Characterization For Micro-structure And Compressional Behaviour Of General Fibrous Assemblies

A fibrous assembly is formed by friction and entanglement between fibers which make it more stable and it belongs to typical soft matter.Compressive properties are one of the most important mechanical properties which mainly depend on the structure including packing density,fiber arrangement and interactions.The changes in the structure caused by compression include the overall density and the gradient of density along the compression direction,the reorientation and arrangement of fibers,and the increase of interactions between fibers.When the external force is removed,general fibrous assemblies will recover nonlinearly,and an obvious hysteresis can be observed.The dominant phase is the change in structure in this process.Characterizations of microstructure and its relationship with compressive behavior are challenging work both in the actual measurement and theoretical interpretation.Although the classical theory has been established by Van Wyk,a series of simplified assumptions make theoretical results deviate from the actual situation.Microstructure and compressive behavior of general fibrous assemblies as well as their relationships are studied in this paper.An advanced nondestructive testing technology,Micro-CT,was used to scan general fibrous assemblies.Structural parameters are obtained based on the reconstruction algorithm and volume rendering technology.A software for image processing and fractal dimension calculation named Image Analyzer 1.0 was developed based on C++ language.The numbers of fractal dimension within a two-dimensional fault were between 1 to 2 and a high correlation between area fraction and fractal dimension has been found.Microstructures of general fibrous assemblies are quantitatively analyzed and characterized by scientific visualization calculation including area fraction,pore structure and the orientation of fibers.Fiber area fraction fluctuates slightly along the direction of height and is kept in a small range when fibers packing is relatively uniform.The area fraction increases along the direction of hight after compression.The area fraction of fibers fluctuates along the direction of height and increases after the compression.The degree of non-uniformity increases,that is,the uneven signal is amplified.In a two-dimensional plane,most of the pores exist in the form of opening.Only a small number of closed pores can be found,and the number is increased after compression,mainly because the numbers of contact points between fibers are increased.Local normalization correlation coefficients and the corresponding orientation parameters are calculated based on fiber automatic segmentation model and tracing algorithm.Local tensor information and similar functions are used to extract the center line and estimate fiber length and orientation angles.A new fiber contact geometry model,Box model,is proposed,and the number of fiber to fiber contacts in a fibrous assembly is calculated based on probability.This model is compared with the contact theory of Komori by adding the end to end and side to end contact boundary conditions.Box model is applied to 2D and 3D randomly oriented fibrous assemblies,and four characteristic parameters are separately calculated,namely the average number of contact points on an arbitrary fiber,the number of fiber to fiber contacts in unit length,the mean length between two neighboring contact points on a fiber and the total contact points.For a randomly oriented fibrous assembly,the average number of contact points on a single fiber is related to fiber aspect ratio and volume fraction.Probability of contact was optimized by considering the theory of steric hindrance proposed by Professor Ning Pan.Uniaxial compressional experiments were carried out by the mechanical behavior and the conductive in situ comprehensive measurement system designed by TMT team of Donghua University.The characteristic curves and values related to the compressional and recovery properties were extracted.The compression process can be divided into three parts:near linear region,transitional region and linear increase region of modulus.Fibers are moving and rearranging during compression,and the pressure passes through the contact point between the fibers,and the transfer depends on the time,so the active force is always slightly larger than the passive force.Force-displacement,stress-strain and modulus curves can be used as recognition curves for fibrous assemblies whose single fiber has a wide range of densities.The characteristics of curves are closely related to fiber volume fraction.The eigenvalues in force-volume fraction curves,the initial fiber volume fraction,can well distinguish fibrous assemblies whose single fiber has a wide range of densities.The force-volume fraction curves can still be used as recognition curves for fibrous assemblies whose single fiber has a small range of densities.The difference of volume fraction increases with the increase of force value.The compressional and recovery curves are monotonic and concave,and do not coincide with each other.There are obvious hysteresis and energy loss in each cycle because of viscoelasticity of fibers.During a successive compression-recovery cycle,the compression work and energy loss are constantly attenuated,the recovery work is basically kept unchanged,the recovery rate of work is increasing,and finally a stable state can be reached.Dyeing fibers can be used to observe the movement and morphological changes of the fibers on the surface of fibrous assemblies during compression.The compression process of fibrous assemblies with low packing density was observed.It was found that the displacement mainly comes from the whole shift of the fiber segments between contact points,and the contact points were relative firm.The assembly tended to be a stable elastic system.Density gradient distribution could be observed in an assembly with high packing density.The relationships between compressional properties and structural characteristics were studied.The relationships between pressure and fiber volume fraction were investigated theoretically and experimentally from work and energy.It was found that compressional work was an equation of volume fraction,and the pressure was proportional to the third power of volume fraction.The relationships between the pressure and fiber volume fraction were discussed under the assumptions that the fiber section was circular or non-circular.The measured curves did not satisfy the linear correlations at the beginning of compressions.When the compressions were carried out to a certain stage,the measured curves were linearly correlated,which were related to the initial packing state of fibrous assemblies.