Applied Composite Materials 8: 371–384, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. 371 Properties of Weft Knitted Composites Affected by Preform Stretching GORAN DEMBOSKI and GORDANA BOGOEVA-GACEVA Faculty of Technology and Metallurgy, University St. Cyril and Methodius, Rudjer Boskovic 16, 91000 Skopje, Macedonia (Received 8 February 2000; accepted 22 August 2000) Abstract. The influence of preform stretching on mechanical properties of weft knitted PP/glass fibre composites was estimated. Stretching of Rib 1:1 weft knitted fabric was performed prior to consolidation in a wale or course direction and biaxially. Preform stretching increased tensile and bending strength of composite as a result of additional orientation of the reinforcing fibres in the stretching direction. Stretching in a wale direction was shown to be most effective due to the spe- cific inherent structure of Rib 1:1 fabric. The impact toughness of composites was not significantly changed as a result of decreased preform elasticity in the direction of stretching. Key words: glass fibre/polypropylene composites, weft knitted preforms, stretching, mechanical properties. 1. Introduction Application of textile structures from hybrid yarn as semifinished preforms has successfully solved problems associated with high matrix viscosity and reduction of processing steps in the production of thermoplastic composites [1–3]. Woven, braided, nonwoven and warp knitted structures have been most used textile struc- tures in composites application [4]. Weft knitted structures also initiated research interest. There are many advantages that favor using of weft knitted preforms for composite application such as: high productivity of weft knitted technology; easy handling of the preform; convenient for application of deep drawing techniques; high drapeability; possibility of forming double curved structures with no folds; po- tentiality of minimizing waste by producing fully fashioned and integrally knitted 3-D forms [5–7]. However, weft knitted thermoplastic composites possess modest mechanical properties due to a curved structure of reinforcing fibres [8]. A number of authors have investigated the possibility of influencing weft knitted composites mechan- ical properties by variation of textile structure. It was found that plain weft knit structure with floating loops could increase the tensile properties of composites in the course direction. Such a structure provided isotropy in composite properties and the course-to-wale ratio in tensile strength was changed from 0.6 to 1.0 [9]. Several