Fatigue behavior of fine-grained magnesium under tension-tension loading at 0 °C
Introduction
There is a strong motive to reduce the weight of transportation tools, so that fuel efficiency can be improved and greenhouse gas emission can be mitigated. As the lightest structural metal, magnesium-based materials were utilized in various automobile parts such as the wheels of racing cars. To broaden the practical applications of magnesium-based materials, it is necessary to enhance the specific strength and ductility of the materials and establish a comprehensive knowledge database on various properties of the materials. Therefore, extensive research efforts were devoted to the material development through different processing and synthesizing techniques [1], [2], [3], [4], [5], [6] and the material characterization under a wide range of service and loading conditions [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. One widely-used and effective processing technique is grain refinement. The produced materials with fine grains generally exhibit much higher strength than the corresponding materials with coarse grains. Lin et al. [20] reported that yield strength and ultimate tensile strength were 100 MPa and 160 MPa respectively for as-cast AZ31 magnesium alloy with the average grain size of 75 µm, while the strength values increased to 217 MPa and 282 MPa respectively for AZ31 magnesium alloy with the average grain size of 0.7 µm after the material experienced 8 passes of equal channel angular pressing. Similarly, yield strength and ultimate tensile strength were reported to be 150 MPa and 330 MPa respectively for as-cast AZ91 magnesium alloy with the average grain size of 40 µm, while the strength values increased to 260 MPa ad 375 MPa respectively for AZ91 magnesium alloy with the average grain size of 1.2 µm after the material experienced 8 passes of equal channel angular pressing [21].
It is well-known that fatigue failure is one critical type of engineering component failure mode since most engineering components experience cyclic loadings during their service life. When automobiles are driven enroute, the loadings on many of their parts are continuously changing. The wheels, as one of the essential parts in automobiles, are also under cyclic loadings when they are in service. Thus, there are a large number of research papers reported cyclic mechanical properties and fatigue behaviors of a variety of magnesium-based materials [11], [12], [14], [17], [19], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35]. The testing temperature of these investigations was primarily focused on room temperature [11], [12], [14], [17], [19], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [33], [35], although many studies were performed under the elevated temperatures higher than room temperature [27], [32], [34]. Only very few research results were obtained from fatigue testing at testing temperatures such as −183 °C, −253 °C, and −268.8 °C that are lower than room temperature [36], [37], [38]. Room temperature represents the service temperature for many engineering applications. However, there are also a lot of situations with the service temperature lower than room temperature. According to the average winter temperature values for the 50 states in USA [39], three states (i.e., Alaska, North Dakota, and Minnesota) have the average temperatures below −10 °C, 20 states have the average temperatures between −10 °C and −1°C, and 7 states have the average temperature between −1°C and 1 °C, during the winter season. To safely employ magnesium-based materials in automobiles that can be exposed to the winter environment in these states, it is important to have the knowledge on fatigue behavior of the materials under the low service temperature. To acquire the needed knowledge, this study investigated fatigue properties of fine-grained magnesium through tension–tension fatigue testing under a range of applied loadings at the testing temperature of 0 °C. In addition, this study conducted fatigue life analysis for fine-grained magnesium through the Smith-Watson-Topper (SWT) model and the Jahed-Varvani energy-based (JV) model.
Section snippets
Materials and experiments
In this study, commercially pure magnesium plates were processed through multi-pass rolling at about 200 °C [40], [41] to obtain magnesium sheets with refined grains and a thickness of about 1 mm. The average grain size in the material is on the order of several microns as reported in the reference [41]. The chemical composition of the studied material is ~ 99.93% magnesium, < 0.005% aluminum, 0.005% cobalt, < 0.005% copper, < 0.001% iron, 0.024% manganese, 0.03% silicon, 0.001% zinc, < 0.001%
Quasistatic tensile mechanical behavior
The true stress σ-true strain ε curve as shown in Fig. 3 was obtained from quasistatic tensile testing of fine-grained magnesium at the testing temperature of 0 °C. The yield strength σy and ultimate tensile strength σUTS are 105 MPa and 193 MPa respectively based on the engineering stresses. The curve shows that there was significant hardening of the material after its yielding and the hardening effect reduced with the increase of strain based on the concave down shape of the curve. The curve
Conclusions
Quasi-static tensile testing and tension–tension fatigue testing were performed on fine-grained magnesium samples at the testing temperature of 0 °C. The quasi-static tensile data indicates that the material possessed considerable ductility and there was significant hardening of the material after yielding. The hardening effect reduced with the increase of strain and the strain hardening exponent was derived to be 0.142 for the studied material.
Based on the quasi-static mechanical behavior, six
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the Basic Energy Sciences Office at the US Department of Energy (No. DESC0016333).
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