An Investigation Of Multi-scale Damage Mechanical Behaviors On Short Fiber Reinforced Composites

Different from continuous fiber reinforced composites,the mesoscopic failure mode in short fiber reinforced composites present a high degree of uncertainty.At present,it is hardly to reveal their failure mechanisms by employing the theoretical mechanical models,which always ignored the local stress influences.A new modeling scheme is urgently needed to effectively solve the mechanical behaviors of short fiber reinforced composites with full consideration of their meso-structure characteristics.To this end,a high-precision Computed Tomography(CT)characterization technology is proposed to effectively extract the mesoscopic features of the composites by combining data reconstruction algorithms with image processing technology.On this basis,a novel multi-scale modeling strategy is presented to reveal the mesosocpic damage to macroscopic fracture for the short fiberreinforced composites,which represents a great scientific theoretical significance and engineering application value.In this paper,the experimental methods and numerical simulations are both employed to investigate the relationship between the macro-crack propagation and the meso-failure modes.Moreover,the proposed multiscale damage mechanical model is verified by the in-situ CT tensile test.The parametric element is established,and a three-dimensional Finite-Volume Direct Averaging Micromechanics(FVDAM)with a high calculation accuracy is presented.The orientation and length of fiber geometrical information are firstly characterized by CT technology.According to the distribution characteristic of short fibers,a simplified ellipsoidal inclusion model is presented by evaluating the three-axis ratio relationship with respective to the spatial orientation and length of the short fibers.Compared with experimental data,numerical results showed an obvious inconsistency.This is attributed to the ignorance of the pore defects during the numerical modeling.To improve the predicting accuracy of the mesoscopic mechanical model,a two-step strategy is proposed with full consideration of the internal pore defects to evaluate the anisotropic elastic modulus.According to the volume fractions of pore defects and short fibers,the equivalent calculation is performed on the first step by employing the three-dimensional simplified FVDAM.By defining the obtained anisotropic mechanical parameters to the spherical inclusion phase on the second step,the modified model is verified by comparing with the experimental data.The proposed two-step modeling method represented the high computational efficiency and accuracy.To investigate their nonlinear mechanical behavior,the Bodner-Partom unified visco-plastic constitutive model is used to describe the nonlinear response derived from the matrix.By comparing with the test results,the accuracy of the model is validated.To further reveal the fiber failure mode,a modified Kelly model is presented to evaluate the critical fiber length with consideration of the inevitable pore defects.Utilizing the ex-situ CT tensile test to characterize the fiber length change around the fracture surface,the fiber critical length acquired by theoretical method is verified.Furthermore,regarding the mesoscopic failure modes of short fiber reinforced composites,CT and scanning electron microscopy are used to observe the fiber fracture,interface debonding,matrix cracking and pore damage.The local stress field influences on mesoscopic failure behavior and the synergistic coupling effect between different failure mechanisms are analyzed,which lay a foundation for establishing the multi-scale damage model.A multi-scale damage model is presented by combining with the framework of FEM(Finite element method)and the FVDAM.For the undamaged region,the simplified mechanical model is called through Gauss integration points in the elements to simulate the nonlinear mechanical response of the composites.For the fracture area,CT technology and the postprocessing Avizo software are used to obtain the meso-geometric morphology of internal constituents.Combining with the tracking algorithm of the fiber centerline,a parameterized modeling is conducted on the dispersed short fibers.After the Boolean operation,the whole damage mechanical model is constructed,and the damage progress from the mesoscopic failure to the macro-crack propagation can be revealed.The influences of mesoscopic characteristics of short fibers on interface degradation,plastic evolution of matrix and macroscopic crack growth path are also analyzed.