PSI - Issue 2_A

Timothy Crump et al. / Procedia Structural Integrity 2 (2016) 381–388 Cru p / Structural Integrity Procedia 409 (2016) 000–000

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After showing above that non rate-dependant loading produces an accurate analytical and experimentally verified crack speed and profile, the DCB was then subjected to a small step loading above to increase the reflected energy, as in Song (2008). This is to increase the kinetic energy within the DCB for which the crack will propagate to be able to observe the point where a crack may branch. This has been seen to be where the first reflected strain wave interacts with the crack tip causing an unstable decay of the cracks energy state as two or more cracks. The point of macro-crack bifurcation/branching time of 39µs was observed to occur at the maximum crack speed 0.72Cr, which agrees with dynamic theory, see Fig.6(b).

(a)

(b)

Fig. 6. (a) Steady state DCB under static loading compared to experimentally observed speed and analytical solutions; (b) DCB under a step loading predicting branching point.

3.2 Homolite-100 3D DCB To observe the bifurcation of the crack, the small step loaded model in Fig.6(b) was extended into 3D for a 1 mm thick specimen in Fig.7. A resultant stress plot for this can be seen in Fig.8(a). It was found from the model that the bifurcation occurred at K Ib =3.16 MPa√m with a bifurcation crack speed of 1657.3 ms -1 , which is in good agreement with the observations in the experiment. The bifurcation half angle measured was θ b =4.1 o also in agreement with 8 o observed, however full acro-crack branching is yet to be implemented and is subject to further work.

Fig. 7. Thin 3D DCB plate with notch; (b).

To observe the effects of reflective waves on crack shape two other values of G c = 0.3Jm -2 and G -2 were also considered, see Fig.8(b)(c). It can be seen in that with reduced crack tip momentum that the macro-crack shape alters with the reflective waves having more influence on the crack shape. In Fig.8(b) the ‘s-shape’ of the crack becomes more prominent and in Fig.8(c) the crack bifurcates again due to the influence of the end of the DCB. c = 0.03Jm