Abstract
The effect of tarnish films on corrosion fatigue crack growth behavior of hard drawn, tough pitch copper in dilute ammoniacal solutions of various concentrations (0.02M-0.07M) at 55°C was investigated by measuring the rate of fatigue crack growth by the use of a cantilever type fatigue tester. The threshhold stress intensity factor range ΔKth, below which no detectable air-fatigue crack growth is observed was estimated at 7.4kg·mm-3/2 from crack growth curves in air. Scanning electron microscopy was employed to study the fractography of corrosion fatigue crack surface of specimens tested in ammoniacal solutions. The crack surface was all covered with tarnish films as thick and black as those on the surface of the specimen. The rate of crack growth in ammoniacal solutions is found to be nearly equal to that in air at the early time of crack growth, but from a certain crack depth it becomes smaller than in air. This critical depth is strongly dependent on the growth rate of tarnish films in cracks. As a result of decrease of this depth in accordance with increase of the tarnish film growth rate in cracks, the decrease of crack growth rate occurs at the earlier time of crack growth. These phenomena can be explained from the wedge action of tarnish films formed in fatigue cracks. From the above results, it is considered that the plastic insulated cable of hard drawn copper can be used in tarnishing environments on the basis of fatigue crack growth characteristics in air.
