Water Infusion on the Stability of Coal Specimen under Different Static Stress Conditions

Underground coal mines are frequently subjected to water infusion, resulting in many mining hazards. This study investigated the effect of water infusion on the stress and energy evolution characteristics of coal specimens representing isolated pillars under different initial axial stress conditions using the discrete element method. A water infusion distribution model was developed, in which random functions were employed to describe water distribution for the purpose of realizing the dispersion of results for a better reliability. Based on the results, a stress-level classification was presented to evaluate the water effect on pillars’ instability. For the investigated coal specimens, the water weakening effect on stress and energy remains stable when the axial geo-stress on pillars is less than 65% of uniaxial compressive strength (UCS). In contrast, when the axial stress coefficient is greater than 65%, pillars become unstable eventually. A higher axial stress coefficient is more likely to introduce a lower critical instability point of the water saturation coefficient for pillars in the process of water infusion. However, the instability point remains random to some extent for specimens following the same water distribution rule under the identical test condition. Two instability types, which also happened randomly, were observed in the numerical results for damaged coal specimens under different water saturation coefficients and axial geo-stresses, namely free-falling and step-falling.

Nonlinear Numerical Investigation Around Instability Point of the Flexible Rotor Supported by the Journal Bearing

Rotating machinery supported by the journal bearing may be subject to instability. This instability is classified as the Hopf bifurcation. Either a sub-critical bifurcation or super-critical bifurcation appears depending on the parameters of rotor systems. Conventionally, an infinitely short journal bearing model was used for the investigation of this bifurcation. In this paper, the force transmitted by the oil film of the journal bearing is represented by the finite length model. Nonlinear numerical simulation is performed at around instability point of the flexible rotor supported by the journal bearing. The bifurcation characteristic and the change of bifurcation type are investigated when the ratio L / D, which is the ratio of bearing length L to the bearing diameter D, increases.

Experimental Nondestructive Test for Estimation of Buckling Load on Unstiffened Cylindrical Shells Using Vibration Correlation Technique

Nondestructive methods, to calculate the buckling load of imperfection sensitive thin-walled structures, such as large-scale aerospace structures, are one of the most important techniques for the evaluation of new structures and validation of numerical models. The vibration correlation technique (VCT) allows determining the buckling load for several types of structures without reaching the instability point, but this technique is still under development for thin-walled plates and shells. This paper presents and discusses an experimental verification of a novel approach using vibration correlation technique for the prediction of realistic buckling loads of unstiffened cylindrical shells loaded under axial compression. Four different test structures were manufactured and loaded up to buckling: two composite laminated cylindrical shells and two stainless steel cylinders. In order to characterize a relationship with the applied load, the first natural frequency of vibration and mode shape is measured during testing using a 3D laser scanner. The proposed vibration correlation technique allows one to predict the experimental buckling load with a very good approximation without actually reaching the instability point. Additional experimental tests and numerical models are currently under development to further validate the proposed approach for composite and metallic conical structures.

Graph Degree Sequence Solely Determines the Expected Hopfield Network Pattern Stability

We analyze the effect of network topology on the pattern stability of the Hopfield neural network in the case of general graphs. The patterns are randomly selected from a uniform distribution. We start the Hopfield procedure from some pattern v. An error in an entry e of v is the situation where, if the procedure is started at e, the value of e flips. Such an entry is an instability point. Note that we disregard the value at e by the end of the procedure, as well as what happens if we start the procedure from another pattern [Formula: see text] or another entry [Formula: see text] of v. We measure the instability of the system by the expected total number of instability points of all the patterns. Our main result is that the instability of the system does not depend on the exact topology of the underlying graph, but rather only on its degree sequence. Moreover, for a large number of nodes, the instability can be approximated by [Formula: see text], where [Formula: see text] is the standard normal distribution function and [Formula: see text] are the degrees of the nodes.

The Influence of a Mistuned Blade’s Staggle Angle on the Vibration and Stability of a Shaft-Disk-Blade Assembly

The influence on coupling vibrations and stability among shaft-torsion, disk-transverse and blade-bending of a rotor system with a mistuned blade's staggle angle was investigated analytically. A shaft-disk-blade system has been found existing two types of coupling vibrations, disk-blade (DB), and blade-blade (BB) modes when the shaft was assumed rigid. If the shaft's torsional flexibility was taken into account, an additional type of coupling modes, shaft-disk-blade (SDB), appeared. When an angle-mistuned blade existed, the blades periodicity was destroyed and it was found to change not only the natural frequencies but also the types of modes. Due to blade's mistune, the shaft torsion had to participate to balance such that DB modes vanished and replaced by SDB modes. A mistuned staggle angle was numerically found to alter the natural frequencies in an almost linear trend. At last, the rotational effects were found to merge frequency loci and eventually reached an instability point. Very interestingly, a mistuned blade diminished the possible instability caused by blade-dominating modes, which existed in a perfect and periodic rotor. In words, the rotor might benefit from a mistuned blade from the stability viewpoint. The shaft-dominating mode, yet, was unaffected by the mistune and retained a possible instability.