Application Of Digital Speckle Correlation Method To Research On Micro-Mechanical Behavior Of Wood

Timber as a natural polymer composite material with the origin of life,has a very complex mechanical behavior.Even though it has been applied for thousands of years,but the knowledge concerning the complexity of its mechanical behavior is still very limited. Microstructure of wood and its physical mechanism in micro-scale impact on the macro response of wood,and therefore,the research on the micro-structure of wood may reveal the nature of macroscopic properties of wood.Although in the past 10 years,more and more the work of wood physical and mechanical fields has been turned micro-range research,but less concerned about the mutual contact of the macro-mechanical behavior and microstructure evolution.Because of the macro-mechanical properties of wood and its micro-structure changes are closely related,so it has a very important theoretical and practical significance to prove the micro-structure changes and the contact between them and macro-mechanical response after wood loaded,to reveal the deformation of microstructure of the timber,damage, failure mechanism and the law of evolution,to look for some experimental method both reflecting the macro-deformation and showing some of micro-structural changes at the same time,and to give some of the corresponding mechanical parameters and quantitative analysis of mechanical properties.Because of the heterogeneity and anisotropy of the wood,the structure of wood as a precursor in meso-scale is more complicated than that of the general homogeneous material,so the process of its damage initiation and evolution are changing.On wood's own characteristics, it is very difficult to apply the traditional,conventional methods of theoretical analysis to solving the problem above,or describing them clearly.Based on the above idea,in this dissertation,experimental research focusing on micromechanical properties and damage mechanism of small wood samples is carried out by employing advanced optical measurement mechanics experiment ways—digital speckle correlation method(DSCM).DSCM with the audience,non-contact characteristics at measurement of the deformation process can be completely retained in the surface information. This method is a promising solution meso-mechanics quantitative measurement method,and then by calculating the stress of highly concentrated region near the deformation,the failure mode of wood was studied.First of all,this thesis with the use of micro-mechanical testing machine of the environmental scanning electron microscope on a small sample wood tensile test process implemented real-time online monitoring;Secondly,the use of digital speckle correlation method(DSCM) software,this thesis conducted a quantitative analysis for environmental scanning electron microscope filming different stages of picture load.The main research content and conclusions are as follows:(1) In this paper,some of the key technology of the digital speckle correlation method (DSCM) were analysed and measurement system of the digital speckle correlation method (DSCM) was developed.The measurement system can meet the actual measurement requirements and provides the basic conditions for the application of digital speckle correlation method(DSCM) to wood scientific research.(2) This thesis was first proposed a new measurement of the elasticity coefficient on smaller regional experimental methods,radial tensile test of small wood sample was completed in the environmental scanning electron microscope,using Digital speckle correlation method(DSCM) software,a complete specimen central ring has been calculated at a certain stage of elastic deformation under the radial load-strain field,and re-use Hook's law,as well as the calculation of the stress balance equation,four different positions of the radial elasticity(Path,String to the Young's modulus,shear modulus and Poisson's ratio) were derived with the strain field.In order to examine the accuracy of experimental measurement methods, the paper also uses two-dimensional hexagonal cellular model of radial four different locations to carry out the estimation of elasticity,the results are basically consistent.Because the method are the measurement of real materials,so the results are more reliable and more accurate than the simplified model used to estimate the value.(3) Displacement and strain fields were measuered for RT system,TR system,TR45~o systems,as well as LR system at different load levels under proportion limit from Digital Spackle Correlation method.The result showed that digital speckle correlation method can reveal fine details of the nature of displacements in wood.(4) In the growth ring and cell level,failure mechanisms were analysed on small wood samples loaded at different directions to grain.The result reveals the intrinsic regularity between the crack progress and wood micro-structural.In this paper,the corresponding research methods and results lay a foundation for understanding of the nature of timber properties and setting up of the micro-mechanical model, as well as design and use of materials.