Papers by Author: Zhi Ping Tang

Abstract: The brittle to ductile failure mode transition and the formation of adiabatic shear bands (ASB) of polypropylene were observed at high strain rate impact loading. The dynamic experiments were conducted using a split Hopkinson pressure bar (SHPB) set-up with hat-shaped specimens. The post-test observations of the recovered specimens were performed by polarized light microscopy. The mechanical behaviors of specimens are strongly influenced by temperature, the strength of specimen decreases with increasing temperature. Furthermore, the specimen fractures as a brittle solid at room temperature (20°C), while at a high temperature (100°C), the specimen fractures on a ductile model. At high temperature impact tests, shear bands are observed in the shear zone of the hat-shaped specimen, and the cracks formed at the corners propagate along the shear bands.

Abstract: A new type of shock absorber was designed. The shape memory alloy (SMA) bars were used as the kernel components for energy dissipation and restoration in the stress mode of pure tension. Their initial deformation was enlarged by the hydraulic system with two pistons of different sizes. In this way, high material utilization and high relative displacement were guaranteed together. The prototype device was fabricated, analyzed and experimental tested. The results show that the shape and amplitude of the force-displacement curve of the device could be easily changed by varying the hydraulic system, the number and/or the characteristics of the SMA bars. The new device can be separately or combined used as shock absorber and/or vibration isolation component for vehicles or other machines.

Abstract: The desired compression-shear loading were applied by the beveled ends with different angles, and this new technique based on split Hopkinson pressure bar can be used to investigate the dynamic response of materials. A series of experiments of NiTi shape memory alloy were performed at different impact velocities and different loading angles, and the dynamic equivalent pressure curves were given. The initial phase transition points under different impact velocities were plotted on σ-τ stresss space, and the phase transition surfaces predicted by a dynamic phase transition criterion were given.

Abstract: The impact shear response of crystallized polypropylene under combined compression and shear loading was studied by using an inclined gas gun and electro-magnetic particle velocity gauges. The experimental results show that the transverse wave velocity increases nonlinearly with the impact velocity, indicating its shear behavior is strongly related to the hydrostatic pressure. Remarkable shear wave attenuation occurs near the impact surface when the impact velocity and inclination angle reach the critical value. The micro-observation of recovered samples with a polarized optical microscope reveals that there exists a melting layer of about 2-3μm thick, i.e. adiabatic shear failure layer, very near the impact surface (about 5μm) which causes the shear wave attenuation.

Abstract: Shock-induced phase transition under intensive impact loading can strongly change the dynamic behavior of materials and wave propagation features. Dynamic constitutive modeling is one of the key problems in this area. We established a 3D dynamic incremental constitutive equation of phase transition with N transforming phases considering pressure, shear stress and temperature based on the Gibbs free energy. We also proposed an evolution equation for the product phase taking account both the over driving force and the growing space. The dynamic stress-strain response predicted by the model for TiNi specimen undergoing martensitic transition fitted the experimental result well, which demonstrates the present model can describe the dynamic deformation behavior of materials during phase transition process under impact loading.

Abstract: In this paper, the quasi-static axial compression of the pseudo-elastic phase transformation cylindrical shells (PTCS) is systematically investigated experimentally and numerically. Some interesting phenomena and regularities are found and in-depth understanding is gained: (i) static buckling instability properties of the PTCS with different sizes; (ⅱ) the formation and development regularities of the phase transformation hinges (PTHs). The length-diameter ratio (L/D) of the PTCS is closely related to its buckling modes.

Abstract: In this paper, radial impact responses of TiNi phase transformation cylindrical shell were experimentally studied on two-directional constraint. The nominal load-displacement curve was gained by advanced SHPB; meanwhile, the deformation process of the cylindrical shell was recorded by the high speed camera (30000 fps). The effect of stress wave on the whole structure during load process was as follows: the load-displacement curve fluctuated fiercely and rose gradually in the beginning period, whose frequency was basically the same as that of the stress wave propagation in the Striker 2; After that, the load-displacement curve tended to flat, the shell structure became to be ellipse as the compression increases, and the whole unload process slowed down. Finally, the surface strain was obtained by processing the shell deformation image, and the dynamic effect of the material phase transition and phase transition hinge on the cylindrical shell was examined.

Abstract: In order to accurately analyze the dynamic damage process of three-point-bending concrete beam with offset notch, the multiscale numerical model of time and space by coupling Discrete Element Method (DEM) and Finite Element Method (FEM) of three-point-bending concrete beam with offset notch was established. Using the multiscale numerical model of three-point-bending concrete beam, the generating and the extending processes and rules of cracks in concrete beam under condition of different offset distances of notch were analyzed at macro- and meso-levels. The simulated results are in accordant with the experimental ones. Comparing the computation time of the multiscale model and that of the DEM model in the example of three-point-bending concrete beam, the former accomplished the heterogeneous mesoscopic simulations of the latter, at the same time, the former was much more efficient than the latter.

Abstract: Some preliminary experiments are conducted to investigate the responses of granite， marble and red stone in the states of natural，water-saturation and oil-saturated under the shock of short pulse laser. The 3D contour of ablation pit and the preliminary relations of the ablation contour to the number of laser pulses are obtained. The results show that some cracks generate at local places for the shock of laser，and the degree of ablation of the oil-saturated rocks is more severe than that of the natural and water saturated rocks. However，further work should be conducted to control the direction，sizes and number of cracks generated under the shock of laser.