I. Introduction Connection is a nonlinear link in structural systems and a weak link in the study of large-scale engineering structural dynamics. The connection contributes 90% of the energy dissipation of the structural system, causing discontinuities and non-linearities in the stiffness of the system. Connection is the most important part of the engineering structure and the "most vulnerable" link in the engineering structure. Bolted connections are the most commonly used detachable connections in engineering structures. At the moment when the preload is applied, the preload is "offset" from the expected value of the design. Ordinary torque wrenches can vary by as much as 20-30%. After that, various use environments such as creep, relaxation, vibration, overload, temperature, etc. will cause loss of pre-tightening force. The loss of pre-tightening force may further lead to changes in contact pressure and friction state of the joint interface, thereby reducing load transfer at the interface. The ability to separate, cause seal failure, wear or corrosion, etc., changes in mechanical and physical properties, which is the problem of structural joint damage. Second, the bolt flange connection structure overview The bolt flange connection refers to the detachable connection of the flange, the gasket and the bolt as a set of combined sealing structures. The pipe flange refers to the flange used for piping in the pipeline device, and the device refers to the device. The inlet and outlet flanges of the equipment. The holes are evenly distributed on the flange, and the bolts make the two flanges tightly connected. If the structure requires sealing, a gasket is usually added between the flanges. Typical bolt-flange joint structures, contact and friction The application of the finite element method to handle contact problems began in the 1960s. Erson and Wilson proposed a finite element method for contact in a two-dimensional elastomer without friction. Subsequently, Chan and Tuba, Ohte, etc. extended the finite element solution to the two-dimensional and axisymmetric elastic contact problems with Coulomb friction. Since the contact process usually depends on time and is accompanied by material nonlinearity and geometric nonlinearity; the contact area, shape and contact surface force and motion state constantly change with load, so the contact problem is usually solved by incremental method. Tusta et al. proposed a finite element method based on load increment theory in 1973. The irreversibility of the loading process with frictional contact problems is initially solved. In 1976, Fredriksson established the incremental governing equation for contact with elastomers through theoretical derivation. In 1979, Okamoto and Nakazawa established the incremental equation and the finite element method based on the principle of virtual work. In 1983, Mazurkiewicz et al. established a stiffness equation for force and displacement between pairs of contact points to solve the problem of frictional contact with gaps. At present, the numerical calculation method based on the equation variational framework is the main means to solve the problems of bolt-flange structure in practical engineering problems. Figure 1 Typical bolt flange connection structure Third, the connection damage Bolted joints are the main source of local nonlinearity and passive damping of engineering structures. Connection damage will inevitably lead to changes in structural characteristics such as stiffness and damping. Firstly, the modal parameters of the structure, such as natural frequency, mode shape, and mode slope, should be considered as the characteristic parameters for characterizing the connection damage, and these parameters have been successfully applied in the field of health monitoring of large civil engineering structures. However, as pointed out by the MD Todd Institute, modal parameters that reflect the overall dynamics of the structure are insensitive to joint damage unless the damage evolves to an extent sufficient to cause a significant change in the structural modality. This is because during most of the service structure, the connection damage usually only occurs on a few bolts of a few joints, which has obvious local characteristics; the fastening force of the overall connection is not significantly reduced, and the damage level is small. Not enough to cause significant changes in the overall dynamics of the structure. Therefore, it is not feasible to use the overall modality of the structure as a characteristic parameter to monitor the damage of the bolted joint, and there are still great difficulties in the excitation and measurement of the high-frequency local modal. Therefore, the health monitoring method based on structural modal parameters has poor applicability to bolting health monitoring problems. In addition, the process of identifying modal parameters using structural dynamic response information actually implies the assumption of linearization of the system, ignoring the nature of the nonlinearity of the connected structure, and the method itself is insufficient. Fourth, thermal-contact coupling considerations Thermodynamic analysis of heat transfer-contact coupling is a challenging subject in the field of computational mechanics. Wriggers et al. studied the thermodynamic friction properties and constitutive relations of the interface; Johansson et al. gave a mathematical description of the thermal contact problem model; Wriggers et al. gave the finite element solution model for the thermal friction contact problem, using iterative analysis techniques for both types of problems. Solving; Junark solves the thermal contact problem in a complementary relationship, which is characterized by the complementary relationship between the heat flow and the temperature difference as a variable, but on the other hand, the heat transfer conditions of the contact surface are too strict and conservative. The heat exchange pattern that may be performed by the contact gap medium cannot be considered. Firstly, the contact surface heat exchange and temperature relationship model is established, and then the thermal conductivity of the medium in the gap is simulated. The iterative process is used to solve the coupling problem. The actual calculation shows that the iterative process has good convergence characteristics under normal conditions. Five, linear and nonlinear Studies of stiffness nonlinearity in connections are in large and major locations. In the early days, the discontinuity of the stiffness distribution caused by the connection was first considered. The method of modeling the connection structure with massless springs was successfully established and used. The elastic stiffness parameter can be obtained by static test, finite element contact static analysis or parameter identification using structural vibration characteristics. The main difference of such models is focused on what form of elastic stiffness equation is used, either linear or nonlinear, and nonlinearly uses a piecewise linear model. The joint stiffness models in all directions are generally decoupled, with independent elastic simulations. After obtaining the model parameters, such models and the rest of the overall structure model (connection quality is taken into account in the nearby connected structure) together constitute a linearization of the overall structure or a computational model containing local spring nonlinearity, which can be used for the overall structure static, Analysis of dynamic behavior. Summary and outlook The study of connection structure dynamics can be divided into two main ideas: one is to pay attention to the real physical behavior of the connection interface, and to find out how to happen in the kinetic environment by the theory, experiment or numerical method. One is to pay attention to the overall dynamic response of the connected structure from the perspective of the dynamics of the connected structure, and to investigate the influence of the connection on the overall dynamic behavior from the perspective of overall dynamics. The former wants to build a model that describes physical phenomena; the latter requires a mathematical model of materialistic phenomena. The connection problem is a mechanical interface problem and is a traditional problem in the field of mechanics and mechanical engineering. From a practical point of view, the dynamics of the joint structure that are urgently needed to be solved are: (1) dynamic modeling of mechanical interface friction behavior; (2) mechanical interface gap/collision dynamics modeling; (3) mechanical interface Wave propagation modeling; (4) Dynamic modeling of multi-directional coupling of mechanical interfaces; (5) Nonlinear dynamics of connected high-dimensional systems; (6) Connection model and existing linear structure dynamics finite element calculation program Comprehensive problem; (7) condition monitoring of the connected structure under vibration overload combined environment; (8) health monitoring of the connected structure. The study of connection problems requires the tribological research in the field of mechanical engineering and the solid state contact mechanics research in the field of mechanics. It is awaiting further research on new ideas and methods. references: [1] B. Parson, EA Wilson. A method for determining the surface contact stresses resulting form interference fits [J]. ASME Journal of Engineering for Industry, 1970. [ 2 ] SK Chan, IS Tuba. A finite element method for contact problems of solid bodies-- Part I; Theory and validation[J]. International Journal of Mathematic Science, 1971, 13. [3] S. Ohte. Finite element analysis of clastic contact problem [J]. Bulletin JSME, 1973, 16. [ 4 ] Wang Yucheng. Finite Element Method [M]. Beijing: Tsinghua University Press, 2003. Floor Drain Strainer,Shower Drain Cover Strainer,Anti Odor Floor Drain Strainer,Stainless Floor Drain Strainer Kaiping City Jinqiang Hardware Products Co.,Ltd , https://www.jmpowerdrain.com