The Application Of Second Law Of Thermodynamics In Metal Structures Of Elastic-plastic Deformation

As one of the most basic theory of physics in the nature, the laws of thermodynamics are much instructive significance for the rapid development of science and technology. According to the theoretical physics, the process of development and change of any system and thing in nature world must follow the principle of conservation of energy and entropy increase principle.The deformation under loading of metal material is a simple thermodynamic process, therefore,the deformation process of material can be completely determined using the thermodynamic theories.In this article, using the laws of thermodynamics, the elastic-plastic deformation of metal materials is re-recognized, the amount and distribution of the elastic-plastic stress field are got,pointing out the stress distribution of the current elastic-plastic theory is unreasonable. And the following new understandings are obtained:1. From the corresponding physical mechanism and energy field of the elastic deformation and plastic deformation, we can know that elastic deformation and plastic deformation of metal material are two physical phenomena of different mechanisms and different thermodynamic processes. To the structures with free form deformation, such as pure bending beam and circular shaft, even if in the stage of elastic-plastic deformation, only the elastic strain energy corresponding to elastic deformation is stored in the material and the work done by plastic deformation is dissipated in the form of heat. The distribution of elastic strain energy in the structure still follows the minimum internal energy principle energy, and the stress distribution in the structure also follows the minimum internal energy principle energy, and there is no residual stress in the stable equilibrium structure in the absence of any external forces.2. Through exploring the production condition of plastic deformation in the elastic restraint structures such as the indeterminate bar-system structure,the thick wall cylinder and the thick wall spherical shell, we can conclude that production of plastic deformation in the elastic restraint structure is limited to the adjacent elastic deformation area, which makes the plastic deformation is hard to produce, the plastic deformation produces only in the outer wall.The stress distribution form of thick wall cylinder after plastic deformation still comply with the minimum internal energy principle similar to the elastic stage.3. Through the theoretical analysis of production conditions of plastic deformation and requirements for wall thickness of high pressure pipe and yield strength ratio of high strength material, a new understanding is presented for the brittle fracture phenomenon of high pressure pipeline and selection of material. It is demonstrated that increasing the wall thickness and improving the strength of material cannot increase the bearing capacity of the structure. It is realized that the size and yield ratio close to a certain proportion can improve the utilization rate of the pipeline steel, and different steels corresponds to different critical wall thickness when the metal materials is chose for pipeline steel.