Research On Stress Wave Propagation Model In Log Based On Finite Element Method

The internal defects of logs will seriously threaten the integrity and effective use of logs,so it is of great significance to detect the defects of logs quickly and effectively.At present,the stress wave detection method is a relatively common non-destructive testing method for wood,its basic principle is to judge the health condition of wood by measuring the change of stress wave propagation velocity.However,the velocity equation of wood stress wave is a high order differential equation,which carnot be solved easily by mathematical analysis.Finite element method is a numerical method for solving differential equations and partial differential equations.Therefore,the finite element model can be established to solve the characteristic frequency of log internal stress wave signal,and then the wood defect detection can be earied out from the perspective of frequency domain.In this paper,the stress wave vibration signal in raw wood is taken as the research object.According to the wave theory,the transfer equation of stress wave propagating along the radial or chordal direction is discussed.On this basis,the cross-section finite element models of healthy and hollow pinus sylvestris specimens were established by using the top-down modeling method in ANSYS software with pinus sylvestris as the prototype.According to the orthonormal anisotropy of log,the element type and material attribute of the model should be set first.Then the model is meshed by automatic meshing.Finally,modal analysis type is selected to perform modal analysis on the model.From the result post-processing module,the first 5 natural frequencies and modal shapes of the healthy and hollow pinus sylvestris finite element models can be obtained.In order to verify the correctness of the model,the experimental platform was built and the stress wave signals of the cross section of the healthy and the hollow pinus sylvestris specimens were collected.When using stress wave processing technology to analyze the collected signals,the selection of wavelet threshold de-noising parameters(the choice of wavelet basis,the choice of maximum decomposition layer and the choice of threshold)is mainly studied.By Matlab simulation results,the effects of different parameters on the denoising effect can be intuitively compared,and an optimal combination can be selected for the denoising of stress wave signals.Then the frequency spectrum of the stress wave signal is analyzed by FFT,and the first five natural frequencies of the stress wave signal are obtained.By comparing and analyzing the first five frequencies of the healthy wood specimen and the hollow wood specimen,it can be seen that when the wood is defective,the corresponding natural frequency will decline,which provides a basis for analyzing the wood defects from the perspective of frequency domain.By eomparing and analyzing the experimental data and the simulated data,their relative errors are within 10%and highly consistent,which verifies the correctness of the finite element model established for pinus sylvestris specimen,and provides a basis for using the finite element model to detect wood defects.