Issue 36

R. H. Talemi, Frattura ed Integrità Strutturale, 36 (2016) 151-159; DOI: 10.3221/IGF-ESIS.36.15 157 stress distribution extends along the loading direction till the middle of the specimen and mode I fracture occurs. Then the mixed mode behaviour governs the failure mode. 0 200 400 600 800 1000 0 1 2 3 4 5 Displacement [mm] ܨ [kN] J FE = 2 kJ J Exp. = 1.94 kJ Crack propagation  0 200 400 600 800 1000 1200 0 1 2 3 4 5 Displacement [mm] ܨ [kN] Simulation Experiment (a) (b) Figure 3 : Comparison of force against hammer displacement and absorbed energy between simulation result and experimental observation. Figure 4 : Maximum principle stress (MaxP) distribution during crack propagation steps. From the simulation result, it was noticed that, the XFEM-based cohesive segment approach can be a suitable methodology to model brittle fracture behaviour of API X70 pipeline steels. Nevertheless, due to the strong discontinuous behaviour of the XFEM crack propagation process, the possibilities of facing numerical convergence issues are very high. Up to ABAQUS 6.13 version this convergence issue is related to the XFEM implementation inside ABAQUS, which is not responding perfectly under dynamic loading conditions. Moreover, an enriched element cannot be intersected by more than one crack. Dynamic stress intensity factor calculation The dynamic stress intensity factor ( K ID ) can be determined using different experimental approaches. Nishioka and Atluri [11] have introduced an optimum technique for determining the dynamic stress intensity factor through the measurement of the Crack Mouth Opening Distance ( δ CMOD ), applying the well-known relationship as in static conditions. Following the same approach as proposed by Nishioka and Atluri [11], the dynamic stress intensity factor was calculated from the FE results as: 1 2 ( ) ( ) CMOD ID E C K C a       (12)