The fatigue crack growth behavior of the high strength Al 7039 alloy was investigated in this study. Crack growth behaviors of CT specimens under five different aging conditions were investigated and crack closure phenomena in specimens with different thickness were studied.
The aging conditions were peak-aging (24 h. at 120℃), over-aging (36 h. at 120℃), over-aging (48 h. at 120℃), double aging (16 h. at 77℃ and 14 h. at 160℃), and double aging (8 h. at 107℃ and 16 h. at 150℃) respectively. The microstructures of the specimens were examined by the optical and electromicroscopy to find the relationship between the microstructural features and their fatigue properties. Precipitate free zones (PFZ) were observed near the grain boundary. The threshold stress intensity range, $ΔK_{th}$, was largest for peak-aging condition where the width of the PFZ was smallest. The fatigue crack growth rate, however, for peak-aging condition appeared to be rather high.
Kikukawa-Compliance method using a conventional clip on gauge was employed to investigate the effect of thickness on crack growth and crack closure. At R=0.05, the fatigue crack growth rate tended to increase with increasing thickness. However, at R=0.5 thickness did not appear to affect the fatigue crack growth rate. In addition it is found that $K_{op}/K_{max}$ suddenly decreased with increasing $K_{max}$ in region I, while constant in region II. In region II, fatigue crack growth rate independent of thickness could be well represented by $ΔK_{eff}$. On the other hand, in region I the growth rate was dependent on the thickness.
