Creep Behavior of an Oxide/Oxide Composite with Monazite Coating at Elevated Temperatures

This study focuses on experimental investigation of stress-rupture behavior (creep response) of an oxide/oxide composite in a cross-ply (0/90) lay-up at elevated temperature. The test material, Nextel 610/monazite/alumina composite, employs monazite, an oxidation-resistant interfacial coating designed to improve performance at elevated temperatures. The experimental program included monotonic tensile tests to failure and creep-rupture tests at elevated temperatures. Tensile tests served to establish an ultimate tensile strength (UTS) for the material. The ensuing creep-rupture tests involved stress levels at varying percentages of the UTS. Stress-rupture curves at 900 and 1100 degrees C were established. A family of creep curves for various constant stress levels at 900 and 1100 degrees C was produced. Composite microstructure, as well as damage and failure mechanisms were also investigated

The stabilization of slip on a narrow weakening fault zone by coupled deformation-pore fluid diffusion

The transient stabilization of rapid slip on a very narrow weakening fault zone by the coupling of the deformation with pore fluid diffusion is investigated. More specifically, the fault zone is assumed to be so narrow that it can be idealized as a planar surface and the constitutive law is specified as a relation between stress on the fault τ_(f/t) and relative slip δ. The study considers only the stabilizing effect due to the time dependent response of the fluid-infiltrated elastic material surrounding the fault: the response is elastically stiffer for load alterations which are too rapid to allow for fluid mass diffusion between neighboring material elements (undrained conditions) than for those which occur so slowly that the local pore fluid pressure is constant (drained conditions). Calculations are performed to determine the length of the precursory period (the period of self-driven accelerating slip prior to dynamic instability) by assuming that the near-peak τ_(f/t) versus δ relation is parabolic and that the far-field tectonic stress rate is constant. An important result of the calculations is that the duration of the precursory period is predicted to decrease with increasing fault length for a plausible range of material parameters. Although this appears to disagree with results based on simple dimensional considerations, the result is due to the dependence of the constitutive law on a characteristic sliding distance necessary to reduce τ_(f/t) from peak to residual value. Calculated precursor times are very short, typically less than a few days for fault lengths of 1 to 5 km, a tectonic stress rate of 0.1 bar/year, and field diffusivities of 0.1 to 1.0 m^2/sec. The results are, however, sensitive to details of the τ_(f/t) versus δ relation which are, at present, poorly known

Computational Modeling of the Time-dependent Behavior of Cementitious Materials

Finite element procedures combined with microstructure development modeling are integrated to quantitatively predict the viscoelastic/viscoplastic relaxation of cement paste due to intrinsic calcium silicate hydrate viscoelasticity/viscoplasticity and microstructure evolution. The combined models are implemented in a computational routine to predict time-dependent stress and strain fields in cement paste. Besides predicting the time-dependent viscoelastic/viscoplastic properties of cement paste, the early-age desiccation shrinkage of cement paste is also computationally simulated utilizing this modeling approach. The model simulations suggest that inherent viscoelastic deformation caused by calcium silicate hydrate might not necessarily be the primary mechanism leading to the overall early-age time-dependent behavior of cement paste. The effect of time-dependent dissolution of load-bearing phases due to either the hydration reaction or the application of stress/strain can be substantial and should be considered as a significant mechanism for the apparent viscoelasticity/viscoplasticity of cement paste

Computational Modeling of the Time-dependent Behavior of Cementitious Materials

Finite element procedures combined with microstructure development modeling are integrated to quantitatively predict the viscoelastic/viscoplastic relaxation of cement paste due to intrinsic calcium silicate hydrate viscoelasticity/viscoplasticity and microstructure evolution. The combined models are implemented in a computational routine to predict time-dependent stress and strain fields in cement paste. Besides predicting the time-dependent viscoelastic/viscoplastic properties of cement paste, the early-age desiccation shrinkage of cement paste is also computationally simulated utilizing this modeling approach. The model simulations suggest that inherent viscoelastic deformation caused by calcium silicate hydrate might not necessarily be the primary mechanism leading to the overall early-age time-dependent behavior of cement paste. The effect of time-dependent dissolution of load-bearing phases due to either the hydration reaction or the application of stress/strain can be substantial and should be considered as a significant mechanism for the apparent viscoelasticity/viscoplasticity of cement paste

The influence of load holds on the fatigue behaviour of drawn Ti-6Al-4V wires

Dwell sensitivity in titanium alloys is generally attributed to the phenomenon of load shedding, which is a time dependent redistribution of stress from weak grains, with their c-axis perpendicular to the loading direction, to strong grains, with their c-axis approximately parallel to the loading direction. This leads to the formation of internal quasi-cleavage facets on the basal planes of strong grains. In this paper, the effect of load holds on the fatigue behaviour of drawn Ti-6Al-4V wires is investigated, because these wires do not contain strong α grains. It has been found that this leads to a different dwell fatigue behaviour compared to what has been described in literature. In the case of drawn wires, introducing load holds promoted crack initiation at the surface, through the formation of a facet on a prismatic plane of a surface grain that was oriented for very easy prismatic slip. This was confirmed by sectioning facets, using focused ion beam milling, and electron backscatter diffraction measurements. Because of the specific crystallographic texture of drawn wires, the phenomenon of load shedding is less pronounced, and there is no formation of internal facets. The amount of cycles to failure was reduced by two to three orders of magnitude compared to fatigue tests without load holds. The time to failure remained similar, and was even higher for some dwell fatigue tests. There was a significant amount of strain accumulation during dwell fatigue tests. The maximum strain increased more rapidly in tests with 30 s load holds compared to tests with 120 s load holds, due to creep recovery during the periods in between load holds. During these periods, the strain decreased, even though a small tensile load was still applied.publisher: Elsevier articletitle: The influence of load holds on the fatigue behaviour of drawn Ti-6Al-4V wires journaltitle: International Journal of Fatigue articlelink: http://dx.doi.org/10.1016/j.ijfatigue.2017.01.043 content_type: article copyright: © 2017 Elsevier Ltd. All rights reserved.status: publishe