PSI - Issue 23

Sascha Gerbe et al. / Procedia Structural Integrity 23 (2019) 511–516 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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The fractographic investigation shows for specimens with small pores ( d p,max < 2-3 ∙ λ 2 ) compared to specimens with larger pores, a different crack initiation and propagation behavior in the VHCF regime. On the one hand, crack initiation is shear-stress-controlled and occurs on highly stressed {111}-slip planes (cf. Fig. 4b). On the other hand, due to low SIF ranges (below ∆ K I,th ), microstructural features like the eutectic Si particles are influencing the crack propagation. Figure 4a shows that striations in the vicinity of the crack initiation site are ranging from one interdendritic region to the next. This underlines the barrier characteristic of the eutectic against dislocation movement, which was shown by Gerbe et al. (2019) in crack propagation experiments under pure bending. The fatigue crack initiation and propagation behavior was further studied in intermitted CAT for AlSi7Mg0.3 at a stress amplitude of 140 MPa using CT scans and ACPD technique. The results are plotted in Fig. 5 and the specific projected crack areas are exemplarily shown in Fig. 6 (left: top view; right: side view). For each damage state, the projected crack area perpendicular to loading direction was determined and summed up to the total projected crack area A c,t . A strong relation was determined between changes in AC potential and the total projected crack area A c,t for all damage states. Finally, the total projected crack area A c,t included around 50 % of the total cross-section (  U AC = 80 mV). The fatigue crack initiation was determined at around 10 5 cycles while critical change in AC potential (  U AC = 20 mV) was reached after ≈ 2·10 5 cycles. The fatigue propagation is characterized by an exponential increase in AC potential and fatigue crack area. Due to high fatigue crack propagation rate at 20 mV for  a = 140 MPa, the last two damage states were induced at decreased fatigue loading of  a = 120 MPa (20 to 40 mV) and  a = 80 MPa (40 to 80 mV). Hereby, ex-situ CT scans allow a 3D analysis of the damage evolution within Al-Si cast alloys due to fatigue loading. Multi-crack initiation was determined in the plane of maximum nominal stresses followed by coalescence of two fatigue cracks between  U AC = 10 and 20 mV characterized by a fatigue crack propagation in the plane of maximum shear stresses as shown in Fig. 6.

Fig. 5. Comparison of change in AC potential  U AC and projected crack area A c,t in CAT determined intermittently in CT using contrast agent.

 U AC ≈ 10 mV

 U AC ≈ 20 mV

 U AC ≈ 40 mV

Fig. 6. Exemplarily visualization of fatigue crack within CT at specific changes in AC potential  U AC using contrast agent and 3D-image analysis software - top view of all fatigue cracks (left) and detailed side view of failure inducing fatigue crack (right) (cf. Tenkamp et al. (2019)).

4. Conclusion

Fatigue experiments on two different hypo-eutectic Al-Si cast alloys AlSi8Cu3 und AlSi7Mg0.3 in the HCF and VHCF regime showed significant influences of the appearance and distribution of pores on the mechanisms of fatigue