PSI - Issue 18

Matthias Hell et al. / Procedia Structural Integrity 18 (2019) 823–836 Author name / Structural Integrity Procedia 00 (2019) 000–000

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In order to determine the stress-strain and the strain-life relation, 12 up to 15 strain-controlled tests are required. State of the art strain-controlled tests are conducted using a clip-on strain gage, which detects the elongation over a defined macroscopical measurement section. Fig. 2 shows specimen with standard geometry in accordance with the DIN 50100 and a servo-hydraulic test system with a detailed view of the strain-measurement.

Fig. 2: Standard specimen geometry, servo-hydraulic test system and clip-on strain measurement

For the experimental determination of the material parameters, it is generally assumed, that the behaviour of an infinitesimal material volume at the stress concentration is equal to the material behaviour within a homogeneously loaded cross-section of finite dimensions. It has to be carefully evaluated, if this assumption is valid, as many manufacturing processes including casting, forging, rolling or heat treatment may lead to inhomogeneous property distributions within the component. In order to be able to apply a valid stress-strain behaviour to the fatigue life assessment, the material state has to include all relevant influences of manufacturing processes. A possible way to assess the local material behaviour is the extraction of miniaturized specimens out of material samples with inhomogeneously distributed local properties. Thus, the assignment of a stress-strain behaviour to discretized regions within a component is possible. Fig. 3 shows a micro-specimen including the strain-gage for the strain measurement and the electro-mechanically driven test system.

Fig. 3: Micro-specimen and test system for the experimental investigation on local material properties