Structural Modeling and Perlormance Simulation of Polymer-Mineral Composite
Author:Sheng Pei Yao
Polymer-mineral composite material is a kind of composite in which modified epoxy resin and other polymers are taken as binder,granite and other mineral particles are taken as aggregates,and quartz sand or fly ash are filled as fillers.It has been more and more widely used in the manufacture of machine tool beds for its superior workability,excellent damping characteristics,as well as low thermal conductivity and low thermal expansion coefficient.At mesoscale,it is a typical heterogeneous particle-reinforced composite material,which is composed of aggregates,matrix and the interfacial transition zone between them.The macro-mechanical properties of polymer-mineral composite are influenced by the material properties and meso-structures of components.With the rapid development of computer technology.building effective meso-scale numerical model to research how the components’structures and properties influence the composite’s overall marco-properties is of significant importance to optimize the structure design and assess the security of polymer-mineral composite.The existing meso-scale numerical models as well as the meso-structure and performance characteristics of the components of polymer-mineral composite are investigated deeply in this thesis.To improve modeling efficiency and aggregate volume fraction,an advanced take-and-fall method to build three-dimensional mesoscale numerical model with high aggregate volume fraction is proposed,and the aggregate volume fraction can be well controlled.Based on the characteristics of aggregate shape and preparation methods of specimen,numerical models with aggregates completely surrounded by matrix and aggregates intersecting with specimen boundary are both generated.Considering the influence of interfacial transition zone,the effects of meso-structure and material parameters on the macroscopic properties of composites are revealed.Finally,by building cohesive crack model,the failure process and crack propagation of polymer-mineral composite with different meso-structures are researched.All of these lay a foundation for structure design and property prediction of polymer-mineral composites.The main work and conclusions are listed below:(1)A take-and-fall method is proposed.In this method,loose aggregate model is firstly generated,and then gravitational acceleration is applied to aggregates to make the aggregates fall into the certain space,and finally numerical model with high aggregate volume fraction at meso scale is established.Aggregate volume fraction higher than 50%can be obtained by this method.What’s more,in the case that aggregate volume fractions are 10%and 15%,the take-and-fall method saves 56.9%and 90.91%of the time compared to the traditional take-and-fall method,which indicates that with the improvement of aggregate volume fraction,the superiority in efficiency of the take-and-fall method is more obviously.The minimum gap between aggregates can be controlled by the element shell thickness to guarantee the thickness of matrix surrounding aggregates.A fitting curve and a fitting equation are established to describe the relationship between the shell thickness and the final aggregate volume fraction under the circumstances that fuller curve is used to describe the aggregate size distribution and the maximum aggregate size is 16 mm.Based on the casting method and sawing method for the preparation of specimens in the engineering practice,element deletion method and geometric cutting method of aggregate particles at the model boundary are proposed respectively.Numerical models with aggregates completely surrounded by matrix and aggregates intersecting with specimen boundary are both generated.It is found out by calculation that with the same aggregate grading,the aggregate volume fraction of the latter is about 10%higher than the former,and when the volume fraction is both 49%,the elastic modulus of the latter is about 2.96%higher than that of the former,indicating that the model in which aggregates intersect the specimen boundary is superior to the case that the aggregate is completely wrapped by the matrix in terms of volume fraction and elastic modulus.Based on response surface analysis(RSM),the influence of elastic modulus of aggregate and matrix on macroscopic elastic modulus of materials is discussed.According to the simulated optimized response surface,the elastic modulus of components required to obtain certain properties of composites can be analyzed.(2)According to the geometric features of aggregate particles in engineering practice,new algorithms for generating convex and non-convex particle models are proposed.Judgments of interferences between aggregates are further simplified,and when the aggregate volume fraction is 40%,the calculation time can be effectively saved by 38.54%.The aggregate arrangements are described by the nearest neighbor distribution,and it is found that when the final volume fraction is 25%,the peak value of the nearest neighbor probability density of the aggregate generated by the take-and-fall method is slightly higher than that of the traditional random take-and-place method(about 0.08),which proves that aggregates generated by the former is more evenly distributed.The nearest neighbor distributions of aggregate models generated by take-and-place and take-and-fall methods are basically the same.The heredity of aggregate distribution is verified by analyzing the distribution of coarse and fine aggregates in the loose model and the high-density model in cases that the aggregates are uniformly and nonuniformly distributed.By tracking the falling paths of aggregates and observing the high-density model after falling process,the evolution phenomenon of aggregate interlocks between non-convex aggregates can be observed,which is consistent with the engineering practice.(3)The modeling method of interfacial transition zone between mineral aggregates and polymer matrix is investigated.The effects of thickness,elastic modulus of interfacial transition zone,as well as volume fraction and size of aggregate particles on elastic modulus of polymer-mineral composites are studied by univariate method.The results turns out that,when other variables are constant the elastic modulus of the polymer-mineral composites is inversely proportional to the thickness of the interfacial transition zone,and directly proportional to the elastic modulus of the interfacial transition zone and the volume fraction of the aggregate particles,while the aggregate size has little effect on it.(4)Cohesive crack models for polymer-mineral composite with different aggregate shapes are established,and cohesive elements are pre-inserted into matrix,aggregates and the interface between them to simulate the fracture behavior and crack evolution of the composite.The results show that the more roundness of the aggregate particle shapes,the lower the probability of stress concentration,and the higher of the resulted fracture strength.The tensile strength of polymer-mineral composite decreases with the increase of aggregate volume fraction.When the tensile strength and fracture energy of the interface cohesive elements are 12.5%and 25%of the matrix,failure only occurs in the interfacial transition zone and the matrix.With the improvement of the fracture properties of the interface cohesive elements,aggregates are also broken.When the fracture properties of the interfacial transition zone are same with that of the matrix,the crack pattern becomes relatively smooth.Both the pre-peak stage and the peak stress of macroscopic stress-displacement response are affected by the tensile strength of the matrix cohesive element,while the fracture energy does not affect the pre-peak stage.The improvement of the tensile strength of the matrix cohesive element and the fracture energy can both improve the overall tensile strength of the material.