NEAR THRESHOLD FATIGUE CRACK PROPAGATION BEHAVIOR IN IRON AND ALUMINUM - 3 PERCENT MAGNESIUM ALLOY (CRACK CLOSURE)

Near threshold fatigue crack propagation behavior was investigated for load ratios of 0.05 and 0.5 in iron and Al - 3% Mg at 298(DEGREES)K in dry argon and in Al - 3% Mg at 77(DEGREES)K in liquid nitrogen. The crack closure level and its relations to fracture morphology, load ratio, cold working and temperature were quantified.; While the values of (DELTA)K(,th) decrease as the load ratio is raised at both 77(DEGREES)K and 298(DEGREES)K, the values of (DELTA)K(,eff)('th) are essentially unaffected by the load ratio. The value of (DELTA)K(,cl) increases as (DELTA)K(,th) is approached and changes more rapidly with a change in (DELTA)K at R = 0.05 as compared to R = 0.5. While the mechanism for crack closure at low load ratio in an inert environment appears to be fracture surface roughness coupled with shear mode displacements, at high load ratio plasticity induced crack closure also plays a role due to a high mean stress in annealed specimens.; Near threshold fatigue crack propagation rates and the values of (DELTA)K(,th) in iron and Al - 3% Mg shows an opposite effect of pre-strain, which is the result of a difference in crack closure mechanism. The crack closure stress intensity decreases as (DELTA)K is increased for the annealed specimens, but is insensitive to (DELTA)K for the pre-strained specimens at low load ratio.; Both (DELTA)K(,th) and (DELTA)K(,eff)('th) at 77(DEGREES)K are higher than the corresponding values at 298(DEGREES)K irrespective of the specimen condition or the load ratio. Near threshold fatigue crack propagation rates in moist air are faster than those in dry argon at low load ratio for both iron and Al - 3% Mg.; The areal fraction of shear mode fracture of specimens tested in dry argon (298(DEGREES)K) increases as (DELTA)K(,th) is approached and as the load ratio is reduced, but decreases with pre-strain. The fracture surface of specimens tested in liquid nitrogen (77(DEGREES)K) shows no perceptible difference in fatigue failure mode with (DELTA)K or load ratio. The fracture surface roughness at 298(DEGREES)K increases with (DELTA)K and load ratio. The fracture surface roughness increases in iron, but decreases in Al - 3% Mg with pre-strain.