower surfaces 1-1 lines of measurements were taken at the middle of the surface, also with 1 mm distance along the line of measurement. The biggest variation of the coordinates was in the thickness direction of the specimens. The variation of thickness from corresponding scan line pairs is approximately 0.06 [mm]. Variation of the thickness does not exactly match measurement by caliper. The difference is approx. 0.05 [mm]. For simulations we considered data from coordinate measurements. The measurement is described in detail in Fekete et al (2024).