Abstract:
The corrosion fatigue crack propagation behaviour of a high
strength low alloy steel, N-A-XTRA 70, in a synthetic sea
water solution was tested using S. E. N. specimens subjected
to a loading frequency of 0.1 Hz and a load ratio of 0.6.
In order to simulate the conditions encountered by a thumbnail
type crack several specimens from each of the
microstructural types tested, namely parent plate, heat
affected zone and heat treated material, had their crack
sides covered by transparent plastic covers. Severe overprotection
and slight underprotection conditions were
produced using cathodic protection potentials of -1400,
-1300, -1200 and -700 mV (S. C. E. ).
The Paris relationship da/dN = CLKm was found to be a useful
tool in describing the crack propagation rate data. Results
obtained, presented in the form of plots of log da/dN against
log AK, show that for parent plate, H. A. Z. and heat treated
material, covering the crack sides of specimens produces
enhanced corrosion fatigue crack propagation rates, at
cathodic protection potentials of -1400 and -1300 mV (S. C. E. ),
when compared to non covered specimens. This trend was also
true for H. A. Z. specimens at a potential of -700 mV (S. C. E. ).
For parent plate specimens, however, covering the crack sides
at a potential of -700 mV (S. C. E. ) produced reduced crack
propagation rates over non covered specimens. It is believed
restriced oxygen access may account for these results.
Plots of the Paris exponent m and constant C for the three
microstructures tested produced three lines of the form
m= alnC +b where a and b were found to be dependent upon
material parameters.
Comparison of results with BS 4360: 50D revealed that N-A-XTRA
70 exhibited superior fatigue performance when tested in air
but behaved worse under conditions of free corrosion.
