PSI - Issue 19

Kiyotaka Masaki / Procedia Structural Integrity 19 (2019) 168–174 Kiyotaka MASAKI/ Structural Integrity Procedia 00 (2019) 000–000

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Table 2. Mechanical properties of materials. Material 0.2 % proof stress

Tensile strength  BT [MPa]

Elongation  [%]

Young’s modulus E [GPa]

Hardness [HV]

Tensile  0.2T [MPa] 

Compressive  0.2C [MPa] 

AZ31 AZ61 AZ80

196 221 223

90

231 309 341

19 13 16

43 44 46

54 57 63

134 168

45  m, 17  m and 14  m, respectively. Mechanical properties of all the materials are shown in Table 2. AZ80 presented the highest strength among the three materials. The compressive 0.2 % proof stress of each material was about half of the respective tensile 0.2 % proof stress. 2.2. Fatigue strength measurements Two types of hourglass specimens which minimum diameter were 6 mm and 5 mm were used in this study. Fatigue specimens are shown in Fig. 2. A stress concentration factor of both specimens was 1.074. The 6 mm specimens were mainly used to investigate fatigue strength and the 5 mm specimens were used to investigate scatter of fatigue life. The surfaces of the central part of all specimens were finished by buffing with an alumina solution after #600 to #2000 wet emery papers polishing with machine oil. High cycle fatigue tests were carried out under cantilever type rotary bending loading in air at room temperature and with a frequency of 4000 rpm.

0.8

0.8

a

b

3.2

3.2

R 1 2

R 1 0

φ 1 2

φ 1 2

φ 5

φ 6

3 0

9 0

( 3 7 . 5 )

1 2 5

1 2 0

1 7 0

Fig. 2. Shape of fatigue test specimens (a) 6mm specimen; (b) 5mm specimen.

3. Fatigue results

Fig. 3(a) shows the S-N diagram of AZ31 materials. The S-N diagram is a very gentle curve. Fatigue strength for 10 8 cycles of AZ31 was about 90 MPa, which is the same value as the compressive 0.2 % proof stress of the material. Thus, even if the fatigue stress amplitude is lower than the tensile 0.2 % proof stress, plastic strain is caused by the compressive stress. Fig. 3(b) shows the S-N diagram of AZ61 materials. The S-N data of AZ61 was plotted at a higher stress amplitude levels than the AZ31 data. The reason is that the tensile strength of AZ61 was higher than that of AZ31.The S-N data was approximated to a two-step diagram with a first step stress level at about 190 MPa. Although the fatigue strength at 10 8 cycles of AZ61 was about 150 MPa, fatigue fracture occurred at 8x10 8 cycles under stress amplitude of 130 MPa, the same value as the compressive 0.2 % proof stress of the material. Thus, the fatigue properties of this material are also affected by compressive plastic strain. Fig. 3(c) shows the S-N diagram of AZ80 materials. The S-N curve of AZ80 is simple compared to other materials. Fatigue strength at 10 8 cycles of AZ80 was about 140 MPa. Although the tensile and compressive strengths of AZ80 were higher than the strength of AZ61, fatigue properties of AZ80 were inferior to the properties of AZ61. Only the fatigue strength at 10 8 cycles of AZ80 did not correspond to the compressive 0.2 % strength of material among the three types of materials. It was expected that the fatigue strength at 10 8 cycles of AZ80 is not affected by the compressive plastic strain.