PSI - Issue 13

S.M.J. Razavi et al. / Procedia Structural Integrity 13 (2018) 74–78 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

77

4

a

b

1000

1000

Circular notch, R=0.01, f=10 Hz Scatter index T σ = 1.332 D σ 50% = 213 MPa (N = 1 ∙ 10 6 )

Smooth, R=0.01, f=10 Hz Scatter index T σ = 1.328 D σ 50% = 243 MPa (N = 1 ∙ 10 6 )

Max. stress, σ max [MPa] (log)

Max. stress, σ max [MPa] (log)

k=4.74

k=4.88

100

100

1.0E+04

1.0E+05

1.0E+06

1.0E+07

1.0E+04

1.0E+05

1.0E+06

1.0E+07

Number of cycle, N (log)

Number of cycle, N (log)

c

1000

Blunt V-notch, R=0.01, f=10 Hz Scatter index T σ = 1.158 D σ 50% = 144 MPa (N = 1 ∙ 10 6 )

Max. stress, σ max [MPa] (log)

k=4.15

100

1.0E+04

1.0E+05

1.0E+06

1.0E+07

Number of cycle, N (log)

Fig. 2. Fatigue life curved for the tested specimens; (a) smooth samples, (b) circular notched samples, and (c) blunt V notched samples.

2D numerical simulations under plain stress condition were carried out to calculate the stress concentration factor, K t . Young’s modulus and Poisson’s coefficient were set equal to 110 GPa and 0.34, respectively. By using the obtained experimental data, the notch sensitivity, q can be obtained using Eq. 1.

K

1



(1)

f

q



K

1



t

where K f is the fatigue notch factor and K t is the stress concentration factor. The stress concentration factor defined by maximum local stress to nominal stress ratio and the fatigue notch factor defined by the ratio between fatigue strength of smooth and notched samples were calculated and presented in Table 1. Notch sensitivities of 0.458 and 0.538 were obtained for semi-circular and V notched samples. According to the fatigue results, semi-circular samples with lower notch sensitivity had higher scatter index and V notched samples with higher notch sensitivity had lower scatter index. This can be due to this fact that the samples with lower stress concentration factor are more sensitive to the surface condition meaning that the presence of any defect or high roughness on the surface results in a different fatigue life that expected. This increases the scatter index for semi-circular samples. Generally, two categories of global discontinuities (notch geometries) and local discontinuities (such as defects and surface roughness) are available in materials (including AM materials). For the notched specimens with high stress concentration factor, it is assumed that the global discontinuities govern the failure. While for the notch specimens with low stress concentration factor, the local discontinuities can govern the fatigue failure. The stress concentration factor, the surface condition and the number of defects are the key parameters in defining the governing failure mechanisms. Further experiments are required to illustrate a clear pattern among these parameters.