Numerical Analysis of Ratcheting Under Bending- Membrane Loading Conditions

Ratcheting is a vital failure mode when dynamic loading is present in the scenario and it can lead to fatigue or incremental collapse if not restricted. The purpose of this study is to propose a ratchet diagram for primary bending and secondary membrane loading conditions. For this, a finite element analysis model of a rectangular beam is prepared and solved by the numerical analysis software ‘ABAQUS’. The FEA model is validated by carried out a dynamic nonlinear elastic-plastic analysis with the analytical solution of Yamashita et al. for similar loading conditions. The ratchet occurrence conditions plotted in a non-dimensional stress parameter plot similar to the Bree diagram. The findings suggest that secondary stress rises for the occurrence of ratchet conditions as primary stress decreases. It also found a strong frequency dependency feature. The nature of the input frequency of cyclic loading in the proposed ratchet diagram has been discussed in terms of dynamic displacement over static displacement in the change of non-dimensional frequency of the loading.

From the “Russian Spring” to the Armed Insurrection: Russia, Ukraine and Political Communities in the Donbas and Southern Ukraine

The article is a study of anti-government mobilization in the cities of southern and eastern Ukraine in spring 2014. By closely examining the developments that preceded the outbreak of the armed insurrection in the Donbas, the study seeks to elucidate the various factors that precipitated the veritable collapse of the Ukrainian state in parts of the Donetsk and Luhansk regions and its stabilization elsewhere. The article argues that the armed conflict in the Donbas was hardly a predetermined outcome of the Russian government strategy—which was employed also outside the Donbas—but rather a product of a synergetic confluence of several structural and conjunctural factors that were absent or present to a much smaller degree elsewhere. These included the peculiar political and ethno-cultural profile of Donetsk and Luhansk oblasts; a higher degree of the legitimacy crisis of the interim government; the destabilizing effects of the status quo created by the victory of the Euromaidan—not only in terms of the change of Ukraine’s geopolitical orientation and Russia’s apparently compromised interests, but also in terms of the perceived change of status of different ethno-political communities; the proximity of the Russian border, trans-border ethnic politics, and the activities of nationalist groups from Russia; the residual influence of the once-powerful networks associated with clients of the former president Viktor Yanukovych; the relative weakness of organized pro-Ukrainian groups; and last but not least, the incremental collapse of the law enforcement apparatus, which drastically reduced the capacity of Ukrainian authorities.

Prediction of Ratchet Boundary for 90-Degree Smooth and Mitred Pipe Bends

The present paper attempts to predict ratchet boundary for 90-degree mitred and smooth pipe bends subjected to sustained pressure and cyclic in-plane bending. The methodology utilizes a recently published technique known as the “Uniform Modified Yielding” (UMY) technique, which relies on generation of a virtual structure with inhomogeneous reduced yield strength, whose magnitude and distribution depend on the elastic stress field due to the cyclic load. The collapse load of this virtual structure determines the threshold steady load necessary for commencement of “incremental collapse”. The technique is applied first to predict ratchet boundaries for two benchmark problems possessing analytical descriptions of ratchet boundary and uni-axial states of stress; the two-bar structure problem and the Bree cylinder. Predicted ratchet boundaries exactly coincided with the corresponding published analytical descriptions, and reasons for this correlation were discussed in this paper. The technique was then applied to three 90-degree pipe bends with similar geometries as follows: smooth pipe bend (SPB), single mitred pipe bend (SMPB), and three weld mitred pipe bend (3WMPB). Certain assumptions are adopted to enable treatment of the problem as a quasi-uniaxial one. Conservative estimates are obtained for ratchet boundaries in pipe bends that correlates well with elastic shakedown/ratchet boundary of the same problems as predicted by a recently developed non-cyclic direct technique.

On Dynamic Cycle Collapse of Circular Plates

The upper bound kinematic method, which is based on a reduced kinematic formulation and involves construction of fictitious elastic moment fields and potential incremental collapse mechanisms, is used to evaluate the dynamic cycle collapse loads for a symmetrically loaded circular plate. The respective nonshakedown curves are constructed, A point load effect is discussed.