Influence of Environment and Creep on Fatigue Crack Growth in a High Temperature Aluminum Alloy 8009

Frequency effects on fatigue crack growth rates are examined in aluminum alloy 8009 in sheet and extruded product forms. The investigations show that frequency effects on the fatigue crack growth rates are pronounced in the sheet but minimal in the extrusion. The influence of creep cracking on fatigue crack growth rate is studied through tests with a 60 s hold-time at maximum load at several stress intensity ranges. A 60 s hold-time at maximum load at 315°C tends to retard fatigue crack growth in both the sheet and the extrusion. The mechanism by which this retardation occurs is attributed to stress relaxation at the crack tip. At 204°C a 60 s hold at P_max accelerates crack growth rate in the sheet but not in the extrusion. Vacuum and laboratory air tests show that fatigue crack growth rates in vacuum are lower than in air by about a factor of four. A 60 s hold-time at minimum load has only a minor effect on the fatigue crack growth rates at 315°C and no effect at 204°C, confirming the absence of any strong environmental contribution to crack growth rate. Fracture modes in fatigue, creep crack growth and hold-time at P_max are significantly different. The fractographic results are discussed in relation to the mechanical property data.