Mechanical Properties Of Mo Fiber Reinforced Granitoid Composite For Machine Tool

During the past several years, structure design and control system development of machine tool has grown more matured in China, but most of base components are still made of cast iron or welded steel. Although cast iron or welded steel has the advantage of fledged machining technique and good processability, the preparation period is longer and environmental pollution in production is very serious, which can not conform to the development trend of green manufacturing. And its damping property is close to the limit, therefore dynamic performance and thermal stability of cast iron or welded steel can not satisfy the manufacturing technology requirement of high-speed and high-precision. As a result, it is imminent to develop new materials with excellent damping performance.By using natural granite as aggregate and organic resin as adhesive, granitoid composite (GC) is a new polymer-based multiphase material solidified by polyreaction, which is mainly applied to manufacture base components of high-speed and high-precision machine tools, laser, electron or medical equipments for its outstanding characteristics such as high vibration damping, small thermal expansion coefficient and good corrosion resistance.However, mechanical strength of GC is far less than that of metal materials such as gray cast iron and steel, limiting its popularization and wide application to some extent. As the reinforced phase, molybdenum (Mo) fibers are added to GC in the paper. With the combination of theoretical analysis, experimental research and numerical simulation, take Mo fiber and resin matrix as the research system, the improvement of compression and flexural strength of GC as the research objective to systematically study interfacial stress transfer and distribution mechanisms of the composite in different force conditions. Furthermore, the influence of matrix property, fiber property and fiber microstructure characteristics on meso and macro mechanical performance of the composite is analyzed in-depth, developing Mo fiber reinforced granitoid composite (MFRGC) with excellent properties. Taking precision engraving and milling machine bed as the example to carry out finite element simulation, static and dynamic performance of the bed made of GC and cast iron are comparatively studied, verifying the superiority of GC manufacturing machine base components and the feasibility of replacing cast iron.(1) Based on classic shear lag theory, micromechanical model of Mo fiber in embedded and drawn states is established, and the calculation formula of fiber axial stress and interfacial shear stress is deduced when the composite is in different force conditions. The effect of meso-structure parameters of Mo fiber on axial elastic modulus of the composite is discussed. The influence of embedded length and volume fraction of fiber in completely bonding state, the debonding length of fiber in local debonding state on the distribution of fiber average axial stress and interfacial shear stress is analyzed systematically. The analysis results show that axial elastic modulus increases significantly with the increasing fiber volume fraction, and firstly increases rapidly and then increases slowly to a fixed value with the increasing ratio of length to diameter. For fiber in drawn state, interfacial shear stress decreases with increasing embedded length of fiber, and fiber average axial stress increases with increasing volume fraction. When volume ratio increases to a certain extent, the interface debonding phenomenon will occur at embedded end rather than pulling end. The larger debonding length fiber has, the smaller shear stress debonding position can sustain, and interface is more prone to debonding expansion.(2) Standard tensile sample of resin casting body is prepared in the paper to study mechanical properties of solidified polymer with different mass ratio of epoxy resin and curing agent. The influence of different mass ratio on interface bonding strength between Mo fiber and matrix, wettability and mechanical properties of GC is analyzed respectively. The relationship between interface bonding strength and adhesion work is established based on wettability theory. Strain values of typical measuring points on samples of MFRGC under different loads have been obtained by experiments, and finite element analysis is employed to validate the correctness of experimental results. As mass ratio r increases, surface free energy of solidified polymer first increases and then decreases; while contact angle of fiber-matrix first decreases and then increases, and wettability of fiber-matrix is best when r equals to 4. With the increasing mass ratio r, tensile strength, tensile strain and break stress of resin matrix increase at first and then gradually decrease, reaching the maximum and minimum value at r=4 and r=7 respectively. Both interface bonding strength of fiber-matrix and mechanical strength of GC first increase and then decrease as mass ratio increases, with the best mechanical performance at r=4.(3) Based on different surface treatment process of fiber, changes and differences of structure and properties for modified Mo fiber are comparatively analyzed. The influence of fiber surface roughness and surface properties on interface bonding strength, wettability and mechanical strength of GC is studied systematically too. The relationship equation between interface bonding strength and adhesion work is established, which can be quantitatively characterized by Boltzmann distribution function. Finally, numerical simulation is employed to comparatively analyze the reinforcing effect of new and scrap Mo fiber on resin matrix in strong and weak interface bonding state separately. For surface modified Mo fiber, surface free energy, interface bonding strength and the reinforcing effect on GC show varying degrees of improvement compared with new Mo fiber. Wherein, comprehensive mechanical properties of GC are optimal with the addition of fibers which have undergone combined surface treatment including acidification, gas-phase oxidation and coupling treatment. And interface bonding strength of matrix with scrap Mo fiber is increased by 16.7% on average than that of the new one. Finite element analysis results show that for the same interface bonding state, the ability of scrap Mo fiber to resist deformation of matrix and withstand load is better than the new one; and for the same surface state of fiber, matrix property in strong bonding state is superior to that in weak one.(4) Pull-out theory model of special-shaped fiber is established according to its force condition, and a general calculation equation for maximum pulling force of various special fibers shaped like English letters. Single fiber pullout experiment has been carried out to obtain the influence of fiber shape on interface bonding strength between Mo fiber and matrix, thereby analyzing and comparing pulling and debonding process of ordinary linear fiber and special-shaped fiber according to the output load-displacement curves. The effect of fiber content and fiber shape on mechanical strength of GC is studied systematically and the reinforcement of matrix with special-shaped fiber has been comparatively analyzed by finite element simulation. Experimental results show that interface bonding strength between special-shaped fiber and matrix is clearly greater than that of ordinary linear fiber. By contrast, for special-shaped fibers, the sequence of reinforcing effect on GC is M-shaped followed by N-shaped, U-shaped the third, and V-shaped the minimum. With the increase of fiber content, compression and flexural strength of MFRGC first increase and then decrease. When fiber content occupies 1.2% of total mass, mechanical strength reaches the maximum value. Finite element analysis results show that M-shaped fiber has the best ability to resist deformation of matrix and withstand load.(5) According to the actual working conditions of precision engraving and milling machine tool, bed design and load analysis have been completed. Deformation and equivalent stress distribution can be obtained based on the established bed finite element model of cast iron and GC. Two structure improvement and optimization schemes for GC bed are proposed according to static simulation results. Modal shapes, natural frequencies and amplitude-frequency response curves of corresponding beds have been acquired by finite element analysis, by which finding methods to avoid resonance. The analysis results show that the deformation of optimized GC bed in X, Y and Z direction and total deformation is significantly less than that of cast iron bed under the same load and constraints, demonstrating better static performance. The initial eight natural frequencies of optimized GC bed have increased by 57.5% on average compared with those of cast iron bed, among which the first and second order modes have a more obvious impact on dynamic characteristics, thereby needing local reinforcement on the fore part of bed accordingly. The maximum amplitude of optimized GC bed in X, Y and Z direction is much smaller than that of cast iron bed, with the average reduction of about 89.1%, showing significantly better dynamic performance than cast iron bed.