Crack Paths 2012

The results obtained by the P R Mare presented in Fig. 5. The vicinity of the crack tip is

shown for six different overloads with an overload ratio Rol ranging between 1.6 and

2.4. Apart from the overload ratio Rol, the applied stress intensity factor KI and the

absolute overload ratio N are also given in Fig. 5 for the different overloads.

In the initial stage the crack tip is located at a large graphite particle, which is marked in

the top left image in Fig. 5. The first visible crack advance towards the next graphite

particle can be located by the P R Mat the overload with Rol=1.6. At all overloads before

(Rol smaller than 1.6) there is nothing detectable on the surface of the specimen, where

the P R Mis used. The stress intensity factor at the overload Rol=1.6 is about KI=21.4

0 3 D ¥ PDQd the absolute overload ratio is N=0.67. The comparison to the results

obtained by the crack length foils reveals, that crack extensions have been detected by

the crack length foils also for overloads with Rol smaller than 1.6, particularly a

remarkable crack extension for the overload with Rol=1.5.

The next overload shown in Fig. 5 with Rol=1.8 produces plastic deformations in the

ligament towards the next particle. This ligament is finally damaged when the overload

with Rol=2.0 is applied. This overload ratio results in a stress intensity factor of KI=27.0

0 3 D ¥ PDQG WKH DEVROXWH RYHUORDG UDWLR LV N=0.84. At this overload first plastic

deformations above and below the crack ligament become visible. During the next two

depicted overloads (Rol=2.1 and 2.2), where the critical stress intensity factor for crack

initiation is almost reached (N=0.86 and 0.93), crack branching is visible on the

specimen’s surface. The crack branching starts from the particle where the plastic

deformations next to crack front were detected. Nevertheless, there is still a sharp crack

in the straight (horizontal) direction. Additionally to the crack branching there are also

crack extensions at the sharp crack due to the applied overloads. From now on it must

be taken into account, that the results obtained by the crack length foils are only an

interpolation of the crack length, since there are nowthree crack tips.

Finally, static fracture is reached whenthe overload with Rol=2.4 is applied. As it can be

seen in the crack growth curve in Fig. 4 a very large crack growth increment of about

'a=0.15 m mtakes place. N o wthe crack tip of the sharp crack in horizontal direction is

not fully loaded any more. Instead, the crack branching mechanism gains the upper

hand and leads to final failure of the specimen.

DISCUSSION

First it can be stated that the current experimental findings confirm previous results [1]

[3]. All applied overloads lead to crack growth acceleration emerged within the fatigue

crack growth rate. Furthermore, within this study a simultaneous combination of optical

insitu investigations using the P R Mand crack length measurements by crack length

foils is realized for the first time. With it, a comparison of the two techniques is possible

and previous results can be better understood.

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