Mesoscale Investigation Of Crack Propagation In Carbon Fiber Reinforced Polymer Based On The Phase Field Method

Carbon fiber-reinforced polymer(CFRP)composites have several advantages such as the light-weight,high strength,high elastic modulus,high corrosion resistance,excellent fatigue resistance,beneficial designability and profitable processability.For these reasons,they are widely used in many engineering fields,such as aircraft,aerospace,transportation and civil engineering etc.However,due to the multi-components,anisotropy and high sensitivity to pre-existing defects and external loading of the CFRP,their damage and fracture mechanisms are very complex.Moreover,the CFRP composites inevitably contain different types of defects and damages such as micro-cracks,micro-voids,matrix cracking and fiber breakage etc.in the course of the material processing,transportation and in service.The defects and damages appear as micro-cracks frequently.The micro-cracks may rapidly propagate and merge,and lead to macro-cracks or catastrophic fracture of the components or structures under external loading.Moreover,the fracture processes are often instantaneous and sudden.Those pose a great threat to the safety of the composite structures.Therefore,it is very important to understand the damage and fracture mechanisms and the failure processes of the CFRP,and to establish the corresponding prediction models.The related research has important scientific significance and engineering application value.In recent years,a lot of research has been carried out in this field at home and abroad.However,the relevant numerical methods have their own advantages and drawbacks,and the developed numerical models are not satisfactory.A widely accepted damage and fracture mechanism and the related failure criteria of the CFRP are still lacking.The phase field method can accurately simulate the complex fracture processes such as the crack initiation,propagation,branching and coalescence ect.,and does not require remeshing,presetting the initial crack,tracking the crack path and the additional criteria.The method has been proven to be an important tool in calculating fracture mechanics.Moreover,the macroscopic damages and fractures are mainly caused by the mesoscopic damages and defects.If we only investigate the damage and fracture mechanism of the CFRP at the macroscopic scale,there will be some deviations.In view of the above situation,the phase field model is established based on the proposed method to analyze the influences of the initial damage,the structural characteristics and the external loading on the damage and fracture behaviors of two-and three-dimensional(2D and 3D)CFRP at the mesoscopic scale.Then,the crack propagation mechanism and the mechanical response of the CFRP are systematically inverstigated.The main works and conclusions of the dissertation are as follows:1.Based on the phase field method,a phase field model is derived and established for simulating the crack propagation in a single phase of 2D composite material.The correctness and accuracy of the model are verified by comparing and analyzing the results of numerical examples such as the single edge notched tensile and shear test,the double-crack compression test and the double edge notched tensile test.Meanwhile,the effects of the load type,the boundary condition and the initial crack location on the crack propagation and the mechanical response of the single-phase composites with initial crack are studied.2.Considering the interaction between matrix crack and hole/inclusion in the composite,the phase field model for simulating the crack propagation of the composite with holes/inclusions is derived and developed.The influences of the size of the hole/inclusion and the elastic constant mismatch between the matrix and the hole/inclusion on the crack propagation are discussed under pure tensile loading.Then,the effects of the shape and location of the hole/inclusion on the crack propagation are investigated respectively.The results show that the hole and the soft inclusion will attract the crack to expand,while the hard inclusion will repel the crack propagation,and the larger the area of the hole/inclusion the more obvious effect of attracting and repelling.The hard inclusion is beneficial to the crack-arrest,and the obvious stress shielding effect can be observed when it is in front of the crack.While,the stress shielding effect will decrease or even disappear when the hard inclusion and the crack are far away from the central axis which is perpendicular to the direction of loading.3.According to the characteristics of the longitudinal and transverse sections of the CFRP composites on the 2D mesoscopic scale,the corresponding phase field models are established.The influences of the location and orientation of the initial crack on the crack propagation characteristics and the mechanical response are detailedly investigated in the composite.The effects of the fiber volume fraction,fiber distribution and fiber orientation on the crack propagation are discussed in the representative volume element(RVE).When the fiber is parallel to the load direction and perpendicular to the direction of the initial crack,the crack will propagate perpendicular to the fiber.If there is an intact fiber in the direction of crack propagation,the fiber will break first,and then the matrix around the fiber will begin to fracture.However,if the fiber is not perpendicular to the load direction,the crack initiation usually starts near the interface of fiber and matrix,and then propagate along the fiber direction.The critical failure displacement of the RVE increases with the increase of the angle ? of the fiber.4.The 3D meso-scale phase field model of the CFRP to simulate the crack propagation is derived and established.The influences of the fiber volume fraction,fiber distribution and fiber orientation on the crack propagation of the CFRP are investigated.The results indicate that the stiffness of the unit cell increases with the increase of the fiber volume fraction,but the force and the critical failure displacement decrease,and the effect of the initial crack on the crack propagation process is also reduced.In the case of hexagonal fiber arrangements,the RVE can bear the maximum force.While,the force is smaller and the critical failure displacement is the largest for the random fiber arrangement.