No abstract

Plastic deformation of polyethylene in uniaxial and biaxial loading conditions is studied using molecular dynamics simulation. Effects of tensile strain rates from 1 × 105 to 1 × 109 s−1, and mass density in the range of 0.923–0.926 g/cm3 on mechanical behaviour and microstructure evolution are examined. Two biaxial tensile deformation modes are considered. One is through simultaneous stretching in both the x and y directions and the other sequential stretching, firstly in the x-direction and then in the y-direction while strain in the x-direction remains constant. Tangent modulus and yield stress that are determined using the stress–strain curves from the molecular dynamics simulation show a strong dependence on the deformation mode, strain rate and mass density, and all are in good agreement with results from the experimental testing, including fracture behaviour which is ductile at a low strain rate but brittle at a high strain rate. Furthermore, the study suggests that the stress–strain curves under uniaxial tension and simultaneous biaxial tension at a relatively low strain rate contain four distinguishable regions, for elastic, yield, strain softening and strain hardening, respectively, whereas under sequential biaxial tension, stress increases monotonically with the increase of strain, without noticeable yielding, strain softening or strain hardening behaviour. The molecular dynamics simulation also suggests that an increase in the strain rate enhances the possibility of cavitation. Under simultaneous biaxial tension, with the strain rate increasing from 1 × 106 to 1 × 109 s−1, the molecular dynamics simulation shows that polyethylene failure changes from a local to a global phenomenon, accompanied by a decrease of the void size and increase of uniformity in the void distribution. Under sequential biaxial tension, on the other hand, the density of the cavities is clearly reduced.

In this study, a series of tests on notched polyethylene (PE) plate specimens providing clues to damage and fracture behavior for a wide range of Lode parameters was carried out. Two series of numerical simulations of each test were performed, the first one without taking damage evolution into consideration and the second one taking damage evolution into consideration. Good correlation of experimental and numerical results in terms of the engineering stress-displacement relation has been achieved. The results show that the average Lode parameter increases with the increasing notch radius of the plate specimens. In addition, the damaged and undamaged constitutive equations were determined from the first and second series of finite element (FE) simulation, respectively. The critical damage parameter calculated from the damaged and undamaged constitutive equations was found to decrease when the average Lode parameter is increased. Furthermore, fracture occurs at the center of the minimum cross section, where the maximum damage parameter and plastic strain occur. The fracture strain was found to increase with the increase in the average Lode parameter.

The present paper addresses the mechanical behaviors and failure mechanisms of buried polyethylene (PE) pipes crossing active strike slip tectonic faults based on numerical simulation of the nonlinear response of the soil-pipeline system. The developed finite element (FE) model is first verified through comparing the simulation results with those from large-scale tests and good agreement between simulation and experimental measurements is obtained. The FE model is then applied to investigate the effects of fault crossing angle, pipe and soil properties on the mechanical behavior of PE pipe. The results indicate that the PE pipe crossing negative fault angles is primarily subjected to compression and bending, thus exhibits the phenomenon of buckling. With the increase of crossing angle, there is an increase of the axial strain and the maximum Mises stress in the buckled cross section, and a decrease of the distance between the buckling position and the fault plane. While for positive crossing angles, the PE pipe is mainly subjected to tension and relatively small bending. Increasing the crossing angle causes an increase in bending strain and a decrease in the axial strain. In addition, when the fault moving speed is slower, the axial strain and bending strain are larger, whereas the maximum Mises stress in the buckled cross section and the distance between the buckled position and the fault plane are reduced. Furthermore, the most severe deformation of the pipe is observed when it is buried in the sandy soil, followed by cohesive soil and loess soil.