A stiffness prediction model for threedimensional four directional composites based on mesoscopic structure

The interior unit cell, surface unit cell and corner unit cell of threedimensional four directional composites were established in mesoscopic scale, and the models considered the changes of the yarn cross section and mutual extrusion. Applying the corresponding periodic boundary conditions, and using the stiffness volume average method, the threedimensional four directional composites stiffness prediction models were established. The longitudinal elasticity modulus of trimming and nontrimming threedimensional four directional carbon/carbon composites were predicted. The results show that the predicted values of 8, 13, 16, 21mm width threedimensional four directional carbon/carbon composites and experimental values have 183%, 58%, 42% and 5% errors,respectively. The prediction results have a higher precision when widening the specimens, and the three unit cells model has good precision in longitudinal elasticity modulus which is much better than that of interior unit cell method when the width of specimens is over 13mm.