A micro-mechanics model for non-isotropic, open-celled foams is developed using an elongated tetrakaidecahedron (Kelvin model) as the repeating unit cell. Assuming the cell edges possess axial and bending rigidity, the mechanics of deformation of the elongated tetrakaidecahedron lead to a set of equations for the Young's modulus, Poisson's ratio and tensile strength of the foam in the principal material directions. These equations are written as a function of the cell edge lengths and cross-section properties, the inclination angle and the strength and stiffness of the solid material. This micro-mechanics model employs an elongated Kelvin model geometry which is more general than that employed by previous authors, as the size and shape of the repeating unit cell are defined by specifying three independent dimensions. As a result, the model accounts for an additional variation in the unit cell shape which is not accounted for in the previous models. The effect of this additional shape parameter on the non-isotropic stiffness and strength behavior is demonstrated and the advantages of this more general micro-mechanics model are illustrated. Published by Elsevier Ltd.

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A study was done to determine the fatigue crack growth behavior of a PWA 1484 single-crystal nickel-base superalloy in a temperature range of 427°C to 871°C. Two distinctive failure modes were observed, which were a function of both temperature and frequency. At lower temperatures and higher frequencies crack growth occurred on the {111} octahedral slip planes at an oblique angle to the loading direction. Higher temperatures and decrease in frequencies favored angle to the loading direction. Higher temperatures and decrease in frequencies favored a Mode I type failure process. The failure mode transitions were explained by invoking arguments based on environmental damage mechanisms. The fatigue crack growth rate data were analyzed using three different crack driving force parameters. The parameters investigated consisted of the Mode I stress intensity parameter corrected for the inclined crack trajectory, and two different octahedral Mode II parameters, which are based on the calculation of resolved shear stresses on the {111} slip systems. The Mode I ΔK parameter did a fair job in correlating the data but did not collapse it into a single narrow band. The two octahedral crack driving force parameters, ΔKRSS and a newly proposed ΔKOCT, collapsed all the data into a single narrow band. In addition to correlating the fatigue crack growth rates, the two octahedral parameters also predicted the {111} planes on which the crack growth took place.

No abstract

A study was done to determine the fatigue crack growth behavior of a PWA 1484 single crystal nickel base superalloy in a temperature range of 427°C to 871°C. Two distinctive failure modes were observed which were a function of both temperature and frequency. At lower temperatures and higher frequencies crack growth occured on the {111} octahedral slip planes at an oblique angle to the loading direction. Higher temperatures and decrease in frequencies favored a Mode I type failure process. The failure mode transitions were explained by invoking arguments based on environmental damage mechanisms. The fatigue crack growth rate data were analyzed using three different crack driving force parameters. The parameters investigated consisted of the Mode I stress intensity parameter corrected for the inclined crack trajectory, and two different octahedral Mode II parameters which are based on the calculation of resolved shear stresses on the {111} slip systems. The Mode I ΔK parameter did a fair job in correlating the data but did not collapse it into a single narrow band. The two octahedral crack driving force parameters, ΔKRSS and a newly proposed ΔKOCT, collapsed all the data into a single narrow band. In addition to correlating the fatigue crack growth rates, the two octahedral parameters also predicted the {111} planes on which the crack growth took place.