Abstract:

Objective In order to explore the pore distribution and change of the C/C soft-hard blended preform in the initial and final stages of the compaction process, the problem that the porosity can not be predicted at the completion of the preform was set and research focus.

Method A 3-D four-direction pore model was established based on the C/C soft-hard weaving preform process. The pores from the xoy plane and xoz plane of the unit model in two dimensions and four directions were studied with and without compression load. The influences of fiber shape and cross section shape of the pore of the unit under the load were studied from mesoscopic and microscopic scales. The mapping relationship between the change of fiber size and porosity was solved by using the geometric model and the size coefficient affecting the porosity was proposed.

Results The minimum porosity of the unit preform in the final compaction stage was found to be 26.1% by using the 3-D four-direction preform unit pore model and parameters (Tab. 1). A universal tensile testing machine was used to perform compaction and densification experiments on the preforms of different sizes. Resin curing was performed on the preform under different compression loads, before the pore morphologies were observed. The results show that the compaction process of the preform consists of three stages, i.e. initial linear, nonlinear and final linear. In the initial linear stage, the height of the preform was compressed rapidly by small pressure, the load increases and the height decreases slowly in the nonlinear stage, and in the final linear stage, the load was increased while the height of the preform does not change. The porosity in the compaction process was obtained using the weighing method, and the mapping relationship between the height and porosity of the preform was obtained. The compression load and height variation curve of the three groups of preform with different bottom areas and the same height was showed, and the compression load of the three groups of experiments approximately increased by multiple times (Fig. 10(a)). The porosity decreased with the decrease of the height of the preform (Fig. 10(b)). When the height was kept constant, the porosity of the preform reached the minimum value of 27.4%. The compression load and height variation curve of three groups of preform with different heights and the same base area was reveal, and the compression load of the three groups of experiments was approximately equal (Fig. 11(a)). The minimum porosity of the preform at the compaction and densification stage was 27.9%, 28.4% and 28.7%, respectively (Fig. 11(b)). The experimental results show that the maximum error between the theoretical model and the actual minimum porosity was 2.6%.

Conclusion The experimental results verify the feasibility and correctness of the 3-D four-direction C/C soft-hard blended preform unit pores, which were observed and virtually constructed in two dimensions and four directions of xoy plane and xoz plane. At mesoscopic and microscopic scales, the influences of fiber shape and cross section shape on the porosity of the preform under load were studied. The mapping relationship between fiber size change and porosity was established by geometric modeling, and the size coefficient affecting the porosity was proposed to make the final porosity of the preform designable. At the same time, the compaction and densification process law of the 3-D four-direction preform was revealed, The results show that the final linear stage determines the porosity of the preform. The modeling method can also be applied to the pore model of the preform fabric of the integrated piercing fabric and the 3-D orthogonal fabric, providing theoretical guidance for the regulation and prediction of the final porosity of the preform fabric.

Key words: C/C soft-hard weaving preform, multi-dimension, unit porous model, compaction, porosity