Study And Numerical Simulation Of Fracture Mechanics In Plastic Pressure Pipes By Damege Mechanics

To predict the lifetime under the long-term loading of plastic pressure pipes exactly has great importance to the determination of the examination time of polymer parts and the preventing from the possibility of accidents takes place. However, traditional studies of it were nearly all based on fracture mechanics which were ignored the process before the crack engendered and the evolvement of damage in materials. This paper constructed the physical model and the mathematical model of the mechanical parameters and the evolvement of damage in the long-term creep and fatigue process of polymer material based on damage mechanics.This physical model was based on polymer material science, theory of damage mechanics, rheology and computer graphics, the damage condition under the long-time load was analyzed and the lifetime was predicted by the numerical simulation method use damage mechanics. The damage process of crack initiation was analyzed based on the theory of damage mechanics. The damage process before the engender of crack under the condition of long-time load can be simulated by drawing tests, the material parameters and damage parameters can be obtained from these tests which were subsequently used in the numerical simulation of damage mechanics in polymer parts. The damage processes of the polymer parts were numerically simulated by dynamical quantitative and microscopic methods, which have great importance to the determination of the examine time of polymer parts and the preventing from the possibility of accidents take place. This paper provided the scientific evidence and analytical measures to the damage process of polymer. The main work was as follows:1. The evolvement of damage in the long-term creep and fatigue process of polymer material was analyzed by damage mechanics, One-dimensional principal damage equations were induced to analysis the damage process of polymer under the different loading situations;2. This paper researched the damage process for the fatigue loading and creep loading, the certain physical and mathematical models were established as to each of the loadings of the polymer, the computer program for the damage process was programmed by the Visual Basic 6.0;3. Because some material properties of polymer were nonlinearly dependent on the temperature, we used DMA method to get the dynamics properties in different temperature, from which we can get other temperature-depended material properties by calculation. The damage properties in different temperature of polymer can also be computed by the dynamic properties which were obtained from the DMA experiments;4. The damage process of PP in long-time creep loading was obtained from the numerical simulation based on the creep damage model, the lifetime of PP was predicted which has high reliability to be compared to the data get from DMA tests;5. The damage process of PP and HDPE in long-time fatigue loading was simulated by the VB program which based on the fatigue damage model, and the lifetime of polymer parts were predicted with the fatigue predicting program which has high reliability to be compared to the data get from DMA tests;6. To predict the lifetime of polymer pressure pipes under the real long-life loading exactly, we use the Visual Basic 6.0 program to simulate the composed process of fatigue and creep by the mathematic model based on damage mechanics; 7. The relationship between the lifetime and the parameters in long-time loading was analyzed in this research, and the analytical results were compared with the experimental data. The relationship between temperature of environment and lifetime of polyethylene is investigated. These results demonstrate that the fatigue lifetime tend to be decreased to the increase of the stress range and the temperature under high cycle fatigue loading, and the creep lifetime will be rapidly decreased to the increase of the creep stress and the temperature under creep loading.