Study On Fracture Mechanism Of Fiber Reinforced Composite Based On Phase Field Theory

Fiber-reinforced composite materials have the advantages of excellent mechanical properties and strong designability.They are often used as the main material of the bearing structure and are widely used in aerospace and other fields.However,due to its multiple material component,complex reinforced structures,and large spatial scale span,it is difficult to analyze the fracture process and failure mechanism of materials by traditional macroscopic experimental testing,fracture morphology analysis,and macroscopic phenomenological theory.With the development of meticulous modeling methods for materials at the microscopic level,revealing the multi-scale fracture and failure mechanisms of materials from the perspective of virtual experiments has become a hot research topic.In recent years,the phase field theory is considered to be a very potential calculation method for the virtual characterization analysis of complex fracture morphology because it does not require prefabricated crack initiation locations and propagation paths,and does not require additional crack propagation criteria.To this end,this paper develops a phase field method to study the fracture mech anism of fiber-reinforced composites from a micro-meso scale,and establishes the relationship between the crack propagation process and the macro-mechanical response.For needled C/C composites,a typical layered material,a hierarchical multi-scale scheme with a fiber-based representative unit cell is proposed.The fracture calculation results based on the phase field metho d show that the method can capture well the crack propagation path in multiphase materials,providing effective technical support for the deep understanding of the multiscale complex fracture mechanism of fiber reinforced composites.Based on the Griffith energy criterion and the principle of energy minimization,a phase field method to characterize the fracture mode of mu ltiphase composite materials is developed.The control equations and weak forms of the coupling of the phase field and the displacement field are derived.The new 2D and3 D elements coupling node and degree of freedom of the phase field and displacement field are developed.Calculate and analyze typical 2D and 3D calculation examples such as open and mixed crack growth,sub-interface crack growth affected by bimaterial interface,accurately capture crack nucleation and propagation path,and verify the feasibility of fracture analysis in multiphase materials based on phase field theory.The fiber axial direction is an important bearing direction of fiber-reinforced composite materials.Tracking and analyzing crack nucleation and expansion from a microscopic perspective is helpful for in-depth understanding of the bearing mechanism of the material,thereby optimizing the macroscopic strength and toughness of the material.The criterion of the propagation path of the crack meeting the fiber / matrix interface under the far-field tensile load is established.By studying the effect of the crack path on the toughness and fracture morphology,the mechanical behavior of the typical fiber-reinforced composite material under axial tension is analyzed.The axial tensile fracture modes such as multiple matrix cracks of Si C/Si C composite,fiber pull-out and straight fractures of C/C composite are calculated,and the calculation results are in good agreement with the experimental phenomena.The real-time correlation between the micro-microscopic crack growth process and the macroscopic nonlinear mechanical response is established,which provides an important basis for a deep understanding of the material bearing mechanism.The transverse direction of the fiber is the weak link of the unidir ectional plate,and the transverse fracture is often formed first in the laminate material,which in turn induces other forms of damage.From the microscopic scale,under the transverse tensile load,from the interface cracking to the formation of macroscopic transverse cracks,many complex fracture forms are involved.To this end,a phase field method is developed in which the fracture parameters of the interface change with the fracture mode mixity,and the effects of material and structural parameters such as component material properties,interface thickness,fiber radius,and fiber distribution on the interface strength are analyzed.The three-dimensional numerical calculation of the interface crack intruding into the matrix and forming the tunneling crack along the fiber axis is achieved.The calculation results are in good agreement with the in-situ test results.In the multi-fiber model,the calculation of complete fracture behavior such as interface crack nucleation,propagation,deflection intrusion into the matrix,and the formation of macroscopic main cracks in the matrix is achieved,and the obtained crack propagation path is basically consistent with the experimental observation results.Based on the above research,the multi-scale stiffness and fracture mechanism analysis of needled C/C composites with hierarchical characteristics is carried out.A typical unit cell model considering the fiber morphology changes caused by the needling process is established,and a symmetric periodic boundary cond ition suitable for a rotationally symmetric structure is derived.A hierarchical multi-scale stiffness analysis model based on the scale of the fiber monofilament is proposed.The performance is in good agreement with the experimental results.A two-parameter phase field method considering fiber axial and transverse fracture modes is developed,a macroscopic material performance analysis method considering low-scale nonlinear characteristics is established,and a multi-scale phase field model distinguishing fracture modes and considering low-scale nonlinear mechanical responses is proposed.The calculated mechanical response and fracture mode of the needle-punched C/C materials are in good agreement with the experimental phenomena.