PSI - Issue 13

B. Nečemer et al. / Procedia Structural Integrity 13 (2018) 2261 – 2266 Author name / Structural Integrity Procedia 00 (2018) 000–000

2263

Fig. 2. Selected auxetic porous structures re-entrant (AUX_1); (b) rotated re-entrant (AUX _2)

Cyclic � � � curve ′ ′

Table 1. Material properties of aluminium alloy 7075 T651 (Dowling 2000). Modulus of elasticity [MPa] oisson’s ratio [  ] True fracture strength [MPa] True fracture strain [  ] Strength coefficient [MPa] Strain hardening exponent [  ]

Strain – Life curve σ' f ε' f -0.143 0.262

71000

0.33

744

0.41

827

0.113

977

0.106

1466

-0.619

2.1. Computational analysis

Crack initiation period The crack initiation period (Ni) is determined using strain-life approach in the framework of Fe-safe software (2014). The needed material parameters are taken from Table 1, while the elastic-plastic stress field in the critical cross sections has been determined previously using Abaqus software. Several discrete points (up to 40 points) in the sinusoidal loading cycle of elastic-plastic finite element model are used to form stress-time and strain-time history. The sequence of stress-strain pairs defines a loading block and is then used to determine the fatigue lives using principal strain model and with consideration of Morrow mean stress correction. Fig. 3 shows the fatigue lives presented in log10 scale.

Fig. 3. Crack initiation period at the beginning of the first base cell (a) re-entrant (AUX_1); (b) rotated re-entrant (AUX_2)

Figure 4 shows the schematic procedure of the crack initiation period. It is evident that the crack initiation starts first in the corner of the base cell. Considering the sinusoidal pulsating loading and material properties given in Table 1, the number of loading cycles required for the fatigue crack initiation is numerically calculated for each newly demolished cell. In the next step, the initial crack = 0.2 mm is added at the critical location of the cell and the numerical procedure is continued with the crack propagation simulation. When the fatigue crack reaches the critical