Issue 52

B. Paermentier et alii, Frattura ed Integrità Strutturale, 52 (2020) 105-112; DOI: 10.3221/IGF-ESIS.52.09 111 Again, a thickness reduction at the initial notch and a thickness increase at the end of the fracture is observed in the DT3 specimen. In this case, the stress concentrations in the centre of the fracture are similar to the stress concentrations observed in the DWTT specimen. Figure 4: Fracture surfaces and corresponding stress distributions obtained through numerical simulations for the CVN specimen (a), the DWTT specimen (b), and the DT3 specimen (c). As can be observed in Fig. 3, the peak force prediction of the CVN simulation and DWTT simulation is overestimated in comparison with the experimental data. In contrast, the DT3 peak force estimation does approximate the experimental data rather accurate. This observation suggests that the presence of impact loading in the numerical model increasingly complicates the acceleration and force calculations. As the impact conditions change during the simulation, the strain rate will also be adjusted continuously. Consequently, the explicit integration and the constitutive material model will be affected. The DT3 test on the other hand, only uses a tensile force to introduce the crack propagation leading to an overall more stable simulation. Furthermore, as to fulfil the GTN mesh size requirement, the element dimensions for the differently scaled CVN model and DWTT model were kept constant. This mesh size with respect to the total scale of the model will also influence the explicit solution scheme. Moreover, relatively small elements are required for the GTN model, thus the computational effort significantly increases due to the fine meshing. Therefore, the relatively larger geometry of the specimen does augment the issue regarding the contradictory role of the mesh size as material parameter as well as FE parameter. It is noteworthy that, even though the element size of the DT3 specimen was increased, the ductile fracture propagation does still present a good correlation with the experimental data. C ONCLUSIONS he main objective of this paper was to compare the numerical modelling of dynamic ductile fracture propagation on three different testing scales using the GTN damage model. Based on the obtained data, following conclusions can be drawn:  In this study, the implemented GTN ductile damage model is able to describe ductile fracture propagation on a satisfactory level for the considered different lab-scale fracture tests using high-toughness grade pipeline steels with different mechanical properties.  The presence of impact loading does complicate the force prediction especially when the size of the considered specimen increases. Even though, the test mechanism is similar and stress-strain conditions are the same, it is clear that the simulation of the DWTT is less stable than the smaller CVN test.  Due to the large scale of the DT3 specimen, it currently is challenging to respect the mesh size requirements for the GTN damage model. The mesh size plays a contradictory role as a material property as well as a fundamental T