PSI - Issue 1

FV Antunes et al. / Procedia Structural Integrity 1 (2016) 090–097 Author name / Structural Integrity Procedia 00 (2016) 000 – 000

92

3

2. Numerical model

In this research, a standard M(T) specimen with width (W) of 60mm and length (L) of 300 mm was considered. A straight crack with initial size (a 0 ) of 5 mm (a 0 /W=0.083) was modelled. A small thickness (t=0.1 mm) was considered to simulate plane stress state. Due to the symmetries of the sample in terms of geometry and loading conditions, only 1/8 of the specimen was simulated using adequate boundary conditions. The contact of crack flanks was simulated placing a symmetry plane with frictionless contact conditions behind the growing crack front. This symmetry plane was removed in the cases without contact. Constant amplitude loading cycles were applied to the specimen. In order to have a wide range of loading parameters, sets with constant K min , K max ,  K and R were studied (Table 1). All the load cases were run twice, with and without contact of crack flanks.

Table 1. Load parameters (  K, K max , K min  =MPa.m 1/2 ) Set 1 (K min =0) Set 2 (K max =6.4)

Set 3 (  K=c. te )

Set 4 (  K=c. te )

Set 5 (R=c. te )

R

R

R -2 -1

R -2 -1

R

 K 2.7 3.6 4.6 6.4 8.2 9.1

 K 3.6 5.5 7.3 9.1

 K 4.6 4.6 4.6 4.6 4.6 4.6

 K 6.4 6.4 6.4 6.4 6.4 6.4

 K 2.9 3.6 4.4 5.1 5.8 6.6

0 0 0 0 0 0 0

0.43 0.14 -0.14 -0.43 -0.71 -1.00 -1.29

0.2 0.2 0.2 0.2 0.2 0.2

-0.5

-0.5

0

0

10.9 12.8 14.6

0.25

0.25

0.5

0.5

10.0

The material considered was the 6016-T4 aluminium alloy. Since PICC is a plastic deformation based phenomenon, the hardening behaviour of the material was carefully modelled. An anisotropic yield criterion was considered. The hardening behavior of this alloy was represented using an isotropic hardening model described by a Voce type equation combined with a non-linear kinematic hardening model described by a saturation law (Chaparro, 2008). The finite element mesh was refined near the crack tip and enlarged at relatively remote positions. Square elements with 8  8  m 2 were defined in the most refined region where the crack propagates, while only one layer of elements was considered along the thickness. The total mesh is composed of 6639 linear isoparametric elements and 13586 nodes. Crack propagation was simulated by successive debonding of nodes at minimum load. Each crack increment corresponded to one finite element and two load cycles were applied between increments. In each cycle, the crack propagates uniformly over the thickness by releasing both current crack front nodes. The opening load, F op , necessary for the determination of the closure level was determined considering two approaches. The first consisted in evaluating the contact status of the first node behind the current crack tip with the symmetry plane. In order to avoid resolution problems associated with the discrete character of load increase, the opening load was obtained from the linear extrapolation of the applied loads corresponding to two increments after opening. The second approach uses the contact forces at minimum load to calculate the stress intensity factor required to open the crack (Antunes, 2014). The numerical simulations were performed with the Three-Dimensional Elasto-plastic Finite Element program (DD3IMP) that follows a fully implicit time integration scheme (Menezes, 2000). The mechanical model and the numerical methods used in the finite element code DD3IMP, specially developed for the numerical simulation of metal forming processes, take into account the large elastic-plastic strains and rotations that are associated with large deformation processes. To avoid the locking effect a selective reduced integration method is used in DD3IMP (Alves, 2001). The optimum values of the numerical parameters of the DD3IMP implicit algorithm have been well established in previous works, concerning the numerical simulation of sheet metal forming processes (Oliveira, 2004) and PICC (Antunes, 2008).