[1] In order to investigate the processes responsible for the attenuation of seismic shear waves in the Earth's upper mantle, four olivine polycrystals ranging in mean grain size d from 3 to 23 mm have been fabricated, characterized, and mechanically tested in torsion at high temperatures and seismic frequencies. Both the shear modulus, which governs the shear wave speed V S , and the dissipation of shear strain energy Q À1 have been measured as functions of oscillation period T o , temperature T, and, for the first time, grain size. At sufficiently high T all four specimens display similar absorption band viscoelastic behavior, adequately represented for 1000 < T < 1200 or 1300°C and 1 < T o < 100 s, by the expression] a with A = 7.5 Â 10 2 s Àa mm a , a = 0.26 and E = 424 kJ mol À1 . This mildly grain-size-sensitive viscoelastic behavior of melt-free polycrystalline olivine is attributed to a combination of elastically and diffusionally accommodated grain boundary sliding, the latter becoming progressively more important with increasing T and/or T o . Extrapolation to the larger (mm-cm) grain sizes expected in the Earth's upper mantle yields levels of dissipation comparable with those observed seismologically, implying that the same grain-size-sensitive processes might be responsible for much of the observed seismic wave attenuation. The temperature sensitivity of V S is increased substantially by the viscoelastic relaxation allowing the lateral variability of wave speeds to be associated with relatively small temperature perturbations.

[1] Five melt-bearing polycrystalline olivine aggregates have been newly prepared by hot isostatic pressing and tested at high temperature and pressure with torsional forcedoscillation and microcreep methods. Cylindrical specimens, varying in average grain size from 7 to 52 mm, were annealed and then tested during slow staged cooling under 200 MPa pressure from maximum temperatures of 1240-1300°C where they contained basaltic melt fractions ranging from $0.0001 to 0.037. For temperatures !1000°C, pronounced departures from elastic behavior are evident in strain energy dissipation Q À1 and associated dispersion of the shear modulus G. In marked contrast with the high-temperature viscoelastic behavior of melt-free materials, a broad dissipation peak is observed for each of the melt-bearing specimens -superimposed upon a melt-enhanced level of monotonically frequency-and temperature-dependent ''background'' dissipation. The oscillation period at which the peak is centered decreases systematically with increasing temperature. A ''global'' model comprising an Andrade-pseudoperiod background plus Gaussian peak accounts adequately for the variation of Q À1 with frequency, temperature, average grain size and melt fraction. In the following paper (Part II) a microstructural explanation for the observed viscoelastic behavior is sought and the global model is used to extrapolate the experimental data to the conditions of teleseismic wave propagation in the Earth's upper mantle.

No abstract

No abstract

No abstract

We study the effect of adding localised stiffness, via a spring support, on the stability of flexible panels subjected to axial uniform incompressible flow. Applications are considered that range from the hydro-elasticity of hull panels of high-speed ships to the aero-elasticity of glass panels in the curtain walls of high-rise building in very strong winds. A two-dimensional linear analysis is conducted using a hybrid of theoretical and computational methods that calculates the system eigen-states but can also be used to capture the transient behaviour that precedes these. We show that localised stiffening is a very effective means to increase the divergence-onset flow speed in both hydro-and aero-elastic applications. It is most effective when located at the mid-chord of the panel and there exists an optimum value of added stiffness beyond which further increases to the divergence-onset flow speed do not occur. For aero-elastic applications, localised stiffening can be used to replace the more destructive flutter instability that follows divergence at higher flow speeds by an extended range of divergence. The difference in eigen-solution morphology between aero-and hydro-elastic applications is highlighted, showing that for the former coalescence of two non-oscillatory divergence modes is the mechanism for flutter onset. This variation in solution morphology is mapped out in terms of a non-dimensional mass ratio. Finally, we present a short discussion of the applicability of the stabilisation strategy in a full three-dimensional system.

A state-space model, based upon computational modeling, is used to investigate the hydroelastic stability of a finite flexible panel interacting with a uniform flow. A merit of this approach is that it allows the fluid-structure system eigenmodes to be found readily when structural inhomogeneity is included or a source of external excitation is present. The system studied herein is two-dimensional although the concepts presented can be readily extended to three dimensions. Two problems are considered. In the first, we solve the initial-value, boundary-value, problem to show how the system response evolves from a source of localized excitation. This problem is deceptively complex and has evidenced some very unusual behaviour as demonstrated by theoretical studies based on the assumption of an infinitely long flexible panel. Our contribution herein is to formulate and illustrate the use of a hybrid of theoretical and computational models that includes the effects of finiteness. In the second problem we solve the boundary-value problem to determine the long-time response and investigate the effects of adding localized structural inhomogeneity on the linear stability of a flexible panel. It is well known that a simple flexible plate first loses its stability to divergence that is replaced by modal-coalescence flutter at higher speeds. Our contribution is to show how the introduction of localized structural inhomogeneity can be used to modify the divergence-onset and flutter-onset critical flow speeds.

Nowadays modern high-rise buildings have unique facades which partly rely on the incorporation of curtain walls. A curtain-wall system encloses the building to separate the internal and external environments. It can reduce building weight and it also transfers the wind load to the floor structure of the building. Wind-load codes govern the design of safe curtain wall systems against natural wind forces, considering direct static and dynamic pressure. In this paper aero-elastic considerations are investigated as a potential failure mode of curtain walls. Curtain-wall panels are regarded as comprising a flexible material such as glass and aluminium cladding subjected to an airflow that is parallel to their surface.It is well-known that a flexible panel exposed to increasingly high flow speed will succumb to a divergence, or static buckling-type, instability at a particular critical flow speed. At a higher flow speed the panel will experience violent oscillatory flutter-type instability. Accordingly, we investigate the susceptibility of curtain-wall panels to aero-elastic effects.A state-space model, based upon computational modelling, is used to investigate the aero-elastic stability of each flexible panel in isolation. We briefly present a recently developed approach and its new extension to theoretical modelling of the fully-coupled interaction between a simply-supported flexible panel and a fluid flow. We solve the boundary-value problem to determine the long-time response and investigate the effects on stability of adding localised structural inhomogeneity.Localised structural inhomogeneity is incorporated as an additional single spring type support to the panel. The dependence of instability onset-flow speeds, and the forms of divergence and flutter instabilities, upon the added spring stiffness and its location are then investigated. Results show that the morphology of the unstable solution space significantly differs from that of the oft-studied corresponding hydro-elasticity problem because of the different density ratio between fluid and solid media. Of particular interest is that in the present aero-elastic system flutter occurs through the coalescence of two non-oscillatory unstable divergence modes.The inclusion of a localised spring support to an otherwise unsupported panel is shown to be stabilising with respect to the critical divergence-onset flow speed and the limits to this strategy are identified. This strategy is marginally destabilising with respect to the more damaging flutter instability that occurs at higher wind speeds. However, at a sufficiently high spring-stiffness a sudden change to the solution morphology occurs that yields two unstable non-oscillatory divergence modes and flutter is postponed to much higher wind speeds. We close the paper with an assessment of what these results mean in dimensional terms as applied to different cladding panels. Overall, our results suggest a means to ameliorate adverse aero-elastic effects in potentially disastrous extreme wind-force situations such a...