High frequency corrosion fatigue crack propagation behavior of AISI 316L stainless steel was studied in 1M NaCl and 1M NaCl + 0.01M Na₂S₂O₃, under various anodically and cathodically polarized potentials, and dessicated air at 22°C and 1 atmosphere pressure. Constant load amplitude fracture mechanics techniques employing single edge notch specimens were used to assess the fatigue crack growth rate in the various environments. Unique specimen preparation procedures were developed which allowed near-threshold behaviors to be studied under gradually rising crack tip stress intensity conditions. Polarization studies showed that the presence of thiosulphate catalyzed the dissolution of stainless steel in low pH solutions(pH~1), due to reduction of thiosulphate species to H₂S, but had no effect in the near neutral solutions. Fatigue tests conducted in the neutral NaCl + Na₂S₂O₃ solution at cathodic potentials showed that the presence of thiosulphate had an insignificant effect. This indicated that high frequency fatigue produces efficient exchange of bulk solution with the crack tip environment, which prevented the lowering of pH in the crack by hydrolysis effects and prevented reduction of thiosulphate to H₂S. Fatigue crack retardation phenomena were very pronounced in the near-threshold regions in most of the fatigue tests. The cause of this retardation was attributed mainly to the surface-roughness- induced crack closure effect, which reduced the effective crack tip cyclic stress intensity ΔKth to a lower level. This closure effect only predominated in the near-threshold region where significant Mode II loading was present. The influence of various imposed anodic and cathodic potentials was found to be consistent with the surface-roughness-induced crack closure effects. The observed crack growth accelerating effect of high anodic potentials was attributed to the corresponding high removal rate of surface roughness in the wake of the crack, which kept the effective cyclic stress intensity level near the applied values. Crack fractography was studied by scanning electron microscopy. It showed that the fractography generally consisted of three regions; a crystallographic cleavage-like near-threshold region, a feathery and fibrous transition region, and a striated region. Using an etch pitting technique, it was determined that the crack plane and crack propagation directions in the near-threshold region were mainly those of {111} <110>, {110} <112>, {110} <001>, and other higher indexed planes. These crack orientations were effected by the activation of a single slip system or the alternate activation of two intersecting slip systems. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

A variation of the double cantilever beam specimen has been calibrated and used to study the propagation of stress corrosion cracks as a function of stress intensity in 316 and 310 stainless steels, and a TRIP steel exposed to hot aqueous magnesium chloride solutions. The effects of cold work, temperature and applied potential on both the fractography and cracking rates have been examined. The effects of cold work and crack path on crack branching were also investigated. Bpth stress Intensity dependent (Region I) and stress intensity independent (Region II) cracking were observed. Region II having apparent activation energies from 15.1 kcal/g.mole to 18.1 kcal/g.mole. The crack velocities of 25% cold rolled 316 were found to be independent of applied potential over a range of more than 50mV in Region I and 75mV in Region II. In the same material the crack path changed from solely transgranular at low stress intensities and noble potentials to more than 80% intergranular at high stress intensities and active potentials. The topography of the transgranular fracture was similar to that observed by others except in the case of the TRIP steel where nodular features were observed. These observations have been discussed with respect to mechanisms involving the following (i) electrochemical dissolution, (ii) absorption of hydrogen and (iii) adsorption of a damaging species. Of these, an adsorption assisted process is most compatible with the observations. Qualitatively the adsorbed species are envisioned as modifying the behaviour of the surface atoms at the crack tip. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

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Circumferentially notched rods of three types of austenitic stainless steel were stress corroded under freely corroding conditions at their yield stress in boiling 154°C MgCl2, and boiling MgCl2 with additions of HCl, CoCl2, and FeCl3. Alloy types 304, 316, and 310 were chosen because of their known different stress corrosion susceptibility. The corrosion products formed on the stress corrosion fracture surface were analyzed by electron diffraction and energy dispersive x-ray analysis. Electron diffraction of corrosion products, both in situ and stripped from the fracture surface, showed the corrosion product was a spinel oxide in all cases. Qualitative x-ray analysis of corrosion products, in situ and stripped from the fracture surface, indicated the corrosion product formed on all alloys was enriched in chromium and contained lesser amounts of the elements iron, nickel, silicon, molybdenum, magnesium, phosphorous and chlorine. Observations led to the conclusion that the corrosion product formed in the cracks of the various alloys was similar, being predominantly a chromium enriched oxide spinel with possible traces of metal chlorides or a corundum type oxide. The presence of the spinel oxide was consistent with anticipated E-pH equilibrium within the crack. However, it was felt the variation in s.c.c. behaviour between the different alloys could not be adequately accounted for in terms of the composition of the oxide. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

M.Ing. / Type 3CR12 steel is a corrosion resisting steel which is intended to be an alternative structural steel to replace the use of coated mild steel and low alloy steels in mild corrosive environments. This necessitate the experimental verification of the structural behaviour thereof. The purpose of this dissertation is therefore to compare the experimental structural bending behaviour regarding elastic and inelastic lateral torsional buckling of doubly symmetric I-beams and monosymmetric channel sections with the existing theories for carbon steel beams and to modify or develop new applicable theories if necessary. From the theoretical and experimental results it is concluded that the behaviour of heat treated Type 3CR12 beams can be estimated fairly accurate with existing theories and that the tangent modulus approach should be used for more accurate estimates as well as for beams that are not heat treated.