Steel--Fatigue--Environmental aspects

Experiments have been performed which determined the fatigue crack growth rate (FCGR) of short cracks (a > 0.1mm) for five high strength steels (yield stress 370-570 MPa) in air and in natural seawater with and without cathodic protection. Attention was focused upon Regions I and Il of the classical FCGR-stress intensity range(Delta K) curve with particular consideration of the near-threshold behavior for short cracks. Single edge notch (SEN) three-point bend specimens and a direct current potential drop (DCPD) crack monitoring system were employed, and test parameters simulated offshore structure conditions. The results indicated enhanced FCGR for short cracks compared to macrocracks by 3-20 times in air and 2-6 in seawater free-corroding(FC). Also, the Delta Kth for short cracks was apparently lower than for long ones in both environments. The transition from short to long crack behavior occurred at constant $\Delta$K in each environment (15.6 MPa m in air and 10.0 MPa m in seawater(FC)) irrespective of initial Delta K (Delta K(0)). The transition crack length ranged from 0.25 to 1.6 mm and was inversely proportional to $\Delta$K(0). Scanning electron microscope fractography showed that the mechanism of enhanced crack growth rate was associated with secondary crack (SC) formation in air and SC or inter-granular cracking (or both) in seawater (FC). The enhanced FCGR for short cracks was minimized by polarization to -950 mV(SCE). Through an elastic-plastic fracture mechanics analysis using the J-integral parameter it was found that the influence of plastic deformation at the crack tip was approximately independent of crack length (short versus long), and the linear-elastic fracture mechanics analysis gave a realistic representation for fatigue behavior.

A fatigue machine was rebuilt to investigate the wear of mild steel and
AISI Type 304 stainless steel while undergoing cyclic relative motions
in air and sea water environments. Wear curves were obtained for both
materials by measuring specimen weight loss as a function of the number
of cycles of relative motion, and surface damage of the specimens was
photographed. The results indicate that a sea water environment has a
strong influence upon the wear characteristics of mild steel and 304
stainless steel. For the mild steel, the corrosive effects of the sea
water contributed to severe pitting and cracking of the wear surface
and resulted in a significant increase in wear; after approximately
2.5 million cycles the amount of wear in sea water was approximately
three times that in air. For the stainless steel, the sea water acted
primarily as a lubricant and coolant, and served to reduce the amount
of wear after 2.5 million cycles to about one-tenth of that in air.