13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- material is regarded as a continuum, composed of a ductile matrix with microvoids. There have been a limited number of studies applying this model to ductile materials [9-11]. Fig. 1 Drop weight tear test (DWTT) specimen [2] As opposed to simulations of the DWTT, simulations of Charpy tests are widely carried out to characterize the toughness of line pipe steels. Koppenhoefer and Dodds [12] investigated specimen size and loading rate effects on cleavage fracture of ferritic steels tested in the ductile-to-brittle transition region in pre-cracked Charpy specimens. The probability distribution for fracture of a cracked solid is defined by a two-parameter Weibull distribution [12]. Eberle et al. [13] developed 2D as well as 3D explicit dynamic finite element analyses, in combination with the rate-dependent Gurson model, to simulate Charpy tests. The simulated load-displacement curve and crack front are in close agreement with experimental observations. Tanguy et al. [14] conducted a numerical simulation of the Charpy V-notch test in the ductile-brittle transition regime using a modified Gurson-type model for ductile damage and Beremin model for cleavage fracture. Folch et al [15] also developed a local coupled brittle/ductile fracture approach model to predict either Charpy energy or fracture toughness and to investigate conditions for correlations between them using the Beremin and Gurson models. The modified Beremin model is based on the principles of Weibull statistics for the distribution of the defects and their size as is the standard Beremin model. The modified Gurson model, which incorporates a yield function for porous metal plasticity, was utilized in this work in conjunction with the Lemaitre and Beremin models. Thibaux and van den Abeele [16] reported on the fracture mechanics of instrumented Charpy tests performed on an X70 material. The tests were then simulated using a finite element method and the GTN constitutive model. Damage is represented by an internal variable, f, representing the void volume fraction which is assumed to be isotropic. There have been very few reports on studies of the ductile-to-brittle transition region during a DWTT. Nonn et al [17] performed numerical simulations of a DWTT by applying the GTN model and compared the simulation results with experimental results from an instrumented DWTT. The results show that GTN model gives a reliable prediction of the load level variation with time when considering strain rate dependence. By applying GTN parameters validated on quasi-static fracture mechanics tests, the maximum load level including the beginning of the load drop for a DWTT can be well described quantitatively. The model equations were not provided in the publication. In this paper, we used the GTN model to simulate the fracture behavior during DWTT. The stress at the notch was included as an initial condition in this model for the first time. The equivalent stress, nucleation of voids, void size distribution, etc, were analyzed. We found that the fracture propagates in a triangular shape at the crack tip, and the inverse fracture occurs when the fracture propagated about 3/4 of sample width in current study case. Some of the cases show that the transition during DWTT test is from the brittle to the ductile and then again to the brittle zone. 2. Models and simulation
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