Stress corrosion

Degradation of composite materials in marine environments has been investigated experimentally and with analytical and numerical methods. Basic mechanical properties, fiber volume fraction, moisture absorption curves and transverse tensile properties after water absorption were determined. Transverse fracture surfaces of dry and wet composites were inspected in a scanning electron microscope (SEM). In addition, the edge replication technique was applied. Micromechanical stress analysis of a composite subjected to mechanical, thermal and moisture loading were performed using analytical methods and finite elements. Transverse stiffness and stress levels for interfacial debonding and matrix failure were calculated and correlated with transverse stiffness and strength obtained experimentally.

The stress corrosion cracking susceptibility of austenitic stainless steels SS304L, SS316L and SS904L was studied in an acidified seawater environment by slow strain rate testing at 24, 38 and 66$\sp\circ$C. Fractographic evidence of SCC susceptibility was obtained using scanning electron microscopy. The degree of susceptibility to SCC for each alloy in these environments is discussed based on the mechanical parameters, fractography and anodic polarization behavior. The results showed that SS904L performed better than SS304L and SS316L in the aforementioned environments.

The stress corrosion cracking (SCC) tendencies of several engineering alloys were studied in an acidified seawater environment as a function of applied strain rate and electrolyte temperature. The selected alloys included austenitic stainless steels 304L, 316L, 904L and A-286 (an iron-based superalloy at two heat treatments yielding ultimate tensile strengths of 130 and 200 ksi), Inconel 718 (220 ksi ultimate tensile strength) and Hastelloys C-22 and C-276. The slow strain rate test technique was used to evaluate the SCC strain rate dependency of each alloy at extension rates of 4.7 x 10^-6, 4.7 x 10^-4 and 4.7 x 10^-3 mm/sec. The effect of electrolyte temperature was evaluated at 38C and 60C at a single extension rate of 4.7 x 10^-5 mm/sec. Control specimens were tested in a laboratory air environment at an extension rate of 4.7 x 10^-5 mm/sec. Various mechanical parameters of the specimens tested in the corrosive medium were compared with those of control specimens to quantify the degree of cracking. Fractographic evidence of SCC was obtained using scanning electron microscopy (SEM). An attempt was made to correlate SCC tendency with the alloy's passivation kinetics and microstructure. Atmospheric exposure testing was performed in a simulated space shuttle launch pad environment for selected alloys.

Experiments were conducted to determine the stress corrosion cracking (SCC) susceptibility of various corrosion-resistant alloys which included: 17-4 PH, INCONEL 718 and A286. These alloys were studied for different aging (heat) treatments. Slow strain rate tests (extension rate = 4.7 x 10^-5 mm/s) were performed on each alloy in four different environments; including air and natural seawater acidified with reagent grade hydrochloric acid to a pH of 0.1, 1 and 3. During the experiments, the load versus time and the open circuit potential were monitored. Various parameters such as time-to-failure, energy-to-failure, maximum or failure stress and reduction-in-area were calculated in order to determine SCC susceptibility. Fractography using SEM was conducted to confirm whether any SCC occurred and, if so, to identify its mode (intergranular or transgranular). Limited potentiodynamic studies were also completed to evaluate the passive behavior of these alloys. The results are discussed in terms of the SCC susceptibility and the nature of the cracking. An attempt was also made to correlate alloy microstructure, slow strain rate test parameters and passivation behavior with SCC susceptibility.

Double cantilever beam specimens of 7079-T651 aluminum were
subjected to low constant stress intensities in a sea water
environment to determine the stress corrosion cracking response.
In addition to a constant stress intensity some specimens
were subjected to controlled, constant potentials. Despite
the fact that all tests were in Region I of the crack growth
rate-stress intensity curve, where the former has been projected
to be very dependent upon the latter, a unique relationship
between stress intensity and crack growth rate was not
always indicated. Therefore, some variable other than stress
intensity is assumed to control crack growth, particularly
for the first several hundred hours of exposure. The observed
behavior is discussed in terms of accepted theories of stress
corrosion cracking in high strength aluminum alloys, including
electrochemical dissolution and hydrogen embrittlement.

This study investigated the stress corrosion cracking
behavior of aluminum alloy 7079 - T651 in two corrosive
environments, sea water and a 3% NaCl-distilled water
solution. Self stressing, double cantilever beam specimens
were employed; and these were stressed at various levels,
exposed in one of the two test environments and crack
extension monitored as a function of time. Equations for
the plane strain stress intensity factor (K1) were compared
and evaluated, with respect to stress intensities calculated
from compliance measurements. Results suggest that stress
intensity is not the only important variable controlling
crack growth rate and a time dependent cracking mechanism
may govern a portion of crack growth.

This thesis deals with corrosion problems of underwater turbines in marine environment. The effect of a tensile stress on the uniform corrosion rate of a metal bar is studied, and an analytical model predicting the time of service of a bar under a tensile load in a corrosive environment is proposed. Stress corrosion relationships are provided for different type of alloys, and different types of relationships. Dolinskii's and Gutman's models are studied and extended to a general order polynomial, along with a Least Square and Spline Interpolation of the experimental data. In a second part, the effect of the passive film, delaying the initiation of the corrosion process, is studied. Finally, an algorithm predicting the time of service of a cracked bar is provided, using the stress corrosion assumption, along with a validation using experimental data.