Lijuan Liao

Study of Characteristics of Cloud Cavity Around Axisymmetric Projectile by Large Eddy Simulation

Cavitation generally occurs where the pressure is lower than the saturated vapor pressure. Based on large eddy simulation (LES) methodology, an approach is developed to simulate dynamic behaviors of cavitation, using k - mu transport equation for subgrid terms combined with volume of fluid (VOF) description of cavitation and the Kunz model for mass transfer. The computation model is applied in a 3D field with an axisymmetric projectile at cavitation number sigma = 0.58. Evolution of cavitation in simulation is consistent with the experiment. Clear understanding about cavitation can be obtained from the simulation in which many details and mechanisms are present. The phenomenon of boundary separation and re-entry jet are observed. Re-entry jet plays an important role in the bubble shedding.

A numerical model for the evolution of internal structure of cavitation cloud

Bubble size distributions in cloud cavitation are important in cavitating flows. In this study, a numerical model was developed to study the evolution of the internal structure of cloud cavitation. The model includes (1) an evolution equation of bubble number density, which considers the bubble breakup effect and (2) the multiphase Reynolds-averaged Navier–Stokes equations with a modified cavitation model for background cavitating flows. The proposed model was validated with a flow over a projectile. Results show that the numerical model can predict the evolution of the internal structure of cloud cavitation. Comparisons of the proposed model and Singhal model were discussed. The effects of re-entrant jet and bubble number density on cavitating flows were also investigated.

Study on mechanical properties of polyethylene with chain branching in atomic scale by molecular dynamics simulation

Abstract The effects of length and content of chain branching on the mechanical properties of polyethylene (PE) in atomic scale were examined by molecular dynamics (MD) simulations. Methyl-, ethyl- and butyl-groups were adopted as branched chains to distribute along PE backbones. Plastic flow deformation was captured by providing a uniaxial tensile loading at a given strain rate, which shows the characteristic of rate dependence. Current results are in reasonable agreements with existing experimental data. The statistical results show that the longer length of chain branching induces lower equilibrium density and higher yield strength of branched PE. In addition, higher content of chain branching brings higher equilibrium density and lower yield strength of branched PE. It is assumed that the distribution of dihedral angles influences the deformation of PE definitely. The non-bond interactions contribute to the load-bearing capacity of PE largely. Branched PE shows big differences on mechanical behaviours comparing with the linear one. Chain branching distribution also greatly affects the performance of PE, which needs a further discussion.

Thermal decomposition behaviour of polyethylene in oxygen-free and low oxygen content circumstances by reactive molecular dynamic simulation

Abstract In the present study, the thermal decomposition characteristics of polyethylene (PE) in oxygen-free and low oxygen content circumstances were examined by molecular dynamic (MD) simulations at atomic scale using reactive force field (ReaxFF). Temporal evolutions of species were captured reasonably during the processes of thermal decomposition. The effects of oxygen content, temperature and heating rate were also analysed. In addition, the kinetic properties were predicted with reliable parameters. The results show good agreements with the available ones, which illustrate that the species with two carbon atoms are the vast majority of final products. Higher oxygen content and temperature promote the generation of small molecules with carbon atom number less than or equal to 10. In the presence of oxygen, greater activation energy span and reaction order are calculated with lower adjust R2, which indicates complex reactions according to kinetic analysis. The initial decomposition temperature of PE is proportional to the heating rate owing to heat transfer lag.

Numerical analysis of effect of cohesive parameters on mixed-mode failure of double-scarf adhesive joint subjected to uniaxial tensile loadings

In the present study, the effects of cohesive parameters on the mixed-mode failure of double-scarf adhesive joint (DSAJ) subjected to uniaxial tensile loadings were examined and discussed numerically. For DSAJ with no perpendicular or parallel with the external loading direction, complex stress state (mixture of tensile and shear stresses) occurs at the adhesive interface. In addition, adhesive joint failure, which is a gradually process rather than a sudden transition, is accompanied by energy dissipates gradually at the crack tip. Correspondingly, cohesive zone model (CZM) coupled with finite element method (FEM) was implemented to verify the mechanism of crack from initiation to the complete failure. As the constitutive relation of the adhesive layer, the traction-separation (T-S) law determines the interface damage evolution. Additionally, the shape of T-S curves in mode I and mode II are crucially decided by the cohesive strengths and critical fracture energies in each mode, respectively. Firstly, the non-dimensional-normalized form of ultimate tensile loading of DSAJ was obtained using dimensional analysis. Then, three cases of cohesive parameters (case of constant anisotropy extent & case of constant critical fracture energy in each mode & case of constant cohesive strength in each mode) according to the non-dimensional-normalized form of adhesive properties were designed. Two types adhesives (brittle and ductile) were chosen to examine the effects of adhesive properties on the failure of DSAJ in this study. To avoid the influence of the geometries on DSAJ mechanical behaviors, the thickness of the adhesive layer and the scarf angle θ were held constantly, respectively. In numerical calculations, the change trends of the ultimate tensile loading (Fu), the failure energy (Ef) and the damage level (D) corresponding to Fu with respect to the cohesive parameters were discussed. It can be observed the cohesive strengths in mode I and mode II codetermine Fu of DSAJ with unequal rates. Moreover, Ef of DSAJ, which is the necessary energy for the joint failure, is governed by the critical fracture energies in mode I and mode II with different contributions. Besides, it also obtained that the evolutions of D corresponding to Fu of DSAJ with brittle and ductile adhesives are certain different. Generally, D of DSAJ with brittle adhesive is higher and more uneven than that of DSAJ with ductile adhesive. Accordingly, it can be concluded that DSAJ with brittle adhesive has lower ability to distribute the loading over a smaller cohesive zone with less uniform distribution. In addition, the numerical results revealed that with the increment of ratio in each case set in this paper, D of DSAJ does not rise obviously.Copyright

Analysis on Failure Mechanism of Scarf Joints with Brittle-ductile Adhesives Subjected to Uniaxial Tensile Loads

The failure mechanism of scarf joints with a series of angles and brittle-ductile adhesives subjected to uniaxial tensile loads is analyzed by using a numerical method which employs a cohesive zone model (CZM) with a bilinear shape in mixed-mode (mode I and II). The adopted methodology is validated via comparisons between the present simulated results and the existing experimental measurements, which illustrate that the load-bearing capacity increases as the scarf angle decreases. More important, it is observed that the failure of the joint is governed by not only the ultimate tensile loads, but also the applied tensile displacement until complete failure, which is related to the brittle-ductile properties of the adhesive layer. In addition, failure energy, which is defined by using the area of the load-displacement curve of the joint, is adopted to estimate the joint strength. Subsequently, the numerical results show that the strength of the joint adopting ductile adhesive with higher failure energy is higher than that of the joint using brittle adhesive with lower failure energy.

Effects of Strain Rate on the Stress Propagations in Bonded Shrink Fitted Joints Under Impact Push-Off Loadings

The effects of the strain rate sensitivity on the stress propagations of bonded shrink fitted joint, in which a ring is fitted using an adhesive layer at the middle part of a shaft, subjected to impact push-off loadings are examined. The plastic flow deformation behaviors of structural adhesive under some strain rates are examined experimentally. In addition, the stress wave propagations in the joint are analyzed using finite element method (FEM). The experimental results show that the yield stress of the adhesive increases as the strain rate increases nonlinearly. It is observed that the maximum equivalent von Mises stress occurs in the adhesive layer at the upper interfaces, which causes the rupture of the joint based on the numerical calculations. Furthermore, the strain responses obtained from numerical and experimental methods are compared with each other. A fairly good agreement is obtained between FEM calculations and experiments. In addition, the joint strength is predicted by impact energy using experimental results, which is about 20.85 J in the present study.Copyright

Axisymmetric Analysis of Mechanical Properties of Bonded Shrink Fitted Joints Under Torsional Loads

Shrink fitting combining with anaerobic adhesives (bonded shrink fitted joints) has been widely used for joining cylindrical components in a lot of mechanical structures. This paper deals with mechanical properties (stress analysis and strength estimation) of the bonded shrink fitted joint in which a ring is fitted at the middle part of a shaft subjected to torsion. The stress distributions in the adhesive layer of bonded shrink fitted joint are analyzed using an axisymmetric theory of elasticity when an external torsion is applied to the upper end of the shaft. The effects of the stiffness, the outside diameter and the height of the ring on the interface stress distributions are clarified in the numerical calculations. The maximum shear stress, which shows singular stress property, initiates near the inside upper edge of the adhesive layer interface, where the rupture occurs in the joint under external torsional loads. In addition, the estimation method of joint strength is applied using the interface stress distributions and the modified singularity stresses expressions obtained from the analogous tests. It is found that the joint strength increases as the stiffness, the outside diameter and the height of the ring increase. Furthermore, experiments to measure the joint strength were carried out. Three-dimensional finite element (FEM) method is also adopted to verify the theoretical results. In addition, the strength of the bonded shrink fitted joint is compared with that of shrink fitted joint based on the experimental results, which is found that the strength of the bonded shrink fitted joints is greater than that of shrink fitted joints. The numerical results are in a fair good agreement with the experimental results and FEM results.Copyright

Finite Element Stress Analysis and Strength Evaluation of Bonded Shrink Fitted Joints Under Impact Push-Off Loads

Joints combining shrink fittings with anaerobic adhesives (bonded shrink fitted joints) have been appeared with advantages compared with those with shrink fittings only in light weight and high strength. This paper deals with the stress wave propagations and stress distributions of bonded shrink fitted joint in which a hollow cylinder is fitted at the middle part of a solid cylinder subjected to impact push-off loads with small strain rate. The stress stress wave propagations and stress distributions in bonded shrink fitted joint are analyzed in elastic and elasto-plastic deformation ranges using finite element method (FEM) as a four-body contact problem. The impact loads are applied to the joint by dropping a weight-hammer. The FEM code employed is ANSYS/LS-DYNA. The effects of the stiffness, the outside diameter and the height of hollow cylinder on the stress wave propagations at the interfaces are examined. The strength of the joint is clarified using the stress distributions obtained from numerical calculations. The normal stress near the upper edge of the outside interface of the adhesive layer increases as the rigidity and the outside diameter of the hollow cylinder decrease and the height of the hollow cylinder increases. The shear stress near the upper edge of the outside surface of the adhesive layer increases as the outside diameter and the height of the hollow cylinder increase; while it decreases as the rigidity of the hollow cylinder increases. The strength of bonded shrink fitted joint increases as the rigidity and the height of the hollow cylinder increase and the outside diameter of the hollow cylinder decreases. In addition, the characteristics of the joints subjected to impact loads are compared with those of the joints under static loads. It is observed that the characteristics of the joints subjected to impact loads are opposite to those subjected to static loads. Besides that, the attributes of the contact interface of shrink fitted joint under impact loads are compared with those of bonded shrink fitted joint under same external loads. It is found that opposite properties exist between of them. Furthermore, experiments are carried out to measure the strain response of bonded shrink fitted joints subjected to impact push-off loads. The numerical results are in a fairly good agreement with the experimental results and FEM results.Copyright