PSI - Issue 14

Viswa Teja Vanapalli et al. / Procedia Structural Integrity 14 (2019) 521–528 Viswa Teja Vanapalli/ Structural Integrity Procedia 00 (2018) 000 – 000

524

4

crack growth as shown in Fig. 3a. The cohesive elements initially have zero thickness. The size of cohesive elements in the direction of crack propagation is taken as 0.2mm which is the least crack growth measured. Fig. 2c. shows a schematic diagram of the straight pipe having through-wall circumferential crack subjected to four-point bending load. Due to such arrangement, crack plane experiences pure bending moment. A 3D finite element model using 8-noded solid brick elements is developed using quarter symmetricity. A spider mesh is employed around the crack tip. Six layers of elements along the thickness are taken as shown in Fig. 3b.

Fig. 2. (a) Geometric details of a TPBB specimen; (b) Quarter 3D finite element model; (c) Geometry of pipe with through-wall crack subjected to four-point bending load.

Fig. 3. Quarter 3D Finite element model of (a) TPBB specimen and (b) Straight pipe with through-wall circumferential crack.

2.3. Cohesive zone analyses of TPBB specimens The cohesive parameters for the TPBB specimen have been found by comparing the simulated results with the experimental results. The value of cohesive energy (G) is taken to be the value of initiation fracture toughness for SA333 Grade 6 steel, which is 220 kJ/m 2 as suggested by Schwalbe et al. (2013). The value of peak stress (T) is varied to match the numerical results with the experimental results. This helped to obtain suitable cohesive parameters for a given specimen. The cohesive element gets deleted one after the another and crack extension is calculated by multiplying the size of cohesive element with number of elements ‘killed’ at that particular load step. The load vs CMOD plot can be obtained from the output file. The J-R curves are calculated using the closed form formula given in ASTME1820-15a (2015).

2.4. Determination of ‘q’ parameter of TPBB &straight pipes by elastic -plastic FEA

The stress triaxiality parameter ‘q’ for TPBB specimens is determined by conducting elastic -plastic analysis of cracked specimens. For this purpose, elastic-plastic finite element analyses of all fourteen TPBB specimens have