Abstract
Mechanical fatigue tests of unnotched, notched, and bending twin-roll cast AZ31B magnesium alloy specimens are performed in which strain fields are analyzed with digital image correlation. Clearly, delimited macroscopic bands of twinned grains (BTGs) in which the compressive strain is significantly higher compared to the adjacent regions are observed. Conventional fatigue parameters, e.g., the strain amplitude, exhibited higher values within the BTGs. This findings are confirmed by the fact that for all investigated specimens the initial macroscopic cracks are observed within the BTGs. Consequently, for the presented concept of highly strained volume, fatigue parameters are determined from the highly strained regions with high strain amplitudes. This paper focuses on the application of the effective strain amplitude fatigue parameter decomposed in an elastic and plastic portion, the Smith-Watson-Topper fatigue parameter and energy-based fatigue parameters within the concept of highly strained volume. An extended stress–strain hysteresis model is presented to compute stress–strain hystereses for arbitrary load ratios, required to determine the mentioned fatigue parameters. The application and evaluation of five different fatigue parameters within the concept of highly strained volume demonstrates the accurate description of the fatigue life until failure.
Highlights
Innovative materials for lightweight constructions, e.g., in the automotive industry, will become increasingly important in the future because they can contribute to reduce CO2 emissions [18,22]
This work presents an approach for the fatigue modeling of cyclically loaded structural components made of the wrought AZ31B Mg alloy, as this is an important aspect in the design of lightweight components
The compressive strain in the bands of twinned grains (BTGs) is significantly larger compared to the adjacent regions outside the BTGs
Summary
Innovative materials for lightweight constructions, e.g., in the automotive industry, will become increasingly important in the future because they can contribute to reduce CO2 emissions [18,22]. [10] shows that common fatigue parameters such as strain amplitude, Smith–Watson– Topper or energy-based fatigue parameters, taken from inside BTGs, have higher values, which corresponds to the observation that the initial macroscopic crack is always within BTGs. Further challenges in fatigue modeling of basal textured wrought Mg alloys are anisotropic and asymmetric yield strengths, almost ideal plastic material behavior during twinning and sigmoidal shaped stress–strain hystereses [20,27,31]. The discontinuous strain field (BTGs), important for fatigue modeling [10], has not been taken into account by the mentioned researches To consider all these aspects in one fatigue model, Denk et al [10] introduced the concept of highly strained volume (CHεV), inspired by the highly stressed volume from the early work of Kuguel [17]. The different fatigue parameters are evaluated and compared with each other
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