Jinchun Liu

Experimental And Numerical Investigations Into Responses Of Buried R/C Frames Subjected To Impulsive Loadings

This paper firstly summarizes experimental results on shallow buried blast-loaded WC frames, and then uses finite element procedure to predict the responses of the tested structures. Both experiment results and numerical prediction show that the responses of buried frames under impulsive loadings are quite different from the case of long duration.

An algorithm for the grain-level modelling of a dry sand particulate system

This paper is composed of two parts: the generation of the sand particulate system and insights into the grain-level response under static and dynamic loadings. First, the algorithms for the generation of sand particles are presented, considering the randomness in their shape and distribution. Improvements to the robustness of the algorithms are obtained using controlling parameters. Second, we employ the take-and-place algorithm, placing sand grains into the specimen and checking how they overlap to form the initial model. In order to improve the porosity of the specimen, we develop the compaction algorithm: self-compaction by gravity and artificial compaction by mechanical vibration and pressure. The steps for the generation of a finite element grid are also introduced. Third, the grain-level configurations of the dry sand particulate system (aspects such as porosity, friction and contact) are taken into account in modelling. Results show that the grain-level responses of grains, i.e. deformation, fracture and damage of sand grains, impose significant effects on the mechanical behavior of dry sand under static and dynamic loadings.

AN IMPROVED ANALYTICAL METHOD FOR RESTRAINED RC STRUCTURES SUBJECTED TO STATIC AND DYNAMIC LOADS

Once a RC structure is laterally restrained, both the static and dynamic load resistances will be enhanced due to the membrane action. Despite this known advantage, the apparent lack of systemic and efficient methods of analysis poses a drawback in the design and assessment of blast-resistant RC structures. First, a simplified membrane action theory was presented by modifying the maximum membrane force design method (MMFM) for predicting the total static resistance-deflection curves of restrained beam-slab RC structures. Second, a series of constrained beams were tested to validate the new theory, for which better agreement was observed between the test data, the results predicted by the proposed theory and those by MMFM. The results show that the static load carrying capacity and membrane force increase with increasing restraint stiffness, and the smaller the reinforcement ratio is, the larger the load carrying capacity increases. Third, based on the improved compressive static membrane action theory, a new analytical method was developed to investigate the dynamic responses of restrained RC structures subjected to blast loads, using an equivalent single degree of freedom system that combines the three-parameter elasto-viscoplastic rate-sensitive material model with the proposed static theory. Good agreement is observed between the test data and the analytical results. Finally, it is demonstrated that the dynamic resistance capacity increases with increasing load rate and restraint stiffness and with decreasing tensile reinforcement ratio, but the larger the dynamic resistance is, the larger the plastic deformation of the structure.

Mechanism of anomalous low friction phenomenon in deep block rock mass

Abstract Deep rock mass has the unique “self-stressed” block-hierarchical structure, anomalous low friction (ALF) was one of the typical nonlinear geo-mechanical and dynamic responses in deep block rock mass, which occurred as the result of movements of large-scale geo-blocks under the impact of external pulses (such as a deep confined explosion, earthquakes, rock bursts and etc.). ALF phenomenon obtained its name to describe the curious phenomenon that the friction between interacting geo-blocks quasi-periodically disappears at some discrete points in time along the direction orthogonal to the direction of the external pulse. With the objective to confirm the existence of the ALF phenomenon and study the geo-mechanical conditions for its occurrence experimentally and theoretically, laboratory tests on granite and cement mortar block models were carried out on a multipurpose testing system developed independently. The ALF phenomenon was realized under two loading schemes, i.e., blocks model and a working block were acted upon jointly by the action of a vertical impact and a horizontal static force, as well as the joint action of both vertical and horizontal impacts with differently delayed time intervals. We obtained the rules on variation of horizontal displacements of working blocks when the ALF phenomenon was realized in two tests. The discrete time delay intervals, corresponding to local maxima and minima of the horizontal displacement amplitudes and residual horizontal displacements of the working block, satisfied canonical sequences multiplied by ( 2 ) i . Some of these time intervals satisfied the quantitative expression ( 2 ) i Δ / V P . At last, 1D dynamic theoretical model was established, the analytical results agreed better with the test data, while the quantitative expression drawn from test data was not validated well in theoretical analyses.

3D Numerical Investigation of Cement Mortar with Microscopic Defects at High Strain Rates

AbstractThis paper develops a three-dimensional (3D) microscopic model to investigate the mechanical response of cement mortar with random defects at high strain rates. Ellipsoids with randomness in size, shape, and spatial distribution are used to simulate the defects in mortar matrix. First, we propose the steps to generate the ellipsoid. Second, a “take and place” algorithm is employed to generate a model of cement mortar composed of defects. The mapping algorithm is used to generate a finite-element grid. In finite-element modeling, the material model is used in an advanced general-purpose multiphysics simulation software package to simulate the nonlinear behavior of mortar matrix with strain rate effects. Numerical simulations of the specimen under static loadings agree well with test observations, which reveal that the proposed 3D microscopic model and finite-element analytical approach can give reliable predictions. Finally, numerical studies are conducted, focusing on the effect of the defects on ...

Numerical simulations of shaped charge jet penetration into concrete-like targets

Shaped charge jet has been widely applied in the military and energy sources’ extraction fields; while the related investigations on the shaped charge jet penetration into concrete-like target are still limited, a series of numerical simulation works are conducted in this article. Holmquist–Johnson–Cook and Johnson–Cook models are used to describe the concrete-like targets and metal liner/casing of the shaped charge, respectively. The whole process including the formation, elongation in the air, and penetration into concrete-like target of shaped charge jet is reproduced using the multi-material arbitrary Lagrange–Euler algorithm and fluid–structure interaction method implemented in LS-DYNA. Simultaneously, the striking velocities of the jet (both tip and tail) and the damage of target (diameter and depth of penetration borehole) are derived. The above constitutive models, the corresponding material parameters, and the finite element algorithms are validated by comparing with the available tests’ data. The analyses of parametric influences are further performed. It indicates that for the unfragmented shaped charge jet, the penetration depth increases and the average penetration borehole diameter decreases with the standoff distance increasing, respectively; the compressive strength of concrete target has slight influence on the penetration depth of shaped charge jet; the diameter of shaped charge jet penetration borehole with aluminum liner is larger, while that with copper liner has a deeper penetration depth. It can also be found that the influence of explosive type on the penetration performance of shaped charge jet is negligible at small standoff distance, while the explosive LX-14 performs better than explosives Octol, B, and 8701 at larger standoff distance.

Mechanism of Pendulum-type wave phenomenon in deep block rock mass

Abstract Pendulum-type (μ wave) wave is a new type of elastic wave propagated with low frequency and low velocity in deep block rock masses. The μ wave is sharply different from the traditional longitudinal and transverse waves propagated in continuum media and is also a phenomenon of the sign-variable reaction of deep block rock masses to dynamic actions, besides the Anomalous Low Friction (ALF) phenomenon. In order to confirm the existence of the μ wave and study the rule of variation of thisμ wave experimentally and theoretically, we first carried out one-dimensional low-speed impact experiments on granite and cement mortar blocks and continuum block models with different characteristic dimensions, based on the multipurpose testing system developed by us independently. The effects of model material and dimensions of models on the propagation properties of 1D stress wave in blocks medium are discussed. Based on a comparison and analysis of the propagation properties (acceleration amplitudes and Fourier spectra) of stress wave in these models, we conclude that the fractures in rock mass have considerable effect on the attenuation of the stress wave and retardarce of high frequency waves. We compared our model test data with the data of in-situ measurements from deep mines in Russia and their conclusions. The low-frequency waves occurring in blocks models were validated as Pendulum-type wave. The frequencies corresponding to local maxima of spectral density curves of three-directional acceleration satisfied several canonical sequences with the multiple of √2 most of those frequencies satisfied the quantitative expression (√2)i Vp/2Δ.