In Situ Study On The Mechanical Mechanism Of Deformation And Failure Evolution In Microporous Materials

Microstructure is one of the key factors affecting the macroscopic mechanical properties of materials.Well-designed microstructure can bring extraordinary mechanical properties to materials through simple components.As a simple and typical microstructure,the microporous structure is a direct and effective tool to realize the lightweight of materials.There are microporous structures in many biological materials in nature,such as animal horns,crop straws,bones,honeycomb structures and so on.Learning and imitating natural biological material is one of the effective ways for humans to explore the design of microstructures.In this paper,two typical biomaterials with microporous structures(beetle horn and wheat straw),which have excellent mechanical properties,and functional materials with artificial microporous structures(microcapsule self-healing materials)were selected as the research objects.High resolution synchrotron radiation SR-CT technology was adopted.The characteristics of three-dimensional microporous structure and internal deformation evolution process under external loading were observed by three-dimensional in-situ experiment.The mechanical behaviors,such as the internal crack initiation,crack propagation,fracture,damage healing,structural failure et al.were observed and quantitatively analyzed.Based on the mechanics principle,the micromechanical mechanisms of micropore structure in material loading and energy absorption were also analyzed.Moreover,the mechanical mechanism of various types of pore structure in topological morphology,orientation,three-dimensional spatial arrangement,coupling action between pore and other microstructure,between pore and crack were comprehensively analyzed and discussed.These results could provide valuable optimization strategies for designing lightweight and toughening composites.The main contents and innovations of this paper include the following four aspects:1.The strengthening and toughening mechanisms of the "dense exocuticle—porous endocuticle—crossed lamellar endocuticle" gradient hierarchical structure of beetle horn regulated by brittle fracture and progressive failure were proposed.Based on SR-CT technology,the three-dimensional morphology of the internal microstructure of the beetle horn was obtained,and its deformation and failure process were continuously tracked and analyzed.A lamellar gradient structure of the beetle horn was observed,namely a dense exocuticle,a gradient porous structure and a crossed lamellar structure of the endocuticle.It was found that both brittle fracture and progressive failure in different internal regions simultaneously existed in the layered.structure,which could effectively adjust the stress distribution and failure mode in different regions.This structure can achieve the effective combination of lightweight,load-bearing,and energy absorption.These results may provide valuable optimization strategies for the design of layered composites.2.The lightweight mechanical regulation mechanisms of gradient structure in wheat straw were revealed:the inner layer porous structure for toughing,the outer layer fiber structure for load-bearing,the middle layer "porous+fiber"composite structure for coordinating and transitioning.In situ observation and mechanism analysis of the internal complex gradient microstructural deformation and fracture behavior of wheat straw during the loading process were carried out by the SR-CT technology.Some important deformation and failure evolution phenomena were observed,such as multiple initiations of cracks in the inner layer,crack propagation blocked by fibers,cracks jumped in the middle layer,and deflection and final fracture of cracks in the outer layer.The mechanical behavior caused by the gradient distribution of the internal microstructure,and the interlayer structure which coordinated the mechanical properties of the overall structure,were the essential causes of these deformations and failure.These results have significance for understanding the lightweight design principle of wheat straw and obtaining the optimal design strategy for the microstructure of thin-walled materials.3.The mechanical healing mechanisms of microcapsule self-healing materials under the three-dimension stress field,microcapsule geometric factors and internal defects were explored.The internal microstructure evolution and damage healing process of the rapid and chemical self-healing material of amine-diisocyanate double-microcapsule during loading were investigated by SR-CT technology.Some important internal microstructural evolution phenomena were observed,such as the rupture of microcapsules,the propagation of cracks,the flow and fusion of the repair agent during loading process.The stress field gradient distribution,geometric factors and internal defect-induced rupture and healing mechanisms of microcapsule self-healing materials were proposed.Finally,the optimization strategies during the preparation and service of the material were proposed,which could provide useful guidance for preparing the microcapsule self-healing material,designing the internal microstructure and improving the healing performance.4.Based on the comprehensive analysis of the key parameters of the microporous structures in the materials mentioned above,the mechanical characteristics and their optimization strategies in the lightweight of several typical microporous structures were proposed.Based on the experimental results of the materials with microporous structure in the previous chapter,combined with the method of finite element simulation,the geometric parameters,spatial distribution of the microporous structure,the coupling between the microporous structure and other structures,and the relationship between the microporous structure and the microcrack were explored.The optimization strategy of the microporous structure was explored preliminarily.Some theoretical support and guidance for designing the microporous structure were provided.