A new two-step modeling strategy for random micro-fiber reinforced composites with consideration of primary pores

The presence of primary pore defects inevitably introduced during material manufacturing is non-trivial in evaluating the mechanical properties of composite materials. The pore defects also increase damage uncertainty evolution in composites. Micro-fiber reinforced composites are widely used in bulk molding compounds and composite sheet molding due to their excellent toughness properties. To this end, there has been considerable interest in enhancing their mechanical properties and expanding their application scope. Therefore, considering the effects of primary pore defects when evaluating the mechanical properties of these composites and their implications is worth further investigating.

Existing studies on pore defects can be grouped into two main categories. The first category concerns a wide range of pore sizes synchronously existing in composites. The second one is about studying the mechanical properties of composites at a single geometric scale with a special focus on morphology and pore defect location. Accurate microstructural parameters derived from experimental tests is an important requirement for developing reliable numerical models. Although several models have been proposed to investigate the properties of micro-fiber reinforced composites, most of them are inaccurate due to several deficiencies. Moreover, most do not consider the effects of primary pores.

Among the existing experimental methods for characterizing the three-dimensional (3D) micro-scale morphology of composites, micro-computed tomography (micro-CT) is widely used. Whereas it offers high-precision microstructural parameters and is generally considered effective, it is more time-consuming, especially in obtaining scanning data. With the known effects of pore defects like significant reduction of stiffness, developing reliable and accurate models for studying the properties of micro-fiber reinforced composites is highly desirable.

Herein, a team of Xidian University researchers: Dr. Heng Cai, Professor Junjie Ye, Mr. Jinwang Shi, Dr. Yiwei Wang and Dr. Yang Shi in collaboration with Dr. Bo Huang, Mr. Yonghe Xu, Professor Mohamed Saafi and Professor Jianqiao Ye from Lancaster University developed a new method for evaluating the mechanical properties of random micro-fiber reinforced composites considering the effects of primary pores. First, the 3D micro-scale morphology of the constituent materials making up the composite was explored and detected via micro-CT experiment to enhance the accuracy of the results. Second, a two-step modeling strategy was proposed to predict the nonlinear deformation and effective moduli of micro-fiber reinforced composites. Their work is currently published in the journal, Composites Science and Technology.

The authors demonstrated the capability of the proposed strategy in evaluating the nonlinear stress-strain relations and elastic modulus of micro-fiber reinforced composites. The distribution and morphology of the pores were observed to have a close relationship with the direction of the modeling pressure and matrix flow during the manufacturing process. However, only a few pores were observed on the lower and upper surfaces of the composites. Interestingly, the fibers and the pore defects at the same geometrical scale could be considered during the numerical modeling. Moreover, the spherical analysis showed the isotropic nature of the primary pores. The model was verified by comparing the experimental tests and numerical results. The experimental results agreed well with the numerical results.

In summary, a more reliable, effective and accurate two-step modeling strategy for modeling the properties of micro-fiber reinforced composites considering the effects of primary pores formed during processing was reported. Besides considering the primary pores, it successfully overcame the difficulties associated with existing methods. In a statement to Advances in Engineering, Professor Junjie Ye, the corresponding author stated that their study provided valuable insights that would provide an in-depth understanding of the influence of pores on the mechanical properties of composites.