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Fatigue tests on compact tension specimens of 90-10 copper nickel were conducted in 3.5% NaCl solutions. Anodic or cathodic currents were applied during testing. Anodic currents decreased and cathodic currents increased the fatigue life. Both anodic and cathodic currents changed the fracture mode from predominantly transgranular to intergranular. Constant extension rate tests were performed on similar CT specimens in environments of 3.5% NaCl solution and 3.5% NaCl solution titrated to pH 1.0 with various levels of applied current. The environment had little influence on the monotonic failure of 90-10 copper nickel. Polarization studies were conducted to determine the effects of welding and pH on the corrosion behavior of 90-10 copper nickel. The rate of corrosion was less in the weld and the heat affected zone than in the base metal. As the pH of the environment was lowered, the corrosion rate of 90-10 copper nickel increased due to the retardation of film formation and repassivation. These studies showed that three different mechanisms of corrosion fatigue were likely: localized anodic dissolution, surface film rupture and hydrogen embrittlement. The dominance of one mechanism over the other two depends on the applied current. No evidence of susceptibility to stress corrosion cracking was found, therefore, a true corrosion fatigue process is operative in 90-10 copper nickel. / Master of Science / incomplete_metadata

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1981. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Includes bibliographical references. / by Carl J. White. / M.S.

Mode I and Mode III fatigue tests were performed on copper nickel alloys in helium, salt water environments. The hydrogen, oxygen, two alloys used air and in this investigation were 90-10 and 70-30 copper nickel. Both alloys contained iron which was added to improve the erosion corrosion resistance. The extent of cracking varied with the test environment. Tests showed that oxygen and humid air promoted cracking while salt water helium was used as the baseline retarded cracking when environment. Hydrogen promoted cracking when compared to helium but retarded cracking if comparisons were made with oxygen or humid air. The environmental effects (helium as the base case} in the Mode I tests in gaseous environments were manifested in the form of shorter fatigue lives, easier crack initiation, marginally higher crack growth rates and the development of intergranular fracture at the surface. These effects were accompanied by a change in the near surface deformation characteristics. The increases in fatigue life induced by testing in aqueous environments were greatly extended if the copper nickel was galvanically coupled to steel. Mode III tests showed the same ranking of environmental effects as Mode I tests and also showed multiple initiation, brittle fracture and secondary cracking. Two models were proposed to explain the observed results. One was based on the dilation-aided diffusion of oxygen ahead of the crack tip and subsequent oxidation of internal iron particles. The oxidation caused a volume expansion which produced internal tensile strains and facilitated fracture. The other mechanism was based on dilation-aided transport of hydrogen with subsequent accumulation of hydrogen at interfaces, resulting in a lowering of the interfacial strength and promoting intergranular fracture. The observed increases in life in the aqueous environments were rationalized by the reduced oxygen content available in the stagnant solutions. These observations suggest that the presence of iron accelarates fatigue in copper nickel alloys exposed to aggressive environments. Thus, any application involving fatigue loading with simultaneous exposure to aggressive environments should attempt to ensure that the iron content of the copper nickel alloys is minimized. / Doctor of Philosophy

The goal of this work is to establish a life prediction methodology for thermomechanical loading of the Ni-base superalloy Mar-M247 over a larger temperature range than previous work. The work presented in this thesis extends the predictive capability of the Sehitoglu-Boismier unified thermo-viscoplasticity constitutive model and thermomechanical life model from a maximum temperature of 871C to a maximum temperature of 1038C. The constitutive model, which is suitable for predicting stress-strain history under thermomechanical loading, is adapted and calibrated using the response from isothermal cyclic experiments conducted at temperatures from 500C to 1038C at different strain rates with and without dwells. In the constitutive model, the flow rule function and parameters as well as the temperature dependence of the evolution equation for kinematic hardening are established. In the elevated temperature regime, creep and stress relaxation are critical behaviors captured by the constitutive model. The life model accounts for fatigue, creep, and environmental-fatigue damage under both isothermal and thermomechanical fatigue. At elevated temperatures, the damage terms must be calibrated to account for thermally activated damage mechanisms which change with increasing temperature. At lower temperatures and higher strain rates, fatigue damage dominates life prediction, while at higher temperatures and slower strain rates, environmental-fatigue and creep damage dominate life prediction. Under thermomechanical loading, both environmental-fatigue and creep damage depend strongly on the relative phasing of the thermal and mechanical strain rates, with environmental-fatigue damage dominating during out-of-phase thermomechanical loading and creep damage dominating in-phase thermomechanical loading. The coarse-grained polycrystalline microstructure of the alloy studied causes a significant variation in the elastic response, which can be linked to the crystallographic orientation of the large grains. This variation in the elastic response presents difficulties for both the constitutive and life models, which depend upon the assumption of an isotropic material. The extreme effects of a large grained microstructure on the life predictions is demonstrated, and a suitable modeling framework is proposed to account for these effects in future work.

A model was created to predict the thermo-mechanical fatigue crack growth rates under typical engine spectrum loading conditions. This model serves as both a crack growth analysis tool to determine residual lifetime of ageing turbine components and as a design tool to assess the effects of temperature and loading variables on crack propagation. The material used in the development of this model was a polycrystalline superalloy, Inconel 100 (IN-100). The first step in creating a reliable model was to define the first order effects that influence TMF crack growth in a typical engine spectrum. Load interaction effects were determined to be major contributors to lifetime estimates by influencing crack growth rates based upon previous load histories. A yield zone model was modified to include temperature dependent properties that controlled the effects of crack growth retardation and acceleration based upon overloads and underloads, respectively. Multiple overload effects were included in the model to create enhanced retardation compared to single overload tests. Temperature interaction effects were also considered very important due to the wide temperature ranges of turbine engine components. Oxidation and changing temperature effects were accounted for by accelerating crack growth in regions that had been affected by higher temperatures. Constant amplitude crack growth rates were used as a baseline, upon which load and temperature interaction effects were applied. Experimental data of isolated first order effects was used to calibrate and verify the model. Experimental data provided the means to verify that the model was a good fit to experimental results. The load interaction effects were described by a yield zone model, which included temperature dependent properties. These properties were determined experimentally and were essential in the model's development to include load and temperature contributions. Other interesting factors became apparent through testing. It was seen that specific combinations of strain rate and temperature would lead to serrated yielding, discovered to be the Portevin-Le Chatelier effect. This effect manifested itself as enhanced hardening, leading to unstable strain bursts in specimens that cyclically yielded while changing temperature.