Instability Phenomena in the Neogene Deposits of Northern Peloponnesus, Greece

 The geomorphic and instability conditions of the Neogene formations in Northern Peloponnesus are examined and related to the lithologies and hydrological data. Instability phenomena caused by erosion are briefly described and a qualitative analysis of the run-off conditions within the Finikas and Meganeitos catchments is presented. Received: 31 July 1997 · Accepted: 12 January 1998     

射流参数对加热条件下液膜射流破碎不稳定性的影响(2)--射流参数对液膜对称模式破碎的影响

利用对具有温度梯度的粘性液膜射流模型进行的不稳定性计算结果，研究了实际射流参数（射流速度）气体密度，液体粘性，液膜厚度，温度梯度，液体种类等）对对称模式扰动作用下液膜射流大，小尺度破碎模式破碎特征的影响规律，并探讨了决定液膜射流破碎尺度的实际因素。     

The effect of roll and clad sheet geometry on the necking instability during rolling of clad sheet metals

Experimental evidence is presented showing that the rolling strain required to produce internal necking in clad sheet depends both on the volume fraction of each constituent in the clad and the geometry of the rolls. This result was not predicted by earlier localization analysis that approximates the stress/strain field in rolling with that of plane strain compression. The evolution of hardness during rolling and complementary finite element results point to redundant shearing of the soft phase as the reason for this behavior. Such redundant shear differentially strengthens the softer component of the clad and reduces the induced tensile stress in the harder component, delaying the localization. Therefore, unwanted strain localization in rolling of clad sheets can be delayed significantly by the use of small radius rolls in configurations such as a Sendzimir mill.     

Variations of instability in a rotating spindle system with various bearings

To drive the speed of spindle faster and faster, especially for micro-via-drilling, the gas bearing–spindle is a must. However, most investigations of the dynamic characteristics of the spindle system are limited to the ball bearing type of spindle. This work examines the dynamic instability of a rotating spindle system with various bearings to elucidate the difference between the ball and gas bearing–spindle systems. A round Euler–Bernoulli beam is used to approximate the spindle. The Hamilton principle is applied to derive the equation of motion for the spindle system, and the multiple scales perturbation method is employed to solve the instability solution of the system. The effects of bearing types and speeds of rotation on the dynamic characteristics and instability of a rotating spindle system are further studied.     

Evidence of scuffing initiation by adiabatic shear instability

A mechanism for the initiation of scuffing based on adiabatic shear instability was assessed experimentally through the measurement of dislocation densities by X-ray diffraction. Scuffing was predicted to occur when the rate of local thermal softening exceeded that of work hardening in the tribological contact. Since these rates were dependent on microstructure, the theory was tested by performing scuffing tests on SAE 4340 steel subjected to five different heat treatments. The scuffing resistances were consistent with the theory, and thermal events during the tests and the resulting microstructural morphologies were consistent with a locally adiabatic environment. The dislocation densities measured in samples from tests stopped before, during, and after scuffing were consistent with the theory. Use of the Advanced Photon Source (APS) at Argonne National Laboratory was supported by the U.S. Department of Energy under Contract W-31-109-ENG-38.     

Prediction of dimensional instability resulting from layer removal of an internally stressed orthotropic composite cylinder

When a layer of cylindrical composite component containing an axisymmetric residual stress state is removed from the inner or outer surface, the dimension of the remaining material changes to balance internal forces. Therefore, in order to machine cylindrical composite components within tolerances, it is important to know dimensional changes caused by residual stress redistribution in the body. In this study, analytical solutions for dimensional changes and the redistribution of residual stresses due to the layer removal from a residually stressed cylindrically orthotropic cylinder were developed. The cylinder was assumed to have axisymmetric radial, tangential and axial residual stresses. The result of this study is useful in cases where the initial residual stress distribution in the component has been measured by a non-destructive technique such as neutron diffraction with no information on the effect of layer removal operation on the dimensional changes.     

Instability detection of ARMA systems based on AR system identification

The ability of parametric autoregressive (AR) system identification methods to detect the instability of an autoregressive moving average (ARMA) system of an unknown order is investigated. The collection of least squares AR estimators of various orders is shown to have the capacity to detect the instability of the underlying system. Necessary information is not the order of the system but, instead, an upper bound of the number of unstable poles with the maximal magnitude outside the unit circle.     

Prediction of longitudinal combustion instability in a solid-propellant rocket motor

The main objective of this technical paper is to demonstrate a capability which can predict the linear stability in a solid-propellant rocket motor using approximate stability analysis result which is based on spatial and temporal averaging of the equations for two-phase flow. The stability history of a rocket motor during burning can be showed along the time. To do this, a performance prediction of a rocket motor should be also carried out. The results derived from two sample calculations are presented and the limits of this analysis are discussed. This analysis is only for the longitudinal mode acoustic combustion instability in solid-propellant rocket motor.     

The relation between the state indices and the characteristic features of undrained behaviour of silty sand

The characteristic features of a series of isotropically and K₀-consolidated undrained triaxial compression behaviour of a silty sand were investigated for a range of initial void ratio ($e)$ and mean effective confining stress ($p^{\prime }$). The silty sand used in this study contained about 10% natural fines. The critical state line (CSL) of K₀-consolidated specimens, K₀U was slightly lower than the CSL for isotropically consolidated specimens, CIU. The respective CSLs for K₀U and CIU were used to define state indices, such as state parameter ($\psi )$, state index ($I_s$), state pressure index ($I_p$) and modified state parameter (${\psi }_m$), within critical state soil mechanics (CSSM) framework. It was found that each state index exhibit a unique relation with liquefaction potential, irrespective of consolidation type, however different relationships were observed between state indices and the stress ratio at the triggering of liquefaction, ${\eta }_{\mathit{IS}}$ or the liquefaction resistance, $q_{\mathit{IS}}$. The correlation of characteristic features of undrained shearing (i.e., liquefaction potential, ${\eta }_{\mathit{IS}}$ and $q_{\mathit{IS}}$) and drained shearing (maximum rate of volume change, ${\left(d{\epsilon }_v/d{\epsilon }_a\right)}_{\mathit{m}\mathit{a}\mathit{x}}$) with state indices were compared statistically in terms of root mean square deviations (RMSD). All characteristic features of undrained shearing generally showed the best correlation with $I_p$ in term of RMSDs, however $\psi$ showed comparatively wider scatter for the specimens showing flow and limited flow behaviour.