Issue 38

T. Sawada et alii, Frattura ed Integrità Strutturale, 38 (2016) 92-98; DOI: 10.3221/IGF-ESIS.38.12

α = 1/0

125

125

125

α = 1/0

α = 2/1

α = 1/0

100

α = 2/1

100

100

α = 1/1

α = 2/1

75

25 Torsion strength τ S [MPa] 50 75

25 Torsion strength τ S [MPa] 50 75

25 Torsion strength τ S [MPa] 50

α = 1/1

α = 1/1

α = 1/2

α = 1/2

α = 1/2

α = 0/1

α = 0/1

α = 0/1

0

0

0

0 25 50 75 100

0 25 50 75 100

0 25 50 75 100 125

Axial strength σ L

[MPa]

Axial strength σ L

[MPa]

Axial strength σ L

[MPa]

(a) 0.0 V f (c) 0.5 V f Figure 2 : Multiaxial static strength evaluation with Tsai-Hill failure rule. (b) 0.2 V f

Fatigue strength properties Fatigue strength in uni-axial loading is generally evaluated by using the S-N diagram. S-N diagrams are associated with the relationships among the number of cycles to failure and stress range, stress amplitude, or maximum stress in uni-axial loading. However, we have to deal with stress states of combined axial stress and torsion stress in this paper. Therefore, S N diagrams are organized by using principal stress applied on specimen surfaces σ p1 , and the number of cycles to fracture N f . σ p1 was calculated as follows:

2

2           1 1 12 2 2  



(5)

p1

where σ 1

and τ 12

are the axial stress and shear stress applied.

Figs 3 and 4 show the relationships between σ p1,max as fracture cyclic numbers on a double logarithmic chart obtained as S-N diagrams. Arrows in the diagrams show that the fatigue tests were terminated after reaching 1×10 6 cycles. as cyclic maximum principal stress and N f

0.5Vf, α=1/0 0.5Vf, α=2/1 0.5Vf, α=1/1 0.5Vf, α=0/1 0.2Vf, α=1/0 0.2Vf, α=2/1 0.2Vf, α=1/1 0.2Vf, α=0/1 0.0Vf, α=1/0 0.0Vf, α=2/1 0.0Vf, α=1/1 0.0Vf, α=0/1

Static strength

100

[MPa]

Peak stress σ p1, max

10

0 10 -1

1 10 0

10 10 1

100 10 2

1000 1 3

10000 1 4

100000 1000000 1 5 1 6

Fracture cycles N f

[cycles]

Figure 3 : Relationships between σ p1, max with various α in C-SGP. Since the value of the fatigue life in C-SGP are scattered widely, fatigue life in C-SGP can no longer be expected to sort in V f and α by using the S-N diagram. Meanwhile, S-N diagram in I-SGP indicates the characteristic to decrease σ p1, max with increasing N f without dependence on fatigue test conditions. Comparing the fatigue properties in molding processes, they and N f

95