The Numerical Simulation And Testing Of Vibration Modal Of Plywood

Non-destructive Testing technology is developing rapidly in recent years, which is widely used in research field of wood material because of its characteristics of low cost, convenient, accurate and fast. The nondestructive testing methods can be roughly divided into two categories at present:one is to determine the dynamic Modulus of Elasticity on the basis of elastic wave velocities of acoustic signals in the wood; another kind according to the natural frequency, damping ratio and modal shape of wood to make modal analysis. By contrast, owing to short measurement time, simple, without coupling agent and strong anti-interference, the modal analysis is more suitable for rapid and continuous detection on production line.Modal analysis includes test modal analysis and calculating modal analysis. Calculating modal analysis, which is finite element modal analysis, is a kind of computer simulating technology. Unlike traditional physics experiment method, FEM is very suitable for the development of modal characteristics detection of wood materials, because of no limit to the materials and conditions, simple process, low cost. It will be an important means as wood nondestructive testing technology. The paper establishes physical model of plywood, explores the influence of defects on the dynamic properties of plywood by comparing natural frequency and modal shape. The paper provides the reference for the use of the modal test to determine the size and the location of defects. The main contents and conclusions include as follow:(1) It makes plywood physical model using three different ways, and then to make reasonable and feasible plywood physical model by comparing the FEM analysis results. Results showed that, the natural frequencies of three kinds of model has high fitting degree, the maximum relative error is about5%. It turns out that the analysis results of the three models are correct and the modeling method is feasible. These models all can be used for simulation of plywood.(2) To select model according to the different characteristics of three kinds of finite element model based on the content. SOLID model has advantages of simple operation and quick calculating, but not can be analyzed to the plywood each layer, so we can use it to analyze whole mode; SHELL model is very suitable for simulation of plywood sandwich structure; although the operation of Adhesion model is relatively complex, but also has its unique advantage, so it can be used to analyze the vibration characteristics of plywood with defects.(3) Finite element software is utilized to investigate the influence of the bonding defects and cracks on the vibration characteristic of plywood. It is found that the existence of bonding defects and cracks would affect the modal frequency and modal shape of the plywood, which dependent on their sizes and locations. Therefore, the sizes and locations of defects in plywood may be predicted by vibration testing.(4) Using the transverse vibration test comparison with FEM analysis, the paper analysis feasibility of finite element simulation. Results show that the vibration characteristics of the plywood can be approximately simulated with finite element numerical calculation method.