PSI - Issue 13

Markus Könemann et al. / Procedia Structural Integrity 13 (2018) 914–919 Markus Könemann/ Structural Integrity Procedia 00 (2018) 000–000

916

3

the toughness values is to be expected. For quick execution of the test, the sample can be fixed in the sample holder before starting the test. This makes it possible to temper the sample outside the pendulum impact tester, so that the temperature of the sample deviates only slightly from the pre-set temperature of the chamber during impact. In addition to determining an energy value by looking at the pendulum stroke after the impact, it is also possible to carry out an instrumented test. The test force is determined by a piezoelectric load cell built into the impact tensile block. The displacement is determined by an angle sensor, which is linked to the pendulum. Alternatively, it is also possible to calculate the displacement from the deceleration of the pendulum. By adapting the test setup in this way, tests can be carried out on sheet material with a product thickness of about 1.5 mm. Even thinner sheets can be examined. In instrumented tests, it must be ensured that a maximum force of 10 kN is not exceeded in order to avoid damage to the load cell.

Fig 2. Comparison of stress triaxiality of Charpy-V-notch specimen and tensile impact test standard geometry.

3. Standard geometry and specimen catalogue To compare different sheet materials and steel classes, a standard geometry was developed which transfers the ideas of the Charpy impact test to tensile loading. In Charpy impact test, the specimen is examined in a stress state, which is particularly unfavorable for the material. This refers to a particularly large multi-axis stress, which favors or accelerates the failure of many steel materials (Bai & Wierzbicki, 2008). For this purpose, a notch is inserted into a specimen with a cross-section of 10x10 mm. The notch has a depth of 2 mm and an opening angle of 45°. The radius at the notches ground is 0.25 mm. As in the Charpy impact test, a notched specimen is examined for the tensile impact test with a challenging stress state for the material. The specimen is notched on both sides, whereby an opening angle of 45° is selected for the notch and a radius of 0.25 mm is provided in the notch base. The resulting stress state is less critical compared to the Charpy impact test, since the stress triaxiality cannot reach the values of the Charpy specimen. However, since more uncritical stress conditions are consistently achieved in designs made of sheet materials, the existing stress triaxiality is sufficient for considering sheet materials. Fig. 2 shows a comparison of the stress triaxiality of Charpy specimen and tensile impact test specimen. The viewing direction is always perpendicular to the notch, i.e. in the direction of crack propagation. It can be seen that the values achieved in the tensile impact test are about 45% below the values of the Charpy specimen. The reason for this is the reduced thickness of the sheet material, which prevents a further increase in triaxiality. In addition to the standard geometry, which tests a particularly unfavorable stress state in the material, a sample catalog is also provided in this article. This catalog makes it possible to check different stress states corresponding to those of the components to be used. In this way, it is possible to reduce over-dimensioning due to an unjustifiably critical view of the material, since the material is only checked in the areas of interest. In order to cover a wide range of stress states, a method was developed which allows the targeted selection of individual stress states. A distinction is made between two different Lode angle parameters (0 and 1) and the stress triaxiality is continuously adjusted by adjusting the specimen geometry.