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The nonlinear mechanical responses of tetrachiral honeycombs under large deformation are theoretically analyzed by employing elliptic integrals, and the results are verified by numerical simulations. The nonlinear relationships between normal stress, shear stress, Poisson's ratio and the strain are derived for the tetrachiral honeycombs. It is found that the Poison's ratio of the tetrachiral honeycombs under large deformation is totally different from that under small deformation. The Poisson's ratio can achieve to a large magnitude of 0.8 in the former case, while remains 0 in the latter case. Moreover, the tetrachiral honeycombs exhibit positive Poison's ratio under large tensile-deformation, whilst showing negative Poisson's ratio under large compression-deformation. The magnitude of Poisson's ratio depends on both the honeycomb's deformation and the ligament relative length, while it is insensitive to the ligament thickness. Besides, the coupling effect between tensile (or compression) and shear is observed for the tetrachiral honeycombs. In other words, shear deformation appears within the honeycomb under uniaxial loading. The coupling effect depends on both the honeycomb's deformation and the geometric parameters of the cell structure. And the quantitative relationships among them are derived. This special coupling effect of tetrachiral honeycombs provides a new idea for designing more metamaterials.

Billiards is one of the most fashionable indoor sports around the world. The choice of billiard cues is important for players to win a game. Up to now, the evaluation on the performance of billiard cue almost merely depends on the experience of the famous players, which is subjective and controversial. Aiming at both the objective evaluation and the testing method on the performance of billiard cue, a set of experimental device was designed to measure the dynamic behaviors of both the cue and the ball. Based on theoretical considerations, a force-transferring coefficient is proposed to quantitatively and objectively evaluate the performance of a cue in the efficiency of hitting balls. Both experiments and numerical simulations indicate that those cues with moderate stiffness exhibit the most advantageous force-transferring performance, better than the softer and the harder cues. Moreover, further numerical simulation results indicate that the force-transferring performance of the cue can be enhanced by reducing the anisotropy of the shaft material. For the spinning-ball skills, the rotational velocity of the ball is derived and can be calculated from the ball's translational velocity measured by the device. Experiments showed that a softer tip could make a larger rotating velocity of the ball compared to the harder tip. It is expected that the present work will facilitate the objective and quantitative characterization of the performance of billiard cues, so as to assist players in evaluating and selecting billiard cues. It also offers a technical suggestion for selecting proper row material of cues in order to improve cues’ performance.

Vibration of flange joints and force transmission characteristics of connection interfaces are important due to concerns over the structural strength and signal identification in engineering structures such as missiles and rockets. However, nonlinear factors such as clearance, damping and discontinuous interfaces make it difficult to predict the responses under specific excitations such as shock impacts. This paper proposed a typical configuration of a bolted flange joint and investigated the response of vibration and dissipation of the joint under a series of impact loadings. The amplitude of excitation was 120 MPa, and the duration of the loading ranged from 20 μs to 200 μs. The excitations of high amplitude and frequency lead to a vibrational response coupled to a shock response of the joints. The connecting interfaces contributed to the distortion of the shockwave and changed its frequency, which can be characterized by the factor α. Those interfaces were also sources of energy dissipation, and their effect decreased as the frequency of excitation decreased. This methodology is helpful for predicting the local behavior of joints in a complex engineering structure under extreme conditions.

The shock behaviour of NiTi shape memory alloy is investigated by using molecular dynamics simulation. The nano-pillar samples of the alloy are subjected to the impact of a piston with a velocity of 350 m/s at initial environment temperatures of 325 and 500 K. At 325 K, we observe two different pathways of the formation of BCO phase, the gradient twins, and the detwinning phenomena, strongly depending on the local stress and the deformation state. As the initial temperature increases to 500 K, the plasticity is dominated by the dislocation movements rather than the twinning at 325 K. The phase transformation and plasticity result in stress attenuation when the stress wave propagates through the nano-pillar. Furthermore, it is interesting to note that multiple stress peaks occur due to the formation of local complex atomic structures with various wave speeds, leading to the catch up and overlap of the stress waves.

In the present research the dynamic energy absorption behavior of lattice material filled with shear thickening fluid (STF) is studied by the modified split Hopkinson pressure bar (SHPB) apparatus. The nominal engineering stresses versus strain curves are measured to analyze the dynamic energy absorption behavior of the combined material. The dynamic behavior of lattice material is also measured as a comparison. The results show the dynamic strength of the lattice material is about 3 MPa, which is a little higher than the static strength of about 2.3 MPa because of the inertial effect during impact. However, when filled with STF, the combined material shows extraordinary energy absorption behavior when compared to the empty lattice material. The stress of the lattice material filled with STF increase almost linearly to about 9 MPa and then increases slowly with the increase of strain. The excellent energy absorption behavior of the lattice material filled with STF could be explained by the interaction between the lattice cores and the STF during compression. For the lattice material, while some of cores buckling, it will lose the load capacity. While filled with STF, if the initial buckling of some of the lattice cores happens during impact, the strong lateral drag force is generated by the ambient STF due to the fast lateral velocities of the cores, leading to the great increase of its dynamic energy absorption behavior.

Experimental and numerical simulations were undertaken to estimate the effects of imperfect conditions on stress waves in split Hopkinson pressure bar (SHPB) experiments.ThephotonicDopplervelocimetry(PDV)mea- surement results show that the rise and fall times of an incident wave increases with an increasing inclination angle; also,thefluctuationsoftheincidentwavedisappeargradually with the increase of inclination angle. The following charac- teristics for various defects in the SHPB were obtained by numerical simulation: (1) the influence of a curved bar was negligible; (2) misalignment modestly affects the fluctuation characteristics, and bending waves were generated at this condition; (3) inclination and indentation of the impact end- surface had a great impact on the incident waves, and both of themincreasetherisetimeoftheincidentwavebyincreasing the degree of defects. In view of the results, misalignment, inclination, and indentation in SHPB experiments should be minimized.

Molecular dynamics simulation was conducted to investigate the phase transformation behaviour of nickel-titanium (NiTi, 50%-50% at.%) nanopillar under uniaxial compression at loading rates varying from 3.30x10(7) to 3.30x10(9)s(-1) and at temperatures varying from 325 to 600K. The phase transformation of NiTi was observed to be sensitive to loading rates and temperatures. The phase transformation stress of B2 -> B19 increased with increasing temperature while it was insensitive to loading rate. The phase transformation stress of B19 -> B19 '-> BCO increased with increasing strain rate and decreasing temperature. In addition, reverse phase transformation was observed during compression due to the interaction between the phase transformation of B19 -> B19 '-> BCO and the deformation twinning/dislocation slide-induced plasticity of the BCO phase, leading to different residual crystal structures after loading. Moreover, a diagram for the phase transformation behaviour of NiTi in the simulated ranges of strain rate and temperature was obtained, from which the contrary experimental observations on the phase transformation behaviour of NiTi from the studies of Nemat-Nasser et al. (Mech. Mater. 37 (2005) p.287) and Liao et al. (J. Appl. Phys. 112 (2012) p.033515) at various strain rates could be well explained.

A scanning acoustic microscope (SAM) and a scanning electron acoustic microscope (SEAM) have been used to study domain structures in ferroelectric BaTiO3, Bi4Ti3O12 and ferroelastic NdP5O14 single crystals. In SAM, domain walls act as scattering boundaries for leaky surface acoustic waves and thus sources of image contrast; While in SEAM, different signal generation and image contrast mechanisms may arise, mainly thermal elastic and piezoelectric effects. Further insight into the properties of domains was approached by discussing and comparing the image contrasts involved in both techniques.