During tensile creep of a hot isostatically pressed (HIPed) silicon nitride, the volume fraction of cavities increases linearly with strain; these cavities produce nearly all of the measured strain. In contrast, compressive creep in the same stress and temperature range produces very little cavitation. A stress exponent that increases with stress (k x IT", 2 < n < 7) characterizes the tensile creep response, while the compressive creep response exhibits a stress dependence of unity. Furthermore, under the same stress and temperature, the material creeps nearly 100 times faster in tension than in compression. Transmission electron microscopy (TEM) indicates that the cavities formed during tensile creep occur in pockets of residual crystalline silicate phase located at silicon nitride multigrain junctions. Small-angle X-ray scattering (SAXS) from crept material quantifies the size distribution of cavities observed in TEM and demonstrates that cavity addition, rather than cavity growth, dominates the cavitation process. These observations are in accord with a model for creep based on the deformation of granular materials in which the microstructure must dilate for individual grains to slide past one another. During tensile creep the silicon nitride grains remain rigid; cavitation in the multigrain junctions allows the silicate to flow from cavities to surrounding silicate pockets, allowing the dilatation of the microstructure and deformation of the material. Silicon nitride grain boundary sliding accommodates this expansion and leads to extension of the specimen. In compression, where cavitation is suppressed, deformation occurs by solution-reprecipitation of silicon nitride.
This report documents a model to represent the true stress-strain, σ − , behavior of structural steel. It is based on combination of data from the NIST World Trade Center collapse investigation and many other evaluated literature sources. Unlike other models for stress-strain behavior of structural steel, such as the Eurocode 3 formulation [1], the model explicitly describes the time-dependent nature of the strength of steel at high temperature. For untested steels, it predicts the stress-strain behavior using only the measured room-temperature yield strength, S y. The relative deviation between the model of this report and the actual data for the steels is generally less than 25 %, and is always less than 50 %. On subset of eight steels, the model predicts the stress-strain behavior slightly better than the equally complicated Eurocode 3 model. For three literature structural steels, not analyzed as part of the model, the model of this report and the Eurocode 3 model predict stress-strain behavior with similar quality.
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