Crack Paths 2009

higher than 0.6, however, sudden acceleration appeared. The acceleration was about

thousand times. The data were re-plotted against maximumstress intensity factor Kmax

in Fig.6. The remarkably accelerated propagation rate data fell in a unique data band.

The sudden acceleration was governed by Kmax, which indicates that the time-dependent

crack growth mechanism is involved. The Vickers hardness of 843Ktempered material

was 325 which was lower than the usually accepted critical hardness for delayed failure.

The fracture surface is shown in Fig.7. The morphology of uncharged material (Fig.a)

Kmax=79MPam1/2

Kmax=50MPam1/2

K=73MPam1/2max

(b) Charged, t=1800s

(c) Charged, t=1800s

(a) Uncharged, t=60s

10μm

Fig.7 Fracture surface of 843Ktempered material tested at R=0.6

10-876543

10

100

200

was striation mode. On the contrary,

SCM440H,R = 0.6 Uncharged Tempe84r3(K)

20

50

H charged, Temper (K) 443 873 80 90 4 2

the morphology of hydrogen charged

material which showed medium

acceleration (Fig.b) as well as sudden

acceleration

was quasi

(Fig.c)

reat , d

cleavage.

The effect of material hardness on

propagatio n

the occurrence of sudden acceleration

was examined using materials

tempered at different temperatures.

C r a c k

Test were done at R=0.6. Test results

are shown in Fig.8. In all materials

except 923K tempered material,

Maximumstress intensity factor, Kmax (MPam1/2)

sudden acceleration was observed. The

Fig.8 Effect of tempering temperature on

fracture surfaces are shown in Fig.9. In

acceleration of crack propagation at high

the case of 403K tempered material

stress ratio (triangular stress, R=0.6)

1028