Applied Mechanics and Materials Vol. 782

Abstract: Steel fiber reinforced polymer high strength structural concrete (SPHSC) is a new composite material which is used in long-span bridge structures invented by this research team. Based on the new composite material, the experiments of stress-strain full curves under uniaxial compression were carried out. The stress-strain full curves were achieved from the simple improved method. By the method of theoretical derivation and experimental data fitting, the constitutive equation of SPHSC which contains only two undetermined parameters of compressive strength and parameter of steel fiber was obtained. The study is the theoretic basis of the new material which was widely use in civil engineering.

Abstract: In this study, high temperature tensile test was conducted on the specimen of superalloy GH4169 coated with wt8%-YSZ under the conditions of 950°C and 80MPa. By using finite element (FE) simulation method, the failure initiation and evolution of the metal substrate and YSZ coating were predicted. It was found that stress concentration originally occurred in the YSZ ceramic top coating, causing an axial stress and triggering severe debonding failure in the center region of the specimen at t=180s. With increased load, further interfacial debonding failure of the residual coating occurred due to the presence of oblique tensile stress and at t=900s, and only a few residues can be seen at the arc transitional region. Subsequently, the metal substrate was subjected to uniform tensile deformation and finally ruptured at t=76min with apparent necking. In addition, it was notable that YSZ coating can relieve stress significantly (nearly 40% lower), thus helping prolong the substrate's service life in the same environment. Simulated results were consistent with observed behavior.

Abstract: MAT224 is a tabulated version of the Johnson-Cook model in LS-DYNA. Compared with the original Johnson-Cook material, MAT224 was developed to simulate the dynamic response of a material by just defining the effective stress as a function of effective plastic strain at different strain rates and temperatures, thus avoiding the tedious parameter fitting procedures in the traditional Johnson-Cook model. However, the stability and precision of solution is strongly dependent on the effective stress versus effective strain curves in MAT224, and unreasonable curve data will lead to warnings or errors in the process of solution. In the current study, a two-dimensional axisymmetric finite element model for the Ti-6Al-4V titanium alloy under dynamic compression was built, and MAT224 was employed. By investigating the effects of the curve numbers, strain ranges, data points, as well as changing tendencies, on the simulation results, the stability and the reliability for MAT224 are systematically studied.

Abstract: Modified Hopkinson pressure bar apparatus is widely used to investigate the dynamic fracture behavior of materials at higher rate loading. While using a small sample for fracture toughness testing, plane strain conditions are compromised. In the current work, a large diameter two-bar/ three-point bend fracture setup is used to analyze stress wave propagation behavior within a larger cracked specimen. The experimental setup model consists of striker, incident bar, loading pin, cracked three-point specimen, span and transmission bar. The model is prepared using ANSYS software and the transient dynamic analysis technique is used to simulate the dynamic load. The effects of increased transient time on the stress wave propagation behavior within the cracked sample and the stress and strain values at the crack tip of the three-point bend specimen are analyzed. In addition, the effects of the hollow striker, the hollow incident bar and the specimen span are studied. It is found that during large specimen testing, an increase in the transient time results in the lower stress and strain values in the specimen crack-tip. The relationship of the specimen span, the striker and the incident bars with the strain values in the specimen is analyzed and a method for the three-point bend specimen testing at the higher strain rates is also proposed.

Abstract: Pose estimation is a thoroughly studied problem in computer vision. But in some realistic scenarios, reference points cannot lie within camera’s field of view (non-intervisible). In this article, planar mirror is placed in front of body, allowing the camera to observe reflection of reference points which can characterize body coordinate. We can form an equation system related to transformation between camera coordinate and body coordinate. We propose an unattached and linear approach to solve and optimize transformation of camera-to-body without any prior information of planar mirror configuration. Additionally, we analyze the sensitivity of our algorithm. We present a number of simulations and experiments to prove that our formulation significantly improves accuracy and robust.

Abstract: Multi-scale grid is an essential deformation carrier in optical methods for multi-scale deformation measurement. In this study, several new-type multi-scale grids were designed and fabricated by electron beam lithography. Each pattern includes several periodically distributed dots with the same spacing but different sizes. As a consequence, the grayscale of the whole grid pattern periodically changes. The peak parts of the grayscale generate a secondary grid, i.e., the large-scale grid. The ratio of the large-scale grid pitch to the small-scale grid pitch can be easily adjusted according to the requirement. The natural integration between the small-scale grid and the large-scale grid works well in eliminating the mutual disturbance between the different-scale grids. Besides, this type of grid has a very high success rate in fabrication owing to the small differences in size between the big dots and the small dots. The proposed multi-scale grid pattern is expected to serve as the deformation carrier in moiré methods and geometric phase analysis for multi-scale deformation measurement.

Abstract: Co-continuous ceramic composites have a complicated topology structure which makes it much more difficult for finite element model reconstruction. In this paper, the two-dimensional co-continuous ceramic composites finite element model is reconstructed by a modified quartet structure generation set method which modified the generation parameters based on quartet structure generation set (QSGS) method, and a numerical simulation at high strain rate is accomplished. The content mainly contains: (1) The distribution features of metal phase and ceramic phase of real co-continuous ceramic composites SEM image is calculated by mathematical statistics to determine the parameters that control the reconstruction such as volume fraction, core distribution probability and directional growth probability; (2) Two phase volume fraction and 2-point correlation function of the reconstructed finite element model is calculated as the quality assessment parameters, which verify the reconstructed finite element model are in allowable error range compared with the real SEM image; (3) Numerical simulation at high strain rate is carried out using the reconstructed finite element model. The failure behavior of co-continuous ceramic composites at high strain rate is analyzed, validates the reconstructed finite element model meets the requirements of numerical calculation.

Abstract: The software AUTODYN is used to investigate the damage mechanism of explosion in the brick and concrete buildings with different mass of charge in this paper. The results show that the mesh size of 5mm is very appropriate to study internal explosion; the maximum pressure area in the junction between roof and wall is broken up first and has the highest velocity, where is far from explosion source; the internal explosion damage in brick and concrete buildings appears in the boundaries between walls and roof.

Abstract: In this paper, we study practical generalized synchronization of uncertain chaotic system with a given manifold Y = H(X). We construct a class of the bi-directionally coupled chaotic systems with impulsive control, and demonstrate theoretically that the bi-coupled systems could realize practical generalized synchronization on the basis of stability theory of impulsive differential equations. Numerical simulations with super-chaotic system are provided to further demonstrate the effectiveness and generality of our approach.

Abstract: In finite element numerical simulation, the calculating results of geometrical models with different mesh density tend to have obvious differences, especially when material damage and fracture are considered. In order to study the mesh dependency of the finite element calculation, models of a projectile penetrating into the target with different mesh densities are constructed by using LS-DYNA. A nonlocal approach is investigated and the results without and with MAT_NONLOCAL are compared. It is found that the nonlocal model with a variable characteristic length parameter provides a relatively accurate and stable result, and the calculation cost can be reduced.