PSI - Issue 25

A. Gryguć et al. / Procedia Structural Integrity 25 (2020) 486– 495 Andrew Grygu ć / Structural Integrity Procedia 00 (2019) 000–000

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3. Results & Discussion The strain-controlled fatigue behaviour of various forms of wrought AZ80 Mg have been previously explored by several researchers. It is well documented that wrought forms of Mg respond to fatigue loading with pronounced tension-compression asymmetry due to strong activation of the twinning-detwinning cyclic deformation mechanism [31][30]. Previous work by Gryguć et al. examined the cyclic properties of extruded-forged [28] and cast-forged [26] AZ80 Mg in a variety of different forging configurations and temperatures. Wang et al. investigated the low cycle fatigue (LCF) behaviour of AZ80 extruded at 380 ° C and an extrusion ratio of 35 [32]. Xiong et al. explored the effect of material orientation on the cyclic deformation characteristics of AZ80 rolled plate that was post-homogenized at 340 ° C for 2.5 hours [33]. Figure 1 illustrates the strain-life ( ε -N) curves for the aforementioned wrought varieties of AZ80 Mg that have been previously investigated. It can be seen that in the intermediate range of ε A = 0.4 – 0.6% all of the various forms of material have similar number of cycles to failure. Also shown in the legend of Figure 1 is the monotonic yield strength for each material, interesting to note is the cast-forged material has the lowest yield strength, which at strain amplitudes greater than 0.6% (i.e. LCF), results in a longer life for a given strain amplitude. This observation is an artifact of the fact that in a strain-controlled fatigue experiment, lower strength materials will have lower cyclic stresses resulting in lower damage per cycle. In contrast to this, at strain amplitudes less than 0.4% (mid to high cycle regime) where the response is mainly elastic, the cast-forged material has the lowest life for a given strain amplitude, which agrees well with the fact that it has the lowest yield strength of the presented materials.

1,2%

1,0%

0,8%

0,6%

0,4%

Extruded-Forged T=250°C, σys = 286 Mpa Cast-Forged, T=350-450°C, σys = 177 Mpa Extruded T=380°C, R=35.0, σys = 236 Mpa Rolled (ND) T=340°C, 2.5 hrs, σys = 203 Mpa [32] [28] [26]

Strain Amplitude [%]

0,2%

[33]

0,0%

10

1.000

100.000

Number of Cycles to Failure [N f ]

Figure 1 – ε -N curve for fully reversed strain-controlled fatigue tests for a variety of different wrought forms of AZ80 Mg Alloy. The relevant process parameters and resulting monotonic yield strength are shown for reference for each form of wrought AZ80.

3.1. Cyclic Response The well-known “kink” in the strain life curve of Mg alloys is well documented, its exact threshold is related to the material’s yield strength and can vary between 0.45-0.5% for various Mg alloys in the AZ family [34], [35]. At strain amplitudes above this kink, twinning-detwinning dominate the cyclic response (and plastic deformation) whereas below the kink, dislocation slip is most active [34][28]. Figure 2 illustrates the cyclic response in both the axial and shear directions for multiaxial loading. Important to note is the fact that the axial contribution to the loading is Δε/2 = 0.4% which is below the kink value and thus the role of twinning-detwinning are not significant. This