Issue 48

S. Kikuchi et alii, Frattura ed Integrità Strutturale, 48 (2019) 545-553; DOI: 10.3221/IGF-ESIS.48.52

tested at  a = 160 MPa and observed after a given number of loading cycles. Fig. 5 reveals that a 17-µm-long fatigue crack was initiated from a pore after 8.5×10 4 cycles (Fig. 5(b)). Subsequently, multiple cracks were initiated from pores and gradually propagated (Figs. 5(c)-(g)). Fatigue cracks, which were initiated from pores, coalesced at several instants and the final fracture occurred at 2.08×10 5 cycles. Fig. 6(a) plots the crack growth rate, d a /d N , of long cracks for the untreated and MM series against  K . In both series, d a /d N decreases with decreasing  K and the d a /d N values for the MM series are consistently higher than those for the untreated specimens at comparable  K levels. Furthermore, the  K th value for the untreated series is higher than that for the MM series. The long fatigue crack growth resistance of SUS304L is reduced owing to the presence of the harmonic structure.

Figure 5 : Optical fractographs of untreated series that failed at a stress amplitude of 160 MPa ( N f

= 2.08×10 5 cycles).

10 -7

10 -8

(a)

Sintered at 1223 K R = 0.1 10 Hz (b)

Untreated (  a

= 160 MPa)

 K

 K eff

Untreated MM

10 -8

10 -9 Crack growth rate d a /d N , m/cycle

10 -9 Crack growth rate d a /d N , m/cycle

 K eff for long crack (Fig.6(a))

Sintered at 1223 K R = 0.1 10 Hz

10 -10

10 -10

2 3 4 5 6 7 8 910 Stress intensity range  K , MPam 1/2

1

5

10

Stress intensity range  K , MPam 1/2

Figure 6 : Relationship between (a) crack growth rate and stress intensity range for the untreated series and MM series, and (b) crack growth rate and stress intensity range for the untreated series with a long crack and a small crack.

Furthermore, the d a /d N against  K eff

for the untreated and MM series are also plotted in Fig. 6(a), which reveals that the

 K eff,th value for the untreated series is higher than that for the MM series. This indicates that the effect of the harmonic

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