Fracture mechanics study of corrosion fatigue

Abstract

In this investigation, corrosion fatigue crack growth studies were conducted on 7075-Alclad on an MTS closed-loop, servo-controlled hydraulic testing machine, where the maximum and minimum loads were regulated and the corrodent was drip-fed on the surface. The use of linear compliance fracture mechanics specimens for crack growth measurements gave constant stress intensity over approximately 75 mm of crack extension for a given load. The specimen design is such that cladding is retained on one side and its effect is therefore apparent on the crack propagation of the composite material. Constant amplitude fatigue crack propagation tests were conducted over a K range of 5.5 to 12.5 MN/m³/² in air as well as in 3.5% NaCl at test frequencies of 5 and 1 Hz and a stress ratio of 1/3 under free corrosion conditions.

Results indicate that adjusting the bulk pH from 5.4 to 1 retards fatigue crack growth rate. Lower pH of 1 results in increased corrosion activity per cycle which blunts the crack and/or builds up corrosion residue on the mating fracture surfaces, increasing the crack closure forces and retarding the crack growth. The addition of 1.7 N NaNO to 0.2 N NaCl solution reduces the crack growth rate; this specific ratio of nitrate/chloride ions is said to displace Cl adsorption on the aluminium oxide film.

Changing the test frequency has little influence on crack growth rate in air. In the presence of corrodent, not only were the crack growth rates higher than in air, but also decreasing the frequency increased crack growth rate, because at lower frequencies, more time is available for environmental interaction than at higher frequencies. A Haversine load at a K value of 5.5 MN/m³/² results in faster crack growth rate because of the greater activation under those conditions.

The difference in crack growth rate in air and in 3.5% NaCl is found to decrease with increase in K because of the obvious changes in environmental interaction conditions. Fractographic studies by SEM provide some evidence that the cracking is controlled, at least in part, by environment. The presence of 3.5% NaCl at the crack tip changes the topology of the fatigue striations from ductile to brittle. In the peak-aged and two-step aged conditions, particle/matrix interface decohesion was not observed.

The crack growth rate at K of 12.5 MN/m³/² decreases both in air and in 3.5% NaCl progressively from peak-aged to two-step aged to over-aged temper. In peak-aged temper, the semicoherent precipitates lead to planar deformation which tends to become localized on a few active slip bands, whereas widely spaced large precipitates in over-aged condition do not cause intense strain localization and the moving dislocations bow out by the Orowan mechanism. The two-step-aged temper gives a large number of duplex precipitates, consisting of small, closely spaced precipitates which raise the yield strength, and large, widely spaced precipitates which cause uniform plastic deformation and increased rate of work hardening, leading to increased microstructural stability.

Current fracture mechanics-based retardation models neglect the microstructural effects brought about by different thermal treatments. Observations in this investigation, however, reveal that microstructure, particularly precipitate morphology, plays a major role. The effect of precipitates on fatigue crack growth rate may be attributed to their influence on cyclic strain hardening and softening, as well as to their effects on ductility or toughness. Two parameters appear to characterize the observations, viz., , the measure of material strength, and _c*, the measure of ductility. These reflect the ability of the structure to distribute high strains in the crack tip region.

These parameters have been used to predict crack growth rates in terms of crack tip opening displacement (CTOD) and ductility. In terms of these parameters, the CTOD models suggest that the product of strength ( ) and ductility or toughness ( _c*) must be maximized to minimize crack growth rates in materials, under all conditions of environment, be it corrosive or not.