Papers by Author: Li Qun Tang

Abstract: Deformation and failure of meso-structures take great effect on the loading and energy absorption of aluminum foam under impact. We designed a split Hopkinson pressure bar (SHPB)-high speed digital camera system to monitor the meso-deformation and failure features, and measure the nonuniformity of deformation of aluminum foam under impact. The meso-deformation and failure of aluminum foam were observed successfully by the system, and it showed that there does exist remarkable nonuniform deformation along the specimen. In order to expand the experimental results, the specimen of aluminum foam with meso-structures is modeled by 3D Voronoi technique. The numerical results show that the FE model can simulate the experiment well, and shows that nonuniformity of deformation appears in aluminum foam specimen significantly. The analysis indicates that the assumption of uniform deformation of specimen in SHPB cannot be strictly satisfied for the material of aluminum foam.

Abstract: Based on Biot’s elastodynamic theory for poroelastic media, the dynamic response of a poroelastic half-space due to a time-harmonic concentrated vertical load applied at the free surface is investigated. Different from previous treatments of the free surface as either fully permeable or fully impermeable, the free surface of a pororelastic half-space is treated in this study as a more realistic semi-permeable boundary condition, i.e. the permeability of the free surface is considered. The governing equation for axisymmetric motion of a poroelastic half-space is solved by applying the Hankel integral transform. Numerical results are presented to show the effects of semi-permeable boundary condition on the dynamic response of poroelastic half-space.

Abstract: In this paper coupling damage behaviors of Liquid Rubber Based Concrete (LRBC) are studied experimentally. Compressive fatigue and impact alternant loading tests were carried out on cylindrical LRBC specimens. The elastic moduli were recorded before and after fatigue and impact tests. The quasi-static compressive stress-strain curves after fatigue and impact tests were obtained. According to the definition of dissipated energy, the cumulating of dissipated energy was used to define damage. The analysis on impact and fatigue damages during the loading processes show that fatigue and impact loading lead to the formation and development of inner damage. During the alternant loading process, impact and fatigue damages are coupled mutually, with the former affects the evolution of fatigue damage evidently.

Abstract: In the light of the mechanism of early cracking in a batch of precast reinforced concrete hollow slabs of four certain bridges in construction whose midspan appears transverse cracks, investigation ranging from method of construction to distribution of cracks are conducted in situ. The possibility of the occurrence of cracks is discussed in detail. Theoretical calculation as well as numerical analysis reveals that the main reason of formation early cracking in slabs is self-weight loading. In order to assess the performance of the bridge deck system which appears cracks and evaluate the effect of early cracking on structural behavior, numerical analysis is performed to conclude that the bridge deck is satisfied with the serviceability requirements. In view of the fact that this kind of slab is widely used in bridge projects for convenience in construction and advantage in cost, effective and economical crack control measurement and technical proposal are recommended with regard to the design, construction and amendment of design specifications.

Abstract: Flexure behaviors of plain concrete (PC), steel fiber reinforced concrete (SFRC), polymer modified concrete (PMC), steel fiber reinforced and polymer modified concrete (SFRPMC) and hybrid fiber reinforced concrete (HFRC) with steel fiber and polymer fiber are studied in this paper, flexure tests were carried out and flexure strengths of the five different materials with different mixture ratios were measured and compared. Flexure ductility of PC, PMC, SFRC, and SFRPMC were calculated and compared. In addition, considering performance and cost estimation comprehensively, HFRC is recommended, preliminary tests show that HFRC may be one of the potential materials for bridge pavement.

Abstract: Damage behaviors of plain concrete (PC), steel fiber reinforced concrete (SFRC), steel fiber reinforced and polymer modified concrete (SFRPMC) are studied in this paper by use of a Split Hopkinson Pressure Bar (SHPB). Three kinds of concrete materials appear obvious strain rate strengthening effects. SFRPMC appears a better resistance and energy absorption ability. A rate-dependent damage model is suggested to depict the impact damage evolution of three kinds of materials under different impact velocities. The simulation results showed the theoretical model could well describe the dynamic behaviors of the three kinds of materials, and steel fibers attribute more to resist crack develop in early stage, “bridge effect” of steel fibers slow up the damage evolution in SFRC, with the addition of polymer, the internal structures of SFRPMC were modified, SFRPMC gains better ductility, and appears a kind of “softening effect”, which makes the damage in SFRPMC develop more slowly than that in PC and SFRC under impact loading.

Abstract: The constitutive relation for open-celled metal foams with random characteristics of cells was constructed based on the mechanical behavior and the distribution of the cells, which implied the effect of the mesoscopic characteristics of the cells on the macroscopic behavior of the foam. The constitutive relation was able to represent the whole three phases of the stress-strain curve of the open-celled metal foam with merely one expression. Besides, the explicit expressions for the foam’s yield strain and yield stress were supplied. Experimental data was employed to check the constitutive relation. It was found that the constitutive relation was able to represent accurately the whole compression process of the foams, and the calculated yield points had a good agreement with the experimental results.

Abstract: The experimental studies on the static and dynamic mechanical properties of aluminium foam material are presented first. Finite element models of four structures, including circular tube filled and bonded with aluminium foam, circular tube filled but unbonded with aluminium foam, single aluminium foam column and empty aluminium tube, under dynamic transverse compression are established by FEMB code. The dynamic mechanical behaviors of the structures are analyzed using LS-DYNA finite element code. The simulating results at certain cases are compared with experimental measurements and the satisfying consistency confirmed the validity of the model. The further numerical simulations are carried on the dynamic mechanical behaviors of four structures with outer tubes of different wall-thickness. It is found that aluminium foam filling can greatly improve the load-bearing capacity and energy-absorbing efficiency of structures. On the other hand, the effect of the aluminium outer tube on the structure is obvious compared with single aluminium foam column, in spite of the foam core and the tube are bonded together or unbonded. Another result can be seen that the bonding between the foam and outer tube affects the structure weakly for both thinner and thicker tubes. Finally, the simulating results show that the thicker wall of tube can improve the load-bearing capacity and energy-absorbing ability of the structure.

Abstract: Impact tests were carried out by use of a 74-mm-diameter split Hopkinson pressure bar to investigate the impact damage behaviors of plain concrete (PC), steel fiber reinforced concrete (SFRC) and steel fiber reinforced and polymer modified concrete (SFRPMC). The results show that all three kinds of materials appear strain rate strengthening effects, and SFRPMC appears a better impact resistance and energy absorbing ability than PC and SFRC. Based on the analysis of experimental results, a rate-dependent damage model is suggested to depict the dynamic behaviors of SFRC and SFRPMC, which derives the impact damage evolution of the materials. It shows that the damages in SFRPMC develop more slowly than that in SFRC.