Issue 33

J.M. Vasco-Olmo et alii, Frattura ed Integrità Strutturale, 33 (2015) 191-198; DOI: 10.3221/IGF-ESIS.33.24

Figure 5 : Estimated K op

and K cl

Figure 6 : P op

and P cl

values as a function of the crack

at different crack lengths for the specimen

length for the specimen tested at low R -ratio.

tested at low R -ratio.

From top plots, it can be observed a change in the slope of experimental data for load values between 100 and 110 N, which indicates that crack starts opening. However, from bottom plots, it is observed that for load values higher than 100 N (for loading branch) and 115 N (for unloading branch) the strain offset is almost 0, which indicates that the crack is completely open. According to this, P op and P cl values can be estimated for all the analysed crack lengths (Fig. 8). In Fig. 8 it can be observed that P op and P cl values are between 75 N and 125 N, with no apparent differences between P op and P cl values. Therefore, these results show great level of agreement with those obtained from CJP model, so the experimental methodology proposed to estimate P op and P cl values from K F trend can be considered as validated.

Loading branch

Unloading branch

600

600

raw data smoothed data no closure

raw data smoothed data no closure

500

500

400

400

300

300

200 load (N)

200 load (N)

)

100

100

)

(a)

(b)

0

0

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

-0.05

0

0.05

0.1

0.15

0.2

0.25

0.3

COD (mm)

COD (mm)

600

600

smoothed data

smoothed data

500

500

400

400

300

300

200 load (N)

200 load (N)

(d) d)

c)

100

100

(c)

-0.02 -0.015 -0.01 -0.005 0 0.005 0.01 0.015 0.02 0

-0.02 -0.015 -0.01 -0.005 0 0.005 0.01 0.015 0.02 0

strain offset

strain offset

Figure 7 : Results obtained from strain offset method for a crack length of 35.47 mm corresponding to the specimen tested at low R ratio. a) Applied load versus COD for loading branch, b) applied load versus COD for unloading branch, c) applied load versus strain offset for loading branch and d) applied load versus strain offset for unloading branch. In a similar way, results from the specimen tested at high R -ratio have been also analysed. K F and K R values obtained as a function of the loading cycle at crack lengths of 34.64 and 41.09 mm are shown in Fig. 9. It is observed that K F values show total level of agreement throughout the whole loading cycle with those calculated from Eq. 6. Therefore, there is no change in the slope of K F as in the case of the specimen tested at low R -ratio, which indicates the absence of the shielding effect and consequently, the crack remains fully open during the loading cycle. In addition, this behaviour is also observed

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