The role of friction in tow mechanics

Abstract

Friction plays and important role in the processing of fibrous materials: during production of tow materials, during textile manufacturing and during preforming operations for composite moulding processes. One of the poorly understood phenomena in these processes is the dynamic frictional behaviour of the fibrous tows. This thesis addresses the characterisation of this behaviour during the production of Continuous Fibre Reinforced Polymers (CFRPs). The dynamic frictional behaviour of fibrous tows is studied in a multi-scale context. Both experimental and modelling work were performed to gain insight into the mechanism of fibrous tow friction. Experimental work on carbon, aramid and E-glass tow material provides the relevant parameters that form the basis for contact mechanics modelling approaches of tow friction for tows on metal counterfaces and tows in direct contact with each other. The model is based on the assumption that frictional forces are determined by the the product of the real contact area between the contacting bodies and an interfacial shear strength. The multi-scale nature of friction clearlymanifests itself in the analytical-statisticalmodelling approaches. Surface characteristics down to the sub-microscopic level of asperities on themetal counterfaces and small ridges on carbon filaments were taken into consideration for the determination of the real contact areas. The bottom-up approach of the tow-metal and tow-tow modelling procedures provides the means to predict the experimentally observed frictional behaviour of tows. Finally, this study provides a coupling with the macroscopic ply or laminate scale by linking friction measurements on carbon fabric to those on carbon tows. The main conclusion of this study is that the sub-microscopic friction at the level of contacting asperities and filament ridges determines the final macroscopic behaviour of dry arrangements of fibrous tows to a large extent. This study provides a coupling between the multiple length scales. As such, it forms a basis for further modelling and experimental work.