FEM Simulations On The Three-point Bending Fatigue Of3-D Braided Composite From Unit Cell Approach

The3-D braided composite is a kind of product which has good performance of no suturing and mechanical processing. This structure has high strength and modulus, its damage tolerance and the fracture toughness is also very high. The product can mold complex parts and components. And it can form with the third phase as excellent composite products which have good mechanical properties. The3-D braided composites also own the property of impact resistance, stratification and resistance to cracking and fatigue. These advantages fully overcome shortcomings of traditional fiber reinforced polymer. Therefore, studying the bending fatigue performance of the3-D braided composites is of great significance.The unit cell model that based on the micro structure of the3-D braided composite was established to investigate three-point bending fatigue behavior and damage. In the unit cell model, the composite was simplified into a combination of resins and yarns to define the stiffness matrix and failure evolution of the material. UMAT (FORTRAN user-material) subroutine which based on the unit cell model has been developed and connected with commercial FEM code ABAQUS to calculate the deflection, stiffness degradation and fracture morphology of the composite. The experimental and FEM results of three-point bending fatigue behavior were compared.And the results show that good agreement between experimental and FEM proves the unit cell model and the UMAT subroutine are reasonable for three-point bending fatigue design of the3-D braided composite.The main researches are as follows:(1)3-D braided fabric with carbon fiber tows was designed to manufacture in a tradition loom. Epoxy resin was injected into the3-D braided fabric with Vacuum assisted resin transfer molding (VARTM) technique.(2) All tests were conducted on MTS810.23at a constant velocity of2mm/min. Deflection curve and stiffness degradation curve were recorded. The fracture morphologies were photographed to reveal the fatigue damage mechanisms.(3) Based on unit-cell model of the3-D braided composite, a user-defined materials subroutine was established. UMAT (FORTRAN User-defined Material Subroutine) connected with commercial FEM code ABAQUS to calculate the three-point bending fatigue behavior and damage of the composite. The experimental and FEM results of three-point bending fatigue behavior of the composite were compared. Good agreements between experimental and FEM calculation prove the validity of the unit cell model and the subroutine UMAT.