Open Access

Josef Denk

• The present thesis treats the mechanical behavior of wrought magnesium alloys on the example of the twin-roll cast AZ31B alloy and presents a new fatigue lifetime model. The main objectives are the investigation of the material's mechanical properties and behavior and the modeling of the fatigue lifetime of arbitrarily shaped and loaded specimens. In this context, attention is paid to the microstructure. State of the art fatigue lifetime models are inappropriate for wrought magnesium alloys, which is why this work continues on this issue. Fatigue tests on different shaped notched and unnotched specimen for uniaxial loading and for bending loading form the basis for the analysis of the mechanical properties and behavior and for the modeling of the fatigue lifetime. In situ optical strain field measurements provide detailed information about the deformations. Accompanying microstructural investigations, conducted by optical microscopy and scanning electron microscopy, characterize the deformation behavior during mechanical loading. The known nearly zero strain hardening rate and sigmoidal-shaped stress-strain hystereses in case of twin formation are observed for the investigated strongly textured AZ31B alloy. Aside from these specific characteristics, more properties with significant effect on the fatigue behavior are uncovered. First, twins are found to form clearly delimited bands of twinned grains, rather than being distributed homogeneously. Within these bands, most grains reveal at least one twin as soon as these bands are formed. On the contrary, adjacent regions contain no twins. These bands of twinned grains, solely composed of {10-12} tension twins, end abruptly. Furthermore, the compressive strain at the lower load level and the strain amplitude are significantly larger for the twinned regions compared to the twin-free regions and so do all other fatigue parameters. Hence, the macroscopic deformation behavior is strongly inhomogeneous. In addition, it is shown that the specimens always fail inside the twinned regions. Also, the first macroscopic cracks are observed inside the twinned regions that cause the fatigue failure. Available fatigue lifetime models are not suitable because of the mentioned specific characteristics. Therefore, a new fatigue lifetime description method, the concept of highly strained volume, is developed and presented. The main approach is the consideration and evaluation of exclusively the twinned regions (highly strained regions) of a specimen. The fatigue parameters (different strain amplitudes, Smith-Watson-Topper fatigue parameter, different strain energy density parameters), as a function of the number of cycles to failure and the geometrical size of the highly strained portion of a specimen, are appropriate to model a clear relation between the load and the fatigue lifetime. A double power function is used for regression of the fatigue test results of 76 specimens. Best modeling accuracy is achieved with the fatigue parameters strain amplitude, Smith-Watson-Topper fatigue parameter and combination of elastic and plastic strain amplitudes with coefficients of determination of 0.85, 0.85 and 0.86, respectively. The used fatigue tests for model testing cover a large range of numbers of cycles to failure (100 to 360000), sizes of the highly strained volumes (≈ 0.1 mm3 to ≈ 1000 mm3), different load cases (uniaxial and bending), different load ratios and different specimen shapes (i.a. notched and unnotched). Overall, modeling of the fatigue lifetime with the concept of highly strained volume is appropriate, which is why it can be used for estimating the fatigue lifetime of wrought magnesium components. Modeling the high cycle fatigue regime, the concept of highly strained volume is out of its bounds because no twins occur. To consider this, the concept of highly stressed volume is successfully applied.