Study of changes in 3D-woven multilayer interlock fabric preforms while forming

Multilayer woven reinforcements are increasingly employed in the domain of composite materials. Delamination occurrence and resultant failure of a laminated composite piece subjected to high vibrations, is an issue of much concern in aeronautics. The situation becomes more complex in case of bended/curved pieces. In order to improve through the thickness mechanical properties, 3D-woven multilayer interlock fabric is used as composite reinforcement. Structural changes, i.e. thickness change, relative slippage of layers, change in tow aspect ratio and change of orientation of the tows columns, etc. occur in such fabrics during the forming process. These changes may lead to the gradient of the resin amount in composite, internal stresses and variations of mechanical properties in the piece. No significant research has been conducted on this aspect. Lack of knowledge or neglecting these changes may lead to prejudicial estimations of ultimate mechanical properties and fracture analysis. In the present article, the changes that occurred in 5-layer and 13-layer 3D-woven multilayer interlock fabrics have been studied, when moulded at five different angles and two different bending radii. A significant change in thickness, tow aspect ratio, tow orientation and relative layer slippage is observed.