Abstract
The crack growth rate of type 5056 aluminum alloy in reversed bending is much reduced in vacuum from that at atmospheric pressure, being independent of pressure in the range 10-4 to 10-6mmHg. On the contrary, the crack rate increases in high vacuum of 10-7mmHg under lower stress amplitude due to vacuum outgassing effect by prolonged exposure to high vacuum. The enhanced crack growth due to the vacuum outgassing appears more clearly in the ultra-high vacuum of 10-9mmHg irrespective of applied stresses. The plastic size generated at crack tips increases with the decrease of vacuum pressure, extraordinal plastic deformation being observed at 10-9mmHg. The acceleration of fatigue crack growth due to high vacuum outgassing can be remarkably observed on the surface crack extension of thin sheet specimens, which grows greater in the case of more prolonged exposure to higher vacuum.
