PSI - Issue 19

Yukio Miyashita et al. / Procedia Structural Integrity 19 (2019) 604–609 Author name / Structural Integrity Procedia 00 (2019) 000–000

605

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1. Introduction In order to reduce a weight of transportation structure, non-combustible magnesium alloy adding Ca has been developed (S. Akiyama, et al. (2000) , H.Ohara (2016)). Fatigue reliability of weld part is important in an actual structure, however, fatigue strength of weld part possibly changed with change in conditions of welding process. One way to overcome this problem could be to establish rational fatigue design method based on fatigue mechanism of weld part. In this study, fatigue strength tests and fatigue crack propagation tests were carried out with TIG weld specimens in non-combustible magnesium alloy welded at different institutes in order to investigate influencing factors on fatigue strength characteristic of the welds. Moreover, relationship between fatigue crack growth behavior and welding process was studied by carrying out fatigue crack growth tests with TIG weld specimens welded with different welding conditions in order to propose fatigue design method at welding part of non-combustible magnesium alloy.

Nomenclature 2 a

Total crack length Width of a specimen

W

Stress range

Δσ ΔΚ ΔΚ eff ΔΚ th

Stress intensity factor range

Effective stress intensity factor range

Threshold stress intensity factor range ΔΚ eff, th Effective threshold stress intensity factor range

2. Material used and experimental procedure

Material used in this study was Mg-6%Al-1%Zn-2%Ca alloy. Chemical composition, mechanical property and microstructure observation of the material used are shown in Table 1, Table 2 and Fig.1. Average grain size was 6 μ m. An extruded plate with thickness of 3 mm was used. A base material specimen with the loading direction parallel to the extruded direction is called LD and perpendicular to the extruded direction is called TD. Weld specimens were prepared and applied for fatigue test. Butt weld joint with gap length of 0 mm was produced by TIG welding. Welding direction was parallel to extruded direction of the base material. Weld rod with diameter of 3 mm in the same alloy with the base materials was used. Other welding conditions were indicated in the followings. In the present study, weld bead was removed in specimens used for fatigue strength test and fatigue crack growth test. Specimens used for fatigue strength test is shown in Fig.2(a). Fatigue strength test was carried out under load control. Cyclic loading with sinusoidal wave form and stress ratio of 0.1 were applied at frequency of 20 Hz in fatigue strength test. The test was terminated when a specimen was survived until 10 7 cycles. Testing environment was laboratory air.

Table 1 Chemical compositions for material used.

Al 6.3

Zn

Ca

Mn 0.34

Si

Fe

Ni

Cu

Mg bal.

0.58

2.06

0.019

0.003 <0.002 <0.002

Table 2 Mechanical properties for materials used.

Tensile strength, MPa

o.2% proof stress, MPa

Young’s modulus, GPa

Elongation, %

LD TD

302 295

230 166

41 42

7 9

Fig.1 Microstructure of the base material.