M.Ing. / Please refer to full text to view abstract

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

D.Ing. / In mechanical design and analysis the mechanical properties of the material used are crucial to achieve effective design or analysis. In designing structures that are susceptible to dynamic loading different mechanical properties of the material may be needed than those used for quasi-static situations. Usually when one refers to the dynamic properties of a metal one refers to the notch toughness of the material. That is the resistance of the material to crack propagation under dynamic loading. Another less well known dynamic property of a metal is strain rate sensitivity. This implies that mechanical properties like yield strength, tensile strength and rupture strain varies according to strain rate. Typical applications where these properties are of use are in impact situations such as vehicle collisions and cold and hot working of metals in the manufacturing industry. The mechanical properties of certain metallic components or structures may change when the component or structure are subjected to dynamic loading that causes permanent deformation. The purpose of this investigation is to investigate the strain rate sensitive behaviour of certain stainless steels. The steels investigated are AISI Types 304, 316 and 430 stainless steels, 3CR12 corrosion resisting steel (a proprietary alloy also known as Type 1.4003) and mild steel which acts as a reference. The strain rate sensitivity of the above mentioned steels are investigated experimentally at room temperature for strain rates between 10' to approximately 100 s -1 . The steels are all tested in as delivered sheet form and testing is conducted in both rolling directions. The testing at the medium strain rates necessitated the design and construction of a dynamic tensile tester, the design of which, is also presented. The implementation of strain rate sensitive material properties into structural design and analysis are investigated and a constitutive model is proposed. The implementation of the proposed constitutive model into numerical methods analysis tools such as the finite element method is discussed and presented. The practical implementation of the proposed constitutive model is illustrated by numerically analysing the problem of a clamped beam struck transversely by a mass and comparing this with available experimental data. The validity of a typical constant velocity tensile test that is used to determine strain rate sensitive material properties is also investigated numerically to place the experimental results obtained into perspective. All the steels tested are found to be strain rate sensitive. Their behaviour is satisfactorily described by the constitutive model presented. No general trend regarding strain rate sensitivity is found when the results of the two rolling directions are compared. The importance of including strain rate sensitivity into structural design and analysis is illustrated by the analysis of the clamped beam struck transversely by a mass. The numerical results compare well with the available experimental data. It transpires from the numerical analysis of a typical constant velocity tensile test that it is difficult to obtain a constant strain rate throughout the gauge length of a typical test specimen. It also shows that there exists an optimum specimen geometry where the strain rate variation in the gauge length is at a minimum.

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

Graduation date: 1997

In recent years, the engineering community has focused attention on selecting durable and low maintenance materials. As a result of recent advances in steel fabrication technologies, stainless steel has risen as a valuable alternative to regular carbon steel for heavy structural elements in addition to the traditional light gage structural elements of common use. The objective of this investigation is to summarize the existing literature concerning on the behavior of cold formed and hot rolled, annealed stainless steel members undergoing axial compression forces. Research related to the subject will be summarized as well as available design practice codes, from where applicable expressions will be investigated and used to perform practical examples.

Thesis (M.S.)--Virginia Polytechnic Institute, 1962. / Vita. Abstract. Includes bibliographical references (leaves 36-38). Also available via the Internet.

Thesis (M.S.)--Brigham Young University. Dept. of Mechanical Engineering, 2004. / Includes bibliographical references (p. 27-40).

Thesis (Ph. D.)--Brigham Young University. Department of Mechanical Engineering, 2004. / Includes bibliographical references (p. 49-50).

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