Fracture simulation using a nonlocal particle model

Extensive researches have been done to simulate crack initialization, propagation, branching, and coalesce from different engineering disciplines. Various numerical methods have been developed, which can be generally grouped into two categories: the continuum-based methods and the discontinuous approaches. For cracking problems, the classical finite element method (FEM) uses mesh matching and remeshing techniques which is computationally very expensive. The cohesive elements does not require very dense mesh near the crack tip region, but usually requires the crack path as a priori knowledge for the computational efficiency. eXtended FEM (XFEM) treats the discontinuity via level sets method and enrich the crack tip elements with analytical solution for the stress or displacement from linear elastic fracture mechanics. Arbitrary crack branching and coalesce is still challenging in the XFEM framework. The discontinuous approaches, such as lattice spring models and peridynamics, can handle fracture problems very efficiently. No additional criteria are needed as the crack growth is a natural outcome of the system evolution. As the elongation of the connecting bonds exceeds the critical value, it breaks and the crack propagates automatically. However, there are some other issues with the discontinuous approaches, such as restriction on effective Poisson’s ratio and crack path preference. A Volume-Compensated Particle Method (VCPM) was proposed by Chen et al. to solve these issues within the discontinuous framework. In the VCPM, both pairwise and nonlocal potentials are used to describe interactions among particles. One unique issue in the regular lattice particle method is the directional preference of the crack propagating path due to the regular lattice topology. The objective of this study is to investigate a general formulation using the VCPM concept to eliminate/reduce the crack path preference in the fracture simulation

Delamination detection in composite beams using a transient wave analysis method

In this paper, delamination detection of composite beams is investigated through a transient wave analysis method. A higher-order beam theory is proposed to model the Lamb wave propagation behavior. Wavelet Transform (WT) is used to localized the delamination. The reflection ratios and transmission ratios are found to depend strongly on the frequency of the incident flexural waves, as well as the size of the delamination. So it can be well used to detect the small size of the delamination, which is important for the Structural Health Monitoring (SHM). The numerical results show that the localization and identification of the size of the delamination are feasible by the proposed approach, which is the essential first step for the enhancement of safety and reliability of composite structures. The results are being verified by the experiments

Unified framework for microstruture evolution and property quantification

In this study, a novel unified framework for the study of both microstructural evolution and the mechanical property quantification is proposed. The multistate Potts model is used to simulate the microstructural evolution, whereas a volume-compensated particle method (VCPM) is used for the mechanical property quantification of the steady state microstructure. The VCPM proposed by Chen and colleagues was originally developed for the investigation of the fracture phenomenon of solid materials. The model was also successfully extended to study the elastoplastic properties of solids by introducing a volume conservation scheme. In the VCPM framework, the domain of interest is discretized into regular unit cells according to the triangle and square packing for 2D and simple cubic, body centered cubic and face-centered cubic packing for 3D. Both local pair-wise and nonlocal multibody potential are proposed to account for the interactions between particles. The multistate Potts models have been used extensively to model a variety of microstructural phenomena, such as the grain growth in a single or multiple-phase system, recrystallization, solidification, and many others. The space-filling array of regular cells is used to represent the Potts domain which is the same as the one used in VCPM. The microstructure evolves such that the system Hamiltonian is minimized. To consider different external effects on the states of the microstructure, different energy terms can be introduced into the system Hamiltonian, such as surface energy to account for the interface effect and strain energy for the grain orientation. Once the final steady state is obtained using the multistate Potts model, usually some other techniques, such as FEM, are used to calculate the effective properties of the microstructural system. It requires the mapping between the FEM meshes and the microstructure. The mapping is very difficult, especially for the interface mapping when the microstructure is very complex. In this study, the VCPM is coupled with the multistate Potts model to simulate the microstructural evolution and quantify the effective mechanical properties of the system within one framework. No mapping between these two models is required since they share the same underlying domain structures. The nonlocal potential proposed in VCPM is introduced into the multistate Potts model as an alternative of the original strain energy term. By doing this, effective simulation of the microstructure evolution for multiple-phase materials can be achieved. Given the final microstructure, the VCPM simulation is carried out to calculate the effective properties of the obtained system

Reductive dechlorination of polychlorinated biphenyls is coupled to nitrogen fixation by a legume-rhizobium symbiosis

Chlorinated persistent organic pollutants, including polychlorinated biphenyls (PCBs), represent a particularly serious environmental problem and human health risk worldwide. Leguminous plants and their symbiotic bacteria (rhizobia) are important components of the biogeochemical cycling of nitrogen in both agricultural and natural ecosystems. However, there have been relatively few detailed studies of the remediation of PCB-contaminated soils by legume-rhizobia symbionts. Here we report for the first time evidence of the reductive dechlorination of 2,4,4&#39;-trichlorobiphenyl (PCB 28) by an alfalfa-rhizobium nitrogen fixing symbiont. Alfalfa (Medicago sativa L.) inoculated with wild-type Sinorhizobium meliloti had significantly larger biomass and PCB 28 accumulation than alfalfa inoculated with the nitrogenase negative mutant rhizobium SmY. Dechlorination products of PCB 28, 2,4&#39;-dichlorobiphenyl (PCB 8), and the emission of chloride ion (Cl-) were also found to decrease significantly in the ineffective nodules infected by the mutant strain SmY. We therefore hypothesize that N2-fixation by the legume-rhizobium symbiont is coupled with the reductive dechlorination of PCBs within the nodules. The combination of these two processes is of great importance to the biogeochemical cycling and bioremediation of organochlorine pollutants in terrestrial ecosystems.</p

The influence of viscous liquid to the propagation of torsional wave in pipes

This paper deals with the torsional waves propagating in a pipe contacting with viscous liquid. The expression describing the movement of viscous liquid near the pipe is presented. Both the pipe is filled with and immersed in the liquid are considered and both the isotropic and transversely isotropic pipes are investigated. Numerical calculations were carried out to investigate the influence of viscosity of liquid on the dispersion and attenuation curves. The first torsional wave mode is investigated individually by varying the viscosity and the density of the liquid. It is concluded that the viscosity of the liquid has little influence on the dispersion curves, while its main effect is the attenuation. When the frequency and the thickness of the pipe are not large, the attenuation ratio between the two cases of pipe immersed in liquid and pipe filled with liquid is approximately a constant, which is determined by the ratio of inner radius and the thickness

The Geotechnical Hazard Induced by 8.1 Earthquake in West Pass of Kunlun Mountatin in China in 2001

On November 14, 2001, a great earthquake with a magnitude of Ms=8.1 occurred in the Qinhai-Tibet Plateau, China, which is called as the west pass of Kunlun mountain Ms8.1 earthquake. This earthquake is the largest and the first one with a magnitude greater than 8.0 in mainland China in the recent 50 years. It caused a large-scale deformation zone with a length of 426 km in frozen soil deposit in the plateau with a height of 4000-5500 meters above sea level. Some geotechnical hazards induced in the meizoseismal area, such as compression and tension failure of soil deposit, shaking landslides (collapse of slope and cliff), seismic settlement, sliding of glacier and snow. Unexpectedly, liquefaction occurred within the melting sand layer in frozen soil deposit on the banks of lakes and rivers. All the hazards were investigated in the field by the authors. This paper presents the results of the field investigation and tests made