PSI - Issue 39

Aljaž Litrop et al. / Procedia Structural Integrity 39 (2022) 41–46 Author name / Structural Integrity Procedia 00 ( 2019) 000–000

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1. Introduction Every material should be thoroughly tested before use in industrial applications. In the sheet metal forming industry, the material is first tensile tested to determine its mechanical properties. [1, 2] For more advanced applications, such as a deep-drawing process, the forming limit curve (FLC) of the material needs to be determined, which gives us much more knowledge about the material behavior under different loading directions up to fracture. [3] For fatigue properties, low-cycle fatigue (LCF) and high-cycle fatigue (HCF) tests are performed. However, even if the above-mentioned tests are performed, there is still not enough data about the material behavior under shear loading, [4, 5] especially when the shear load is dynamic (alternating or pulsating). For this purpose, a special shape of the specimen was designed, which has a predictable location of crack initiation and a homogeneous field of shear stress and shear strain. To achieve homogenous shear stress in the specimen, a special fixture system was designed to perform shear fatigue tests at elevated temperatures. Due to the small shear area, the optical non-destructive method Digital Image Correlation (DIC) was used, which enabled observing the strain and displacement components on the surface of the shear specimen. [6, 7] The main objective was to observe the crack’s initiation and further growth, using the DIC method to calculate the field of strains and displacements. [8, 9] This method enabled observing shear strain discontinuities near the tips of cracks growing every cycle. 2. Material and methods Our tests were carried out using a fixture system specifically designed for dynamic shear tests. The standard solutions are intended for static tests with V-notched specimens. [10] According to the requirements, the fixture system was designed to be used at elevated temperatures and for dynamic loads (pulsating and alternating). The fixture system is suitable for the MTS test rig with an additional fixture for the extensometer and space for using a camera and suitable lighting on the specimen surface in the fixture system. The background for the development of the fixture system was the ASTM D7078 standard. [11] According to the fixtures, the specimens were optimized for dynamic shear loading. The standard solution is to use a simple V-notch with a high-stress peak at the end of the notch. For this reason, additional fillets with different radii were added. The specimen geometry was optimized using numerical simulations (FEM), where different notch geometry types were analyzed. [12, 13, 14] The final notch geometry is shown in Figure 1. b. In the present study, DIC was used to contrast the crack areas on the surface of the shear specimen. The gradient in surface strain was measured with DIC highlights in the regions of strain localization where a crack is likely to occur. This arrangement allows us to track crack propagation. Since the objective was not to measure strain around the crack but only the trend of crack growth, a less labor-intensive DIC routine was used. [15]

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Fig. 1. a) Fixture system with DIC setup and b) captured image for further DIC analysis